Understanding how DC voltages combine is quite simple. They add linearly when we wire sources (like batteries) in series. But understanding how AC waveforms combine is complicated. AC waveforms don’t always sum intuitively unless the peaks and valleys align perfectly. We see many examples where people interchange the terms phase and polarity. In reality, those words refer to different phenomena. Let’s take a close look at audio signal phase and polarity.

What Is an Audio Signal?

We will use sinusoidal waveforms as our example audio signals for this discussion. The sounds we hear are combinations of infinite frequencies. If you look at music on an oscilloscope, it would look something like the image below.

Phase refers to the relative timing between two waveforms. A single waveform can’t have phase. We must compare it to something else. As such, we will need a reference waveform for this article. You can see that below.

In Phase

Let’s start with the basics. When we describe two waveforms as “in phase,” we infer that the peaks and valleys are aligned. The starting point of the waveform should happen at the same time. The image below shows two waveforms that are “in phase.”

Two waveforms can be in phase but have amplitude differences. The image below shows that condition.

Reverse Polarity

We often see amplifiers with a switch that inverts the polarity of the audio signal passing through the devices. These are frequently and mistakenly labeled as “Phase.” Let’s explain.

Phase is the relative difference in time between two signals. Polarity refers to a contrasting waveform direction. In DC voltage, we know that if we put a voltmeter’s red lead on the battery’s positive terminal and the black lead on the negative, we’ll see a positive voltage. If we reverse the leads and put the red on the negative and the black on the positive, we’ll see a negative voltage on a digital meter. We call this “having the polarity backward.”

In AC waveforms, the same thing happens. If two waveforms have the same starting point and frequency, but one goes positive while the other goes negative, we refer to one being in the opposite polarity.

Here are our two audio waveforms in the oscilloscope. The yellow waveform shows the opposite polarity to that of the pink waveform.

Let’s consider this in terms of audio signals going to a speaker. The speaker driven by the yellow signal would start from rest and then initially move forward. The speaker driven by the pink signal would begin moving toward the magnet. Activating the polarity switch has the same effect as swapping the positive and negative speaker wire connections.

The image above shows an amplifier with the polarity control switch incorrectly labeled as phase. As a bonus, the amplifier has the infrasonic filter incorrectly marked Subsonic. This labeling is a fairly common error.

If the above amplifier could adjust the phase of the waveform of one signal to be 180 degrees from the other, the waveforms would look like this:

Phase

When discussing phase shifts, we include the peaks and valleys of a waveform and the starting time. This start point or time is especially crucial to discussing audio signals that are never sinusoidal. The starting times move if there is a phase shift between two signals. A difference in starting times implies a delay on one of the waveforms. We can do this quickly and deliberately with a digital signal processor. Adding delay is common in “time aligning” the left and right speakers in a stereo sound system.

Electricians and engineers often discuss phase angles in AC waveforms when driving inductive loads like electric motors. In these cases, the voltage applied to the motor may be “out of phase” with the current flowing through the motor. The same thing happens with speakers and passive crossover networks. These components’ reactive elements (inductance and capacitance) can cause the current and voltage to be “out of phase.” These phase shifts are why we can’t accurately measure the power out of an amplifier without measuring the phase angle between current and voltage. If that was confusing, don’t fret. These are concepts that engineers and technicians learn about in college.

We often talk about degrees when discussing the phase of two waveforms. One complete cycle of a sine wave is 360 degrees. A half-cycle is 180 degrees. The angle between when the sine wave initially moves from the 0-volt level to the first peak is 90 degrees. Here is an example of a 90-degree phase shift between two waveforms:

Here’s what a 360-degree phase shift looks like:

Understanding that the starting points of the two waveforms differ is crucial to understanding how audio waveforms interact. If you were to “align” the peaks and valleys, we’d think these signals were “in phase.” This practice is a common issue for people who try to “time align” a subwoofer to a midbass speaker by reversing the polarity of one relative to the other, then adjusting a delay so there is a sizeable acoustic null shown on an RTA at the crossover frequency. This practice doesn’t consider the start time of the audio signals. Considering phase and start time is required to deliver a system where the sound from the subwoofer arrives at the listening position at the same time as the midbass or midrange drivers.

Why Are Signal Phase and Polarity Important in Car Audio Systems?

For people who’ve upgraded their vehicles with a radio and two pair of speakers, the primary concern is that the acoustic polarity of all the speakers in the system is the same. When a speaker produces a sound, it should combine acoustically with the output of other speakers in the system. You can test this quickly without any special tools.

Play music with a reasonable amount of bass. Use the balance and fader controls to listen to one speaker at a time. Start with the front left speaker. Now, use the balance to add the front right speaker so both play. The relative bass level should increase when you add the second speaker. If it doesn’t, the wiring likely has a polarity problem.

Repeat the test with the front left speaker playing, then use the fader control to add the left rear speaker. Once again, the bass should increase. If it does, fade rearward so only the left-rear speaker is playing. Now, use the balance control to add the right-rear speaker. Once again, we should get more bass. If the bass decreases when a second speaker plays, drop by a local specialty mobile enhancement retailer and ask them to check the wiring.

Signal Summing and Cancellation

Sometimes the distances between speakers cause incomplete summing of all frequencies. Let’s go back to looking at audio waveforms for a second. If two signals have the same amplitude, frequency content and starting times, how they add together (called summing) is predictable.

In the example above, the signals combine perfectly and the total amplitude doubles. An example would be listening to music at home on a set of stereo speakers and sitting equidistant from both speakers. When it’s perfect, or at least pretty good, signals that are equal in amplitude and in phase in both channels appear to come from a spot between the speakers. Some call this a “phantom center image.” It’s just how stereo recording works and doesn’t require a fancy name.

Let’s look at how signals with different phase relationships sum. First, let’s look at two waveforms, of which one is reversed in polarity.

If one speaker moves outward and the other moves inward, their output should cancel. If you have a pair of subwoofers and one is wired backward, the result is that the system produces no bass.

Now, what happens when one signal arrives before another? Here are the waveforms if one signal lagged the other by 90 degrees:

A few bad things happen when signals don’t start simultaneously. In terms of the combined waveforms, the amplitude only increases by 25% instead of 100%. This happens if you’re at home, sitting much closer to one stereo speaker than the other. The same happens in your car, where the left door speaker is maybe 30 inches away, and the right is 45 inches away.

Audio Content and Signal Summing

Remember, music is not a single frequency, as we’ve shown in the oscilloscope plots. It’s a combination of thousands of frequencies. We don’t talk about the phase between audio signals; we talk about delay times. Every frequency has a different wavelength. So if there is a timing difference between the signals, some frequencies combine, and others cancel. The result produces an uneven frequency response.

Let’s combine some signals in Adobe Audition to demonstrate this.

I created two sine sweeps. These are 20-second tracks that start at 20 Hz and increase in frequency by 20 kHz. The image below shows that the left and right channels are equal in amplitude and phase.

Let’s look at the frequency response of the two signals:

The frequency response graph isn’t perfectly flat because I don’t have control over the speed at which the sweep occurs. In short, the bass is present for longer than the highs. What you need to know is that the response is flat and smooth.

Let’s add about 1.7 milliseconds of delay to the right channel waveform. This is the time it takes sound to travel about 24 inches. That’s not an uncommon path length difference between a vehicle’s left and right speakers.

As you can see, the results are dramatic. The frequency response is full of dips. We call this comb filtering, which happens when two sound sources are at different distances from the listening or measuring position. This could be attributed to path length differences between speakers or to a speaker’s sound reflecting off a surface and recombining with the direct-path sound. If there are path length differences, your installer can use a digital signal processor to delay the signal to the closer speaker. There isn’t much you can do about reflections other than to relocate the speakers.

Audio Signal Phase and Polarity

We hope this serves as a primer to help you understand the concepts of audio signal phase and polarity. A solid understanding of these concepts is crucial to designing and calibrating car audio systems that sound amazing. If you want better sound from your car audio system, drop by a local specialty mobile enhancement retailer today. Ask to hear one of their demo vehicles and discuss the options for improving your vehicle’s stereo.

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 will always be debates about whether a Class D amplifier can sound as good as a Class AB. We’ve proven that it all depends on who designs the amplifier. We also see mumblings about single-ended amps versus full-bridge designs. STMicroelectronics created a different and unexpected way to deliver efficient power. These amplifiers are categorized Class SB and Class SBI. Let’s see if we can explain how they work.

Amplifier Output Device Topologies

Before we dig into the Class SB amplifier, we need to clarify a few things. There are two ways to configure the driver devices (transistors or MOSFETS) to provide large amounts of current to speakers. In a Class A configuration, the output device is halfway on when no audio is playing. The audio signal can modulate the output from its resting point at 50% up to nearly 100% and down to nearly 0%. While extremely linear, these designs are notoriously inefficient and waste large amounts of energy. A Class A amplifier circuit has a maximum efficiency of 25% and only gets worse as the output level decreases from full. Playing nothing at all, a true Class A amplifier wastes half the maximum current delivery capability of the power supply as heat.

In a Class B amplifier, we dedicate one transistor to the positive half of the audio waveform and a second to the negative half. When no music is playing, the output devices are functionally off.

We have to add a little voltage to the output devices so that the transition from one to the other is smooth. Many low-quality amplifiers don’t do this well. In those instances, we’ve seen a little step in the waveform through the transition. This step is called crossover distortion. When executed smoothly, there is no step. This mode of operation is commonly called a Class AB output device configuration. In reality, it’s just a voltage-biased Class B, but there’s clearly no going back on the name now.

In this context, Class A and Class B are called output device topologies. These are the only two ways output devices can be wired.

Class D Operation

When we talk about Class D amplifiers, we aren’t discussing how the output devices are configured. Class D is a description of the signal used to drive the output devices. Most Class D amplifiers use a Class AB output device configuration.

As you can see in the image below, the analog audio waveform is chopped into little pieces by the Class D driver IC. The width of the spikes relative to the switching frequency represents the output level. Very narrow spikes produce small amounts of output, and very wide spikes produce high output levels.

The benefit of Class D operation is that the output devices are switched fully on or off. They spend very little time part-way on. In essence, they block all current flow or allow it all through. Transistors and MOSFET devices are least efficient when they allow half the current through – as we saw in a Class A configuration. The result is a dramatic improvement in efficiency. Many well-engineered Class D amplifiers have 92% total efficiency.

Amplifier Integrated Circuits

Before we go off the rails with Class SB, let’s look at dedicated amplifier ICs. Almost every car radio for the last few decades uses a single chip as an amplifier. These chips typically have four channels of amplification and all the protection circuitry required to prevent DIYers from blowing up their radios. Most IC amplifiers provide 16 to 21 watts of power into 4-ohm loads from each channel.

Sony took things further with their High Power radios and used a Texas Instruments IC called the TAS5414C. In their head unit applications, I’ve measured over 42 watts of power from each channel. In what Sony calls their Subwoofer Direct mode, I’ve measured over 76 watts of power.

Bridge-Tied-Load Amplifiers

Most head units have only the vehicle’s battery voltage available to drive the speakers. They don’t typically have step-up power supplies like an amplifier. As such, we can only get about 13.5 or 14 volts across the speaker terminals. This means we are limited to a theoretical maximum of 24.5 watts. In reality, we see a few watts less as some voltage is wasted in the amplifier circuitry.

If your installer were to look at the output of a car radio at full power on an oscilloscope, you’d see the following waveforms on the speaker wires.

If we turn the volume down to almost nothing, we’d see the following:

As you can see, both speaker wires have a DC offset voltage. When no audio is playing, there’s about 6 to 6.5 volts present on the speaker wires. Because the voltage is common to both wires, the speaker doesn’t move. The speaker only responds to differences between the wires.

Now, let’s look at a single-ended amplifier. This would be an example of one channel of a typical car audio amplifier.

You can see that the probe with the green trace rests at the ground voltage. The probe with the purple trace shows the output voltage swinging from positive to negative and back. The speaker will move forward and rearward to follow the purple waveform.

STMicroelectronics’ Class SB and Class SBI

OK, now you should understand how output-switching devices can be configured and how different waveforms can be used to increase efficiency. STMicroelectronics combined things in the Class SB amplifiers to create something unique and, to put it mildly, creative. STMicroelectronics uses the SB abbreviation for “single-ended bridged” and SBI as “SB improved.” Their claim is Class AB sound quality with a 50% improvement in efficiency.

Class SB amplifiers function as single-ended amplifiers at low to moderate power levels. The waveform sweeps from negative to positive on one speaker lead while the other rests at ground.

If this were a typical Class AB amplifier, the output waveform would clip if we increased the signal to the amplifier such that the output tried to exceed the rail voltage limits. That would resemble the waveform below.

The Class SB amplifier gets creative when it runs out of rail voltage at high output levels. Check this out.

When the main output reaches clipping, the “normally at ground” output increases voltage in the opposite direction. The result is a net increase in amplitude. That’s very creative. In application, having full control over every component in the amplifier is crucial to this design functioning properly. With everything housed in a single IC, STM has that control.

Is a Class SB Amplifier Better?

Before claiming that STMicroelectronics has reinvented the car audio amplifier, consider that these ICs have specific applications. They are designed to be compact and efficient. As such, amplifiers created with them at the core can be compact with smaller heatsinks than an equivalent Class AB amplifier. Further, these amplifiers are available with digital inputs. This connectivity makes them ideal for integrating into a closed-network infotainment system, like MOST, AVB or A2B offer. The amplifiers produce about 45 watts of power. This might be good for the main channels of a factory-installed car audio system but won’t be adequate for high-power aftermarket solutions or subwoofers.

What Do Consumers Need to Know about Class SB Amplifiers?

So, what does the typical consumer need to know about Class SB amplifiers? The answer is not much. That said, knowing what new technologies are in use is always good. Class SB amplifiers are primarily a solution for vehicle manufacturers and low- to mid-power audio systems. Installers and technicians must understand how to recognize audio signals from a Class SB amplifier when integrating digital signal processors and new amplifiers into a vehicle. Many, but not all, aftermarket audio upgrades work with Class SB-powered factory-installed source units. If you want more performance from your car audio system, drop by a local specialty mobile enhancement retailer and ask them about the amplifiers, speakers, source units and subwoofers that will deliver the sound you want.

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.

Over the last decade, the popularity of under-seat truck subwoofer upgrades has exploded. At least a dozen companies offer vehicle-specific solutions with shallow-mount subwoofers to add bass to these vehicles. If you’re perusing the enclosure options for your truck, you’ll notice two distinct options: enclosures with drivers that face upward into the seat bottom and those that face down into the floor. Is one design better than the other? Let’s discuss.

Truck Subwoofer Enclosures

Before we dive into the potential differences between up- and down-firing truck enclosures, let’s talk about the purpose of an enclosure. First and foremost, a subwoofer enclosure separates the sound coming from the back of the cone from the sound coming from the front. If you took a subwoofer out of its shipping carton and held it in your hand while playing music, you’d find it produces no bass. The lack of output is because the sounds coming from the front and back of the cone cancel each other out. When we put a subwoofer in an enclosure, it traps the sound coming from the back of the cone, and we only hear it from the front.

The second purpose of a subwoofer enclosure is to act as a high-pass filter to limit subwoofer cone excursion at very low frequencies. Without an enclosure, driver damage is possible with even moderate amounts of power. The air in the enclosure changes the compliance of the subwoofer suspension. More accurately, it makes it harder for the cone to move. Yes, this does seem counterproductive. Subwoofers need an enclosure to play 35 to 65 hertz loudly while remaining controlled at lower frequencies. Getting the enclosure design right is very important to the performance of the subwoofer system.

Down-Firing Subwoofer Enclosures

Down-firing subwoofer enclosure designs that aim the subwoofer into the floor are not new. There are many myths about down-firing subwoofers, though. Theories about low-frequency information reinforcement and “tightness” changes are prevalent. At extreme excursion levels, the proximity to the floor might result in a small amount of pressure pushing back on the cone. The closer to the floor the subwoofer is, the more this is a possibility. Yet it won’t significantly affect performance.

If you’ve read the BestCarAudio.com series of articles on Understanding Subwoofer Quality, you know that all subwoofers (and speakers) add moderate amounts of harmonic distortion to the signals they produce. These harmonics are typically second- and third-order, so twice and three times the fundamental frequencies. If a subwoofer plays a 50 hertz tone, the second harmonic is 100 hertz, and the third is 200 hertz. One benefit of firing a subwoofer down into the floor is that these higher frequencies might be somewhat attenuated. The attenuation amount depends on the floor’s proximity and the materials under the subwoofer (carpet, etc.).

Up-Firing Truck Subwoofer Enclosures

With an up-firing enclosure, we have a seat cushion to filter higher-frequency harmonic content. The difference between up- and down-firing subwoofer enclosures might not vary significantly in this context.

With a down-firing enclosure, the designer must provide adequate space between the subwoofer and floor to ensure that the cone and surround will never touch anything. There should also be sufficient space so that the sound produced by the subwoofer can escape into the vehicle. These criteria might result in the subwoofer being 1.5 to 2 inches off the floor. The drawback here is that this wastes some enclosure volume.

If the subwoofers are firing upward into a seat cushion, a grille over the subwoofer will allow the seat to sit directly on top of the driver. Bass frequencies can pass through the seat cushion with minimal obstruction.

If you’re shopping for a subwoofer enclosure for your truck, ask the product specialist you’re working with about the enclosure volumes. The number one issue with truck enclosures is volume. Most of them are too small, which results in boomy and sloppy bass reproduction. Choose the enclosure that has the most internal air volume.

But Wait, There’s More!

In researching this article, I read a few dozen discussions about down- and forward-firing home theater subwoofer enclosures. A common issue repeatedly demonstrated a misunderstanding in system design and calibration. There were many comments about how a subwoofer blended better with the main speakers firing backward into a wall versus out into the room. People had taken measurements with Room EQ Wizard to further the matter to corroborate their observations.

Of course there will be a difference! The subwoofer and the midbass speakers must be in phase at the crossover power for the transition to be smooth. Turning a home theater sub around without changing settings will reverse the effective acoustic polarity. Unless you’re unlucky and your sub is 90 degrees out of phase with the main speakers, one direction will work better than the other. Most home theater subwoofers have a polarity control switch on the amplifier panel. Better units will have a phase control knob.

What’s the takeaway from this second consideration? System calibration is crucial to getting the most from your car audio system. If you don’t have a digital signal processor (even though you should), then the technician installing the system should focus on speaker polarity and levels to produce the smoothest response possible. If you have a DSP, getting the phase between the subwoofers and the rest of the speakers right is crucial. This alignment can be fine-tuned with polarity and delay adjustments. When appropriately configured, and you have high-quality speakers and subwoofers that don’t add a lot of distortion, the bass will seem to come from the front of the vehicle. Midbass frequencies will be tight and dynamic with fantastic impact.

Should I Choose an Up- or Down-Firing Truck Subwoofer Enclosure?

The final answer to choosing an up- or down-firing truck subwoofer enclosure is to select the solution that allows the subwoofers to sound their best. In most cases, this will mean going with an enclosure that offers the largest volume to reduce unwanted resonance. You may want to read the BestCarAudio.com article about enclosure stuffing to reduce resonance in sealed subwoofer enclosures. Having your installer pack the enclosure with Dacron or something similar might offer a significant improvement in sound quality.

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.

You should be aware that copper-clad aluminum wire is the least-preferred option for connecting an amplifier to your vehicle’s battery or alternator. Copper-clad aluminum wire has more resistance for a given diameter than an all-copper cable. As a result, it wastes precious voltage and transforms it into heat when it could be allowing your amplifier to make more power. Why do many high-quality speakers and subwoofers use CCAW in their voice coils? Read on for an explanation.

Speaker Parameters and Frequency Response

No single specification can describe how a speaker or subwoofer will perform. To create a product that delivers flat response through its designed operating range, speaker design engineers need to balance several factors to achieve their design goals. These factors include the speaker’s size, resonant frequency, cone mass, suspension compliance, inductive properties and target power-handling capabilities.

The graph above shows the predicted frequency response of a Rockford Fosgate T4652S woofer in red. The low-frequency response and efficiency are based primarily on the driver’s suspension compliance and the cone’s mass. The speaker’s voice coil inductance is what determines the high-frequency roll-off.

The yellow trace is the same driver, with 5 grams added to the moving mass value. As you can see, the driver is a few decibels louder at frequencies below 80 Hz and a few decibels less efficient above that frequency. The decrease in efficiency happens because the assembly is more challenging to accelerate. A second change is that the resonant frequency is lower.

The green trace is the same Rockford Fosgate driver simulation with 5 grams subtracted from the moving mass. As expected, the driver would produce less bass but be more efficient at higher frequencies.

Speaker engineers work hard to balance all these criteria to produce a product that will blend well with your subwoofers and offer good efficiency. If we take a given speaker design and add mass to the cone assembly, we increase how much bass it can produce but decrease its efficiency. Likewise, reducing the mass results in an increase in the speaker’s efficiency and a reduction in bass output.

Voice Coil Impedance Matters

Another factor that’s crucial to speaker design is voice coil impedance. When an engineer sets out to create a speaker, one of the target specifications will be for the driver to have a specific nominal impedance. Designing a speaker with a 2-ohm impedance doesn’t work if it’s intended for use with an amplifier designed for 4-ohm loads.

Speaker designers have different options for voice coil wire diameter to change the impedance of the winding. More wire means more impedance. Smaller diameter wire also increases the impedance.

However, they must create a voice coil winding tall enough to ensure that the driver remains linear at its excursion limits. The voice coil winding must also be tall enough to handle the heat it will dissipate. Dainty, small-diameter windings, like you’d find in entry-level speakers, don’t have enough physical size to dissipate large amounts of heat. When they get hot, the varnish on the wire melts, and the windings may short together or unravel. The result is a permanently damaged woofer. As such, larger-diameter windings, which use more wire, are necessary.

Another way to increase thermal power handling is to add additional layers to the voice coil. You may have seen four- and six-layer voice coils in some high-power subwoofers. These designs can handle more power than a two-layer coil. However, more copper means more resistance and more mass. The result of these mass increases is decreased speaker efficiency.

The Benefits of CCAW Voice Coils

Speaker design engineers often turn to CCAW to add resistance to a voice coil winding. For example, we might want a speaker to have a nominal impedance of 4 ohms. As such, the voice coil will require a specific amount of copper wire to hit that impedance. The designer could also use 1.62 times as much CCAW of the same diameter wire to reach the same impedance. We also want the voice coil assembly to hit a target mass to work with the cone, dust cap, spiders and surround to achieve a target mass. The choice of material for the voice coil former is also a variable.

CCAW has less mass per unit length than all-copper conductors. It also has more resistance per unit length.

Let’s look at the mass of a CCAW conductor that is 25% copper and 75% aluminum. First, we know copper has a specific gravity of 8.96. Aluminum has a specific gravity of 2.7. If we combine these in a ratio of three parts aluminum to one part copper, we have an alloy with a specific gravity of 4.265. That’s just under half the specific gravity of copper alone. For the sake of simplicity, you can generalize that CCAW weighs half as much as copper. The actual mass depends on the ratio of copper to aluminum, which varies significantly from each supplier.

Next, let’s look at resistance the same way. For a given area, copper has a resistance of 1.7×10-6 ohms per centimeter. Aluminum’s resistance is 2.7×10-6 ohms per centimeter. Most CCAW wiring offers 0.62 to 0.64 of the conductivity of pure copper.

So, a speaker designer can increase the length of the conductor in a voice coil using CCAW wire to hit a target size and impedance. They can switch to CCAW wire to decrease the mass of a voice coil assembly.

There’s No Correlation Between Amplifier Wiring and Voice Coil Design

So the next time you hear someone put down a speaker design because it features a CCAW voice coil, remind them that speaker design and power delivery goals are unrelated. When powering an amplifier, we want the lowest resistance wires possible. We want the lowest resistance wires possible when connecting a speaker or subwoofer to an amplifier. As such, we should use large-gauge, all-copper wiring. When it comes to speaker design, the goals are very different. The engineer wants a voice coil with a specific length, impedance and mass. A conductor with more resistance and less mass per unit length is one of the variables the designer can change to meet their goals.

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.

What happens when a subwoofer is used with an oversized subwoofer enclosure? This article will look at the good, the bad and the ugly in terms of an improper match between the electromechanical characteristics of a subwoofer and the need for a proper enclosure.

Why Do Subwoofers Need an Enclosure?

If you’ve been following our series of articles on understanding subwoofer quality, then you’ll know that the purpose of a subwoofer enclosure is to act as a mechanical high-pass filter. That’s right: The enclosure limits how much low-frequency information a subwoofer produces. Why? Well, below about 250 or 300 Hz, a speaker cone has to move four times as much for every octave lower it plays. Put another way, if you have a subwoofer playing a 100-Hz tone and it’s moving 0.25 mm, it needs to move 1 mm to produce the same output at 50 Hz, and 4 mm to produce the same result at 25 Hz. It would theoretically have to move 16 mm to make the same output at 12.5 Hz.

Why is cone excursion a problem? Well, the suspension components (surround and spider), along with the length of the voice coil and thickness of the top plate, determine how far the cone can move linearly. Linearity is vital because distortion skyrockets when things get non-linear. Think of this like mechanical clipping. It can sound just as bad as an amplifier that’s clipping. In extreme cases, mechanical components of a speaker crash into each other, which can cause damage.

If you’ve ever analyzed the frequency content of popular music, you’ll know that most of it has very little audio information below 35 Hz. The chart below shows the frequency content of several songs. The trace in red is “Dance the Night” by Dua Lipa. The orange trace is “Industry Baby” by Lil Nas X feat. Jack Harlow. The yellow trace is “Paint the Town Red” by Doja Kat. “Slime You Out” by Drake is in green. “Clocks” by Coldplay is in blue. Finally, the violet trace is “I’m Shady” by Eminem. There is nothing below 35 Hz of significance in any of these tracks. That’s not to say that some music doesn’t contain significant amounts of deep bass and even infrasonic information – it’s just less common than you think.

If we can control how much the speaker cone moves at very low frequencies, we can apply a lot of power to a speaker around and above 35 Hz so it will play loudly while preventing the speaker from bottoming out. An infrasonic filter, often mistakenly called a subsonic filter, does the same thing. The electrical filter has one huge benefit: It eases the load on the amplifier, the vehicle’s electrical system and the thermal power handling requirements of the subwoofer.

A Quick Comparison of Acoustic Suspension and Bass Reflex Subwoofer Systems

Two popular types of subwoofer enclosures are used in car audio systems: acoustic suspension (sealed) and bass reflex (vented or ported) designs. An acoustic suspension enclosure is a simple sealed cabinet. The math that predicts the frequency response and excursion of the subwoofer is relatively simple, and the performance is predictable. Notably, there’s no significant change in performance if the volume of an acoustic suspension enclosure varies slightly from the specifications. A bass reflex enclosure is typically larger and includes a vent (or port) of a specific area and length to create a resonating system. At its resonant frequency, most of the sound from the enclosure comes from the vent, and the subwoofer cone moves very little. This is why using a large enough vent with proper radii on both ends is crucial.

The traces in the above image are very typical in terms of this comparison of sealed and vented enclosures. The red trace is the sealed enclosure with a net volume of 1.02 cubic feet. We can see that the low-frequency cone excursion plateaus below 50 Hz. Why? The compliance of the air inside the enclosure adds to the compliance of the subwoofer’s suspension to limit motion. In short, the suspension becomes stiffer.

The yellow trace represents a vented enclosure with a net volume of 1.79 cubic feet and a vent tuned to 40 Hz. Around 60 Hz, the cone excursion on the vented enclosure is slightly higher. This increase happens because the enclosure is larger. Around the tuning frequency of 40 Hz, the woofer cone barely moves. The graph below shows the velocity of the column of air in the vent at the same 400-watt drive level.

The yellow trace shows us that the vent’s air column moves the most at 40 Hz. This makes sense, as this small area produces most of the sound from the enclosure at this frequency.

How Acoustic Suspension Enclosure Volume Affects Response and Excursion

Let’s start looking at how changes in enclosure volume affect frequency response and cone excursion. Considering both of these criteria is crucial since the enclosure’s purpose is to prevent our subwoofer from bottoming out.

Let’s use a roughly 25% variance in enclosure volumes so that the differences in performance are clearly visible. All drivers can work within a range of volumes, so 10% either way just isn’t going to make a huge difference.

In the graph above, I’ve modeled the predicted response of a Rockford Fosgate Punch P2D2-12 subwoofer, which is in the BestCarAudio.com lab for an upcoming Test Drive Review. The red trace is our 1.02-cubic-foot enclosure that matches the acoustic suspension enclosure suggestion on the Rockford Fosgate website. The green trace is a 0.75-cubic-foot enclosure. As expected, using this enclosure reduces output below 65 Hz and increases the resonant peak up to around 80 Hz. Going the other way, we have a 1.25-cubic-foot enclosure in light green and a 1.5-cubic-foot enclosure in orange. The increase in low-frequency output below 65 Hz isn’t significant, which tells us the driver’s suspension plays a more substantial role in cone control.

Regarding outright “accuracy,” the larger enclosures have a lower total system Q. This value is called the Qtc. Lower Qtc values correlate to less resonance and, consequently, tighter sound. The 0.75-cubic-foot enclosure has a Qtc of 0.976, which is considered high. The “optimized” 1.02-cubic-foot enclosure has a Qtc of 0.881, which is still on the high side but gives us a good upper bass kick with some added efficiency. The 1.25- and 1.5-cubic-foot enclosures have Qtc values of 0.826 and 0.781. Many consider a Qtc of 0.707 to be the ideal Q-factor. In reality, it’s a good balance of efficiency and damping. The ideal Qtc is 0.5, but an enclosure with enough volume to produce that value can result in excursion issues. The Qts value of this subwoofer is 0.52, so it’s impossible to get a Qtc value lower than that.

Speaking of excursion, let’s look at a comparison of cone excursion versus frequency for our four sealed enclosures.

As expected, the larger enclosures allow the woofer cone to move more at lower frequencies. Fortunately, this subwoofer has an Xmax specification of 13.3 mm, so we don’t run into any issues at any frequency at this power level.

Let’s look at an extreme comparison of sealed enclosure volumes.

The graphs above show the 1.02-cubic-foot enclosure predicted response in red and the subwoofer’s response in a 3.0-cubic-foot enclosure in violet. Larger enclosures mean more output at lower frequencies. This massive enclosure produces 3.3 dB more output at 30 Hz. That’s not a huge gain, but it represents an increase similar to what you’d hear if you could provide the subwoofer with twice as much power.

We do run into a problem, though. At frequencies below 20 Hz, the driver will exceed its 13.3-millimeter Xmax specification. A nice shallow -6 dB/octave infrasonic filter set to 20 Hz would protect this driver from damage should your music contain audio below 20 Hz. Your installer would have to use a DSP to configure an infrasonic filter like this.

Bass Reflex Enclosure Volume Comparisons

Let’s look at bass reflex enclosures with the same Rockford Fosgate P2D2-12 driver. We’ll start with the 1.79-cubic-foot suggested enclosure as our reference, then go up and down by 25% while maintaining the same 40-hertz tuning frequency.

In the chart above, the yellow trace is our reference 1.79-cubic-foot enclosure tuned to 40 Hz. The green trace represents an enclosure volume of 1.35 cubic feet, the light green trace is 2.24 cubic feet, and the orange is 2.68 cubic feet. The results show that the system’s peak output frequency moves closer to the tuning frequency as volume increases. Let’s look at the vent air velocity graph to see if it corroborates this hypothesis.

The maximum vent air velocity frequency moves closer to 40 Hz as the enclosure volume increases. It’s worth noting that the 4-inch diameter vent isn’t suitable for the largest volume enclosure as the velocity exceeds 34.5 meters per second. The vent must be larger and subsequently longer to function correctly with this enclosure volume. Analyzing vent air velocity is another crucial part of subwoofer enclosure design.

It should be no surprise that the larger enclosures provide less control over woofer cone excursion. Fortunately, the high tuning frequency controls cone motion around 50 to 60 Hz. Below the tuning frequency, we need an infrasonic filter set to 27 Hz for the largest enclosure to keep things under control at this power level.

Effects of Tuning Frequency on Output and Cone Excursion

We put woofers in larger enclosures to get them to play louder at lower frequencies. Let’s look at what happens if we change the tuning frequency of our reference 1.79-cubic-foot enclosure.

The chart above shows our reference enclosure of 1.79 cubic feet tuned to 40 Hz in yellow. The green trace is the same volume tuned to 45 Hz. The light green trace shows the enclosures’ predicted response when tuned to 35 Hz. The orange trace represents a tuning frequency of 30 Hz. There are no surprises here as we trade upper bass output for more output at lower frequencies.

Once again, there are no surprises shown here. The dip in cone excursion aligns with the different tuning frequencies. We don’t have any excursion problems above the tuning, and an infrasonic filter will keep things safe at lower frequencies.

What happens if we combine smaller and larger enclosure volumes with different tuning frequencies? In this example, with the Rockford Fosgate subwoofer, things work out. But that’s not always the case.

In the graph above, the yellow trace is our 1.79-cubic-foot enclosure tuned to 40 Hz. The green trace is 1.35 cubic feet tuned to 45 Hz. The light green trace is 2.24 cubic feet tuned to 35 Hz. Finally, the orange trace is 2.68 cubic feet tuned to 30 Hz.

Knowing that larger enclosure volumes focus the subwoofer system output around the tuning frequency, we should be able to get even deeper bass if we combine a larger enclosure with deeper tuning. Sure enough, the graph above supports this. The 2.68-cubic-foot enclosure tuned to 30 Hz would rumble! Driver excursion limits are very similar to the previous simulations.

Now, can we go too far with variances in enclosure volumes? What if someone doesn’t understand balancing speaker protection with frequency response and constructs a massive enclosure with a very low tuning frequency? Let’s look at that.

The massive 6-cubic-foot enclosure shown above is tuned to 17 Hz and has an F3 frequency of 24.5 Hz. On paper, this looks like a great home theater subwoofer enclosure. However, looking at the excursion graph shows us we have a problem. At 26.5 Hz, the massive enclosure isn’t controlling cone excursion. We are within a millimeter of the driver exceeding its limits. Remember from our subwoofer quality comparisons that distortion increases with excursion. So this solution likely won’t sound all that great, even if it produces lots of output at low frequencies.

Lesser subwoofers used in moderately large enclosures often encounter excursion issues above the tuning frequency. This is common with drivers with very compliant suspensions.

Proper Enclosure Modelling is Crucial

We see people on the internet “designing” subwoofer enclosures with free software daily. We love that enthusiasts want to experiment and build their audio systems. However, the instructions for the software don’t include knowing how to interpret the data these software packages deliver. You’ll find many expert mobile enhancement retailers who know how to combine enclosure designs with digital signal processing and the natural transfer function of a vehicle to deliver impressively compact and efficient subwoofer enclosure solutions that sound phenomenal and operate reliably.

So what happens when you put a subwoofer in an oversized subwoofer enclosure? Well, you might get more deep bass. You might get more controlled bass. You might run into excursion-based power handling issues and damage the driver. Thankfully, subwoofers from top-tier brands like Rockford Fosgate have a balance of suspension design and excursion capability, making them very flexible and resistant to enclosure design and construction errors. If you want the bass in your vehicle to sound the best it can, drop by a local specialty mobile enhancement retailer and have them design a custom enclosure that maximizes the available space in your vehicle and ensures that the subwoofers you’ve chosen will work well with your amplifier.

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.