Building Speaker Enclosures for home speakers.

This document is currently a work in progress. It started out as some basic considerations for building speaker enclosures, and will morph into a full scale document on building speaker boxes for home use, as I find the time to work on it.

    Let's start off by stating that exact enclosure type and dimensions are (normally) very critically matched to a specific driver. You absolutely can not mix and match drivers and enclosures, unlike what is sometimes done with car subwoofers, and get ideal results out of either the driver or the enclosure. There are various reasons for this but one major one is that the home environment is very different than the automobile especially concerning pressurization due to the small internal volume of a car. Another is that the construction of speaker drivers is different for car and home speakers.

I intend to cover various considerations that will allow construction of a speaker system that can provide very high quality sound. This will not be a highly technical document, nor will it be non technical. It is intended to allow those of average ability to be able to construct very high quality speaker systems with a minimum of outside research. Obviously some will be necessary. I do not intend, at this point (this may change later) to cover construction of a specific speaker system, but rather to provide the intrepid do it yourselfer the ability and knowledge to build a system that will fit his or her needs and provide a satisfactory to extremely high level of performance. Of course this will depend on the effort spent and, unfortunately (Reality Bites!) the amount of money available.

The individual will of course require a basic ability to solder and construct speaker cases and crossovers.

Some will ask: Why build it yourself? Are you crazy? The answer is, of course, yes, I think I am. Really, this is a huge amount of work. It does provide great satisfaction from having built it yourself, and if your budget is like mine, you can build a system with better sound quality than you could afford to buy. That's because there are some inexpensive but labor intensive things that can be done to improve performance.

The basic process usually starts from deciding which sort of enclosure makes the most sense for your application, choosing the drivers, then designing the system. This is because all the design decisions will be made based on the drivers chosen, which need to be chosen with an idea of which style of enclosure is going to be used. When we get into enclosures we will also cover some aspects of enclosureless drivers such as electrostatics and dipole dynamic speaker systems.

If you find any factual errors in this document please e-mail me with the correct information at questions@smartbuyspeakers.com

Why is a box important?

    The major use of an enclosure is to prevent the back wave from a driver from canceling out the sound from the front of the driver. If you have ever listened to a raw driver outside of an enclosure, you will have noticed that the sound is tinny and lacking in bass quality. Using an enclosure to absorb/redirect the back wave opens a whole new can of worms. Due to this fact, there have been advances made in Dipole driver systems, and some speakers are natural dipoles (such as electrostatics) due to the nature of their construction. These are "boxless" speakers, with no enclosure, and as such not subject to these issues. Most of the boxless systems have other issues to overcome such as lack of bass response, and the majority of speakers on the market use a cone type of driver. Dipole systems (with the exception of some systems such as those designed by Linkwitz) usually use a cone driver in a box (subwoofer) to fill in the missing bass. This causes another issue, properly blending the boxy sub with the boxless dipoles. Let's start by examining some enclosure designs and uses.

    One advantage of a properly designed enclosure (an air spring used as a very linear restoring force) is utilized in what is termed an "acoustic suspension" type of enclosure. If memory serves I believe I read that this type of enclosure was invented by Edgar Villchur and manufactured by him and Henry Kloss under the Acoustic Research (AR) name. This design (actually, all of the standard type) relate directly to the structure of a cone style raw speaker driver. A raw driver consists of the speaker cone, a voice coil consisting of wire wound on a (usually) round former, a magnet structure for the voice coil to push against (magnetically), a surround connecting the cone to the basket (frame) of the speaker, and a "spider" which is usually a type of treated cloth that helps center the speaker cone.

    To get back to the Acoustic Suspension enclosure, the idea centers around the fact that the purpose of the spider and the surround is to center the cone at the middle of it's travel, and keep the voice coil centered in the magnetic gap. It was noticed that the surround and spider constructs of the day did not provide the most linear restoring force. The less linear the restoring force, the less linear the performance of the driver. The ideal, of course, would result in perfectly linear and exact reproduction of music. It turns out that air provided a much more linear restoring force than the surround materials of the day. A new style of cone driver was designed  with a much more compliant surround and spider than was normal for the era. Air was used to replace the missing suspension from the rubber and the spider, by sealing the enclosure completely. The air volume inside the box must be matched exactly to the combination of the mechanical compliance consisting of the surround and the spider, the electrical compliance consisting of the force the magnet exerts on the voice coil, and the combined mass of the voice coil, cone, surround, spider and dust cap. Thankfully, these parameters are now available with purchased drivers, listed as the Theile Small parameters. This is how we will refer to them from now on, and this explains why it is utter folly to try to replace a driver without either using an exact replacement or one with identical parameters. Not all drivers are compliant enough (many are not made for) acoustic suspension enclosures. Acoustic suspension enclosures can have the smoothest performance, and potentially the best transient response, but may suffer a little in efficiency and overall bass output compared to a Bass Reflex (ported) enclosure. The Acoustic Suspension bass rolls off slower than that of a bass reflex, so can potentially provide lower total usable frequency response, but at a reduced level than that of a bass reflex.

Overview of Speaker Enclosure Types:

Later on, we will cover actual design parameters for each type of enclosure. The goal now is to decied which type of system you want.

The Acoustic Suspension Speaker Enclosure:

As mentioned above, this type of enclosure was invented by Edgar Villchur. Villchur earned a Masters in Education at New York University. At the time that he discovered the principles of the Acoustic Suspension Enclosure, dynamic speakers had to be very large in order to reproduce deep bass frequencies. A bass speaker capable of a 40 Hz tone was approximately 14 feet tall!

As Villchur put it in a January, 2005 interview with Stereophile magazine, “The problem was the mechanical suspension that held the cone in place. When the cone moved a large distance, the suspension would stretch. It wouldn't allow the cone to travel the full distance in response to low-frequency waveform peaks. I thought, Well, what we need is a linear restoring force, one that doesn't bind, one that allows the cone to move a large distance and brings it back elastically. That's when the light went on. This linear spring had been there all the time. The cushion of air in the cabinet was exactly the kind of spring I wanted.”

Villchur reduced the stiffness of the mechanical cone suspensions, then reduced the size of the air cushion by reducing the size of the cabinet. “You end up with a linear restoring force, radically reduced low-frequency distortion, and—as an extra dividend, not a primary dividend—a small cabinet.”

Edgar used a cannibalized Western Electric 12 inch driver. The entire rim suspension was cut away and replaced with mattress ticking. Part of the spider was also cut away. There was just enough suspension left to center the voice coil.

Villchur's measurements showed the prototype had better bass and less distortion than aything on the market, and it was one fourth the size. Edgar decided to sell the design to a loudspeaker manufacturer, as he did not want to get into manufacturing. The response from the manufacturers was typical of the shortsightedness that most big businesses demonstrate sooner or later. Edgar called someone at Altec. According to Edgar in the Stereophile interview mentioned above, his contact there said “You know, Ed, we have a pretty good staff of engineers here. If there were something around such as you describe, I think they would have found it.” A friend of his also approached Rudy Bozak (A designer and technician in the field of music reproduction) who turned him down because “what you describe is impossible."

He then talked about the design to Henry Kloss who suggested the use his loft to begin manufacturing the new speaker. This was the beginning of AR (Acoustic Research) which went on to manufacture some of the most successful loudspeaker systems in history. The first model was the AR-1. Edgar Villchur went on to design the first dome tweeter, which is now ubiquitous but was not imitated by other designers for about 10 years.

The driver is placed in a sealed and airtight enclosure, generally with the front of the driver facing outwards, but is not restricted to this method only. For proper construction a very small hole should be placed in the box to allow a slow equalization of internal and external air pressure. 

    Positive aspects of the acoustic suspension enclosure include:

• good bass out of a small enclosure.
• Better transient response than ported types enclosures Transient response is a measure of how accurately a speaker responds to brief transient signals such as the first instant of a drumstick striking on the drum head. 
• Bass rolloff is slower than bass reflex types, approx 12 db / octave. This can extend useful bass deeper for a small size driver.
• Simple to construct

 Disadvantages of the acoustic suspension enclosure include:

• Less efficient than some other types of enclosure (About 3db down. Each 3db equals a halving. A speaker 3db less efficient will take double the amplifier power to acheive the same volume level

    The Bass Reflex enclosure can provide more output than the acoustic suspension, and if built properly (usually big) can negate the theoretical downsides such as the 24 db / octave rolloff rate below F3. A bass reflex enclosure is simply an enclosure with a port tuned (usually) to the precise free air resonance frequency of the driver to be used in it. The port may be anywhere on the speaker. The function of the port turns the box into what is known as a Helmholtz resonator. At the tuning frequency the port damps the resonance of the woofer and physically reduces the woofer excursion. At this frequency the output of the port is at maximum, and the output of the woofer at a minimum. Below the port tuning frequency the response drops off rapidly, making this system a high pass filter. However, if the tuning frequency is below the range of human hearing, say 19 Hz, the rapid drop in response is of no audible significance. One downside to a Bass Reflex is if high level frequencies below the port tuning frequency are fed to the woofer, it may bottom the driver out. This is because this system provides no useful damping below the port tuning frequency. Again, this is of minor significance if the tuning frequency is below the frequencies fed to the woofer. One exception is if a turntable is used that may produce low frequency rumble below 20 Hz. In that case a sharp cutoff high pass filter may be used below the tuning frequency to attenuate unwanted signals. There is not a lot that can be done to improve the transient response of a bass reflex enclosure, naturally worse than that of an acoustic suspension (transient response refers to how fast the cone starts and stops during a signal). This is in part due to a ringing effect associated with the sharp roll off below the port frequency. Let me point out, however, that many highly acclaimed and expensive (more than $8000 a pair) speakers have been designed using this type of enclosure, for example some models of the highly acclaimed Wilson brand of loudspeakers. Adjusting enclosure "q" (quality factor - a measurement of enclosure damping on the woofer primarily based on the internal volume of the enclosure) closer to .5 improves transient response at the expense of bass response. Much of the information following pertains to many types of enclosures, so I will just briefly mention some other enclosure types. There is infinite baffle, which mounts the driver in a large baffle such as a wall. There are also transmission line and acoustic labyrinth enclosures, both which are ported with a long, meandering port filled with stuffing. The idea is to absorb most or all of the back wave from the transducer (speaker) without raising the resonance of the system as is done with acoustic suspension or bass reflex systems. These are less common. Another type of enclosure, or lack thereof, is the dipole system, in which the back wave from a driver is put to use to decrease the amount of room reflections through cancellation from the driver back wave. These typically give a figure 8 radiation pattern, radiating to the front and back, canceling at the sides. Many dipole systems do not use a standard driver, but can be flat panels such as planar speakers or electrostatics. These do not use an enclosure as such, so we will focus on some other basic aspects relating to standard driver configurations.

The whole can of worms

    Now that we have an enclosure, more problems crop up. Usually a system requires more than one driver in a box to properly cover the entire frequency range of our hearing. There are some exceptions, such as enclosures using Fostex and Lowther drivers, but these are a special case and as of yet have difficulty covering the entire spectrum. They also require very carefully designed and, if you want real bass, may need very large (think 6' high and 5 feet across) enclosures. Enthusiasts say these speakers have a unique and addicting naturalness to the sound, and are usually very efficient. This lets them be driven by low powered amps, keeping more parts out of the signal path. Since every electronic part in the signal path unavoidably degrades the signal by a small amount, there are advantages to keeping the parts count as low as (but not lower than) possible.

    Since almost all systems have multiple drivers, they require a crossover to separate the frequencies, and send the high frequencies only to the treble driver, etc. If we don't do this the bass frequencies will likely burn out our tweeter (treble driver) and cause various other problems including distortion and system inefficiency. Cross over design is an art in itself, and various problems occur including the different drivers partially canceling each other out. This is called comb filtering, and can be reduced by steeper slopes on the crossover and physically moving the drivers closer together. Moving the crossover slopes steeper however means you move away from the transient perfect first order crossover, but increase power handling and reduce the range each driver must cover. Moving the drivers closer together can be difficult because of the physical size of the driver. Some manufacturers have attacked this problem by making concentric drivers, I:E the treble driver (tweeter) being physically small is placed in the center of the woofer (bass driver). This can create other problems (of course) one being that the woofer cone acts as horn loading for the tweeter, and can create a hollow sound. To hear an example of this effect talk into your cupped hands. Again, with proper construction this can be overcome (see the Theil line of speakers).

    Another enclosure problem is that the walls of the box vibrate with the driver movement, and the vibrating cabinet colors the sound. In addition, the inside of the cabinet will reflect sound off the walls back out through the driver cone. Extra bracing is required in the cabinet to reduce resonances, and sound absorbing material such as wool or fiberglass is usually used inside the cabinet. The bracing should be such as to move the resonances above the operating frequency of the drivers so that it is not excited by driver movement. Some manufacturers have tried to use this resonance to work for them, and tune the boxes to give a pleasing resonance or one that peaks at a natural dip in system response. This is based on a the idea that musical instruments, such as a violin, are tuned to a pleasing resonance. Never having heard one of these, I will reserve my opinion on this method, but I suspect I would not care for it based on my personal preferences. An enclosure can also be designed using constrained layer damping, which makes a sandwich of a sound absorbent material between two harder layers. This has proven to be a very effective method of vibration control.

    Reflections from inside the enclosure can be addressed by suitable use of sound dampening material inside the enclosure such as fiberglass or wool. Non parallel walls also help, and are often used to combat another enclosure problem, standing waves in the enclosure. This is when the size or shape of an enclosure happens to match a physical wavelength of sound, and the wave tends to continue instead of die off, again resonating and creating an objectionable addition to the sound. This is usually combated by designing the proportions of the enclosure to the so called golden ratio of 1.618. Resonances can also be fought by other methods, such as the Arc port in Polkaudio speakers. Arc stands for Anti Resonance Control, and uses an out of phase back wave from the driver, ported to the front, to cancel a resonance. This requires a rather precise port tuning.

    Speaking of port tuning... several issues need to be addressed here, also. A port is usually tuned to an exact frequency of resonance with the cabinet. A port should not be too small of a diameter or it can cause unmusical noises as the air vibrates. The back of the port should not be too close to a speaker wall, and if both ends are provided with a flare the performance is much better. This flared design is what BIC America terms a 'Venturi' port. Another method is used on Polk's speakers, which they call a 'Power Port'. This design has a port flared at both ends, but add a Hershey kiss shaped device in the center of the output side of the port. Essentially this helps distribute the bass from the port and enables the port side of a speaker to be placed directly against a wall or floor surface without hindering performance. 

    Without going into great detail about this right now, the problem occurs when the sound wave emanating from a driver encounters a sharp angle. This causes a type of reflection again messing up frequency response and causing comb filtering. The solution is to round all corners and edges on the cabinet, and flush mount the drivers. It is also helpful to use a layer of felt or foam on the front baffle of the cabinet to reduce reflections and diffraction effects.

    As you can see, cabinet design is not trivial, however some very good diy designs can be made. Most commercial designs have to make trade offs among all these factors, not to mention all the other myriad aspects such as crossover designs, driver impedance compensation, vertical and horizontal treble dispersion, driver matching and an endless amount of other issues. Since all systems are designed to a certain price point, it is indeed possible for a talented and informed diy'er to come up with an outstanding design that is better than a system you could otherwise afford.

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