Loudspeaker Design Basics

Mar 25 2007

 

Loudspeaker Design Basics

Loudspeaker Design Basics

Today we will take a look at the basics of loudspeaker design. Specifically, we will cover general concepts, terminology, design parameters, and enclosure styles which are essential knowledge for a successful build.

Loudspeaker design is commonly perceived as somewhat of a “black art” and not really approachable by DIY audiophiles. Sure, it is not uncommon to see someone slap together a sub box for their car stereo. Due to the forgiving nature of the car environment, the builder can be off in their calculations and still end up with a relatively decent sounding solution. Of course, “decent” is in the ears of the beholder. With a little extra time spent calculating, testing, and making minor adjustments to the final product, any DIY user can produce a loudspeaker worthy of critical review.

Today, we will do an initial foray into the world of home audio sub loudspeaker design. We will look at the various enclosure topologies and briefly discuss their advantages and disadvantages. We will cover some of the terminology of the trade and highlight any potential problem spots a novice is likely to encounter.

Speaker Driver Basics

SPEAKER DRIVER BASICS

driver_cutaway.jpg
Speaker Driver Cutaway

Let's start by talking about the various parts that make up a typical speaker driver. The most prominent part is the cone. It is usually constructed from treated paper, but some are made from more exotic materials such as aluminum, woven Kevlar, and plastic. Ideally, the cone should be as stiff as possible so it never changes shape, but also extremely light so it can quickly respond to the audio signal. It has become popular among certain manufacturers to mold interesting designs into their cone. Some have even produced a square cone with slightly rounded corners. While these marketing gimmicks look flashy, the end result is compromised sound production. Plain old smooth circular cones may be boring to look at, but they are the best possible shape to get the job done.

The voice coil is attached to the center of the cone. This is a paper bobbin with several hundred wraps of magnet wire. The principle acts just like an electromagnet. When a positive voltage is applied, the voice coil and cone travel one way. When the voltage is reversed the cone switches direction. Thus, a pure sinusoidal signal will make the cone travel smoothly from one extent to the other. A large permanent magnet is used to increase the efficiency of the system.

The cone is supported at the outer edge by the surround. This is usually made of a thin closed cell foam material. The surround should provide uniform support to the cone holding it centered within the driver, but also allow it to linearly oscillate in and out. This means that the surround should provide the same resistive force against the cone throughout its usable travel distance. If the surround does not uniformly support the cone as it moves, it will result in something other than pure sinusoidal motion. The inside of the cone is supported by the spider which is made of an accordion folded cloth material. The spider mainly keeps the voice coil centered within the driver motor assembly. The dust cap is bonded to the cone and keeps roaming fingers out of the voice coil area. It also provides a handy place for the manufacturer's marketing department to display their logo. Painted logos are fine, but molded logos can be another source of sonic distortion.

Holding everything together is the frame. This is usually a rugged piece of stamped or cast metal. It needs to be very rigid to resist the stresses due to constant vibration and general abuse. The positive and negative terminals for the voice coil are usually attached to the back of the frame.

speaker.jpg
Visibly Well Designed Speaker Driver

The speaker driver pictured above appears to be structurally well designed. The magnet is large which should improve its efficiency and the frame looks sturdy enough to support it. In this case, the spider is a brownish colored stiff cloth material which should work fine. The cone looks to be made from a compressed wood pulp which was treated to prevent decay and also gives it a nice shine. The single roll rubber surround seems to be well attached to the cone and is large enough to allow for long cone excursions. Before we rush out to buy it, let's read on to find out about speaker enclosures.

Speaker Enclosure Basics

SPEAKER ENCLOSURE BASICS

So, why do we require an enclosure at all? Why not just suspend the speaker driver in free air? The answer to this question is rather complicated. First, as the speaker cone travels back and forth it is generating both a front sound wave and a back sound wave. These two waves are 180 degrees out of phase with each other meaning that if they were to mix, they would essentially cancel each other out. One way to prevent this from happening is to place a barrier between the front and back of the speaker driver thus isolating the two waves. In its simplest case, think of an infinitely large sheet of MDF particle board with the driver mounted in it. This is referred to as an “infinite baffle” as it forever divides the front wave from the back. Unfortunately, infinite sheets of MDF are expensive and difficult to fit into most home theater environments (to say nothing of the WAF), so wrapping the baffle into a box is the next best thing. It is not ideal as sound waves distort whenever they encounter any sharp surface irregularities such as the enclosure corners. Fortunately, most people cannot detect this phenomenon, and it can also be reduced by rounding off all enclosure edges.

The second reason to use an enclosure is because the speaker driver is not perfect in design. Like all objects, it has a natural frequency that it will resonate at in free air. While operating in this region, the speaker driver exhibits some very bad electrical and physical properties. Because the driver wants to resonate at this frequency, it requires very little input signal to make the voice coil slam into the metal back plate. This is often referred to as “bottoming out” or “unloading” and besides sounding awful, it will eventually bend and permanently damage the voice coil. A properly tuned enclosure acts as a spring or shock absorber at this critical frequency supporting the driver and preventing it from unloading prematurely.

Speaker enclosures should be constructed from very stiff materials. These materials should also be well damped such that they do not vibrate when exposed to audible frequencies. Natural solid woods have been used as a raw material for enclosures for many years. It is appealing to look at and can be cut quite easily, but it tends to warp and distort due to climate changes. Cross laminated wood products help reduce the warping, but also introduce vibrational noise from the laminated panels. The most practical material that can be found at any home improvement store which fits the bill is MDF particle board. It is a composite of wood dust and a glue resin which resists warping and vibration, while being quite inexpensive and easy to work with. Four by eight foot sheets can be found in a variety of thicknesses to suit the project. Be sure to bring a friend to the lumber yard as one sheet of 1/2" MDF is around 80 pounds. Once the cabinet is complete, it can be finished with either paint, felt carpet, or even a high quality wood veneer.

Theile-Small Parameters

THEILE-SMALL PARAMETERS

In the early 1970's A. N. Thiele and Richard Small individually published several articles in the Journal of the Audio Engineering Society which outlined how speaker drivers in an enclosure could be characterized by a handful of relatively simple equations. This was groundbreaking as it allowed the average DIY designer to measure the key properties of any speaker driver, plug them into the formulas, and calculate the appropriate enclosure parameters. Most driver manufacturers provide the user with these properties, but the serious builder will take the time to verify they are correct. For our purposes, we will assume that the manufacturer's processes are well controlled and the published specs can be believed. Below is a list of the most common Thiele-Small parameters that are required to work the formulas along with a brief description.

Fs – Measured in hertz (Hz), describing the frequency where the combination of the moving mass and suspension compliance allows the driver to resonate in free air. A more compliant suspension or a larger moving mass will cause a lower resonant frequency, and vice versa. Usually it is less efficient to produce frequencies below Fs. Woofers typically have an Fs in the range of 13–60 Hz.

Qts – A unitless measurement, describing the total electric and mechanical damping of the driver. Most drivers have Qts values between 0.2 and 0.8.

Vas – Measured in litres (L), describes the stiffness of the suspension with the driver in free air. Larger values mean lower stiffness, and generally require larger enclosures.

Xmax – Measured in millimeters (mm), describes the maximum linear peak (or sometimes peak-to-peak) excursion of the cone.

Basic Enclosure Styles

BASIC ENCLOSURE STYLES

Sealed (acoustic suspension)
sealed.gif The easiest enclosure to design is the standard sealed enclosure. As the name implies, a sealed enclosure is essentially a closed box with a driver mounted in the front. If built correctly, the box should be air tight. This gives a uniform cushion of air inside the box to help control cone motion. The volume of the enclosure is relatively flexible and there are very few equations required in its design, so this makes an excellent project for the novice speaker builder. If the final product is slightly off, the addition or removal of filler material can compensate. The downside of all this simplicity is a box which is very inefficient. All of the rear sound wave energy is absorbed within the cabinet.

 

Vented (ported, bass reflex, tuned, or tuned ported)
ported.gif The vented enclosure is a modification on the sealed enclosure. Rather than trying to make the enclosure air tight, we instead install a length of pipe allowing air to move freely in and out of the cabinet. This has two positive effects. First, since the rear sound wave can now reinforce the front wave, the overall speaker efficiency is increased. Second, by carefully calculating the diameter and length of the port, the natural resonance of the cabinet can be tuned. Why is this important? By tuning the cabinet to a point just below Fs, the speaker driver is given a bit of a boost allowing it to reach lower frequencies. If this is done well, it can sound quite natural, but this effect can also be abused creating an artificially boomy bass.

 

Passive Radiator
pr.gif Some speaker drivers are not well suited to vented designs because their parameters demand an overly long port tube which may be impractical. In this case, a compromise between sealed and vented enclosure types can be made. It involves replacing the port tube with a dummy speaker called a drone cone or passive radiator. This is essentially a speaker driver without a voice coil or magnet assembly. The enclosure is still built to be air tight, and the mass and size of the passive radiator combine to form the natural resonance of the cabinet. Again, this will help control the driver cone motion and extend its bass capabilities. This type of enclosure is more efficient than a sealed box and not as boomy as a vented box.

Advanced Enclosure Styles

ADVANCED ENCLOSURE STYLES

The following enclosures are included more for completeness and the desire to increase the reader's knowledge rather than suggesting that they be attempted by a novice speaker builder. Once a few basic designs under your belt, return here for an extra challenge to keep you on your toes.

Isobaric
iso.gif The key to this variation is to mount the two drivers so that the sealed air cavity between them is as small as possible without risk of mechanical interference. Then the drivers are wired so that the cones move in phase with each other. This effectively divides the Vas of the driver by 2 allowing for enclosures to be one half their normal size. The down side is now the amplifier has to power two drivers rather than one causing this design to operate 3 dB less efficient than a single driver. This design topology can be utilised in any of the other enclosure styles. If there is a need for a smaller box and there is amplifier power to spare, then this might be the solution.

 

Bandpass
bandpass.gif This enclosure combines either a sealed and a vented box or two vented boxes to form a bandpass cabinet. The design is patented by Bose and is quite complex to model due to the various interactions in the system. The end result usually becomes a trial and error scenario that stretches out over months. Unfortunately, this box is large and has a narrow operational frequency range. Due to the forces enacted upon the cone it is not uncommon to destroy a driver used in this box.

 

Transmission Line
tl.gif This enclosure has the most promise of all. In its simplest form the premise is to tune the cabinet just below Fs as if it was a long tube. The tuned frequency is solely dependent upon the tube's length. Low frequencies would typically require exceptionally long tubes, but through the liberal use of acoustic stuffing the sound waves are "slowed down" allowing for a shorter tube. The also provides for a quick way to adjust the box tuning after it is built by either adding or removing filler. A straight pipe usually doesn't fit within the home theater decor, so instead it is folded inside a standard box. The DIY builder who is comfortable with difficult wood working projects will be rewarded with a speaker which will have an excellent transient response and deeper bass. In my opinion, this is the pinnacle box style to work towards.

Conclusion

CONCLUSION

We have covered many of the terms and concepts commonly used when building speaker cabinets. One area that has not been covered is the electrical crossover circuity. Due to the complexity of this topic, it will need to be handled in a separate guide. I also plan to investigate several of these enclosure types in more detail giving you the formulas and hints to get the project done right. Until then, if you are anxious to get started, take the time to read "Loudspeaker Design Cookbook" by Vance Dickason. I have found it to be an invaluable resource in my journey as a speaker builder. Also, visit Parts Express and MCM Electronics who both cater to the DIY speaker builder community. There you will find a large variety of drivers and other products needed for a professional appearance.

While this topic may seem daunting at first, it can be very rewarding to settle in to a movie supported by speakers that you have designed and built by hand. Besides, it's good to get your head out of the inside of a PC for a change.

 

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