Subwoofers. Yay for subwoofers! No upgrade to a car audio system will deliver a more noticeable improvement in performance and realism. Adding a properly designed subwoofer system to your car stereo is often one of the first upgrades we recommend. The challenge is finding a solution that will look and sound great while you make sense of myriad specifications that might not be helpful.

Subwoofers and High-Frequency Performance

The motivation for this article was a story a friend shared about a client who had downgraded their selection of subwoofers based on the published frequency response of two solutions. Subwoofer A claimed to offer output up to 600 hertz. Subwoofer B, which is the model the client switched to, claimed output to 2 kHz. The client theorized that he could use the sub to fill in midrange frequencies if needed, and as such it was, therefore, a better solution.

On paper, the logic isn’t wrong. But, in practice, that’s not how subwoofers work.

Why Subwoofers Have Low Crossover Frequencies

We typically run subwoofers with a low-pass filter set between 60 and 80 hertz in car audio systems. If the car has smaller door or dash speakers, the crossover might need to be set as high as 100 hertz. With the typical crossover slope of -24 dB/octave, the sub’s output would be attenuated by more than 50 dB by 400 hertz. The ability to play to 1 kHz isn’t essential.

Why do we cross subs over so low? Well, we don’t want to hear vocals coming out of them. Most subwoofers aren’t designed to handle midrange frequency reproduction well. Most of us want the vocals to come from the front speakers in our cars or trucks. Since male voices extend to around 100 hertz, it makes sense for this information to be played by the door- or dash-mounted woofers in the system, not the subwoofer.

Why can’t subwoofers play higher frequencies? There are two reasons. The first limiting factor is cone mass. A typical 10-inch subwoofer cone assembly weighs between 125 and 175 grams. That’s a lot of mass to move back and forth 1,000 times a second. In fact, it just doesn’t work. The cone can’t switch directions fast enough to track the input signal at that frequency, so the output is attenuated significantly.

The second issue is inductance. The voice coil assembly on a subwoofer also acts as an inductor. As frequency rises, so does impedance. The result is less high-frequency output. You can learn more about inductors in this article (Link to BCA inductor article once published).

“Needs More Midbass”

While midrange performance isn’t important for a subwoofer, midbass performance is crucial. Many subs on the market have cones heavy enough to limit their output at frequencies just above 100 hertz. This mechanical high-frequency filtering can make it very hard to get the phase response between the sub and the door speaker right. If the sub has some built-in mechanical attenuation and the technician working on your audio system adds some electrical filtering, the net acoustic result might not be ideal.

A subwoofer that can play an octave or two above the crossover frequency is important. Without that extension, the bass might sound disconnected from the rest of the system. Properly configured car audio systems deliver a smooth transition between the subwoofers and the woofers, which is crucial to reproducing music accurately.

Vague Frequency Response Specs Are Useless

We’ll state in no uncertain terms that any frequency response specifications published without tolerance values are as helpful as trying to make a painting with a brush but no canvas or paint. For example, a manufacturer could state that a speaker will play from 20 Hz to 20 kHz. Most would think that’s ideal, right? What if the output was down 40 dB at those frequencies relative to 1 kHz? Without a response tolerance, the information is useless. If you want to look at frequency response specs, a tolerance of 1 or 3 dB combined with low and high-frequency limits is required.

What Matters When Choosing a Subwoofer?

When choosing a subwoofer, the predicted frequency response is important. As we’ve explained repeatedly, a giant subwoofer in a small enclosure might not produce as much low-frequency output as a smaller subwoofer in the same space. Thankfully, we can use computer simulation software to predict the subwoofer’s performance. Let’s take a look at two subwoofers similar to what this client was considering.

Based purely on the Thiele/Small parameters of Subwoofer B, here’s the subwoofer’s response in a 1-cubic-foot sealed enclosure.

As you can see, the voice coil’s inductance attenuates the high-frequency response of the driver. By 1000 Hz, it’s down 17 decibels from its peak output at around 85 hertz. So stating that this driver plays up to 1.5 or 2 kilohertz is misleading and defies the laws of physics. What should matter is how much low-frequency information this subwoofer can produce. On the bottom end, it’s down 3 dB at 50 Hz and 10 dB at 29 hertz.

OK, let’s look at the original driver with the narrower published frequency response specifications.

The first thing our intrepid amateur car audio system designer should notice is that this subwoofer has a much flatter response through the midbass region. Why? This driver has an aluminum shorting ring built into the motor. The shorting ring helps to reduce inductance dramatically. The shorting ring also reduces cone-position-based changes in inductance that all speakers experience. Ultimately, the shorting ring dramatically reduces distortion. Both drivers deliver very similar output in this enclosure regarding low-frequency output. Does this mean they sound the same? Absolutely not.

How Loudly Does It Play?

A key component in designing a proper subwoofer system is ensuring adequate power handling based on cone excursion. To get a better understanding of the topic, you might want to read the BestCarAudio.com article on cone excursion vs. distortion.

If we look at the cone excursion vs. frequency graph for Subwoofer B, we see that it exceeds its rated Xmax specifications at all frequencies below 30 hertz when driven with 400 watts. The suspension components (spider and surround) are typically selected based on the voice coil geometry Xmax specification, so distortion is likely to become significant if pushed hard with a 400-watt amplifier. A power level of 275 would be safe at all frequencies in this enclosure, and keeping things under 200 watts is likely a good suggestion.

On the other hand, Subwoofer A has a much more significant Xmax specification. It’s good at all frequencies at 400 watts and can handle 775 watts without the voice coil leaving the gap. This increased excursion capability allows Subwoofer A to produce significantly more output. It also means that Subwoofer A likely sounds clearer and more accurate when driven with 400 watts than Subwoofer B.

What Do We Need To Know About Subwoofer Frequency Response Specifications?

When buying subwoofers, frequency response specifications like 20-200 Hz or 25 Hz to 1.5 kHz are useless unless there is an amplitude tolerance specification. An applicable specification would be 25 to 300 kHz (±1.5dB). As mentioned in other articles (https://www.bestcaraudio.com/when-it-comes-to-subwoofer-specifications-some-numbers-dont-matter/), efficiency specifications like 85dB@1W/1M are also irrelevant, as they don’t take into account how the enclosure affects low-frequency performance.

Suppose you want to know how a particular subwoofer will perform in your vehicle. In that case, the specialty mobile enhancement retailer you’re working with should model the driver in the enclosure they will be using with BassBox Pro, Term-Pro, LEAP, WinISD or something similar. You can then look at the driver options to see how the predicted response and effective efficiency will change. Sadly, in the case of Subwoofer A vs. Subwoofer B, the client chose incorrectly. He missed out on a great subwoofer because he was misled by irrelevant information.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

Few people in the car audio industry seem to grasp that speakers are typically the weakest link in audio systems, in terms of adding distortion to what we hear. Whether it’s a poor design with improper voice coil centering in the magnetic gap or poor magnetic or compliance linearity, speakers add significant amounts of unwanted information to what we hear. This article will take a deep dive into explaining how increased cone excursion affects distortion.

Understanding Car Audio Speaker Cone Excursion

Speaker cones move back and forth to excite air molecules and produce sound. They function in the same way that hitting the skin of a drum, blowing through a horn or vibrating the string of a guitar creates pressure waves in the air. If we apply more voltage to a speaker, the cone moves more. Reproducing low-frequency information requires that air molecules be displaced further, requiring more cone excursion (and more voltage) to produce bass frequencies. Larger instruments like an upright bass, concert grand piano and timpani also produce more low-frequency information than a banjo, spinet piano or bongo drum.

Unfortunately for speakers, the more their cones move forward and rearward, the more chances there are for the cone to not track the electrical signal perfectly. When this happens, unwanted harmonic information is added to the audio signal. We call this distortion. If the cone, dust cap or surround resonates, this also adds unwanted distortion. It’s not uncommon for speakers playing at moderate output levels to reach well over 1% distortion. This means that more than 1% of the sound they produce doesn’t follow the input signal accurately.

Measuring and Understanding Speaker Distortion

To help explain this concept, I took a popular 6.5-inch PA-style speaker that’s used in car audio systems and mounted it in my test enclosure. I set up my Clio Pocket with the microphone a few millimeters from the cone and performed a series of frequency response sweeps at different power levels. The Clio system can analyze the measurement and display second- and third-order harmonic information. Let’s look at the first measurement in detail.

The graph you see above shows three pieces of information. First, the red trace is the frequency response of the speaker. This trace tells us how much energy the speaker produces at different frequencies when fed with a chirp signal. The chirp signal is a sine wave sweep that starts at 20 Hz and ends at 40 kHz. I adjusted the output of the amplifier for this test such that it produced right at 1 volt of output, which is 0.25 watt into a 4-ohm load.

The perfect speaker (which doesn’t exist) would produce a perfectly flat frequency response from the lowest bass frequencies to the highest of high frequencies. This speaker was within about 5 dB of flat from 200 Hz to 3000 Hz. Remember, this measurement is with the microphone right at the cone, so the sound pressure level numbers on the left don’t directly correlate to what you’d hear in a car or truck unless you installed the speaker in your headrest. Uh, please don’t do that.

The blue trace is the second-order harmonic distortion trace. To explain what this information means, let’s look at a specific frequency, 200 Hz. The speaker is producing about 88 dB SPL of output at 200 Hz. This is called the fundamental frequency. The blue trace tells us that it’s also producing a second harmonic (which would be 400 Hz) at a level of 38 dB SPL. Again, the absolute numbers don’t matter, but we need to know that the distortion is 50 dB below the fundamental. That works out to 0.316% for the second-order harmonic.

The green trace is the level of the third-order harmonic, which for a 200 Hz signal is 600 Hz. We have an output of about 29 dB SPL, 59 dB below the fundamental and representing a distortion level of 0.112%.

I’ll reiterate and rephrase this to be precise: If you feed this speaker a 200 hertz signal at a level of 0.25 watt, it will also produce output at 400 hertz and 600 hertz (and many more multiples). This is how speaker distortion works, and it’s common to every speaker of every design, at every price point and from every manufacturer. Finally, better speakers add less distortion – that’s a key part of what makes them better. I deliberately chose this PA-style speaker because it has an extremely short voice coil, so it will be easy to push it into high levels of distortion at low frequencies with minimal power. The purpose is to quantify how distortion increases with cone excursion, not to “test” this speaker.

More Power Means More Distortion

For the next test, I increased the output of the amplifier to 2.83 volts, which works out to 2 watts of power. This added power should correlate to a 9 dB increase in output.

The first thing to notice is that the shape of the frequency response trace (red) didn’t change. Second, the speaker did increase its output by exactly 9 dB. You have to love the laws of physics! What matters in this measurement is that the harmonic distortion has increased significantly. The increase isn’t linear with the increase in output from the speaker. Looking at 200 Hz again, the first harmonic is now at a level of -44 dB relative to the fundamental, which is 0.631% THD. The third harmonic is at 50 dB below the fundamental, which is 0.316% THD.

Let’s double the power again to 4 watts and see what happens.

The fundamental has increased another 3 dB as expected. The first-order harmonic content at 200 Hz is at -42 dB relative to the fundamental, which is 0.794% THD. The third-order is 47 dB below the fundamental, which is 0.446%.

Let’s double the power again to 8 watts and repeat the measurements.

The fundamental is right at 103 dB SPL at 200 Hz, and the second harmonic is 40 dB lower at 63 dB SPL. This represents almost exactly 1% total harmonic distortion. The third harmonic is down 44 dB, which is 0.631% THD. We won’t get into the math here, but the total distortion caused by all harmonics wasn’t measured in this test, and you can’t add the numbers directly (e.g. 1.631%).

OK, let’s bump up the power again to 16 watts.

While we continue to focus on the 200 Hz calculations, look what’s happening to the third-order harmonic distortion down below 100 Hz – it’s getting louder very quickly and is actually catching up to the fundamental information. Nevertheless, at 200 Hz, the second-order harmonic output is at 37 dB below the fundamental, which is 1.412% distortion. The third-order distortion is at -40 dB relative to the fundamental, which is 1.0 % THD.

Hopefully, you’re starting to see a pattern. Let’s double the power again to 32 watts and see how the speaker behaves.

We picked up another 3 dB of output across the board. Our fundamental is at 108 dB at 200 Hz and the first harmonic is down only 34 dB, which is right at 1.995% THD. The third-order harmonic output is down 38 dB at 1.259% THD. If you’re getting the feeling the speaker would sound terrible attempting to reproduce audio information at 200 Hz at a drive level of 32 watts, you are right.

OK, one last time. Let’s double the power again to 64 watts and analyze the frequency response and harmonic content.

With the fundamental output at 111 dB at 200 Hz, we have 79 dB of output at the second harmonic of 400 Hz, which represents 2.511% THD. The third harmonic output at 600 Hz is at 76 dB, which is 35 dB below the fundamental, or 1.778% THD.

The chart above summarizes the increase in distortion relative to the increase in output. It’s easy to see how the second and third harmonics continue to get louder relative to the fundamental frequency.

Look at what’s happening down at 100 Hz and below. The third-order harmonic output is as loud or louder than the fundamental. When you feed this speaker 64 watts of power at 50 Hz, it produces 93 dB SPL of output (with the mic in this position) and 97 dB of output at 600 Hz. That’s audio information that wasn’t in the music. If you want to do the math, or, more accurately, if you’d like me to do the math, that’s 158% distortion.

What Have We Learned about Speaker Distortion?

There are two takeaways from this first look at car audio speaker distortion. First, the amount of distortion produced by a speaker increases as the cone excursion increases. We should already know from other articles that cone excursion increases at lower frequencies. Putting together these pieces of information tells us that we don’t want to push the smaller speakers in our vehicles to reproduce the bottom two or three octaves of the audible music range. Adding a subwoofer system with a dedicated amplifier and a speaker designed to reproduce low frequencies allows for more bass and can dramatically improve the clarity of the midrange speakers in your audio system.

Second, in general, 6.5-inch PA-style speakers aren’t good at reproducing audio below about 300 hertz. If you understand speaker enclosure design and have modeled this type of speaker using software like BassBox Pro, Leap or Term-Pro, you’ll know that most of these drivers have a -3 dB frequency in the 150 to 200 Hz range. So, pushing this type of speaker to produce audio information below 250 hertz is asking for trouble, or at the very least, lots of distortion.

Choose Your Car Audio Speakers Wisely

If you’re shopping for new speakers for your car or truck, drop by your local specialty mobile enhancement retailer and listen to the options they have available. Suppose you’re the type who likes to correlate features with performance. In that case, drivers that use aluminum or copper shorting rings, feature flat spiders or have a copper distortion-reducing cap on the pole piece are likely to add less distortion than models without.

Look for speakers with cone materials that balance mass with rigidity and damping characteristics – getting any of these wrong is a recipe for trouble. Finally, trust your ears. The speaker should sound smooth and natural with no emphasis anywhere in the frequency range, especially in the bass region. If they sound good on a display compared to the rest, they are a good choice for your car or truck.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.