RTA Reference Data
Purpose
The purpose of this paper is to provide comparative acoustic measurements between
SmaartLive, and the real time analysis (RTA) and auto-EQ functions of the DriveRack
260 (DR260) under various conditions. Data is collected to explore several issues
through the measurements. These issues include the repeatability of the data, effects of
microphone position, the effects of the room and a comparison to an FFT based analysis
program. This data will be gathered through four measurements.
1. Repeatability measurement of the DR 260 auto-EQ
2. Direct on axis speaker frequency response measurement in close range (near
field)
3. Direct on axis speaker frequency response measurement at greater distance
(far field)
4. Frequency response with the measurement microphone on the floor
Multiple speakers are not considered since there are so many combinations of issues that
could be measured. These include, on-axis, off-axis, centered, off center, with and
without subs, different distances between subs/LF drivers, crossover slopes, etc. All of
these issues become important and the combination is just too demanding of the number
of measurements to get timely and valuable data.
The setup
In the interest of having a familiar set of working parameters, some very common and
relatively inexpensive components were chosen for testing. The test data is taken with
the following equipment.
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Mackie SRM 450 speaker(s)
Audix TR-40 reference microphone
DriveRack 260
Crest XR-20 Mixer
Data collected by computer (Dell Inspiron 8200) through a Sound Devices USB
Pre 1.5
SmaartLive v.5.4
All audio cable is Canare L-2T2S and hand terminated by me
The DriveRack PA was not used in this test for three reasons. The first is that the limited
amount of time available to take the data prohibited testing more than one device. The
second is based on the assumption that the auto-EQ algorithms are the same. The final
reason is that the DR260 has the ability to show the RTA data in real time.
The entire procedure takes place in a 3 car garage that is approximately 35’x16’x8 ½’.
The garage doors were closed during the measurements. The division between the LF
and HF driver on the speaker was approximately 5 feet from the concrete floor. The
speaker was also approximately 8 feet (roughly equal between the two walls) from one
wall and 12 feet (about 1/3 of the way into the space) from the other.
Pink noise is used as the measurement signal. That signal was generated by the device
doing the measuring and was approximately 95 dB SPL, A weighted at the measurement
microphone for each of the measurements.
All PEQ, compression, limiting and other features and functions were bypassed or set to
very high thresholds as to not have an effect on the data. To allow for a minimum of 6dB
crest factor for the pink noise and still have ample headroom, an input level at least 12dB
below clipping into the DriveRack was used. Crossover filters were disabled so that the
signal was, essentially, unaltered through the DR 260 except for the auto-EQ effects
where appropriate for test data.
The Data
The results of the tests are shown here without discussion. The purpose is to simply
present some information that can be used for further discussion by the group. Data is
data. In no way was any data manipulation done to change the outcome. Every effort
has been made to NOT make this a scientific study. It is, instead, a “close to real life”
scenario that has quirks and anomalies. The expectations that some of us are mixing in
“perfect spaces” is not assumed, likely or even possible. Bottom line-this is real life
stuff.
Test 1. Repeatability of the DR 260 auto-EQ function.
The purpose of this test is to determine how repeatable the DR 260 auto-EQ is. For this
test, the concern is not with how accurate the data is. That is, whether the result actually
produces a “flat” EQ for the speaker is not important. That premise will be tested in Test
2. This is a measurement of precision, or repeatability only. The question is “does the
DR 260 auto-EQ create the same solution every time under identical conditions?”
With the measurement microphone approximately four feet from the speaker and directly
between the HF and LF drivers in height, the DR 260 was set to auto-EQ the speaker.
The parameters were high precision and a flat EQ curve, Response 0, as the target.
Below are the screen shots of three different trials of auto-EQ. The measurement
microphone and speaker were not moved for each of the trials.
Figure 1.1. Results of first auto-EQ
Figure 1.2. Results of second auto-EQ
Figure 1.3. Results of third auto-EQ
Test 2. Direct on-axis frequency response measurement at close range (near field)
The first purpose of this measurement is to use various methods to establish a frequency
response baseline of our speaker. Essentially, data is from four different views of the
same speaker’s frequency response. Since the measurement is taken at a distance of 4
feet, the proximity will mitigate the effects of reflections from other surfaces in the room.
The direct signal should be stronger than the first reflected signal by a considerable
amount. All of these measurements are taken after the completion and application of the
last auto-EQ from above.
This measurement also compares the
information provided through three
different measurement systems. The first
(Figure 2.1) shows the screen from the
RTA of the DR 260. Essentially, it is a
1/3 octave RTA using the reference level
set when adjusting the gain control as the
“zero” line for measurement.
Figure 2.1. On-axis, near field, frequency
response based on the screen in the DR
260 RTA
The second measurement system is the RTA provided with SmaartLive. There are two
versions shown. The first (Figure 2.2) is the 1/3 octave screen for a direct comparison to
Figure 2.2. On-axis, near field, frequency response screen shot of SmaartLive’s 1/3
octave RTA
the DR 260 RTA, the second (Figure 2.3) is a 1/24 octave screen to have a second point
of comparison with RTAs in general.
Figure 2.3. On-axis, near field, frequency response screen shot of SmaartLive’s 1/24
octave RTA
The third measurement system is the SmaartLive transfer function. While much more
data is available through the SmaartLive program to determine the validity and problems
Figure 2.4. On-axis, near field, frequency response screen shot of SmaartLive’s
transfer function. Phase data and coherency is also indicated.
associated with the data shown, this is a sample screen shot used for reference (Figure
2.4).
The third function of this measurement is to examine the accuracy of the auto-EQ based
on the results of running the auto-EQ function in Test1. The final screenshot, Figure 1.3,
is the EQ applied to determine the readings in Test 2.
Test 3. Direct on-axis frequency response measurement at greater distance (far field)
This measurement is designed to reveal the effects of the room on the frequency response
measurement. It is a direct comparison between the RTA in the DR 260 (Figure 3.1) and
the transfer function from
SmaartLive (Figure 3.2).
Additional information in the
SmaartLive screen is now
available as a point for
discussion. This
measurement was taken 12
feet from the speaker and onFigure 3.1. Results of the DR 260 RTA in the far
axis. The same auto-EQ
field room response.
curve is still applied to the
speaker.
Figure 3.2. Screen shot of SmaartLive’s transfer function in the far field room
response. Note the addition of coherence data (red) in the frequency response box
and the phase data in the box above it.
As an additional reference (Figure 3.3), the impulse response from the SmaartLive
measurement is also shown.
Figure 3.3. Impulse response from SmaartLive associated with data in Figure 3.2.
Test 4. Frequency response with measurement microphone on the floor
This simple measurement demonstrates the effect on frequency transfer to the
microphone by moving the measurement microphone on the floor (Figure 4.1). There is
approximately 12 feet between the speaker and the microphone. It is the same setup as
Test 3, except for microphone placement.
Figure 4.1. Smaart Live transfer function measurement of off-axis measurement
microphone on the floor