Parviflorus -­‐ UP906 Loudspeaker Design Christopher Trevino 1/5/2015 FA4740 Christopher Plummer 1 Table of Contents Functional Description ............................................................................................................ 4 Visual Design .......................................................................................................................... 5 Technical Goals ...................................................................................................................... 6 SPL Output ........................................................................................................................................ 8 Speaker Bandwidth ......................................................................................................................... 10 Driver Size and Spacing ................................................................................................................... 11 Directivity ....................................................................................................................................... 11 Technical Details ................................................................................................................... 12 Baffle Step ...................................................................................................................................... 12 Construction Materials ................................................................................................................... 13 Diffraction and Speaker Shape ........................................................................................................ 13 Driver Analysis & Selection .................................................................................................... 14 Tweeter Analysis ............................................................................................................................ 14 Tweeter 1: SEAS Excel T25CF-­‐001 (E0006) Tweeter ........................................................................... 15 Tweeter 2: ScanSpeak Classic D2008/8512 20 mm Dome Tweeter ................................................... 16 Tweeter 3: Audax TW025A28 1" Gold Dome Tweeter ....................................................................... 17 Tweeter 4: SB Acoustics SB26ADC-­‐C000-­‐4 Aluminum Dome Tweeter ............................................... 18 Tweeter 5: Fostex FT28D 28 mm Dome Tweeter ............................................................................... 19 Tweeter 6: Morel MDT30S 1" Textile Dome Tweeter ........................................................................ 20 Tweeter 7: Eton 25SD-­‐1, 1" Soft Dome Tweeter ................................................................................ 21 Tweeter 8: Fountek NeoX 2.0 Ribbon Tweeter with Round Silver Faceplate .................................... 22 Woofer Analysis .............................................................................................................................. 23 Woofer 1: Peerless 835026 8" Aluminum Cone HDS Woofer ............................................................ 24 Woofer 2: Hivi d88-­‐8” Poly Bass Midrange Shielded ......................................................................... 25 Woofer 3: SEAS Prestige CA22RNX (H1288) 8" Coated Paper Cone Woofer ..................................... 26 Woofer 4: Seas Prestige 8" Woofer CD22RN4X H1192 ...................................................................... 27 Woofer 5: SB Acoustics SB23NACS45-­‐8, 8" Aluminum Cone Woofer ................................................ 28 Woofer 6: Morel MW266 8" Woofer Polymer Coated Paper Cone ................................................... 29 Woofer 7: SEAS Excel W22EX-­‐001 (E0022) 8" magnesium Cone Woofer .......................................... 31 Woofer 8: Tang Band W8-­‐2022 8" RBM Subwoofer 8 Ohm ............................................................... 32 Driver Selection .............................................................................................................................. 33 Crossover Design ................................................................................................................... 34 Initial Design ................................................................................................................................... 34 Testing & Tuning ................................................................................................................... 34 2 Initial Performance and Tuning ....................................................................................................... 34 Enclosure Optimization ................................................................................................................... 39 Final System Documentation ................................................................................................. 41 Anomalies from Measurement Microphone ................................................................................... 41 Final Measurements ....................................................................................................................... 45 Bibliography .......................................................................................................................... 51 Appendix A: Drafting Documentation .................................................................................... 53 3 Functional Description The Parviflorus UP906 (UP906) will be the primary monitors in either a professional or home studio setting. They will be used as mid-­‐field mixing monitors with portability being a moderate to low concern. To cater towards my unknown future living circumstances, each loudspeaker should be able to be carried by one person up five flights of stairs and should be transportable by a standard sedan or SUV. The speakers will be placed in a medium or small-­‐sized room, likely between 8’x10’ to 10’x15’, that has moderate to sparse acoustic treatment and a standard ceiling height. “Listening backwards”, as David Moulton would put it, or faithfully reproducing sounds is more important than translatability to other systems for the UP906s.1 These will act as the primary monitors in my studio, with cheaper consumer monitors to give an A-­‐B, forward-­‐listening perspective. Creating a good sense of stereo depth and width, without over-­‐exaggeration, is more important to me than frequency extension into the low end. Because these speakers will be used for mixing, accurate transients and a flat frequency response are essential. When I am not using these speakers for professional purposes, these speakers will allow me to enjoy material on a reference system. The UP906s will need to produce THX standard output, in order to share content with clients at professional standards. As a result of this, they must be able to produce 85dB/1m with a crest factor of 20dB. The speakers will cater towards a single primary listener (sound engineer) with there sometimes being secondary listeners (clients), which makes high SPL output beyond the mixing position a low priority. The UP906s will have low listener fatigue to enable them to be used for prolonged periods of time, both professionally and for personal enjoyment. The UP906s will have a sleek, classic look that should draw a little attention to them upon first sight but not seem out of place in a professional setting. These speakers should look and sound gorgeous. The system is being designed to use MiniDSP’s PWR-­‐ICE 125 2-­‐channel plate amplifier and digital signal processor in an active 2-­‐way system. I prefer to use digital instead of analog SPL components because of time efficiency in tuning Low End the system. Digital components also do not have Extensio
unknown inconsistencies and do not degrade over n time—unlike their analog counterparts. Size/
Speaker PrioriXzaXon Weight 1
David Moulton, Total Recording, KIQ Productions, 2000, 313. 4 Visual Design I want these speakers to both sound and look great. I chose to go with a two-­‐tone approach, similar to Paul Kirby’s Aeternus Lux speakers2, with the front baffle being a different color than the rest of the enclosure. A cherry wood front baffle compliments the silver aluminum woofer and solid black tweeter that I have chosen. As the cherry wood matures, the front baffle will take on the glow of a ripened thimbleberry—the speaker’s namesake. The rest of the enclosure will be finished with a clear shellac to attract people towards the front baffle while still highlighting the grain of the Baltic birch. Paul Kirby’s Aeternus Lux Loudspeakers 2
AreteAudio, Tech Loudspeakers, https://areteaudio.wordpress.com/2013/05/11/aeternus-­‐lux-­‐2-­‐2-­‐modular-­‐
system/ Accessed: Dec. 16, 2014. 5 Technical Goals To achieve the best transient response, the UP906s will be a sealed enclosure with a Q of .7. At a Q of .7, a sealed box does not color bass frequencies and keeps transients clear.3 A reflex enclosure gives better bass extension than a sealed box, but at the cost of frequencies associated with the port ringing out over time.4 Clear transients that do not “boom” over time are more important for the UP906s to have than the lower bass extension and small box size that reflex enclosures afford. Bass extension is still important, though. I’ve chosen to use an 8” woofer instead of smaller counterparts in order to achieve a lower bass extension in the UP906s. Attaining a low Q value requires a larger volume of air inside of the speaker enclosure. Q value is also dependent upon the woofer being used. To get a .7 Q with the woofers I’ve chosen, loudspeaker cabinet modeling software Winspeakerz shows that I need a minimum internal volume of 1.25 cubic feet. The UP-­‐906s final internal dimensions will be 11 7/8” x 8” x 24.5” (LxWxH), or 1.35 cubic feet, in order to achieve the .7 Q. Extra acoustic space was factored in to accommodate for the braces and acoustic damping material. The internal acoustic dimensions closely match the golden ratio which, when it is applied to loudspeaker enclosures, is said to decrease the likelihood of dominant standing waves from forming.5 Dominant standing waves create colorations in the frequency response, which is generally unwanted. I modeled the UP906s internal dimensions off of the golden ratio in hopes of achieving a flatter frequency response through preventing dominant standing waves from forming. MiniDSP’s PWR-­‐ICE 125 2-­‐channel plate amplifier and digital signal processor enable to me digitally crossover my drivers. This limits my system to either being a purely active 2-­‐way or an active/passive hybrid wherein I actively crossover between a low woofer and the mid woofer, and then use a passive crossover between the mid woofer and tweeter. I want to avoid using analog crossover components because of their inconsistencies and tendency to degrade over time. In order to have an all-­‐digital crossover, the UP906s will need to be an active 2-­‐way system. A digital crossover enables me to have greater flexibility and control in my system, which meets my functional goal of having these speakers be adaptable. The UP906s will be used in rooms that have standard ceiling heights with moderate to no treatment on the walls and floors with similar acoustic issues. The speakers’ vertical dispersion will 3
Philip Newell & Keith Holland, Loudspeakers for Music Recording and Reproduction, Focal Press, 2007, 68. 4
Philip Newell & Keith Holland, Loudspeakers for Music Recording and Reproduction, Focal Press, 2007, 327. 5
John L. Murphy, Introduction to Loudspeaker Design, True Audio, 1998, 88.
6 therefore be minimized to reduce ceiling and floor reflections. Their horizontal dispersion will be moderate so as to give a broad apparent source width while still maintaining some spatial definiteness. 6 The UP906’s tweeter will be offset from center to break up diffraction off of the front baffle. This should lead to flatter frequency response, since it prevents a single frequency from being highlighted by baffle diffraction7. The corners of the front baffle will also be rounded to help further decrease the effect of baffle diffraction. 6
Floyd E. Toole, Sound Reproduction: The Acoustics and Pschyoacoustics of Loudspeakers & Rooms, Focal Press, 2008, 96. 7
Philip Newell & Keith Holland, Loudspeakers for Music Recording and Reproduction, Focal Press, 2007, 92. 7 SPL Output In order to determine what the ideal SPL output for the UP906s would be, I performed some research to find out my own personal listening preferences. Over the course of several weeks, I observed and recorded the SPL output of when I typically listened to music at an ambient level, at a mix level and what my maximum comfortable listening level was in the morning, afternoon and evening. It is important to check listening preferences at different times of day because our ears naturally become more fatigued as we go about our day. Therefore, we could reasonably expect to listen at lower SPLs earlier in the day than we do in the evening. Max Comfortable Evening Mixing Akernoon Morning Ambient Listening 60 63 66 69 72 75 78 81 Upon doing personal research, I found that I tend to mix between 75-­‐80dB but would like my system to be able to handle mixing to THX standards so that I can share material with clients by a professional standard. The UP906s will therefore need to produce a minimum of 85dB at one meter from the loudspeaker with a crest factor of 20db8. Most listening will occur between one and two meters from the loudspeakers, which would make it unnecessary for a louder baseline SPL at one meter from the loudspeaker. We can derive from the inverse square law that the SPL output two meters from the loudspeaker would be 79db, with a crest factor of 20db. This is still well within my own mixing preferences and, although it does not fit THX standards, it is something I am still comfortable with. Since I am using the Mini DSP PWR-­‐ICE125, I can expect the amplifier to add 20dBW to my system. I therefore need to use a speaker that has a minimum sensitivity of 85db/1m to achieve my goal of attaining THX standards. The loudspeaker needs to be able to continually function for at least 8 hours in any climate found in the continental United States, which would make a medium to high-­‐efficiency system ideal. A larger enclosure size will decrease the strain upon the speaker drivers, thus lowering the wattage and 8
Digital Domain, Level Practices (Part 2) (Includes K-­‐System). http://www.digido.com/how-­‐to-­‐make-­‐better-­‐
recordings-­‐part-­‐2.htmlAccessed January 4, 2015. 8 thermal output from the speakers and creating a higher-­‐efficiency system than a smaller enclosure would. This will allow the speakers to operate for prolonged periods of time without issue.9 9
David Moulton, Total Recording, KIQ Productions, 2000, 209. 9 Speaker Bandwidth The desired 50hz-­‐20,000hz bandwidth of the UP906s was determined through performing research of personal listening preferences. I listened to a variety of genres of music on a system that reproduces 30hz-­‐20,000hz with +-­‐1.3db variation while routing through an EQ with aggressive high pass filters at 5 hertz increments starting at 35hz and going up to 80hz. Then, I charted out when I perceived the high pass filter, the point at which I thought the high pass filter affected my enjoyment of the music, and the point that I considered to be the compromise between the two. These subjective measurements are useful to discover individual listening preferences. I found that I loved the low end of some songs at 40hz, but was satisfied with 50-­‐60hz with the majority of the music that I listened to. Size, which is the biggest limiting factor for bass extension in sealed enclosures, was not a major issue in this design. By creating a larger enclosure, I was able to surpass my goal and almost achieve my ideal bandwidth. The UP906s extend to an F3 of 43hz. 10 Driver Size and Spacing Initially, I played with the idea of using two smaller 5.5” drivers in parallel, stacked horizontally on top of each other. This design would have increased the efficiency of the system by taking strain off each driver as well as improving the horizontal off-­‐axis response of the UP906s. In addition to this, it would have created what I consider a better visual aesthetic. I am not usually a fan of large speakers that consist solely of one large woofer and a tweeter because there are peculiar proportions of empty space left on the front baffle. Using two small drivers with a sealed enclosure, though, was both more expensive than using one large driver and didn’t give me as low of an F3 as I wanted to have. Despite my initial reticence, I decided that using an 8” woofer/tweeter two-­‐way would be the best route. An 8” woofer in a sealed enclosure allowed me to get a lower F3 than it’s smaller counterparts. A vented enclosure would have allowed me to reach my goal F3 with smaller woofers but preserving the transient response of my speakers was more important to me. The tweeter is offset from the woofer to break up diffraction caused by high frequencies bouncing off of the front baffle. From center, the woofer and tweeter are placed 8 inches apart from each other to decrease the time delay between each driver. Directivity The UP-­‐906s should have good axial response and cater towards one or a few listeners at a time. Given that the speakers will be used in a variety of treated and untreated space, it would be ideal to either have a tighter dispersion to lessen the effect of early reflections in the room or have a non-­‐
colored off-­‐axis response to decrease overall coloration from the room. 11 Technical Details Baffle Step The baffle step, or the point at which frequencies being outputted by a speaker begin acting in a 2π (finite) space instead of a 4π (free/infinite) space, is dependent upon the width and shape of a speaker’s front baffle. Factoring in for baffle step is important because sound behaves differently in finite and free space. Drivers producing frequencies in free space, below the baffle step, have a 6dB loss in efficiency.10 Since I chose to create a rectangular front baffle, the baffle step is easy to calculate. Martin J. King provides the following formula to determine the baffle step: f3 = 4560 / WB
where
WB = width of the baffle in inches
My front baffle is 11 inches wide, which puts my baffle step around 410hz. I chose to compensate for the baffle step by placing a high shelf filter in my woofer, starting around 400hz and going up to the crossover point, and then decreasing the output of my tweeter by 2dB. In a quasi-­‐anechoic space, this leads to a frequency response shown on the graph below. Once placed in a physical space, frequencies below the baffle step will be boosted because of room acoustics. This could lead to issues, depending upon how much of a boost occurs, but the MiniDSP software would allow me to compensate for this later on. Frequency Response of Right UP-­‐906 in a quasi-­‐anechoic space. 10
Martin J. King, “Simple Sizing of the Components in a Baffle Step Correction Circuit” 2005, 7/23/05 Revision, th
www.quarter-­‐wave.com/General/BSC_Sizing.pdf, Accessed December 27 , 2014. 12 Construction Materials The UP906s feature a triple layering of materials in its cabinet construction. From external to internal material, the front baffle is made of 7/8” cherry wood, a layer of 1/8” mass-­‐loaded vinyl and then a layer of ½” MDF. The rest of the enclosure has the same construction, except that it uses ¾” 13-­‐
ply Baltic birch instead of cherry wood. The cabinet has a false back to house the MiniDSP plate amplifier as well as two braces in the front part of the enclosure to increase rigidity. The braces are offset from the thirds to prevent dominant standing waves from forming11; the braces have also been cut to decrease weight and increase airspace within the box. Every edge of the box, except for the top plate, is made with 90-­‐degree angles to increase the rigidity and strength of the box. The top plate was made with 45-­‐degree angles as an aesthetic choice to hide the plywood layers. Drafting documentation for the UP906s is included in Appendix A. The triple-­‐layering of the cabinet’s construction dampens the box more effectively than a single material construction would. Each material absorbs and reflects sound waves differently. Within the enclosure, black hole patches and ½” pink insulation have been added, as per personal preference, to give extra damping and increase the perceived box size through absorption of sound waves. Diffraction and Speaker Shape Initially, I wanted to design a diamond shaped speaker but decided to go with a standard rectangular box for ease of construction. Diamond shaped enclosures are far better acoustically because they increase the distance travelled and reflections necessary for sound to escape the enclosure. Rectangular enclosures are much simpler to construct, though they do have some disadvantages. Rectangular speaker enclosures have a distinct baffle step, or diffraction off of the edges of the front baffle. Rounding the edges of the front baffle will lessen the effect of the diffraction. Rectangular enclosures also have a shorter and simpler path to exit the enclosure than their diamond-­‐
shaped counterparts. Keeping the box shape simple was important to me because I knew that the other complexities in my box construction would require quite some time to do well. 11
George Short, North Creek Cabinet Handbook, Old Forge: North Creek Music Systems, 1992, 12. 13 Driver Analysis & Selection Tweeter Analysis The two most important factors for me in choosing the ideal tweeter were frequency response and the low-­‐frequency roll-­‐off for crossing over. Since the UP-­‐906s will be reference-­‐mixing speakers, achieving an accurate frequency response is one of the most important goals in both of my drivers. Off-­‐
axis response and break-­‐up frequencies are also important considerations. 14 Tweeter 1: SEAS Excel T25CF-­‐001 (E0006) Tweeter Specifications: Price: $122.50 Sensitivity: 91db 2.83v/1m F(s): 700Hz Recommended Crossover: 1000Hz Weight: 2.42 lbs Frequency Abnormalities: 2000Hz-­‐25000Hz +-­‐2db Notes: The price point and anomalies in the on and off-axis
frequency response made this driver look unappealing to me. The
tweeter has a great natural roll-off, though, which would make it
crossover easily with my woofer.
15 Tweeter 2: ScanSpeak Classic D2008/8512 20 mm Dome Tweeter Specifications: Price: $72.40 Sensitivity: 88 db 2.83v/1m F(s): 800Hz Recommended Crossover: 1200Hz Weight: 1.2 lbs Frequency Abnormalities: +2.5db from 2000Hz to 20,000Hz Notes: The design of this tweeter creates sharp off-­‐axis roll-­‐
off shortly above 12,000Hz, which would be useful for creating tight dispersion. The anomalies in the frequency response, though, make this a less than ideal candidate. I like that the tweeter is relatively protected and the price point is agreeable.
16 Tweeter 3: Audax TW025A28 1" Gold Dome Tweeter Specifications: Price: $115.75 Sensitivity: N/A F(s): 1180Hz Recommended Crossover: 1500Hz Weight: 1.5 lbs Frequency Abnormalities: Climbs 6db from 2000Hz to 20,000Hz Notes: This dome tweeter has a cool look to it, but the smoothing on its frequency chart, as well as its frequency anomalies and price, are all turn offs. I don’t feel that I could create a naturally flat response system with such an aggressive climb in sensitivity over its high frequencies. I also was not able to find the overall sensitivity of the driver, which is also disconcerting. 17 Tweeter 4: SB Acoustics SB26ADC-­‐C000-­‐4 Aluminum Dome Tweeter
Specifications: Price: $52.90 Sensitivity: 90db 2.83v/1m F(s): 680Hz Recommended Crossover: 1400-­‐3000Hz Weight: 1.65lbs Frequency Abnormalities: Major frequency break-­‐up just shy of 20,000hz Notes: Overall, this tweeter is a nice option. There are no major colorations in its frequency response and it has a modest price. The break-­‐up frequency may be of some concern, though it doesn’t affect audible frequencies on-­‐axis. This could be a problem if the system were to be placed in a room where the reflections off-­‐axis were pronounced. I like that the tweeter is protected from unwanted poking.
18 Tweeter 5: Fostex FT28D 28 mm Dome Tweeter Specifications: Price: $78.40 Sensitivity: 90db 1w/1m F(s): 1000Hz Recommended Crossover: 2000-­‐2500Hz Weight: 1.54 lbs Frequency Abnormalities: Off-­‐axis very irregular Notes: This driver appears to have a moderately flat frequency response on-­‐axis from 2000Hz to 20,000Hz. The manufacturer liberally used smoothing in their specification sheet, though, which always makes me leery. The off-­‐axis response is much lower, though irregular. Again, much like Tweeter 4, a speaker using this driver may find issues with odd colorations from off-­‐axis. I plan on creating a near/mid-­‐field set of monitors, so off-­‐axis colorations are of some concern to me. The high F(s) would force me to cross over above 2000Hz, which could limit my choice in woofers.
19 Tweeter 6: Morel MDT30S 1" Textile Dome Tweeter Specifications: Price: $77.70 Sensitivity: 90db 1w/1m F(s): 650Hz Recommended Crossover: 1000hz-­‐3000Hz Weight: 1.76 lbs Frequency Abnormalities: Anomalies between 1000-­‐2000Hz; dip at 15,000Hz Notes: This tweeter has a moderately flat frequency response, though the dip at 15,000Hz is somewhat concerning. This fabric dome tweeter doesn’t have an erratic break-­‐up frequency, something which is common in metal drivers. The off-­‐axis response is pretty even as well.
20 Tweeter 7: Eton 25SD-­‐1, 1" Soft Dome Tweeter Specifications: Price: $87.15 Sensitivity: 90db 1w/1m F(s): 1000Hz Recommended Crossover: 1500-­‐3000Hz Weight: 2.2 lbs Frequency Abnormalities: 3db rise from 10,000-­‐20,000Hz Notes: This tweeter has a moderately flat frequency response with +-­‐2db from 1000Hz-­‐10,000Hz. The 3db rise from 10,000-­‐20,000Hz is something that I think DSP could manage. The off-­‐axis response from this tweeter is well controlled. The high F(s) could limit my choice in woofer, since it would require me to crossover at a higher frequency.
21 Tweeter 8: Fountek NeoX 2.0 Ribbon Tweeter with Round Silver Faceplate Specifications: Price: $108.50 (on sale at $73.00 at time of writing) Sensitivity: 94db 2.83v/1m F(s): N/A Recommended Crossover: 2500-­‐3000Hz Weight: 2.25 lbs Frequency Abnormalities: 3db rise between 8,000-­‐30,000Hz Notes: Although I didn’t think that a ribbon tweeter would be ideal for the system I wanted, I at least wanted to look at one to consider how it might work in my system. Ribbon tweeters tend to have a characteristic high-­‐
end which would undermine my desire for a flat frequency response reference system. This tweeter has about +-­‐2.5 frequency response full-­‐
range, which isn’t ideal. It is very loud, which would dispel any fear of reaching my target SPL levels.
22 Woofer Analysis There were four things that were important in my woofer selection: the driver’s F3, the enclosure size needed to achieve a Q of .7 in a 2nd order sealed box, frequency response, and high-­‐end roll-­‐off for crossing over. My goal with the UP-­‐906s was to have the flattest frequency response possible while also extending the bass as low as I could in an enclosure that could be comfortably carried by a single person. 23 Woofer 1: Peerless 835026 8" Aluminum Cone HDS Woofer Specifications: Price: $95.67 Sensitivity: 86.8 db 2.63v/1M Break-­‐up Starts at: 5000Hz Break-­‐up Peak Amplitude: 98db Recommended Crossover: 1000-­‐2000Hz Weight: 5.4 lbs. Other Abnormalities: Notes: This woofer has a great low end, with an f3 of 53hz.
The box size, at 2.3 cubic feet, is right around where I’d like
it to be. The woofer features a natural 1st order roll-off at
1,000hz and has a break-up frequency around 5000hz. It
has a lower power rating, though, which means it couldn’t
get as loud.
Speaker Modeling in 2nd Order Closed Box 24 Woofer 2: Hivi d88-­‐8” Poly Bass Midrange Shielded Specifications: Price: $199 Sensitivity: 86db 1w/1m Break-­‐up Starts at: 2000 Hz Break-­‐up Peak Amplitude: 6dB Recommended Crossover: 500-­‐1000Hz Weight: 16.3 lbs Frequency Abnormalities: 6db dip at 1Khz with a large Q. Notes: This speaker offers an F3 of 53hz while also having a small box volume of .63 cubic feet. The natural 2nd
order filter at 2000hz would help with crossing over, though the large dip in frequency response at 1000hz is
disconcerting leads me to think that it would be more prudent to start crossing over at 500-1000hz. These speakers
are very heavy as well as two times more expensive than most other options, which makes me shy away from tor my
final choice. This would work well in a 3-way system, not my 2-way one.
Speaker Modeling in 2nd Order Closed Box 25 Woofer 3: SEAS Prestige CA22RNX (H1288) 8" Coated Paper Cone Woofer Specifications: Price: $103.80 Sensitivity: 89.5db 2.83v/1M Break-­‐up Starts at: 3000Hz Break-­‐up Peak Amplitude: 6dB Recommended Crossover: 1000-­‐2000Hz Weight: 4.5 lb Frequency Abnormalities: Notes: This speaker drives down to an F3 of 50hz in a 1.7 cubic foot
box. It has a good, non-colored off-axis response and is just overall a solid driver. There’s an upward slope in
frequency going up to the break up frequency, but in a completely manageable way. The enclosure size may be a bit
large for what I’d like, but overall I like this driver because it has a tame break-up frequency.
nd
Speaker Modeling in 2 Order Closed Box 26 Woofer 4: Seas Prestige 8" Woofer CD22RN4X H1192 Specifications: Price: $120.40 Sensitivity: 87dB 2.83v/1m Break-­‐up Starts at: 3000hz Break-­‐up Peak Amplitude: 6dB Recommended Crossover: 500-­‐2000Hz Weight: 4.9 lbs Frequency Abnormalities: There is an odd 6dB level drop at 600hz and then a 3dB bump at 1200hz. This was likely designed for a 3-­‐way system. Notes:
This driver has an F3 of 60hz, which is higher than most of the other drivers I’ve been looking at. On the plus side, to have a Q of 7, the required enclosure size is only .5 cubic feet. Given the frequency response anomalies in this driver, I feel that this driver wouldn’t work with the 2-­‐way active system I plan on building. Speaker Modeling in 2nd Order Closed Box 27 Woofer 5: SB Acoustics SB23NACS45-­‐8, 8" Aluminum Cone Woofer Specifications: Price: $129.80 Sensitivity: 87.5db 2.83V/1m Break-­‐up Starts at: 2000Hz Break-­‐up Peak Amplitude: 10dB Recommended Crossover: 1000-­‐2000Hz Weight: 6 lbs Frequency Abnormalities: The break-­‐up frequency is particularly rough, especially off-­‐axis (as is characteristic of metal cones) Notes:
This driver has an F3 of 49hz in a 1.25 cubic foot box, which is the second lowest out of all of the drivers I’ve
looked at. I really like this driver because it accomplishes my goal of bass extension while also maintaining a
reasonably small size box. It has a lower power handling, though, which makes it have decreased potential SPL
output. Even with that, though, it would be able to operate for sustained periods at 105db at mix position.
Speaker Modeling in 2nd Order Closed Box 28 Woofer 6: Morel MW266 8" Woofer Polymer Coated Paper Cone Specifications: Price: $132.20 Sensitivity: 89 1w/1m Break-­‐up Starts at: 1000Hz Break-­‐up Peak Amplitude: 6db Recommended Crossover: 500Hz Weight: 3 lbs Frequency Abnormalities: Notes: This driver is the only driver I found that could reach my target in bass extension. When mounted in a 5
cubic foot box, this driver has an F3 of 39hz. Unfortunately, that enclosure size is much too large for what I want to
design. Even when sacrificing the Q factor of the system to .9, this driver still requires too massive of an enclosure
for it to work with my other design goals. These drivers would work phenomenally as subwoofers in a reflex
enclosure.
Speaker Modeling in 2nd Order Closed Box -­‐ .7 Q 29 Speaker Modeling in 2nd Order Closed Box -­‐ .9 Q 30 Woofer 7: SEAS Excel W22EX-­‐001 (E0022) 8" magnesium Cone Woofer Specifications: Price: $275.30 Sensitivity: 88dB 2.83v/1m Break-­‐up Starts at: 3000Hz Break-­‐up Peak Amplitude: 14dB Recommended Crossover: 1000-­‐2000Hz Weight: 4.5 lbs Frequency Abnormalities: 2nd order roll-­‐off between Notes:
This driver is prohibitively expensive, but I wanted to include it as a point reference with the other drivers I was looking at. In 1.05 cubic foot sealed enclosure, it attains an F3 of 52Hz. With a consistent first order roll-­‐off from 200hz to 1000hz and a second order roll-­‐off 1500hz to 3000hz, this driver is very friendly to work with in three-­‐way systems. Unfortunately, the low-­‐end roll-­‐off is not desirable for my design. Speaker Modeling in 2nd Order Closed Box 31 Woofer 8: Tang Band W8-­‐2022 8" RBM Subwoofer 8 Ohm Specifications: Price: $134.00 Sensitivity: 83db Break-­‐up Starts at: 2,000Hz Break-­‐up Peak Amplitude: Negligible Recommended Crossover: 1000-­‐1500Hz Weight: 16.3 lb Frequency Abnormalities: Notes: This driver accomplishes the lowest F3, 48hz, and second smallest enclosure size (.6 cubic feet) out of all of
the drivers I’ve looked at. Its frequency response is relatively flat and it has a very gentle break-up frequency. The
one downside is its weight. Clocking in at 16.3 lbs, this driver is massive. The driver’s frequency response is +-2.5
db from 50hz-2500hz
Speaker Modeling in 2nd Order Closed Box 32 Driver Selection My final choices for drivers were: Tweeter 4: SB Acoustics SB26ADC-­‐C000-­‐4 Aluminum Dome Tweeter Woofer 5: SB Acoustics SB23NACS45-­‐8, 8" Aluminum Cone Woofer These SB Acoustics drivers crossed over between each other well. Each had good frequency responses and sensitivities that would help me achieve my goal SPL output of 85dB with a crest factor of 20db. The SB Acoustics SB23NACS45-­‐8 woofer offered one of the lowest F3s in my target size enclosure, 43hz with a required 1.25 cubic feet to attain a .7 Q. Although there is a bad break-­‐up frequency on the woofer starting at 2,000kHz, the on-­‐axis response is well handled and the break-­‐up frequency doesn’t become a major problem until frequencies higher than I plan on crossing over at. The SB Acoustics SB26ADC-­‐C000-­‐4 tweeter has a break-­‐up mode that becomes noticeable right around 20,000Hz. This isn’t an issue though, since the Mini-­‐DSPs have a natural high-­‐end roll-­‐off starting around 16,000Hz. Aesthetics was also an important factor in choosing these drivers. The black and silver finishes of the drivers work well with the detailed grain and dark body of the oiled cherry wood I used on the front baffle. The three support struts on the tweeter’s cover has strong angles that draws attention towards the tweeters. Whereas, the large silver cone on the woofer adds a contrasting accent to the front baffle. By using Tung oil, I was able to bring out the natural grain of the cherry wood and bring a gorgeous finish to the rest of the front baffle. 33 Crossover Design Initial Design My initial crossover design had both the woofer and tweeter crossing over at 1500Hz, with 2nd Order Linquist-­‐Riley crossovers. This seemed to be a good choice, given how the two drivers naturally roll off. I wanted to use 2nd order Linquist-­‐Riley filters to keep my crossover as simple and symmetric as possible to avoid phase issues and other problems that are inherent in more complicated crossover designs. The MiniDSP PWR-­‐ICE125’s digital crossover allowed me to make fast changes to my design, so I was not overly concerned in having a precise initial crossover design. Testing & Tuning The initial testing was done with a Rational Acoustics RTA-­‐420 reference microphone approximately 2½’ feet from the speaker. The microphone was centered on the gap between the woofer and tweeter. The speaker itself was placed in a quasi-­‐anechoic space on a wooden box approximately four feet off the ground. Four 2’x4’x4” acoustic pads made from a combination of insulation and luan supports with a transonic fabric cover were stacked on the floor directly between the microphone and speaker. Please note that the RTA-­‐420 has a distinct increased sensitivity between 3,000Hz and 20,000HZ. Calibration files for the RTA-­‐420 were unavailable for initial testing. Initial Performance and Tuning The first crossover (Figure 1) had both drivers crossing over at 1500Hz. The 6dB drop around 1200Hz was not something I anticipated, as neither driver had a significant response drop at that point. This led me to experiment with time-­‐aligning my driver to see if that would correct the drop. Figure 1: Initial Crossover; Tweeter/Woofer 1500hz 2nd Order LR Crossover 34 Figures 2 and 3 show my first tests in time-­‐aligning the drivers. I started by delaying the woofer by .1ms, which produced an even greater drop in frequency at 1200Hz. In Figure 4, I did the same .1ms time delay on the tweeter. This delay completely eliminated the response drop. Figure 2: .1ms Delay on Woofer; Tweeter/Woofer 1500Hz 2nd Order LR Crossover Figure 3: .1ms Delay on Tweeter; Tweeter/Woofer 1500Hz 2nd Order LR Crossover 35 After time-­‐aligning the drivers, I noticed that there was a 4dB gain in the system starting at 500Hz. This led me to experimenting with a high-­‐shelf filter on the woofer as well as lowering the overall level of the tweeter. At this stage, I settled on a -­‐3dB high-­‐shelf filter starting at 500Hz on the woofer and dropped the level of the tweeter by -­‐2dB. Figure 4 shows the frequency response after these adjustments. Figure 4: Woofer High Shelf Filter Adjustment/ Tweeter Level Adjust; Tweeter/Woofer 1500Hz 2nd Order LR Crossover Figure 4’s frequency response was close to what I wanted. Except for a couple points, the system shows a +-­‐2dB deviation from 80Hz to 20,000Hz. I thought that the frequency response anomalies at 200Hz and 300Hz were a result of the floor bounce from the test conditions or were box resonances from having a non-­‐damped box. Either way, I decided to program this crossover setting into both speakers and listen to music through them. My first impression of the system was that it was harsh. Metal cone drivers tend to be harsh when they are first used, so I ran the pink noise through the system at 80-­‐85dB for 8 continuous hours on two different nights, as well as listened to several hours of music from different genres, in an attempt to break them in.12 After these break in sessions, I listened to the speakers again and thought that they were a bit honky in the mid-­‐high frequencies. Honky-­‐ness of this sort can be caused by the tweeter being strained by producing too low of frequencies13. My next step in tuning the system, then, was to adjust the crossover frequencies of my drivers to try to eliminate this honky-­‐ness. 12
Christopher Plummer, Lecture, Michigan Technological University, Houghton, MI, December 4, 2014. Christopher Plummer, Lecture, Michigan Technological University, Houghton, MI, December 4, 2014. 13
36 Figure 5 shows my first attempt in adjusting the crossover frequencies. I adjusted the tweeter to crossover at 2000Hz in this example, while still maintaining 1500Hz woofer crossover and 2nd order Linquist-­‐Riley filters. This created a big cancellation at 1500Hz. I tried to correct this by crossing over both drivers at 2000HZ, shown in Figure 6. By keeping the drivers crossing over as close in frequency as possible, I hoped that there would be greater summation than cancellation. Figure 5: Tweeter 2000Hz; Woofer 1500Hz 2nd Order LR Crossover Figure 6: Tweeter/Woofer 2000Hz 2nd Order LR Crossover 37 The cancellation was not as pronounced when both drivers crossed over at 2000Hz, but it was still noticeable. This led me to try crossing the woofer over at 2500Hz, shown in Figure 7. This created a frequency response comparable to what I had achieved in Figure 4. I decided to drop the woofer’s high shelf filter to -­‐5dB to try to correct for the slight gain in the woofer at 500Hz and up (Figure 8). This configuration accomplished the frequency response I hoped for. Figure 7: Tweeter 2000Hz; Woofer 2500Hz 2nd Order LR Crossover Figure 8: Woofer High Shelf Filter Adjust to -­‐5dB; Tweeter 2000hz; Woofer 2500Hz, 2nd Order LR Crossover 38 Enclosure Optimization The UP906s are intended to be a reference monitor system without exaggeration in frequency response or in spatial imaging. My intent for the final dampening adjustments to the enclosure was to maintain as much accuracy in the system while giving the speakers a slight breath of space. To accomplish this, I used a combination of ½” pink insulation and 1” black hole foam. To describe the function of these materials in simple terms, insulation acts to increase the perceived spatial depth of speakers while decreasing the discrete spatial accuracy; converts acoustic energy into heat and doesn’t decrease the enclosure’s internal acoustic space. Black hole foam dampens box resonances and can work to create less exaggerated spatiality and a more “true” reproduction of sound at the cost of potentially sounding “surgical”; converts a broader range of acoustic energy into heat, as a result of a more complex construction, but decreases an enclosure’s internal acoustic space because of its density.14 At all steps of this process, I left one speaker empty of damping materials, as a control, and then changed the materials in the other speaker. To keep as many factors constant as possible throughout the optimization process, I also used a single playlist of uncompressed music from different genres. Each song in the playlist highlighted a different characteristic of the speakers, such as accuracy and depth of sound staging, distortion, and reproduction of natural and digital reverbs. The UP-­‐906’s enclosure has two braces that separate the internal space into three areas; I will refer to these areas as the top, middle and bottom chambers. There are holes cut into the braces, which make each of these chambers a unified whole acoustically. There were different considerations for each chamber, though, which is why I will refer to them separately. I was most concerned about the middle chamber, which houses the woofer, because that is the origin point of all of the acoustic energy entering the enclosure; my tweeter is sealed and doesn’t contribute any substantial acoustic energy into the enclosure. I started my optimization process by going to the extremes in using insulation and black hole foam, so that I could understand what each material did. For this first test, I filled the enclosure 50% with black hole, concentrating predominantly on lining the back wall of the cabinet as well as the sides of the middle chamber directly behind the woofer. My thought was that, by deadening as many of the direct and early reflections emitted backwards into the box, I could reduce frequency coloration and weaken box resonances. This test produced the anticipated “surgical” sound of clear response and narrower sound stage while also drastically decreased the working output of the speakers. Next, I filled the enclosure 50% with insulation. I loosely packed insulation into the top and bottom chambers and created a double-­‐thick wall of insulation on the back wall. In the middle chamber, I lined the sides and a made an arc behind the woofer. This yielded better spatial depth while only slightly affecting spatial accuracy. I had expected this test to create a more drastic change than it had. 14
th
Christopher Plummer, Lecture, Michigan Technological University, Houghton, MI, October 27 , 2014. 39 The end result was a slightly exaggerated sound stage but was still close to how I wanted the speakers to sound. After these baseline tests, I began to mix the two materials. For the first mix, I used 30% insulation and 10% black hole foam. I followed the same methodology for the insulation as I had done before, loosely packing the top and bottom chambers while creating an arc in the middle chamber, but used less insulation. Instead of a double layer insulation on the back wall, I placed a 4-­‐inch strip of black hole foam there that went the full height of the box. I also put a 4-­‐inch strip in the top chamber, on the sidewall opposite of the tweeter, and another strip on the opposite side of the bottom chamber. In the middle chamber, I ran 4-­‐inch strips along both sides. This created a balance between accuracy and spatial depth. My first mixture test sounded good enough that I packed my control speaker the same way and then began fine-­‐tuning material ratios. In the end, I found that the amount of black hole foam was good but that I wanted slightly more insulation in the middle chamber. 40 Final System Documentation The UP906s have a +-­‐2dB response from 53Hz-­‐20,000Hz, with an average 2% harmonic distortion. There is a 4% distortion spike with a small Q around 220Hz as well as two 3% distortion spikes at 900 and 700Hz. Visual graphs are included at the end of this section. Anomalies from Measurement Microphone All of my final measurements were taken with a Behringer ECM8000 reference microphone. Calibration files were not available for these tests to compensate for anomalies in the microphone’s response. Frequency response charts from the manufacturer show conflicting information. According to the documentation available on Behringer’s website, the microphone is almost +-­‐1dB flat from 20Hz-­‐
20,000Hz, save for three 4-­‐decibel bumps at 5,000Hz, 6,500Hz, and 85000Hz. The same microphone on PartsExpress doesn’t show this anomaly, but rather shows a 2dB bump starting at 2,000Hz, peaking at 7,000Hz and then zeroing off at 20,000Hz. 15
Behringer ECM800 Frequency Response (as posted on PartsExpress) Parviflorus UP9-­‐6 Frequency Response using Behringer ECM8000 15
Behringer ECM8000 Frequency Response Chart, http://www.parts-­‐express.com/pedocs/specs/248-­‐625-­‐
behringer-­‐ecm8000-­‐specifications-­‐44477.pdf, Accessed December 27, 2014. 41 16
Behringer ECM800 Frequency Response (as posted on manufacturer website) Parviflorus UP9-­‐6 Frequency Response using Behringer ECM8000 Upon looking at the frequency response of my speakers, it seems that PartsExpress offers a more accurate response for the microphone. Please note that the frequency response graphs for my speakers show an almost +4dB deviation in the high frequencies, particularly 6,000HZ and up. Upon doing my final calibration, I had a collection of trained listeners judge my speakers, none of whom noticed these discrepancies. A frequency deviation greater than 3dB should have been perceptible by this audience, especially considering the frequency band in question. I also did a comparison frequency response test using an uncalibrated Rational Acoustics RTA-­‐
420 reference microphone. As you can see on the chart on the following page, the RTA-­‐420 has a gradual boost starting at 3,000Hz as well as a major 9dB boost around 13,000Hz. The frequency response plot of the UP906s, when taken with the RTA-­‐420s, doesn’t exhibit as much deviation, which leads me to believe that most of the abnormal deviation above 3,000Hz is as a result of microphone more so than the actual speaker. I chose to use the ECM8000 over the RTS-­‐420 microphone for final testing because the ECM8000 has less coloration in its frequency response. 16
Behringer ECM8000 Frequency Response Chart, http://www.behringer.com/assets/ECM8000_P0118_S_EN.pdf, Accessed December 27th, 2014. 42 Rational Acoustics RTA-­‐420 Microphone Frequency Response17 Parviflorus UP906 Frequency Response using uncalibrated Rational Acoustics RTA-­‐420 Parviflorus UP906 Frequency Response Comparison between ECM8000 (Blue) and RTA-­‐420 (Red) 17
Rational Acoustics RTA-­‐420 Frequency Response Chart. th
http://www.rationalacoustics.com/store/catalog/product/gallery/id/97/image/339/ Accessed December 27 , 2014. 43 Having compared the effects of two different microphones with predictable frequency responses on the UP906s final frequency response, I feel comfortable saying that the actual frequency response of the UP906s is at least 2db lower starting around 3,000Hz. Both of the microphones I used for testing exhibited extra sensitivity in the same range where my graphs show an abnormal frequency response deviation. My suspicion that the microphone’s used for the tests misrepresents the actual frequency response of the UP906s was further confirmed by trained listeners who did not perceive the abnormalities. If all of the above is true, then the UP906s would have a frequency response around +-­‐
2dB from 53Hz to 20,000Hz. 44 Final Measurements Figure 1: Overall Frequency Response Figure 2: Overall Integrated Frequency Response Figure 3: Harmonic Distortion Percentage 45 Figure 4A: Minimum Phase Response of Woofer (CO at 2500Hz) Figure 4B: Minimum Phase Response of Tweeter (CO at 2000Hz) 46 Red: 0 Degrees Green: 15 Degrees Blue: 30 Degrees Yellow: 45 Degrees Purple: 60 Degrees Figure 5: Vertical Frequency Response Red: 0 Degrees Green: 15 Degrees Blue: 30 Degrees Yellow: 45 Degrees Purple: 60 Degrees Figure 6: Horizontal Frequency Response 47 Figure 7: Difference Between L/R Speakers Figure 8: Integrated Step Response Figure 9: Impulse Response 48 Figure 10A: Waterfall Plot View 1 Figure 10B: Waterfall Plot View 2 49 Figure 11A: Woofer Full Frequency Response Figure 11B: Tweeter 500Hz-­‐20,000Hz Frequency Response 50 Bibliography AreteAudio, Tech Loudspeakers, https://areteaudio.wordpress.com/2013/05/11/aeternus-lux-22-modular-system/ Accessed: Dec. 16, 2014.
Behringer, Behringer ECM8000 Frequency Response Chart,
http://www.behringer.com/assets/ECM8000_P0118_S_EN.pdf Accessed: December
27th, 2014.
Vance Dickason, The Loudspeaker Design Cookbook, 7th edition, Audio Amateur Press, 2006.
Digital Domain, Level Practices (Part 2) (Includes K-System). http://www.digido.com/how-tomake-better-recordings-part-2.htmlAccessed January 4, 2015.Bob McCarthy, Sound
Systems: Design and Optimization, Focal Press, 2007.
John Eargle, The Loudspeaker Handbook, Kluwer Academic Publishers, 2003.
Klippel, Loudspeaker Nonlinearities,
http://www.klippel.de/uploads/media/Loudspeaker_Nonlinearities%E2%80%93Causes_
Parameters_Symptoms_01.pdf, Accessed: December 29th, 2014.
David Moulton, Total Recording, KIQ Productions, 2000.
John L. Murphy, Introduction to Loudspeaker Design. True Audio, 1998.
Philip Newell & Keith Holland, Loudspeakers for Music Recording and Reproduction, Focal
Press, 2007.
Neumann, Harmonic Distortion, http://www.neumann-kh-line.com/neumannkh/glossary.nsf/root/F77C48111116FFBDC12578B20039968C?Open&term=THD+N,
Accessed December 27th, 2014.
Parts Express, Behringer ECM8000 Frequency Response Chart, http://www.partsexpress.com/pedocs/specs/248-625-behringer-ecm8000-specifications-44477.pdf
Accessed December 27, 2014.
Rational Acoustics RTA-420 Frequency Response Chart,
http://www.rationalacoustics.com/store/catalog/product/gallery/id/97/image/339/,
Accessed December 27th, 2014.
Serene Audio, Harmonic Distortion, http://www.sereneaudio.com/harmonic_distortion, Accessed
December 27th, 2014.
George Short, North Creek Cabinet Handbook, Old Forge: North Creek Music Systems, 1992.
51 Floyd E. Toole, Sound Reproduction: The Acoustics and Pschyoacoustics of Loudspeakers &
Rooms, Focal Press, 2008.
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