Enclosure, box or cabinet: Whatever you want to call them, where you install your speaker or subwoofer is critically important to their resulting performance. In this article, we focus on the simplest and most forgiving of enclosures to design and construct – the acoustic suspension or sealed enclosure.

The Laws of Physics

There are a few characteristics to keep in mind about every speaker. The first is that as frequency decreases, cone excursion increases. In fact, to produce the same acoustic output, a speaker must move four times as far for every halving of frequency. As an example, if your subwoofer were moving 1 mm at 80 Hz, it would have to move 4 mm to produce the same output at 40 Hz. To produce the same output at 20 Hz, it would have to move 16 mm.

A speaker includes an element called a spider. The spider stores energy when the voice coil of a speaker moves the cone forward or rearward from its resting position. When the cone reaches the end of its travel and comes to a stop, the stored potential energy in the spider wants to be released. This stored energy pulls the cone in the opposite direction. Each transfer of energy includes some losses, and eventually, the cone comes to rest.

Think of the cone motion like a swing at the park. You exert a force on the swing to get it started, and it continues to swing back and forth with a decreasing amplitude until it comes to a stop. Thankfully, a speaker stops moving a lot faster than the swing at the park.

In a speaker, this transfer of energy from the cone to the spider and back is most efficient at a specific frequency. We call this the resonant frequency of the speaker. At the resonant frequency, there is a dramatic increase in impedance because the spider stores a great deal of energy. This energy storage causes the cone to want to continue to move. The movement of the voice coil moving through the magnetic field generates a voltage. This voltage generates a flow of current in the opposite direction to the current flowing from the amp. We represent this opposition to current flow as an increase in impedance.

We also have to consider that every speaker is limited in how far the cone can move. Once we exceed the excursion limitations of the speaker, bad things happen. The voice coil former can hit the back plate. The suspension components may be compromised and start to fail. As a by-product of the cone, dust cap, surround, spider and motor geometry, harmonic distortion also increases as excursion increases.

Our goal in designing any audio system should be to keep distortion as low as possible. Most of the distortion at low frequencies is resonance. These resonances decrease as we move above the resonant frequency of the speaker. The spider and the changing motor force, as the coil moves past the edge of the gap, are the biggest contributors to distortion.

Why Do We Need an Enclosure?

Let’s consider a few additional characteristics. The low-frequency roll-off of a speaker is a high-pass filter. The spider in the speaker is like a capacitor—a spring stores energy and so does a capacitor. The air inside the box is also a spring, and it is in parallel with the spider. The air spring and the spider work together at the same time to do the same thing. The combination of the air spring and the spider increases the high-pass filter frequency. Yes: Contrary to our efforts to produce as much low-frequency information as possible, an enclosure limits low-frequency reproduction.

If that is the case, why do we want to limit cone motion? Consider what we’ve said about how much excursion is required to reproduce low frequencies and about distortion. Limiting low-frequency output from our speaker is not an ideal goal, but limiting some of the really low frequencies to get the right amount of bass at higher frequencies is worthwhile.

There is a benefit to increasing the resonant frequency of the speaker and enclosure system. Let us say we have a subwoofer with a Q of 0.5 and it is our goal to have a total system Q of 0.707. We choose an enclosure air volume that increases the Q, which then increases the system output at the new resonant frequency. Yes, we sacrifice output at lower frequencies, but we gain output around the new system resonant frequency.

I Want More Bass!

Modern speaker designs continue to reduce distortion through computer simulation and modeling of material behavior. Qualified and properly equipped speaker designers can simulate spider, cone and surround behavior to analyze individual resonance and distortion behaviors. They also can model the interaction between the voice coil and the motor structure to predict changes in magnetic field strength and inductance that can further affect how a speaker will sound at moderate to high excursion levels.

These advancements have resulted in speakers that produce less distortion at higher excursion levels. This improvement in performance allows enclosure designers to build speaker systems that will play lower and louder.

Some basic principles govern low-frequency sound reproduction. Cone area is critical. An old article published by the Audio Engineering Society called “The Problem with Low-Frequency Reproduction,” by Saul J. White, included a graph that compared cone excursion vs. frequency vs. system output for a 12- and 15-inch loudspeaker. In the chart, it shows that a 15-inch driver cone only has to move half as much as a 12-inch driver to produce the same output.

To produce sound, we need to displace air. Displacement is calculated by the product of speaker cone area times the distance the cone can travel. In other words, bore times stroke. For the same displacement, more bore requires less stroke.

What is the punch line? If you want it louder, buy more speakers or subwoofers.

Driver Behavior in an Enclosure

The increase in the system Q caused by the addition of air stiffness in the enclosure can cause distortion if the Q is increased excessively. This increase in Q works against our desire for a low-distortion system. Making the enclosure too small increases the Q too much, and we wind up with a system that produces a great deal of output in a narrow frequency range. These undersized enclosures are often referred to as a “one-note-wonders.”

What causes this behavior? The one-note quality is a result of the increased energy storage and transference in the resonant system. The bass just keeps going and going – like our swing at the park.

Power Handling

In an acoustic suspension enclosure, cone excursion increases as frequency decreases. This increase in excursion continues down to the frequency at which the force of the spider and the box exceeds the force of the motor. At that point, the excursion level is limited, and we will not see the increase in excursion . The result: We protect the speaker from physical damage due to cone excursion beyond the design characteristics of the speaker.

Predicting the limits of cone excursion relative to frequency and power is relatively simple for a sealed enclosure. The volume of the enclosure is inversely proportional to the amount of power the speaker can handle when perceived from the standpoint of excursion. A small enclosure limits cone excursion a great deal at very low frequencies, but the system does not produce a lot of deep bass. A large enclosure allows the speaker to move further and produce more low-frequency output, but we cannot drive the speaker with as much power for fear of damaging it.

As we increase the volume of the subwoofer enclosure, the air inside has less “spring effect” on the subwoofer’s motion. This graph shows the increase in driver excursion as air volume increases in four different enclosures.

Acoustic Suspension Overview

An acoustic suspension speaker enclosure reduces bass output at a rate of -12 dB per octave below the resonant frequency. When you combine this roll-off with the cabin gain associated with most vehicles, you can get excellent and linear low-frequency extension well into the infrasonic region. Acoustic suspension enclosures are easy to calculate and to construct. They are very forgiving of minor errors in volume calculation.

Finally, it is worth remembering that acoustic suspension enclosures are not the lowest-distortion enclosure designs available.

When it comes time to design a subwoofer enclosure for your car or truck, visit your local mobile electronics retailer and discuss your requirements. They can help you choose a subwoofer and enclosure design that will give you a solid foundation on which to build your audio system.

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.

Installing mobile electronics systems is not as easy as it used to be. In the “good old days,” cars were made from thick metal and didn’t have many electrical components. Modern vehicle construction makes use of the thinnest, lightest materials possible. These body panels are held together with non-conductive adhesives, and it seems that every square inch of our cars and trucks are packed with computers, sensors and modules. Even a task as simple as wiring an amplifier can become quite complex.

In this Best Car Audio article, we look at the importance of proper wiring techniques and materials, and the effect they have on the performance of an aftermarket audio system upgrade.

Power Delivery

This article is about installing a five-channel amplifier in a vehicle with a factory head unit. There are two important tasks when it comes to installing an amp: getting a signal to it and getting power to it. For an amplifier to produce power, you need to feed it power. The primary source of power in our vehicles is the alternator, followed by the battery. Most people focus on running a large conductor from the positive terminal of the battery to the positive terminal of the amplifier. While this is important, where and how the amplifier is grounded is equally important.

Quality installers will know the best locations on a vehicle to make a ground connection. They will know how to handle a vehicle with an aluminum or composite chassis. They will know how to recognize vehicles that are assembled with adhesives instead of spot welds. If you are installing a large amplifier, they will know how to upgrade the factory wiring under the hood to handle the extra current draw.

Protecting your vehicle is a part of installing power wire. In the unlikely event you get into an accident or something bad happens to your amplifier, the wiring and the equipment installed in your vehicle need protection in the form of a fuse. A good installer will install a fuse as close as possible to each source of power for the system. In most installations, this is near the main battery.

Some systems have secondary batteries and require additional fuses for proper protection. If you have the choice, a fuse provides better protection device than a circuit breaker. Fuses offer more surface area for power conduction. Also, there is absolutely no chance that a properly installed fuse could fail in a condition that may allow current to continue to pass.

Signal Integration

The audio systems in modern vehicles are becoming increasingly complex. Manufacturers (JBL, Infinity and Lexicon) are turning to suppliers like Bose and Harman for increasingly complex audio systems. Each of these companies employs highly trained engineers who spend months tuning each new system.

Trying to source an audio signal that will work with aftermarket equipment is becoming increasingly difficult at the same time. A properly trained installer will have the knowledge and experience to measure the signals in the factory system to determine if they can be used as is. If the signals are not acceptable in level, frequency response or bandwidth, your installer can recommend the correct component to correct them.

Your installer will also know to test each signal source across multiple functions. Ensuring that factory Bluetooth, navigation prompts and parking sensor warnings continue to work as intended is important to you being able to enjoy your upgraded audio system.

Noise Prevention

Dealing with factory computers, sensors, data networks and other high-current components can cause interference with the delicate low-level audio signals we send to our amplifiers. If you are not using good-quality interconnects that offer a twisted-pair design and appropriate shielding, you leave yourself open to picking up all sorts of strange noises. Likewise, choosing the right equipment is critical. You want to ensure that each component in the signal path has differential inputs to reject any noise that might be imposed on your interconnects, or the speaker wires running to the input of your amplifier.

The amplified signal coming from your amp is still subject to noise. Passive crossover networks can be notorious for picking up electromagnetic noise. Where your installer places the passive networks is quite important. An experienced installer knows what to avoid for your system to sound great and be noise-free.

Choice of Wiring Materials

There are two options for power and speaker wire – CCA and what most refer to as OFC. CCA stands for Copper Clad Aluminum. CCA typically is an inexpensive wire that is part aluminum and part copper. Aluminum costs less than copper, but it also does not conduct as well. You will usually see CCA wiring labeled with Gauge or GA when referencing its size. It is also worth noting that, unless the wiring you have chosen says AWG on it, there is no standard for the size of the conductor within the jacket.

Combine that with not knowing the ratio of copper to aluminum, and you run the risk of starving your amplifier for power. We have seen CCA 4 gauge wires that present almost four times the resistance as a proper 4 AWG conductor.

OFC stands for Oxygen-Free Copper. The term OFC has become the accepted slang term for all copper wiring. Properly sized copper wiring (using the AWG standard) offers the best possible power delivery to your amplifier.

Working on modern vehicles has its challenges, but those challenges come with rewards regarding background noise levels, convenience and features. Trust the installation of your audio equipment to qualified professionals. They have the experience to get the job done right the first time. Visit your local mobile electronics retailer to find out more.

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.