Up to this point, we’ve explained the difference in performance between entry level, poorly designed and premium car audio amplifiers. We hope you’ve found this informative, and now it’s time we took a close look at car audio speakers. No car audio component is more crucial than speakers for reproducing music with accuracy and clarity.

This series of articles will analyze the impedance, frequency response, output capability and distortion characteristics of different car audio speakers. The goal is to give those of you who want to upgrade the clarity and performance of your audio system a clear correlation between design features, specifications and, ultimately, performance.

Factory-Installed Honda Civic Speaker

I have a set of door speakers from a Honda Civic for our first subject. This is a woofer (no tweeter) with an effective cone diameter of 125.5 millimeters measured from the middle of the surround on one side of the driver to the center on the other side. The cone is made from a woven yellow fiber which could be of glass or aramid composition. The dust cap is formed from soft textile but is much less rigid. The speaker has a rubber surround, which lasts longer than foam.

Mechanically, the speaker has a relatively small-diameter flat linear spider bonded to a 1-inch voice coil former. There’s no cooling vent on the rear of the magnet or venting under the spider mounting ledge. The basket is formed from injection-molded, glass fiber-reinforced polycarbonate and has six deeply reinforced spokes. As is typical for an OEM speaker, the mounting flange includes a built-in spacer with an integrated gasket that will bring the speaker out near the grille in the interior door trim panel. Overall, aside from a small voice coil and lack of cooling technologies, the design offers nothing of significance to complain about.

Measuring Thiele/Small Parameters

Every speaker of every size can have its low-frequency characteristics modeled by a set of measurements and values summarized as Thiele/Small parameters. These measurements can be used with enclosure simulation software to predict how the driver will behave in an enclosure.

The Thiele/Small parameters quantify the driver’s suspension compliance, resonant frequency, mechanical Q, electrical Q and motor force. The information does not describe any nonlinearities in the suspension or magnetic fields or the excursion limits of the design. Far too many amateur audio enthusiasts think you can quantify the low-frequency sound quality of a speaker using enclosure simulation with Thiele/Small parameters. You can’t.

I’ll use my Clio Pocket with the added mass process to measure this information for the Honda speaker.

Is there anything we can discern in terms of performance from the measured Thiele/Small parameters? The first thing we see is that the driver has a relatively high total Q (Qts) of 0.69. This will add a little resonant bump in output in the lower midbass region. It’s likely a good design trade-off for a speaker designed to be used without a subwoofer, as it will add a touch of warmth to the sound. However, in absolute terms, this will be a bit of unwanted distortion. Lastly, the predicted efficiency is relatively high at 89.04 dB SPL when driven with 1 watt of power and measured at 1 meter. This is also normal for an OEM speaker as they trade low-frequency output for increased output at higher frequencies. The ~10-gram moving mass supports this theory.

Let’s look at what the BassBox Pro enclosure simulation software predicts this driver will do in our 3-cubic-foot test enclosures. I chose this volume as it’s typically large enough to have minimal effect on the driver’s performance and should simulate how the speaker will behave in a door or rear parcel shelf.

As you can see from the graph above, this is more of a midrange driver than a woofer. I guessed at the 30-watt power handling based on the diminutive size of the voice coil and lack of cooling features. In terms of predictions, the driver has a -3 dB frequency of 98 hertz and would greatly benefit from being used with a subwoofer.

Measuring Driver Impedance

Part of measuring Thiele/Small parameters is to make a series of impedance sweeps. Impedance is the opposition to the flow of alternating current (AC) signals. As you can see from the graph below, the driver has a fairly tall, narrow peak around its resonant frequency of 74.7 hertz. You can also see the increase in inductance at higher frequencies as the upward trend to the right.

We can see something else in this graph. Something has caused a noticeable resonant peak at about 700 to 800 Hz, and there are additional wiggles in the response at 2.4, 3.7 and 5.2 kHz. These are likely caused by the cone, dust cap or surround resonating. We’ll see if any of these translate into quantifiable distortion in the acoustic measurements.

Speaker Acoustic Measurements

With the driver loaded into my 3-cubic-foot test enclosure, I placed it on the floor of my lab. The microphone from the Clio Pocket is 1 yard above the top edge of the cone, where it meets the surround. We’ll use this position for all speakers going forward. We’ll begin the testing by taking frequency response measurements at increasing drive levels. While there is no specific standard, we’ll clone what Vance Dickason uses in his transducer tests in Voice Coil magazine with 0.3, 1, 3, 6, 10 and 15 volts. It’s doubtful that the driver will remain linear in output at the 10- and 15-volt levels as those values equate to 25 and 56 watts of power into a 4-ohm load. I will add a 2-volt measurement that equates to 1 watt into a 4-ohm load.

Before we get into the analysis of the speaker, we need to understand a few things about the measurements. First, the information below 30 Hz can be ignored. There is no output of 100 dB SPL at 10 Hz. Second, the dip at 130 Hz is a reflection in the room. It can be ignored as well. We know this is an acoustic cancellation because there is no dip or peak in the impedance or distortion curves. Sorry, I don’t happen to have an anechoic chamber at my disposal. In the meantime, I’ll continue to purchase lottery tickets!

Well, here’s our first look at the Honda speaker. From 160 Hz through to 1.5 kHz, the response is adequately flat given the non-anechoic characteristics of my lab. From 1.5 through to 5.5 kHz, there is a bump in the output of about 6 dB.

The black trace lower in the graph is the total harmonic distortion (THD) measured by the Clio. Let’s look at a few frequencies and make some percentage distortion calculations. From 200 through to 400 Hz, the harmonic distortion is -49 dB, equating to 0.35% THD. At 80 Hz, distortion is at 1.5%, and the significant bump in distortion around 1.3 kHz represents approximately 0.89% distortion.

Let’s sweep it again with a little more voltage – this time, the signal generator is set to 1 volt RMS.

The first thing to observe at this higher drive level is that the output increases linearly. All frequencies are roughly 10 dB louder. This is good because neither the suspension compliance nor the motor force has become a limiting factor. Something is happening up at 4.5 kHz that’s caused a bump in the distortion curve. Overall, though, it’s not too bad for this roughly 0.25-watt playback level.

Let’s bump things up to 3 volts.

In terms of frequency response, things remain nice and linear. All frequencies are once again about 10 dB louder. What isn’t so good is the harmonic distortion characteristics. A bump appears between 700 and 900 Hz at almost 2% distortion. This would be audible if not buried with other audio information. Distortion in the bass frequencies, 70 Hz, is over 3%. This 3-volt drive level equates to roughly 2.25 watts of power for a nominal 4-ohm speaker.

OK, how about 6 volts from the function generator for the next sweep?

A drive level of 6 volts is roughly 9 watts of power into a 4-ohm load. The graph above shows that distortion at all frequencies has increased by more than the increase in fundamental output. For example, when driven with 3 volts at 900 Hz, the THD was around 2%. Now, with 6 volts, the distortion has increased to 3%. Remember that bump we saw in the impedance graph around 800 Hz? Well, now it’s back as a peak in the distortion graph. You’d be surprised what you can learn from impedance graphs.

Last but not least, let’s feed this driver with a 10-volt sweep that equates to about 25 watts of power.

Though we only picked up about 3 dB more output, the distortion has increased significantly. We have 7% distortion at 800 Hz and over 3.5% at 200 Hz. If we look down in the bass region, 80 Hz is at about 10% total harmonic distortion. In short, this speaker would sound pretty bad when driven with much more than 10 to 15 watts of power and would be screaming at 25 watts.

Better Speakers Offer Better Performance

In terms of establishing a foundation for our measurements and speaker comparisons, we’ll stop here. This article will serve as a benchmark for what looked like a reasonable quality OEM speaker. We’ll test some speakers that might be better and some that might be worse over the next few months. This information should allow us to develop a correlation between design features and performance. In the meantime, if you’re shopping for new car audio speakers, drop by your local specialty mobile enhancement retailer to audition some options for your vehicle.

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.

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.