Originally appeared in the September/October 1988 issue of Car Stereo Review magazine.
When designing a woofer system for a high-end auto sound installation, you should stick to a very logical and straightforward procedure. The most important factors are size, performance, cost, and ease of fabrication.
The first step is to decide how much room you want to dedicate to your woofer system; generally, the bigger the system, the better. This decision will inevitably affect the other parameters of the design process. High-performance systems generally require large woofers and large enclosures. This usually raises the price of the system and lengthens the amount of time required for construction.
After you have decided on how much space your woofer system will occupy, you must determine what type of enclosure is right for your particular application. The simplest type of woofer system is the "infinite baffle." The infinite baffle is nothing more than a barrier that separates the front of the speaker from the rear of the speaker; this barrier is called a baffle board. A typical infinite-baffle installation in an auto sound system consists of mounting the woofer(s) on the rear deck of the vehicle or on the back of the rear seat. In any infinite-baffle design, it is very important that no path exists for sound to reach from the rear of the speaker to its front. If this happens, cancellation will occur and bass output will be reduced.
An infinite-baffle system is probably the easiest way to install woofers in your vehicle. You can expect good performance from such an installation, and since there are no boxes to design or build, installation time and cost are kept to a minimum. Unfortunately, there are several disadvantages to the infinite baffle. When a speaker is mounted in an enclosure, the air inside the enclosure acts like a spring against the speaker cone, and he resulting buildup in pressure provides mechanical damping and prevents excessive cone movement. But an infinite-baffle installation does not use an enclosure, so the mechanical damping is poor, resulting in "hang-over" and lack of definition. The power handling of the speaker will also be compromised since the infinite baffle provides very little control over cone excursion.
The second type of system is the "sealed box," and it is similar to the infinite-baffle design. A sealed box is just that: a completely airtight enclosure. Since the speaker is mounted in one face of the enclosure, there are no paths for sound to travel from the rear of the speaker to its front. Sealed boxes are very easy to design and build, and they generally perform well with almost any driver. Superb damping, good power handling, solid low-frequency response, and simplicity of design and construction make the sealed-box enclosure an ideal choice for many installations.
There are some drawbacks to using a sealed box, however. Using a box thats too small can have adverse effects both on your systems output level and on its deep-bass performance. Efficiency is another matter of concern; typically, sealed-box systems are less efficient than vented systems (see below) and require more power and equalization to achieve comparable output levels.
The best performance will be obtained by mounting the woofer in a "vented box." Vented systems, also known as ported or bass-reflex systems, use a duct to help fine tune the drivers performance. A good vented design provides essentially flat response to just above a low-frequency 3-dB downpoint, with system output rolling off at some rate below that frequency. As frequency approaches that at which the box is tuned, system output from the vent increases and is driven to full acoustic output by an ever-reducing excursion of the woofer itself. At this frequency, output comes almost exclusively from the vent and woofer excursion is at a minimum. This is quite different from a sealed system, where cone excursion increases four times for every octave of reduction in frequency. Reduced excursion means lower distortion, since the vent is not subject to the mechanical suspension limitations of the speaker. The excursion-controlling aspect of a good vented design is of fundamental importance, particularly in high-performance systems where high acoustic output and low distortion are desirable.
The biggest disadvantages of vented enclosures are the increased complexity of their design, poor performance when designed improperly, and unloading of the speaker at frequencies below the tuning frequency. However, note that most speaker manufacturers provide specifications, construction plans, and tuning charts that allow you to construct a vented system rather easily.
Once youve selected the type of enclosure, you can begin the process of selecting the driver(s). For an infinite-baffle system, a low-compliance driver is required. This type of speaker usually has a tight (accordion) surround and a large magnet, both of which compensate for the poor damping inherent in typical infinite-baffle systems.
Sealed systems, on the other hand, require high-compliance drivers. In order to provide good low-frequency output without excessive damping, these speakers almost invariably have loose, roll-edge suspensions. If a small sealed box is to be used, the cone should be constructed of fairly heavy material and be capable of long excursions. Typical heavy cone materials are plastic, polypropylene, and plasticized (treated) paper.
Vented systems, like infinite-baffle systems, work best with low-compliance drivers. The reduced excursion properties of vented systems allow the use of a much tighter cone. This results in less distortion, better transient response, and better cone control. Additionally, these drivers tend to be more efficient because of their lighter paper cones. It should be noted, however, that just about any speaker will work in this type of enclosure – as long as the enclosure is properly designed.
To select the size of the driver(s), you must consider the box volume available and the acoustic output de- sired. Most manufacturers offer charts that specify the recommended volume required for their speakers. Typically, 10-inch woofers require a volume of 1 to 1.5 cubic feet, 12-inch woofers require 1 to 3 cubic feet, and 15-inch woofers require 4 to 10 cubic feet.
As you probably have guessed, large woofers will almost always out-perform small woofers in any installation. The reasons for this are simple. Acoustic output is directly related to the volume of air displaced by the cone. This displacement is similar to the bore and stroke of a piston in an engine. In the case of a woofer, the bore is the area of the speaker cone and the stroke is the excursion. It is obvious that increasing the diameter of the driver or its excursion will increase its maximum output.
Typically, speakers less than 8 inches in diameter are incapable of producing sufficient acoustical power at low frequencies without tremendous excursion. On the other hand, speakers larger than 15 inches in diameter suffer from poor transient response due to the large mass of the cone.
In installations where large woofers are impractical, multiple small drivers can be used. In some instances, multiple small drivers can actually outperform a single, large driver. The advantage of using small drivers is that they require less box volume; the disadvantage is that each driver requires electrical power from the amplifier, and phasing problems can also arise.
The following system relationships can help you to decide whether to use a few large drivers or many small ones.
(A = cone area; E = cone excursion, one way; F = lowest frequency desired; P = acoustical power desired.)
If the cone area is doubled, cone excursion is halved: 2A = E/2.
If the cone area is halved, cone excursion is doubled: A/2 = 2E.
If the frequency is halved, cone excursion increases by a factor of four: F/2 = 4E.
If the frequency is doubled, the cone excursion is reduced by a factor of four: 2F = E/4.
If the cone excursion is halved, the acoustical power is halved: E/2 = P/2.
If the cone excursion is doubled, the acoustical power is doubled: 2E = 2P.
From these equations you can determine the best method for your installation. As a guideline, five 8-inch drivers, three 12-inch drivers, and two 15-inch drivers deliver comparable acoustic output.
Knowing the size and type of the enclosure plus the size, type, and number of drivers required brings you to the fun part: selecting a single driver from the hundreds available. The cost of different drivers varies dramatically, as does their quality. Shop for efficiency in your driver. Efficiency is like mpg: the more efficient your driver, the less power required to drive it. Efficiency is rated two ways. The first is half-space reference efficiency, which is stated as a percentage. The second method is sound-pressure level (SPL) at 1 meter with an input of 1 watt. In both instances, higher numbers are better.
Efficiency is related to several factors. The mass of the cone will drastically affect efficiency. Tripling the cone mass reduces the acoustic output by one half. Magnet size is also a key factor. Theoretically, doubling the magnet size will double the efficiency. The cross-sectional shape of the wire used on the voice coil also plays an important role. Rectangular (edge-wound) wire is more efficient than round wire.
Power handling is also an important criterion when selecting a driver. Be careful when comparing this rating. Some manufacturers give root mean square (rms) ratings; others publish peak-power ratings (or power-handling). Peak-power ratings are usually double the rms rating. The difference in these ratings is easily understood. Since the voice-coil temperature corresponds to the output level of the music, a source with a lot of transient (peak) information will heat the voice coil to a lesser degree than a continuous (rms) sine wave.
The electrical power-handling capability of a speaker is directly related to the voice coils ability to dissipate heat. Large voice coils have more surface area and are able to dissipate heat better than small voice coils. A vented pole piece – usually appearing as a hole in the center of the magnet structure – is also important. Another form of venting is the use of a vented dust cap. This allows air to circulate over the voice coil for cooling. It also keeps excessive pressures from developing inside the voice-coil assembly, which, if unchecked, could blow the dust cap right off the driver.
Obviously, building a woofer system isnt as simple as throwing any old woofer in a box. Adhering to the selection procedures outlined above should result in maximum performance at the best possible price. You might also want to check out Vance Dickasons The Loudspeaker Design Cookbook (Marshall Jones Co.), for some straightforward, no-nonsense direction.