When it comes to expressing the quality of an audio system component, the amount of distortion it adds to a signal is the defining factor. In this article, we’ll talk about how much distortion each part of an audio system adds – so that you can target your budget for the greatest benefit. We’ll rank four categories of audio equipment based on the amount of distortion they add to your music. Ultimately, our suggestion is to optimize your upgrade budget by focusing on the products that will deliver the most significant improvement in perceived audio quality.

What Is Audio System Distortion?

Distortion is the addition of unwanted information to an audio signal. In minimal amounts, distortion can be difficult to perceive. However, above a level of about 0.5%, distortion is audible and can change the music’s tonal balance and perceived dynamics.

It’s become clear that most people think distortion is only present in an audio system when a speaker is overdriven or an amplifier starts to clip. While that level of distortion is very audible, unwanted harmonic and intermodulation distortion is added even at moderate listening levels. For example, a low-quality amplifier providing less than a watt of power to a speaker can add almost 1% distortion to the signal. A poorly designed speaker or one operating outside of its intended frequency range can easily add 3-5% distortion, and a subwoofer driven at moderate levels can be well over 10%.

No. 1 – Speakers and Subwoofers

If the paragraph above wasn’t enough of a hint, let’s make it clear: Speakers and subwoofers are notoriously non-linear. Let’s say you’re feeding a 100 Hz sine wave into an inexpensive woofer. If the design has less compliance (more stiffness) in the forward direction relative to the rearward, the driver’s output will be reduced for half of the waveform. That’s distortion. If the driver has changes in inductance relative to forward or rearward motion, there will be a reduction in output over one half of the produced audio waveform. Distortion is also possible from resonances in the cone, surround and dust cap that add significant unwanted energy.

You need to know that ALL speakers add more distortion as they are driven to higher excursion levels. Aside from not using a properly designed or constructed enclosure for your subwoofers, or trying to play the woofers in your doors at frequencies below 80 Hz, louder means less quality.

When shopping for car audio upgrades, look for speakers that include distortion-reducing technologies. We’ve talked about copper and aluminum shorting rings, copper caps and flat progressive spiders. To minimize power compression (a reduction in output as the speaker is driven at higher levels), you’ll need to choose a solution that balances cooling features like a large-diameter voice coil with acceptable high-frequency performance.

Last but certainly not least, the product specialist you’re working with needs to design, install and calibrate your audio system properly. Proper speaker mounting adapters and spacers made from weather-resistant materials are a starting point for installation. Choosing driver sizes that offer good directivity performance, so your music sounds great from both sides of the vehicle, is another important criterion. Finally, setting crossovers, equalization and signal delays to create a realistic listening experience is crucial. Getting any of these steps wrong will reduce your enjoyment of your favorite music.

No. 2 – Amplifiers

If you’ve read our articles on distortion, then you know that we have a passion for testing amplifiers. At their worst, amplifiers add maybe 1/10th of the distortion to an audio signal that a low-quality speaker does. This doesn’t mean that they aren’t in a solid second place in terms of where you should invest in an audio upgrade.

Amplifiers are supposed to amplify a signal – nothing more, nothing less. They should output what you feed to them without adding warmth, brightness or changes to overall tonal balance.

Low-quality or poorly designed amplifiers can add distortion because of the crossover between the positive and negative output devices. In addition, they can exhibit frequency response issues from poorly designed bass boost and filter circuits. Amplifiers with high output impedance can be affected by changes in load impedance.

A second consideration for amplifiers is noise. If you turn your car stereo system on and hear a lot of hiss, it’s likely from the amplifier. Every amplifier adds a little background noise. Whether noise affects your enjoyment depends on just how much is added. The signal-to-noise ratio lets you know what’s going on behind the scenes.

A few quick tips for buying high-quality amplifiers: As far as Class-D amplifiers have come, they are best suited to powering woofers and subwoofers. The output filter networks can get a little fussy at higher frequencies. As such, Class-AB amplifiers are better for midrange and high-frequency duties. Second, the physical size of an amplifier significantly affects how much noise is added to the output. Small amplifiers are typically noisier. Third, watch how specifications on distortion are provided. The ANSI/CTA-2006-C standard requires that distortion and noise numbers be published when measured at an output level of 1 watt into a 4-ohm load. Measurements at other levels may be misleading.

No. 3 – Source Units

We’ve tested several source units lately, and though their effect on sound quality isn’t as dramatic as the speakers and amplifiers you choose, there are measurable differences from one model to another. While amplifiers have to deal with high voltages and significant current delivery, the signals inside a radio or multimedia receiver are pretty small. These low levels mean that noise from a power supply can affect what you hear.

Back in car audio’s heyday, we had CD units like the Sony Mobile ES CDX-C90, Clarion ProAudio DRZ-9255 and Alpine’s F#1 Status DVI-9990E. Of course, the days of high-end CD players are gone, but there are still a few options for a high-quality source unit.

Picking a good head unit is tough. You’ll likely choose between something mainstream with good performance or an upgrade to something designed to offer better sound quality. For example, we used to suggest 24-bit Burr-Brown D/A converters. Now, those are in almost every multimedia receiver. Our advice is to choose a product that has the features you want and is clearly designed to deliver improved audio quality.

No. 4 – Digital Signal Processors

Putting the DSP family at No. 4 in our list might puzzle anyone who understands how DSPs work and why they are an absolute necessity in a premium car audio system. Let’s make it clear that you need a DSP to equalize and filter the signals going to your speakers for them to deliver realistic performance. As such, a DSP is one of the most important components in an audio system. Its ranking in this article is based on whether or not you need to allocate extra funds to purchase a product in each category to yield better performance. So, having a DSP is a requirement; buying the fanciest one on the market won’t make a huge difference to what you hear.

Having said that, there are subtle differences, especially between entry-level and premium solutions. For example, most digital signal processors use similar DSP chips to perform the audio adjustments. As these are handled in the digital domain, the differences in transparency are more about the analog circuitry and the conversion process.

One tip: Don’t let yourself be fooled by claims about support for high-resolution in a DSP. In most cases, the analog-to-digital and digital-to-analog converters are noisier when running at higher frequencies. Stick with solid performance and ensure that you have an expert configure the system, and you’ll be delighted.

Choose Your Car Audio Upgrades Wisely

Here’s one last thought on buying high-quality car audio equipment: Every brand on the market is competing for your dollar. Some of them offer excellent products that result from hours, if not years, of designing and testing to deliver the best possible performance. On the other hand, some products are just expensive while delivering mediocre performance. So make sure you take the time to audition several options from different specialty mobile enhancement retailers near you.

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.

A while back, we took a quick look at how alternating current (AC) signals in our car audio systems work. While the amount of work done by an AC voltage source can be the same as that of a direct current (DC) source, the changes in current flow direction add complications. In this article, we’ll dive a little deeper into AC signals and explain why a fixed-value resistor can’t directly simulate a speaker.

Ohm’s Law – AC and DC Current Flow

Ohm’s law is a constant for both DC and AC circuits. When 1 amp of current flows through a resistor with a value of 1 ohm, then 1 volt of electric potential will be produced across that resistor. If the resistor value is increased to 2 ohms, and we apply 1 volt across it, then 0.5 amp of current flows through it. The triangle below shows the three ways we can calculate voltage, current or resistance if we have two of the other variables.

In an AC circuit, we know that the current flow switches back and forth. In home and commercial electrical systems, the voltage is in the shape of a sinusoidal waveform. The work done by the voltage or current is an average level of the signal. For a sine wave, the average level, called the RMS level, is 0.707 times the peak voltage referenced to the ground.

Alternating Current Signals Are Complicated

When current flows through a conductor, it creates a magnetic field around that conductor. This phenomenon is known as Oersted’s law, named for Danish physicist Hans Christian Oersted, who discovered this relationship in April 1820.

In a DC circuit, if we have current flowing through a coil of wire, the magnetic field created in that coil opposes changes in current flow. If we remove the voltage source, the current will continue to flow, even if only for a few milliseconds. This phenomenon is why many automotive relays have a diode on the bottom of a socket. The diode shunts the reverse-polarity current spike that results when we remove the voltage source. As a result, the magnetic field in the relay coil collapses and produces a voltage spike. This spike can damage the circuitry driving the relay or cause arcing in a switch.

Audio Signals Are Complicated

The current flowing to a speaker (or more specifically, a tweeter) may change direction as often as 20,000 times a second. All speakers (that we will worry about) use a voice coil that creates a magnetic field that makes the cone move. This same magnetic field opposes changes to the current flow direction. As such, we have more opposition to AC flow than we would for the same amount of DC current.

If we connect a digital multimeter to a speaker, the meter applies a tiny DC current to the voice coil. The number on the meter screen tells us the DC resistance in voice coil winding.

For AC circuits, we need to measure impedance. The Oxford Dictionary defines impedance as “the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.” Since we skipped over it, reactance is the opposition to AC current flow caused by inductance or capacitance. For example, the voice coil in a speaker will act as an inductor at mid to high frequencies.

Let’s look at a sizeable 6.5-inch woofer. If I use a high-quality digital multimeter to measure the resistance, we get a reading of 3.7 ohms.

If I want to know how the speaker opposes the flow of AC current, I need to feed it an AC signal and measure the opposition to current flow at any frequency relevant to the application for the driver. For this article, I’ll use the Smith & Larson Woofer Tester 2. This device can measure DC resistance and inductive and reactive capacitance at any frequency up to 20 kHz. The result is a plot of impedance (AC opposite to current flow) along with a phase plot that tells us whether the load is capacitive or inductive.

If we look closely at the impedance measurement, we can see that the woofer has an impedance of about 30 ohms at a frequency of 42 Hz. This is the driver’s resonant frequency (Fs) and represents the point at which the least amount of current produces the most output.

At higher frequencies, the inductance of the voice coil becomes the primary opposition to AC current flow in the speaker. The Woofer Tester 2 measured the inductance of this driver as having a value of 0.827 millihenry. You can see that the impedance starts to rise at an exponential rate above about 250 Hz. By the time the drive frequency is at 20 kHz, the impedance is approximately 54 ohms.

Driving a woofer or subwoofer is one type of challenge for an amplifier. This task requires significant amounts of power. The load can also be heavily reactive, meaning current and voltage may not be in phase with each other.

Driving a set of component speakers is an entirely different scenario. Even the most minute changes in output level can be readily apparent to the listener. Since speakers can present significantly varying impedances based on frequency, the voltage and current supplied by the amplifier will change as well. These variances in load impedance can result in small changes in output level as the amplifier’s circuitry interacts with the load to create a voltage divider. In systems where there is no equalization, especially in home audio systems, these variances can change how the listener perceives the music.

We created this complex reactive load to challenge amplifiers that cross the BestCarAudio.com test bench. We take frequency response measurements into all the rated loads for the amp and a measurement when connected to this simulated reactive load. Better amplifiers exhibit smaller changes in output level relative to changes in load impedance.

The graph above shows the effective frequency response of an amplifier when fed a 4-ohm resistor (in red) and 2-ohm resistor (violet) and our reactive test load (in black). As you can see, the output of the amp varies depending on what it’s connected to. This phenomenon is more common with Class-D amplifiers than with Class-AB designs.

Why Is Impedance Important to Car Audio Systems?

There are a few takeaways from this article. First, if you measure the resistance of a speaker with a multimeter, you are getting a general feel for its ability to pass current. For example, if the meter measures 3.7 to 4.2 ohms, then you have a nominally 4-ohm speaker.

However, if you intend to design passive crossovers, you need to know the exact impedance around the crossover frequency to choose the correct values. While very few people create passive crossovers these days, we do see many people suggesting that those circuits are interchangeable between different brands, makes and models of speakers. Nothing could be further from the truth. You can’t assume a crossover designed for Brand A speakers will function adequately or sound right with Brand B.

Another consideration about AC signals surfaces when measuring power in a reactive load (like a speaker). The opposition to changes in current means that current peaks lag behind voltage peaks in inductive loads. If you want to measure power from an amplifier when connected to a speaker, you need to measure current and voltage simultaneously. We don’t mean by using a voltmeter and current clamp and taking peak readings. You need to measure both using something like the D’Amore Engineering AMM-1. Those “clamped” power readings the SPL guys talk about are meaningless.

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.

When upgrading your car stereo, speakers are the only link between your electronics and your ears. Unfortunately, low-quality speakers connected to the best amplifiers and source units in the world will still sound mediocre at best. In this article, we’ll take a look at the classes of speakers available for your car or truck. In addition, we’ll provide some tips to help you expedite the process of working with the Product Specialist at your local retailer to end up with the best value and performance for your investment.

Start by Determining Speaker Sizes

The first thing that needs to be determined is the size of the speaker you’re hoping to upgrade in your car or truck. In the car audio industry, the most common speaker sizes are 6.5-inch and 6×9. With that said, there are 3.5-, 4-, 5.25-, 6-, 6.5-, 6.75- and 8-inch round speakers, along with 4×6, 4×10, 5×7, 6×8, 6×9 and 7×10 oval speakers. The Product Specialist you’re working with will likely know what size speaker is in your vehicle or they’ll remove a trim panel to check.

Once you know the speaker size, the next step in terms of physical requirements is to consider your vehicle’s mounting depth and protrusion limits. The mounting depth refers to how much space is available behind the speaker mounting surface for the speaker basket and motor (magnet) assembly. The limiting factor for door speakers is usually the distance to the window when the lower edge is behind the bottom of the speaker. For dash speakers, the limitation is often venting from the climate control system. For rear parcel shelf speakers, torsion-type springs that hold the trunk lid open are the primary concern.

The second consideration is the amount of space available in front of the speaker. If the installation will use the grille included with the speaker, then this isn’t a problem. However, if the speaker will be installed behind the grille that came with the car, then adequate space must be provided for the tweeter, and the woofer at its maximum forward excursion limit.

If there’s room, the technician working on your vehicle may choose to build a spacer out of plastic or a composite to move a deeper speaker outwards. It’s worth repeating (as we have many times) that wood spacers don’t last very long in a car or truck door. They will get wet when it rains, and even when painted, coated in fiberglass resin or sprayed with an undercoating, they will eventually swell and become moldy. This deformation is rarely even and can damage or break high-quality cast-basket speakers. Materials like cast acrylic, ABS, high-density polyethylene and expanded PVC are good options for spacers. Most reputable shops will have these materials in stock and will cut custom spacers for your application. We’ve even seen spacers constructed from composite kitchen countertop materials like DuPont Corian. Companies like Metra offer vehicle-specific speaker spacers and mounting adapters to allow for reliable installations that don’t require any fabrication skills.

Speaker Design

Once you’ve determined the size of speaker that’s suitable for your application, you’ll need to decide on your goals for the audio system. We aren’t quite at the stage where we’ll talk about performance and quality, though. This step involves deciding if you are going to want a coaxial speaker or a component set.

In a coaxial speaker, the tweeter, or a tweeter and super-tweeter are integrated into a single chassis along with the woofer. The benefit of these systems is that they are typically an all-in-one speaker solution. However, if you’re replacing the front speakers in a vehicle, you may want to consider using a component speaker set. In most cases, a two-way component speaker set includes a pair of woofers and tweeters. The woofers would be mounted in the door location, and the tweeters would go in the dash location, in the A-pillars, up high in the doors or in the sail panels. The benefit of a component set is that the higher mounting position of the tweeters may afford better high-frequency delivery, and the source of the sound in the vehicle may be at the dash or windshield level instead of down near your legs.

In the case of component speakers, most include an external passive crossover network. This compact circuit includes capacitors, inductors and often resistors that divide the audio signal from a radio or amplifier into different frequency bands and send the bass and midrange information to the woofer and high-frequency information tweeter.

Component speaker sets are also available as a three-way solution for cars that use a woofer, a midrange driver and a tweeter. In most cases, the mid and tweeter are mounted higher in the door or in the dash to help raise the sound source.

Suppose you’re seeking the best possible performance from your audio system. In that case, a three-way speaker set that’s installed in combination with six channels of amplification and a digital signal processor is the way to go. We won’t get into DSP applications in this article as we’ve covered it many times already.

If you’re going the DSP route, many companies sell their mid- to high-performance drivers in pairs. The Product Specialist can help you chose a woofer size, an appropriate midrange solution and tweeters to create a bespoke configuration for your vehicle’s speaker locations.

Speaker Selection Considerations

Now we get to the part where you decide what level of performance you want from your audio system. Do you need an inexpensive speaker to replace one that came from the factory that’s worn out, or are you trying to upgrade the audio system to deliver clearer, more detailed sound? Part of this consideration includes deciding how much power you have available and your preferred listening level.

For example, suppose you want to recreate the Kiss concert in Ottawa, Ontario, from back in the 1970s. The show reached an output level of 136 decibels. In that case, you need speakers that can handle a lot of power, have significant woofer cone excursion capabilities and include features that will help to ensure they sound good when played loudly.

A quick aside here: Speakers are much more complicated than most people think. For every speaker, as it plays louder, more distortion is added to the output signal due to mechanical and electrical non-linearities, along with cone, dust cap and surround resonances. If you want a speaker that will be clear when played loudly, then choosing something with technologies like a copper or aluminum shorting ring and a copper cap will help. The diameter and length of the voice coil play a prominent role in determining peak power handling. Some 6.5-inch mid-woofers have 2-inch diameter voice coils. Likewise, the suspension should use a flat, progressive spider, and the cone, cap and surround should be designed to eliminate resonance. It’s a complicated balancing act to get this perfect.

Let’s back up here. If you want your music loud, then you’ll need to consider the speaker power handling specifications. If they’re compliant with the ANSI/CTA-2031 standard, those numbers will tell you how much heat energy the speakers can dissipate before they are damaged. Power ratings do NOT define the quality of a speaker. There are terrible-sounding speakers with high power ratings and some that sound quite good with low ratings. Car audio enthusiasts need to stop buying speakers based on power ratings.

Auditioning Speakers Helps Determine Quality

Most car stereo shops have display boards that can be used to audition speakers. Here are a few quick tips. First, when switching from one speaker set to another, try to make the listening level equal. Even slightly louder speakers will sound better because you can hear the quieter parts more clearly. Second, bring one or two songs you know pretty well to the store to listen to the speakers. A dynamic track with a wide frequency range will tell you a lot about the dynamic and damping characteristics of a speaker. A song with clear male and female vocals will tell you a great deal about the clarity and accuracy of the drivers. Lastly, listen to them for a short time at high volume levels. If they sound stressed, they are likely missing distortion-reducing technologies.

When shopping, your goal is to find the speakers that sound the most accurate for a given price point. For a 6.5-inch speaker set, you can pay anywhere from about $55 for an introductory-level two-way coaxial speaker to well over $5,500 for a premium two-way component set. The power handling, output capability, frequency response and, most importantly, the speakers’ clarity increase as you move up the price range. Better speakers reproduce audio signals more accurately, recreating the original sound from the recording studio in your vehicle without adding unwanted emphasis, leaving anything out or adding distortion.

Upgrade Your Speaker Upgrade

You can improve the performance of your audio system significantly by having sound deadening installed in the doors of your car or truck. High-quality deadening products reduce the transfer of noise energy into your vehicle and make it easier to hear the quieter parts of your music.

If you have a vehicle with large openings in the door, sealing those with fiberglass or plastic and then covering the area with deadening can dramatically improve a door speaker’s bass and midbass performance. We’ve seen gains in output of 10 dB from the addition of sound deadening alone.

Another upgrade that can help with the performance of your speakers is to have your installer add a set of rubber or foam gasket rings around the drivers. These rings help to seal the gap between the door and the speaker to prevent sound from bouncing around and potentially causing cancellation.

The Best Way To Improve Your Door Speakers

Finally, if you’re after the best performance possible from a set of door speakers, the best upgrade you can make is to add a subwoofer system to your vehicle. Relieving the speakers’ need to reproduce bass frequencies below 80 hertz can significantly cut the power sent to the speakers. Just as we mentioned earlier, the speakers won’t need to work as hard and consequently will add less distortion to the audio signal. The result will be a system that can be played at higher volume levels with less distortion. That’s a win-win!

When it’s time to replace or upgrade the speakers in your car or truck, drop by a local specialty mobile enhancement retailer and audition the options available for your vehicle. Always remember, when it comes to sound quality and accuracy, the speakers in your vehicle are your most critical consideration.

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.

If you look in the back of a friend’s car or SUV, you’ll find there are a few common traits when it comes to subwoofer system installation. For larger enclosures, the subwoofers are usually mounted behind the seat and fire rearward. For single-sub systems, many installations feature the drivers mounted in an enclosure that fits into the corner of the car. If you don’t want to give up storage space, the spare tire well becomes a popular mounting location. The question is, does the position of the subwoofer affect its performance? Let’s check it out!

The Test System

Not long ago, we set up a test to evaluate the need for equalization in a subwoofer system. We used an old Elemental Designs e12A.22 12-inch subwoofer in a 0.75-cubic-foot sealed enclosure and paired it with the ARC Audio 1000.4 that the BestCarAudio.com team had reviewed in early 2021. As the baseline for this article, we had the subwoofer enclosure positioned against the rear of the driver’s side back seat. The subwoofer was pointed rearward toward the hatch of this 2019 Hyundai Santa Fe SUV. We’ll use the equalized signal as the baseline. We’ve created a map of the cargo area and a representative rendering of the subwoofer enclosure to let you know where each measurement was taken.

Subwoofer Position Testing

The next step was to move the subwoofer around the cargo area of the vehicle and take additional measurements to quantify any changes that might occur.

The second position we tested found the subwoofer in the left rear corner of the cargo area with the driver facing inward. The green trace shows that we lost about 7 dB of output at 23 Hz, but picked up between 1 and 3 dB at all frequencies above 36 Hz.

The third position had the subwoofer flipped around and pointed at the outside of the vehicle. The response, shown with the orange trace, shows some gain at 23 and 26 Hz, along with the same increase above 36 Hz.

We then moved the subwoofer to center of the cargo area, right up against the hatch. We started with the subwoofer pointed toward the rear of the vehicle. The brown trace shows a bit of loss between 21 and 24 Hz and some minor gains above 36 Hz. The dip created at 60 Hz would need to be addressed with an equalizer.

We flipped the enclosure around to direct the subwoofer toward the front of the vehicle and took another frequency response sweep. The blue trace shows very similar changes to those demonstrated with the woofer firing rearward, but the bump at 64 Hz is gone.

The last test location has the subwoofer at the back of the cargo area, once again up against the seats, but this time with the woofer centered in the vehicle. The cyan trace is still close to our reference measurement in this mid-sized SUV, with some loss below 25 Hz and small gains from 36 to 54 Hz. The differences aren’t earth-shattering in any way.

To wrap things up, we tilted the enclosure onto its back with it still right behind the seats and centered in the vehicle. The gains from 36 to 54 Hz disappeared and there was a few extra dB of output above 60 Hz.

If it won’t cause you a headache, here’s a zoomed-in graph with all of the traces visible at once. They are all very close from 27 to 36 Hz and vary by 6 dB at most between 36 Hz and 65 Hz. Given the kind of bass-heads we are, we like the idea of some gain at 23 and 26 Hz, but it’s minute and isn’t going to turn this into an SPL competition vehicle.

It should be noted that these measurements are of just the subwoofer. There are no midbass or midrange drivers operating in the vehicle during the test. In fact, the factory-installed audio system was turned off. If you’re trying to blend the output of the subwoofer with woofers installed in the doors, then phase measurements might be worthwhile. Without repeating all the graphs, we can let you know that having the subwoofer in the back corner of the trunk facing the interior delivered the smoothest phase response of the test group. The second-best in terms of phase was with the subwoofer behind the seat and pointed upward.

We plan to repeat this test in other vehicles to see how a sedan with a trunk performs. We have a theory that things might be quite different. Stay tuned for that!

Upgrade Your Car Audio System with a Subwoofer Today!

When you’re at your local specialty mobile enhancement retailer discussing where to mount the subwoofer system in your SUV, you can refer back to this article as a guide. Ultimately, no particular location was a disaster and none stood out as a big winner. We’d lean toward having the sub in the corner of the cargo area firing toward the center of the vehicle, given the need to choose an optimal solution.

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.

There are a lot of theories around the benefits of adding stuffing to a sealed subwoofer enclosure. Most contend that stuffing makes the enclosure behave as though it is larger, allowing the woofer to play louder at low frequencies. If this is true, then adding stuffing seems like a great idea. While creating many of our articles, we conduct many low-frequency enclosure simulations in BassBox Pro. Let’s see what the software predicts and then translate that into a real-world experiment with an enclosure in our lab.

Why Do Subwoofers Need Enclosures?

Speakers, specifically subwoofers, need an enclosure to limit cone excursion at extremely low frequencies. Without the added compliance of an enclosure, the subwoofer’s cone could exceed its excursion limits at high power levels and be damaged. Therefore, when designing an enclosure for a vehicle, technicians need to balance the requirements of the subwoofer to hit a target Q-factor and low-frequency cut-off point. The enclosure also needs to ensure adequate power handling and have a volume that will fit in the desired location in the vehicle.

Subwoofer Enclosure Specifications

For this experiment, we will use an audiophile-quality 12-inch subwoofer in a sealed enclosure with a net internal volume of 1.358 cubic feet. Before we began any simulations, we measured the Thiele/Small parameters of this particular subwoofer using our Woofer Tester 2.

In terms of specifications, our simulation in BassBox Pro predicts a -3 dB frequency of 40.4 Hz and a Qtc of 0.924. That’s a bit higher of a Q-factor than we’d typically target, but the enclosure will work well in the space we have allotted. Power handling remains well below the 17.6 mm Xmax specification when driven with the 500 watts the subwoofer is rated for.

For reference, we compared the predicted response of our woofer in the 1.35 cubic foot enclosure to the factory-specified 1.02-cubic-foot design. Our larger enclosure will yield 1.6 dB more output at 20 Hz than the smaller design. From that perspective, it’s barely worth the extra space. What we do like is the reduction of the system Qtc from 1.001 down to 0.924. The ideal damping value (Qtc) is considered 0.707. If you’re wondering what speaker or subwoofer Q is all about, give this article a read.

What Does BassBox Pro Predict When We Add Stuffing?

Bassbox Pro includes three options for simulations that include stuffing: Minimal, Normal and Heavy. We repeated the frequency response simulation using these options to see what the software thinks will happen. The results from BassBox show a decrease in the Q-factor of the enclosure

What Happens in terms of the System Q-factor?

In the un-stuffed enclosure, we see a predicted Qtc of 0.924. In the enclosure with the minimal stuffing, the Qtc drops to 0.906. In the normal scenario, it’s predicted to be 0.837. Finally, in the heavily stuffed enclosure, the Qtc is a well-controlled 0.755. As we aren’t looking for boomy bass, adding lots of stuffing should benefit our application.

Any time you get more output from a subwoofer in a sealed enclosure, there will be a correlating increase in cone excursion. This is true for a decrease in output as well. The graph below shows the effects of simulating different stuffing levels in the software.

Testing Our Sample Enclosure

For this experiment, I bought a pair of the least expensive pillows available at the local Wal-Mart. I think they were $3.99 each. They are listed as weighing 0.56 pound each, so it’s reasonable to assume there is 0.5 pound of the specified polyester stuffing in each of them.

Here are the results. Pleasantly, the predictions from BassBox Pro seem to have panned out, for the most part. The chart below shows the changes to the impedance measurements as we added more stuffing.

With no stuffing, the subwoofer presented an Fs of 43.35 Hz and the enclosure Qtc was 0.9532. With a half pillow’s worth of stuffing, those numbers dropped to 42.5 and 0.9148. A full pillow’s worth of stuffing in this 1.3-cubic-foot enclosure is what I’d consider tightly packed and resulted in the numbers dropping more to 42.1 and 0.919, respectively. By adding any more, I was genuinely concerned that I’d put too much in and that the density would negatively affect the performance of the subwoofer. I was wrong. The Fs dropped once again to 41.7 Hz, and the Qts was now a moderate 0.8397. That’s still a bit higher than I prefer, but the change was impressive and quite beneficial. Our simulation predicted that “normal’ stuffing would yield a Qtc of 0.836. I’ve never put that much stuffing in an enclosure. The take-away is, you need to pack it surprisingly tight.

So, does the addition of stuffing simulate a larger enclosure? It certainly seems so! If we want an unstuffed sealed enclosure for our subwoofer with a Qtc value of 0.8397, it would need to have 2.05 feet of airspace. We have a feeling that the larger empty enclosure is a little louder at the lowest frequencies. Nevertheless, we’ll take this compromise any day!

Is Resonance Control Relevant?

Look at the four graphs below, specifically at the dips around 180 and 510 Hz in the “empty” image. The interior length of this test enclosure is 26.5 inches, which is the same wavelength as a 512 Hz tone. There is a standing wave in the enclosure. The addition of more and more stuffing slowly reduces both of those little resonances.

These resonances don’t matter much for a subwoofer system where we already have the signal going to the sub attenuated by 20 or 30 dB by 180 Hz. However, if we were building a full-range speaker, the addition of the stuffing would make a noticeable difference in what we hear through the midrange and midbass regions.

The Theory on Subwoofer Enclosure Stuffing

Having a look through the owner’s manual for BassBox Pro reveals two very relevant statements about adding damping or stuffing to an enclosure. First, “the acoustical absorption inside the box will increase,” and second, “the box damping will increase, making the box seem bigger than it really is.” Our results confirm the second statement as being the most significant benefit. It should be noted that just as with a larger enclosure, the maximum output level drops, but so does the Q-factor of the system. For those interested in optimizing sound quality, this would seem like an ideal solution. For bass-heads, you might be better off leaving the interior of the enclosure empty to gain those extra few tenths of a decibel in output and efficiency.

Drop by your local specialty mobile enhancement retailer to have them design and construct a subwoofer enclosure that works perfectly for the subwoofers you’ve chosen and the available space in 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.