When we talk about any signal, be it audio, video or data, there is an accompanied reality for alterations and errors made to that signal as it passes through different electronic components, conductors or magnetic fields. While we get concerned when we hear that a component introduces distortion or when we read distortion specifications, distortion is part of nature and is simply unavoidable. Until any distortion reaches a significant level in an analog signal, it can’t be heard or seen.

Starting With A Foundation in Audio Distortion

With that in mind, let’s create a foundation for observing and understanding the properties of an audio signal in the electrical and frequency domains. This information will serve as the foundation for understanding distortion in part two of this article.

Any signal, be it Direct Current (DC) or Alternating Current (AC), can be analyzed in two ways – in its time domain or frequency domain. Understanding the difference between these two observation domains will dramatically simplify the life of anyone involved in the mobile electronics industry.

When we observe a signal in the time domain, we are looking at the amplitude of the signal relative to time. Normally, we would use a voltmeter or oscilloscope to look at signals in the time domain. When we consider a signal in the frequency domain, we are comparing the amplitude (or strength) of individual frequencies, or groups of frequencies within the signal. We use an RTA (real time analyzer) on a computer or handheld/benchtop devices to look at the frequency domain.

Direct Current

When analyzing the amplitude of an electrical signal, we compare the signal to a reference; in 99% of applications, the reference is known as ground. For a DC signal, the voltage level remains constant with respect to the ground reference and to time. Even if there are fluctuations, it is still a DC signal.

If you were to chart the frequency content of a DC signal, you would see it is all at 0 hertz (Hz). The amplitude does not change relative to time.

Let’s consider the DC battery voltage of your car or truck. It is a relatively constant value. Regarding amplitude versus time, it sits around a 12.7-12.9 volts on a fully charged battery with the vehicle off. When the vehicle is running and the alternator is charging, this voltage increases to around 13.5 to 14.3 volts. This increase is caused because the alternator is feeding current back into the battery to charge it. If the voltage produced by the alternator was not higher than the resting voltage of the battery, current would not flow and the battery would not be recharged.

Alternating Current

AC Signal – Time

If we look at an AC signal, such as a 1 kHz tone that we would use to set the sensitivity controls on an amplifier, we see something very different. In the case of a pure test tone like this, the waveform has a sinusoidal shape, called a sine wave. If we look at a sine wave on an oscilloscope, we see a smoothly rolling waveform that extends just as much above our reference voltage as it does below.

AC Signal – Frequency

It is now wise to look at this same signal from the perspective of the frequency domain. The frequency domain graph will, if there is no distortion, show a single frequency. In consideration of an audio signal, the amplitude (or height) of that frequency measurement depends on how loud that single frequency is relative to the limits of our recording technology or measurement device.

Audio

When we listen to someone speak or play a musical instrument, we hear many different frequencies at the same time. The human brain is capable of decoding the different frequencies and amplitudes. Based on our experiences, and the differences in frequency and time response between one ear and the other, we can determine what we are hearing, and the location of the sound relative to ourselves.

Analyzing the time domain content of an audio signal is relatively easy. We would use an oscilloscope to observe an audio waveform. The scope will show us the signal voltage versus time. This is a powerful tool in terms of understanding signal transmission between audio components.

A Piano Note

Middle C – Time

Let’s look at the amplitude and frequency content of a sound most of us know well. The following graph is the first 0.25 seconds of a recording of a piano’s middle C (C4) note in the time domain. This represents the initial hit of the hammer onto the string. If you look at the smaller graph above the larger one, you will see the note extends out much further than this initial .25 second segment.

Middle C – Frequency

We know that the fundamental frequency of this note is 261.6 Hz, but if you look at the frequency domain graphs, we can see that several additional and important frequencies are present. These frequencies are called harmonics. They are multiples of the fundamental frequency, and the amplitude of these harmonics is what makes a small upright piano sound different from a grand piano, and from a harp or a guitar. All of these instruments have the same fundamental middle C frequency of 261.6 Hz; their harmonic content makes them sound different. In the case of this piano note recording, we can see there is a large spike at 523 Hz, then increasingly smaller spikes at 790 Hz, 1055 Hz, 1320 Hz and so on.

Sine vs Square Waveforms

Every audio waveform is made up of a complex combination of fundamental and harmonic frequencies. The most basic, as we mentioned, is a pure sine wave. A sine wave has only a single frequency. At the other end of the spectrum is a square wave. A square wave is made up of a fundamental frequency, then an infinite combination of odd-ordered harmonics at exponentially decreasing levels. Keep this in mind, since it will become important later as we begin to discuss distortion.

Noise Signals

Noise is a term that describes a collection of random sounds or sine waves. However, we can group a large collection of these sine waves together and use them as a tool for testing audio systems. When we want to measure the frequency response of a component like a signal processor or an amplifier, we can feed a white noise signal through the device and observe the changes it makes to the amplitudes of different frequency ranges.

White Noise – Time

You may be asking, what exactly is white noise? It is a group of sine waves at different frequencies, arranged so the energy in each octave band is equal to the bands on either side. We can view white noise from a time domain as shown here.

White Noise – Frequency

We can also view it from the frequency domain, as displayed in this image.

Variations In Response

The slight undulations in the frequency graph are present because it takes a long time for all different frequencies to be played and produce a ruler-flat graph. On a 1/3-octave scope, the graph would be essentially flat.

Foundation For Time And Frequency Domains

There we have our basic foundation for understanding the observation of signals in the time domain and the frequency domain. We have also had our first glimpse into how harmonic content affects what we hear. Understanding these concepts is important for anyone who works with audio equipment, and even more important to the people who install and tune that equipment. Your local mobile electronics specialist should be very comfortable with these concepts, and can use them to maximize the performance of your mobile entertainment system.

If you’ve made it this far and want to learn even more about audio distortion, check out part 2 of this article!

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.

 

For decades, there has been discussion after discussion about which of the different subwoofer enclosures are “the best” and why. Let’s take a look at why we need a subwoofer enclosure at all, and how the three popular styles – sealed, vented and bandpass – differ in their design and performance.

Back-Wave Management

If you were to hook any speaker up to an amplifier, hold it in your hand and play music into it, you would find that you don’t hear any bass. That is because the sound coming from the front of the speaker cancels out the sound coming from the back. We need a way to keep the sound coming from the back of the speaker cone from interfering with the sound coming from the front. If you were to cut a hole in the middle of a large, flat piece of wood and mount the speaker in it, you would hear a lot more bass. In fact, until the half-wavelength of the bass frequencies becomes longer than the dimensions of the piece of wood, you will get really good, solid bass. If we put a speaker in an airtight enclosure, none of the sound coming from the back interferes with the sound coming from the front.

Power Handling

The ability of a speaker to use the power produced by an amplifier is limited by two criteria – how far the speaker cone can move and how much heat the voice coil of the speaker can handle. Thermal power-handling limitations are based primarily on the design of a speaker – the size of the voice coil, how airflow is managed around the voice coil and the proximity of the stationary components of the motor assembly to the voice coil are the key contributing factors. The excursion-limited constraints are also part of the speaker’s design – how long the voice coil winding is, how tall the top plate is and how much suspension travel is available are the key factors.

Excursion

When it comes to reproducing bass, a speaker has to move four times as far each time the input frequency is halved. For example, a speaker moving 0.125 inches at 100 Hz has to move 0.5 inches to reproduce the same output level at 50 Hz and 2 inches at 20 Hz. You can see that, for the lowest of frequencies, cone excursion limitations are significant – very few speakers can move 2 inches without significant distortion.

When we put a speaker in an enclosure, the combination of the enclosure and the speaker create a high-pass filter. We are effectively decreasing the low-frequency output of the speaker. Why would we want to do this? The benefit of an enclosure is that we can control the motion of the speaker cone. Looking at a simple acoustic suspension (also known as a sealed) enclosure will be the simplest illustration of this explanation.

Compliance

Each and every speaker – from the biggest of subwoofers to the smallest of tweeters – has a springiness to the cone. We call this the compliance. We measure compliance by comparing it to a volume of air with the equivalent springiness. We call this characteristic of the speaker Vas. In general terms, a speaker with a very small Vas specification has a tight suspension, and a speaker with a large Vas has a softer suspension. There is a lot more to it than that, but for the discussion of enclosure features and benefits, that’s all we need to get into for now.

When we put a speaker in an enclosure, we stiffen the suspension. When you push in on the speaker cone, you are pushing against the speaker’s suspension (which wants to center the cone) and you are trying to pressurize the air in the enclosure. When the cone tries to move outward from rest, you are putting the air in the into a vacuum state – it wants to pull the cone back to its resting position. We do sacrifice low-frequency output, but we gain significant power handling and control over the motion of the speaker cone. For the latter, the combination of the air in the enclosure and the speaker suspension helps to stop the speaker cone from moving once an electrical signal starts it in motion.

Think of it like a shock absorber on a vehicle. You can see that having an enclosure is critical.

Acoustic Suspension Subwoofer Enclosures

The simplest of enclosures is called an acoustic suspension or sealed enclosure. In these enclosures, we are putting the speaker into an airtight box. When we put a speaker in an enclosure, the system resonates at a specific frequency that – we call this Fc. Below that frequency, the output is reduced at a rate of -12 dB per octave. If the system has a resonant frequency of 50 Hz, the output will be 12 dB quieter at 25 Hz.

Acoustic suspension enclosures are amongst the smallest of the different enclosures we will discuss. They are also the easiest to construct, and most forgiving regarding calculation error. If you combine the roll-off of the enclosure and speaker system with the increase in efficiency you get from the relatively small air volume of the vehicle interior (often called transfer function or cabin gain), you can get a very flat in-car response with good infrasonic output. Bass from an acoustic suspension enclosure is very tight and controlled, thanks to excellent transient response.

There is a down side. If you are looking for loud bass, then you need a driver that has a lot of excursion capability, and you need a reasonable amount of power to move the speaker cone back and forth to get the level of output you want. There is another drawback that isn’t talked about as much, and that is distortion. As a speaker increases in excursion, the amount of distortion it creates increases. Likewise, distortion increases near the resonant frequency of the speaker. So, what can you do?

Bass Reflex Subwoofer Enclosures

A bass reflex (also known as ported or vented) enclosure uses a vent to increase low-frequency output by making use of the speakers back-wave energy. The vent, often a round tube or sometimes a rectangular slot, has an area and a length. The specific area and length of the vent and their relationship to the total volume of the enclosure cause the column of air in the vent to resonate at a specific frequency when excited by the speaker. We typically tune bass reflex enclosures quite low to emphasize the bottom octave or so of the audible frequency range. They can be tuned higher to increase efficiency for high-SPL applications. There is always a sacrifice, though – when we tune the enclosure higher, we sacrifice low-frequency performance.

Bass reflex enclosures are typically larger than sealed enclosures. There is no hard-and-fast rule to associate with the size relationship, but 25–50% large is common. The trade-off for that extra volume is two-fold – more efficiency in the tuning frequency and more power handling, at some frequencies.

When the subwoofer used in a bass-reflex subwoofer enclosure produces frequencies around the resonant frequency of the vent/enclosure combination, the driver excursion is reduced to almost nothing and all the “work” is done by the vent. Put more succinctly, around the tuning frequency, most of the music is being produced by the vent. The benefit to this is that power-handling problems caused by cone excursion limitations are dramatically increased. Since the cone is barely moving, very high sound pressure levels can be achieved. Around the tuning frequency, power handling is limited by the thermal capabilities of the subwoofer.

As we mentioned earlier, one factor that contributes to loudspeaker distortion is cone excursion. With a bass reflex enclosure, the driver moves significantly less than with an acoustic suspension enclosure design. As long as the vent itself has enough area and a smooth transition at both openings, the distortion produced by a properly designed bass reflex enclosure can be impressively small.

Nothing is free, is it? A factor in deciding to use a bass reflex design is how fast the output decreases below the tuning frequency. Where an acoustic suspension enclosure rolls off at -12 dB per octave, a bass reflex enclosure rolls off at 24 dB per octave. Below the tuning frequency, the vent acts more and more like a hole in the enclosure, offering increasingly less back pressure as frequency decreases. Designing for, and managing, driver excursion is a fundamental part of bass reflex enclosure design.

Bandpass Subwoofer Enclosures

We will quickly touch on bandpass enclosures to wrap up this article. There are several different designs for bandpass enclosures. Some use a sealed enclosure, and some a vented one. Independent of whether the rear chamber is sealed or vented, the output of the subwoofer plays into a vented enclosure. This enclosure acts as a low-pass filter. Why would we want to design a bandpass enclosure?

First and foremost, all of the output of the enclosure is produced by the vent or vents. This allows a creative designer to build an enclosure in the trunk of a vehicle and have the vent opening play through the rear parcel shelf. There have been some amazing bandpass enclosures build in the front storage area of mid- or rear-engine vehicles. The vent allows the bass to enter the interior of the vehicle. Bandpass enclosures can also offer impressive gains in efficiency over acoustic suspension and bass reflex enclosures, but they do so at the sacrifice of bandwidth and enclosure volume.

A bandpass enclosure has two resonant frequencies – one for each of the enclosures. The resultant management of cone excursion can allow a great deal of bass to be produced from limited excursion drivers. While the speaker cone itself does not move a great deal, the amount of work done by the motor assembly is still significant. You are still putting power into the speaker, and work is being done. Because the front chamber of the enclosure acts as a filter, it can also be very difficult to hear when the speaker is distorting.

Regarding the complexity of design, and forgiveness of construction error, bandpass enclosures are the most complicated to execute perfectly. Unlike an acoustic suspension or bass-reflex design, bandpass enclosure designs must be tailored exactly to the speaker they are being used with. Never trust the concept of a “generic” bandpass enclosure.

Lastly, because a bandpass enclosure includes an acoustic low-pass filter, it has to be used with good-quality, appropriately sized midbass drivers. If not, the bass can sound lost or disconnected relative to the rest of the music.

For More Details On Subwoofer Enclosures, Visit Your Local Specialist

As you can see, there are many ways to install a subwoofer – or any speaker, for that matter. Navigating the available space in the vehicle, as well as different speaker sizes and designs, can be tricky. The design and construction of an enclosure can be complex, especially when complex shapes are involved. Visit your local car audio specialist retailer to explore different enclosure options for your vehicle.

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

Several aspects of the automotive industry have always been embraced by hobbyists and do-it-yourself enthusiasts. Being able to say that you created something with your own bare hands would make anyone feel proud. In the mobile electronics industry, carmakers have been rapidly advancing the technology used in new cars. From computer data networks and advanced vehicle construction materials to elaborate factory audio system tuning, all of these technologies present unique challenges that many people simply aren’t aware of. Failure to compensate for these can wreak havoc with your vehicle’s electrical system, damage the products you are installing or simply result in poor performance from your audio equipment.

Let’s look closely at why it’s best to put the reliability of your vehicle, and the performance of your audio equipment, in the hands of a a shop offering professional installation.

Computers

Automobile manufacturers are always striving to make their vehicles as fuel-efficient as possible while offering the latest technologies and features. One way they save weight is by putting different devices and computers on a computer data network. Rather than running a myriad of control wires from component to component, they run power and ground, and a pair of data wires. Everything from door locks and trunk release modules to ABS brake and traction control systems can talk to each other on the data network.

Where these networks pose a challenge is when you want to add or remove something from the vehicle. Say you have a vehicle that has a secondary radio display in the dash, but you want to upgrade your audio system. The display may get very upset when you take the radio away. Likewise, you typically can’t add new devices to the data network to add new features.

Your car audio specialist retailers have the experience to work with these data networks. They know what interfaces are available for options like a remote starter or backup camera integration, and they have the manufacturer support to program and install them without causing Check Engine or MIL lights.

Reliability

When working on a vehicle, there are many different ways to run wires and make electrical connections. Automotive mechanics know that it’s hard to beat the knowledge and experience that a veteran car audio installer possesses when it comes to working on vehicle electrical systems. They make dozens, if not hundreds, of electrical connections each and every day.

The connections are electrically sound, offering little to no resistance to current flow. Equally, the connections are mechanically sound, ensuring that they will be at least as reliable as the factory connections, if not much better.

Finally, many shops use materials like split loom, nylon sleeves or cloth tape to protect wires as they run through the vehicle. These protective coverings also make the wiring look as if it came from the factory.

Product Warranty

Many mobile electronics manufacturers offer extensions on their product warranties when the products are sold and installed by an authorized dealer. Authorized retailers have been trained on the features of the products they sell. In the case of high-end brands, this training often extends to techniques and methodologies that make the products sound better in your vehicle, and subsequently last longer. The proper installation and configuration of mobile electronic components is the key to their performance and reliability.

Audio Integration

Factory audio systems are becoming more and more complicated. They still don’t rival what is available from the aftermarket, but they have improved. One big step automakers have made in the performance of their audio systems is in their tuning. More and more factory source units and amplifiers include advanced equalization and time correction to maximize the performance of the audio system. When it comes time to upgrade these systems, installers have the techniques and equipment required to test the signals going to the speakers to determine if signal correction is required. If you omit signal correction when installing a new speaker, the tuning that made a cheap factory speaker sound mediocre will work against you, and make a great speaker sound only so-so. Factory audio system signal measurement and correction is now a way of life for a car audio installer.

Modern Vehicle Chassis Design

Did you know that automakers are now using aluminum panels in the construction of their vehicles? In the ongoing battle to reduce weight, the use of aluminum will continue to increase year after year. The downside for the mobile electronics enthusiast is that aluminum is not as good a conductor of electrical current as steel is. In fact, pound for pound, it offers 30% more resistance to the flow of electrical current. This dramatically affects the amount of current we can deliver to high-power audio amplifiers.

Worse than the fact that they are using aluminum is that they have also started using structural adhesives to bond panels together. Spot welds are quick, but they only connect a small area. A good bead of 3M, Loctite or Proform structural adhesive between two aluminum panels connects the entire overlapping surface of the panel together. There is less chance of movement and less chance of corrosion. There’s a downside to this as well: These structural adhesives do not conduct electricity.

If you work with experienced installers, they know how to deal with vehicles that have aluminum chassis and/or adhesive construction. Failure to compensate for these modern construction methods could result in damaged equipment and poor performance.

Choose Professional Installation

For most people, their vehicle is their second-most expensive purchase after their homes. When it comes time to have work done, the knowledge and experience of a trained professional can help ensure that you get all the performance you want from your purchase without any of the headaches of nonprofessional installation. Contact your local car audio specialist retailer today.

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.

Adding a subwoofer to a car audio system is the best bang for the buck regarding audio performance upgrades. You can instantly hear the difference, and it dramatically improves the realism and enjoyment of your system. Let’s look at how to pick a car audio subwoofer for your vehicle.

Factory Audio System Limitations

Before we get into how to choose a car audio subwoofer system for your vehicle, let’s examine why a subwoofer is such a great upgrade. In a nutshell, factory audio systems are designed around a budget – both monetary and weight-related. When automakers start looking at car audio products, price is often their primary concern. Automakers also work hard to reduce vehicle weight as much as possible, thereby reducing fuel consumption and, subsequently, environment emissions from the vehicle. This means that the performance of even these so-called “premium” factory audio systems is mediocre at best, and typically can be quantified as disappointing.

A factory audio systems that does happen to include a subwoofer is often limited in the amount of power available for the sub. The subs themselves are typically small, and the speaker cones can’t move very far to produce sound. Finally, the enclosures are often extremely compact, further limiting low-frequency sound reproduction. These limitations combine to produce bass that, if it seems to exist at all, won’t play very loud, doesn’t play very deep, and lacks clarity and definition.

Retail To The Rescue!

A specialist car audio retailer can work with you to design a subwoofer system that can offer an octave or two more low-frequency output. With the availability of subwoofers designed specifically to fit into small enclosure volumes, these subwoofers can be hidden inside storage compartments, inside your spare tire or in the footwell of almost any vehicle. Gone are the days when all your storage space has to be sacrificed for great sub-bass.

Before we delve into how to pick a sub, don’t let the stereotype of “booming bass” sway you from adding a subwoofer. Just as every home theater system includes a subwoofer to accurately reproduce low frequencies, the addition of a subwoofer to your vehicle will add accuracy and realism. Subwoofers make your music sound bigger and more lifelike by letting you hear and feel frequencies as low as 20 Hz. A sub also takes the load off the smaller door and dash speakers that aren’t capable of making accurate or deep bass.

Starting The Process

Before you head out to your local car audio specialist to inquire about a subwoofer system, there are a few things you should do. Accurately quantify your goals for the subwoofer – are you looking to “warm up” the system, are you trying to rattle your fillings loose or something in between? It’s important that you know what you want. A properly designed system with adequate speaker cone area and power will sound dramatically better than an undersized system pushed to its limits.

As with any purchase, have a rough idea of your budget ahead of time. Consider that there will be some installation cost and additional expenses for wiring for the amplifier and vehicle integration.

Finally, have a look at your vehicle and consider how much space you want to dedicate to the subwoofer system. As mentioned, it doesn’t need to be much these days, but every subwoofer needs a proper-size, properly built enclosure.

Auditioning Car Audio Subwoofers

Listening to a subwoofer in a vehicle is highly useful. The small space inside the vehicle boosts low frequencies more than a larger listening room. Once you have established a reference for how a specific subwoofer in a specific enclosure sounds in the vehicle, then you can listen to different options in the showroom for changes compared to that reference.

As with any listening session, you will want to use music you are familiar with, preferably something you have listened to dozens, if not hundreds, of times. In the case of a subwoofer, the performance of the speakers that are playing with it can affect the perception of its quality, so choose something similar to what you will be using. You will want to focus on three aspects of the subwoofers performance: frequency response, how loud it will play and how accurate it is.

Regarding frequency response, you will want to listen to how loud different frequencies are. Some subwoofers produce a lot of energy in the 50–70 Hz range, some between 30 and 50 Hz, and some play well below 20 Hz. You need a find a balance that accurately reproduces your music and meets your performance goals. High-quality subwoofers can be very musical, accurately defining subtle nuances between each different frequency. A pipe organ, piano, harp, bass guitar, contrabassoon and even a tuba all play into the range of a common car audio subwoofer. In fact, some male voices extend down to 65 Hz or lower. The best subwoofers offer smooth and extended frequency response while remaining tight and dynamic.

Quantifying how loud a subwoofer will play has a great deal to do with what type of enclosure it is installed in, and how much power you have to drive it. The limiting factor on output is cone area and how far the speaker cone can move – we call this excursion. Without accurate measurement equipment, all you can do is decide if the combination of the subwoofer, amplifier and enclosure you have chosen meets your needs.

A Word On Car Audio Subwoofer Enclosures

There are four common ways of installing a subwoofer in a vehicle. If you have a trunk, you can have your installer create a baffle that separates the trunk from the interior of the vehicle, and install the subwoofer in that baffle. That is known as an infinite baffle installation. You need a subwoofer that was designed specifically for that kind of application.

More common is a sealed enclosure, known formally as an acoustic suspension enclosure. These are forgiving regarding their volume requirements and can offer very smooth frequency response when combined with the output gain that most vehicles offer.

Perhaps the “most fun” of all subwoofer enclosures is the ported design, known as a bass reflex enclosure. In these, which are larger than a sealed enclosure, there is a vent of a specific area and length that uses the back-wave (sound coming off the back of the subwoofer cone) to help reinforce the output. You get more bass output for the same size subwoofer.

Finally, the most complex of enclosures commonly used is the bandpass design. In these, the enclosure acts to filter out high-frequency information. These enclosures sound the best when you have great-quality midbass drivers in the system.

An entire article could be dedicated to the benefits of one enclosure design over another, but your car audio retailer can help steer you toward the correct one for your application.

If you are thinking of upgrading your car audio system and you do not already have a subwoofer, drop in at your local car audio specialist right away! There are enclosures not much larger than a shoe box that can make a dramatic difference in the way your system sounds. If you want something more significant, that is easily arranged. Take along your favorite music and have a listen. Adding a subwoofer not only makes a huge improvement, but can be a lot of fun!

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.

For decades, there has been discussion after discussion about which of the different subwoofer enclosures are “the best” and why. Let’s take a look at why we need a subwoofer enclosure at all, and how the three popular styles – sealed, vented and bandpass – differ in their design and performance.

Back-Wave Management

If you were to hook any speaker up to an amplifier, hold it in your hand and play music into it, you would find that you don’t hear any bass. That is because the sound coming from the front of the speaker cancels out the sound coming from the back. We need a way to keep the sound coming from the back of the speaker cone from interfering with the sound coming from the front. If you were to cut a hole in the middle of a large, flat piece of wood and mount the speaker in it, you would hear a lot more bass. In fact, until the half-wavelength of the bass frequencies becomes longer than the dimensions of the piece of wood, you will get really good, solid bass. If we put a speaker in an airtight enclosure, none of the sound coming from the back interferes with the sound coming from the front.

Power Handling

The ability of a speaker to use the power produced by an amplifier is limited by two criteria – how far the speaker cone can move and how much heat the voice coil of the speaker can handle. Thermal power-handling limitations are based primarily on the design of a speaker – the size of the voice coil, how airflow is managed around the voice coil and the proximity of the stationary components of the motor assembly to the voice coil are the key contributing factors. The excursion-limited constraints are also part of the speaker’s design – how long the voice coil winding is, how tall the top plate is and how much suspension travel is available are the key factors.

Excursion

When it comes to reproducing bass, a speaker has to move four times as far each time the input frequency is halved. For example, a speaker moving 0.125 inches at 100 Hz has to move 0.5 inches to reproduce the same output level at 50 Hz and 2 inches at 20 Hz. You can see that, for the lowest of frequencies, cone excursion limitations are significant – very few speakers can move 2 inches without significant distortion.

When we put a speaker in an enclosure, the combination of the enclosure and the speaker create a high-pass filter. We are effectively decreasing the low-frequency output of the speaker. Why would we want to do this? The benefit of an enclosure is that we can control the motion of the speaker cone. Looking at a simple acoustic suspension (also known as a sealed) enclosure will be the simplest illustration of this explanation.

Compliance

Each and every speaker – from the biggest of subwoofers to the smallest of tweeters – has a springiness to the cone. We call this the compliance. We measure compliance by comparing it to a volume of air with the equivalent springiness. We call this characteristic of the speaker Vas. In general terms, a speaker with a very small Vas specification has a tight suspension, and a speaker with a large Vas has a softer suspension. There is a lot more to it than that, but for the discussion of enclosure features and benefits, that’s all we need to get into for now.

When we put a speaker in an enclosure, we stiffen the suspension. When you push in on the speaker cone, you are pushing against the speaker’s suspension (which wants to center the cone) and you are trying to pressurize the air in the enclosure. When the cone tries to move outward from rest, you are putting the air in the into a vacuum state – it wants to pull the cone back to its resting position. We do sacrifice low-frequency output, but we gain significant power handling and control over the motion of the speaker cone. For the latter, the combination of the air in the enclosure and the speaker suspension helps to stop the speaker cone from moving once an electrical signal starts it in motion.

Think of it like a shock absorber on a vehicle. You can see that having an enclosure is critical.

Acoustic Suspension Subwoofer Enclosures

The simplest of enclosures is called an acoustic suspension or sealed enclosure. In these enclosures, we are putting the speaker into an airtight box. When we put a speaker in an enclosure, the system resonates at a specific frequency that – we call this Fc. Below that frequency, the output is reduced at a rate of -12 dB per octave. If the system has a resonant frequency of 50 Hz, the output will be 12 dB quieter at 25 Hz.

Acoustic suspension enclosures are amongst the smallest of the different enclosures we will discuss. They are also the easiest to construct, and most forgiving regarding calculation error. If you combine the roll-off of the enclosure and speaker system with the increase in efficiency you get from the relatively small air volume of the vehicle interior (often called transfer function or cabin gain), you can get a very flat in-car response with good infrasonic output. Bass from an acoustic suspension enclosure is very tight and controlled, thanks to excellent transient response.

There is a down side. If you are looking for loud bass, then you need a driver that has a lot of excursion capability, and you need a reasonable amount of power to move the speaker cone back and forth to get the level of output you want. There is another drawback that isn’t talked about as much, and that is distortion. As a speaker increases in excursion, the amount of distortion it creates increases. Likewise, distortion increases near the resonant frequency of the speaker. So, what can you do?

Bass Reflex Subwoofer Enclosures

A bass reflex (also known as ported or vented) enclosure uses a vent to increase low-frequency output by making use of the speakers back-wave energy. The vent, often a round tube or sometimes a rectangular slot, has an area and a length. The specific area and length of the vent and their relationship to the total volume of the enclosure cause the column of air in the vent to resonate at a specific frequency when excited by the speaker. We typically tune bass reflex enclosures quite low to emphasize the bottom octave or so of the audible frequency range. They can be tuned higher to increase efficiency for high-SPL applications. There is always a sacrifice, though – when we tune the enclosure higher, we sacrifice low-frequency performance.

Bass reflex enclosures are typically larger than sealed enclosures. There is no hard-and-fast rule to associate with the size relationship, but 25–50% large is common. The trade-off for that extra volume is two-fold – more efficiency in the tuning frequency and more power handling, at some frequencies.

When the subwoofer used in a bass-reflex subwoofer enclosure produces frequencies around the resonant frequency of the vent/enclosure combination, the driver excursion is reduced to almost nothing and all the “work” is done by the vent. Put more succinctly, around the tuning frequency, most of the music is being produced by the vent. The benefit to this is that power-handling problems caused by cone excursion limitations are dramatically increased. Since the cone is barely moving, very high sound pressure levels can be achieved. Around the tuning frequency, power handling is limited by the thermal capabilities of the subwoofer.

As we mentioned earlier, one factor that contributes to loudspeaker distortion is cone excursion. With a bass reflex enclosure, the driver moves significantly less than with an acoustic suspension enclosure design. As long as the vent itself has enough area and a smooth transition at both openings, the distortion produced by a properly designed bass reflex enclosure can be impressively small.

Nothing is free, is it? A factor in deciding to use a bass reflex design is how fast the output decreases below the tuning frequency. Where an acoustic suspension enclosure rolls off at -12 dB per octave, a bass reflex enclosure rolls off at 24 dB per octave. Below the tuning frequency, the vent acts more and more like a hole in the enclosure, offering increasingly less back pressure as frequency decreases. Designing for, and managing, driver excursion is a fundamental part of bass reflex enclosure design.

Bandpass Subwoofer Enclosures

We will quickly touch on bandpass enclosures to wrap up this article. There are several different designs for bandpass enclosures. Some use a sealed enclosure, and some a vented one. Independent of whether the rear chamber is sealed or vented, the output of the subwoofer plays into a vented enclosure. This enclosure acts as a low-pass filter. Why would we want to design a bandpass enclosure?

First and foremost, all of the output of the enclosure is produced by the vent or vents. This allows a creative designer to build an enclosure in the trunk of a vehicle and have the vent opening play through the rear parcel shelf. There have been some amazing bandpass enclosures build in the front storage area of mid- or rear-engine vehicles. The vent allows the bass to enter the interior of the vehicle. Bandpass enclosures can also offer impressive gains in efficiency over acoustic suspension and bass reflex enclosures, but they do so at the sacrifice of bandwidth and enclosure volume.

A bandpass enclosure has two resonant frequencies – one for each of the enclosures. The resultant management of cone excursion can allow a great deal of bass to be produced from limited excursion drivers. While the speaker cone itself does not move a great deal, the amount of work done by the motor assembly is still significant. You are still putting power into the speaker, and work is being done. Because the front chamber of the enclosure acts as a filter, it can also be very difficult to hear when the speaker is distorting.

Regarding the complexity of design, and forgiveness of construction error, bandpass enclosures are the most complicated to execute perfectly. Unlike an acoustic suspension or bass-reflex design, bandpass enclosure designs must be tailored exactly to the speaker they are being used with. Never trust the concept of a “generic” bandpass enclosure.

Lastly, because a bandpass enclosure includes an acoustic low-pass filter, it has to be used with good-quality, appropriately sized midbass drivers. If not, the bass can sound lost or disconnected relative to the rest of the music.

For More Details On Subwoofer Enclosures, Visit Your Local Specialist

As you can see, there are many ways to install a subwoofer – or any speaker, for that matter. Navigating the available space in the vehicle, as well as different speaker sizes and designs, can be tricky. The design and construction of an enclosure can be complex, especially when complex shapes are involved. Visit your local car audio specialist retailer to explore different enclosure options for your vehicle.

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