Speaker coil heat dissipation One of the key issues for the speaker coil is that of heat dissipation. With high powers often being applied to loudspeakers, the coil can need to dissipate large amounts of heat. In view of the overall construction of the speaker and the coil, heat is not able to be dissipated easily and speaker coil temperatures can rise. Coils for modern high performance and high wattage speakers - normally bass driver units - are often wound on aluminium formers and high temperature epoxy adhesives are used. These can withstand temperatures of up to 300° or so. It should be remembered that as the temperature rises so the resistance of the coil increases typically by around 0.4% per degree Celsius. This can make a significant difference to the performance. In multi-speaker systems with different speakers used for different frequency bands, the heat dissipation can cause the tonal balance to alter. Not only will there be a different amount of sound content in different frequencies, but the different speakers used for the different bands will respond differently. To remove the heat from the speaker coils a number of measures can be introduced: a large magnet area helps and some coils are blacked to increase the rate of heat loss. In some very high power units the speaker coil may even have heat fins to help. Even so it has been know for some speakers to have the coil wire melted by over driving them for extended periods. Speaker coil impedance The impedance of the speaker coil consists of both resistive and reactive elements. Typically the DC resistance is around two thirds of the stated impedance which can be considered to be a minimum impedance as it varies over the range, rising to a peak at the speaker resonant frequency. This is normally not an issue because the drive amplifier will be designed to have a very low source impedance. It should be remembered that for multi-speaker units, there may be several peaks in the impedance. Also the overall speaker impedance may be mainly reactive at some frequencies resulting in the amplifier voltage and current being out of phase. This may result in high current levels flowing at some frequencies that can cause the amplifier to limit if it does not have sufficient capability - it is always wise to run systems with margin for best operation. Typical speaker coil impedance levels are 4 and 8 Ω for modern systems. Older systems had a standard 3 Ω impedance with high quality speakers having an impedance of 15 &Omega. What Is Speaker Q? If you’re an audio aficionado, then you may have heard of speaker specifications called Thiele/Small parameters. These specifications can be used to simulate and predict the low-frequency performance of a loudspeaker. Three of the specifications relate to a topic called Q. Q (or quality factor) is a unitless number that describes how underdamped an oscillating circuit is. A higher Q value means that the circuit or system has low damping and will ring or resonate for longer. Here’s an analogy that might help you understand. Do you know those springy little door stoppers that kids like to play with? They go boing when you flick them. In the right setting, they’re somewhere between entertaining and amusing. After a few dozen flicks, they become annoying. They have a high Q factor. They bounce back and forth for a few seconds after the initial input (a flick of your finger) is applied. A tuning fork is a perfect example of a resonant system designed specifically to keep ringing after the initial input is applied. If you were to apply a piece of electrical tape to the top of the door stopper, it would act to damp the vibrations. The effect would lower the Q of the spring system, and the resonances (vibrations) would stop faster. The three lines show how different Q factors affect how long a spring will oscillate at its resonant frequency. The damped curve (green) comes to rest quickly. The blue and red curves represent different levels of under-damped behavior. For most mechanical or electrical circuits, a Q of 0.5 is considered to be optimally damped. A Q of 0.3 would be over-damped, and a Q of 0.7 would be under-damped. A tuning fork, for example, has a Q of roughly 1,000. Speaker Q Factor For the purposes of this discussion, we are going to look at a typical door woofer (6.5-inch) to evaluate how different Q factors affect frequency response. The perfect speaker would have a flat frequency response that’s determined by its Thiele/Small parameters. It would look like this: Hypothetical response of a perfect speaker as determined by its Thiele/Small parameters. This speaker has a Qts value of ~0.5. Qts is the Thiele/Small parameter that defines the total Q factor of a speaker. The value takes into account both the mechanical and electrical Q factors of the driver, equally. Most car audio speaker manufacturers don’t fully understand the relationship between Q and frequency response, or they choose to ignore it to deliver a certain “sound.” Unfortunately, when it comes to truly high-end speakers, tailoring the frequency response of the system should be left to an equalizer built into a digital signal processor, and not a characteristic built into a speaker. The red curve shows the theoretical response of another 6.5-inch woofer with a Q factor of >0.7. Here are the benefits of using a higher-Q door woofer: They are more efficient in the upper bass and midbass region. The extra energy stored in the suspension is released and adds to the output, typically in the region focused around 140 to 160 Hz. If you’re designing an audio system without a subwoofer, the extra bass can be of some help in making the system sound fun. The drawback is that the extra energy that is stored and released by the suspension is distortion. It’s sound that wasn’t in the original recording. Remember, the suspension of the speaker is a spring. You don’t want it to continue to resonate back and forth after the signal goes away. That back and forth motion not only affects the frequency domain but the time domain. Sounds continue to ring out after the original input is gone. In subwoofers, this is often what’s described as being boomy. In a door woofer, it’s sloppiness. Conversely, a properly damped speaker is often described as sounding “tight” or “fast.” Since speed can’t change, these descriptions are limited in their accuracy. Shopping for Great Woofers If your car audio system includes a subwoofer (and it really should), then you’ll want to search for a door woofer that has a low Q. Some 6.5-inch woofers like the Audison Thesis TH 6.5 II and Audiofrog GB60 have Qts values under 0.5. The Morel Supremo MW6, Hertz MP 165P.3, and Focal ES 165 KX3 woofers also have relatively low Qts values. Lower values (0.4 to 0.5) are better, and you’ll want to avoid anything above 0.6 if you have a subwoofer and want properly damped midbass performance. Drop by your local specialty mobile enhancement retailer and talk to them about how best to upgrade your car audio system to deliver amazing bass and midbass performance and accuracy. The other reason this rating is important has to do with sound. As a general rule of thumb, a lower resonant frequency indicates a speaker that would be better suited for low-frequency reproduction than a one with a higher frequency. For example, a 55Hz speaker usually offers a deeper, bassier tone with slightly laid back upper mids while a 75Hz model raises the resonant frequency a bit, which affects the deepest lows, but also will tighten things up a bit on the bottom and bring out the upper midrange. It should be noted that this is not always the case though, because other parameters affect the ultimate performance as well. Conclusively, frequency response is the range of range of frequencies that a speaker can play. Some speakers can play between 20Hz to 20 kHz while others can play between 150Hz to 20 kHz. If a source (song/movie) sends a signal for a frequency outside of a speaker’s frequency response, the speaker simply won’t be able to play it. The frequency response of a speaker is how loud a speaker plays at the tones that you can hear. It is usually given as a range of tones or frequencies and the variation of volume either greater or less than the nominal level. The resonant frequency of a speaker, f0 (pronounced: F-naught), is the frequency below which a loudspeaker is increasingly unable to generate sound output for a given input signal.
