bass. Yay subwoofer! Upgrading your car's audio system won't yield more dramatic improvements in performance and realism. Adding a properly designed subwoofer system to your car stereo is usually one of the first upgrades we recommend. The challenge is to find a solution that looks and sounds great, while making sense of the myriad of specifications that may not be useful.
Subwoofer and High Frequency Performance
The motivation for writing this post was a story shared by a friend about a customer who downgraded their choice of subwoofer based on the frequency response of the two published solutions. Subwoofer A claims to deliver output up to 600 Hz. Subwoofer B, the model the customer switched to, has a claimed output of 2 kHz. The customer speculated that he could use a subwoofer to fill the mids if needed, so this was a better solution.
Why do subwoofers have a low crossover frequency
We typically use a subwoofer with a low pass filter set between 60 and 80 Hz in car audio systems. If the car has small door or dashboard speakers, the crossover may need to be set up to 100 Hz. With a typical crossover slope of -24 dB/octave, the subwoofer's output will be attenuated by more than 50 dB at 400 Hz. The ability to play to 1 kHz is not required.
Why did we go so low over the sub? Well, we don't want to hear the noise they make. Most subwoofers are not designed to handle mid-frequency reproduction well. Most of us want our vocals to come from the front speakers of our car or truck. Since the male voice extends to about 100 Hz, it makes sense for this information to be played by the door or dashboard mounted subwoofer in the system, rather than the subwoofer.
Why can't subwoofers play higher frequencies? There are two reasons. The first limiting factor is cone mass. A typical 10" subwoofer cone assembly weighs between 125 and 175 grams. It's a lot of mass moving back and forth 1,000 times per second. In fact, it just doesn't work. The cone cannot switch direction fast enough to track the input signal at that frequency, so the output is significantly attenuated.
The second problem is inductance. The voice coil assembly on the subwoofer also acts as an inductor. As the frequency increases, the impedance also increases. The result is less high frequency output. You can learn more about inductors in this article (link to BCA inductors article, once published).
"Needs more mid-bass"
While mid-range performance is not critical for a subwoofer, mid-bass performance is critical. Many subwoofers on the market have cones that are heavy enough to limit their output to frequencies just above 100 Hz. This mechanical high-frequency filtering can make it difficult to get the phase response between the subwoofer and door speakers correct. If the subwoofer has some built-in mechanical attenuators, and the technician working on your audio system added some electronic filtering, the net acoustic result may not be ideal.
A subwoofer that can play an octave or two above the crossover frequency is important. Without this extension, the bass can sound disconnected from the rest of the system. A properly configured car audio system provides a smooth transition between the subwoofer and woofer, which is critical for accurate music reproduction.
Vague frequency response specs are useless
We will state unequivocally that any published frequency response specification without tolerance values is as useful as trying to paint with a brush rather than a canvas or paint. For example, a manufacturer may state that a speaker plays from 20 Hz to 20 kHz. Most people would consider this ideal, right? What if the output dropped 40 dB relative to 1 kHz at these frequencies? Information is useless without response tolerance. If you want to look at the frequency response specs, you need a 1 or 3 dB tolerance and low and high frequency limits.
Subwoofer Frequency Response
Example of good frequency response information. That's the figure provided by JBL Professional for its 5628 dual 18-inch cinema subwoofers.
What should I pay attention to when choosing a subwoofer?
When selecting a subwoofer, predicted frequency response is important. As we have repeatedly explained, a giant subwoofer in a small cabinet may not produce as much low frequency output as a smaller subwoofer in the same space. Thankfully, we can use computer simulation software to predict how a subwoofer will perform. Let's take a look at two subwoofers similar to the one this customer is considering.
Subwoofer Frequency Response
Red frequency response of Subwoofer B in a 1 cubic foot sealed enclosure.
As you can see, the inductance of the voice coil attenuates the high frequency response of the driver. By 1000 Hz, it's down 17 dB from the peak output around 85 Hz. So stating that the drive plays up to 1.5 or 2 kilohertz is misleading and defies the laws of physics. What matters is how much low frequency information this subwoofer can produce. On the low end, it dropped 3dB at 50Hz and 10dB at 29Hz.
Alright, let's look at the original driver with the narrower frequency response specs posted.
Subwoofer Frequency Response
Yellow frequency response of Subwoofer A in a 1 cubic foot vented enclosure tuned to 35 Hz.
The first thing our intrepid amateur car audio system designers should notice is that this subwoofer has a much flatter response in the low-mid region. Why? This driver has an aluminum shorting ring built into the motor. Shorting rings help to significantly reduce inductance. The shorting ring also reduces the variation in inductance based on cone position that all loudspeakers experience. Finally, the shorting ring reduces distortion significantly. In terms of low frequency output, both drivers deliver very similar outputs in this enclosure. Does that mean they sound the same? Absolutely not.
How loud is it playing?
A key component of designing a proper subwoofer system is ensuring adequate power handling based on cone excursion. To better understand this topic, you may want to read the BestCarAudio.com article on cone offset and distortion.
If we look at the cone excursion vs. frequency plot for subwoofer B, we see that it exceeds its rated Xmax specification at all frequencies below 30 Hz when driven at 400 watts. Suspension components (spider and surround) are usually chosen based on the voice coil geometry Xmax specification, so if pushed hard with a 400 watt amp, distortion can become noticeable. A power level of 275 is safe at all frequencies in this enclosure, keeping things under 200 watts is probably a good suggestion.
Subwoofer Frequency Response
Cone excursion versus frequency (red) for subwoofer B when driven at 400W.
Subwoofer A, on the other hand, has a more significant Xmax specification. It excels at all frequencies at 400 watts and can handle up to 775 watts without leaving a gap in the voice coil. This increased excursion capability allows Subwoofer A to produce significantly more output. This also means that when driven at 400 watts, subwoofer A may sound clearer and more accurate than subwoofer B.
