Enclosure Fabrication and Subwoofer Enclosure Considerations

Courtesy of Clarion Corp.

In the world of car audio installation, we are often asked to install an enclosure in a sound system to augment the lower frequency sound spectrum. This process may include designing and or building custom enclosures to fit a certain vehicle, or to accommodate a specific design due to its desired sound characteristics. As we may have experienced up to this point, mathematics can play a large role in sizing measurements, internal volume calculations, port design and areas, and speaker mounting cut outs.

We can install a well-designed loudspeaker but get poor results in playback because of a poorly designed enclosure for a specific application. Likewise, we can take a much less expensive driver and get great results from it when the designs are applied properly and within the design parameters of the woofers’ intended range of operation.

There are a few common terms used that are important to understand when building enclosures. This information is commonly referred to as Thiele-Small parameters; these are electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. Driver manufacturers publish these parameters in specification sheets so that designers have a guide to selecting drivers for loudspeaker designs.

Below are most of the common electrical terms or references used in determining the proper enclosure design for optimal response.

1.) Free air resonance otherwise known as (Fs) – natural resonant frequency of a woofer.

2.) Compliance or Vas – volume of air having the same compliance of the woofer’s suspension.

3.) Q of the driver or (Qts) – indicates the total strength of the driver’s resonance.

Qts =Qes x Qms / (Qes +Qms)

4.) Qtc = strength of resonance for the speaker system.

5.) Ripple – Determined by the greatest deviation above or below the level at 90Hz, for example, before the driver rolls off.

6.) System cut off or F3 – (measured in Hz) The frequency at which the acoustic power output from a system has fallen to one half of its reference value. Also known as the 3db down point.

7.) Xmax:Xmag-Xmech – (can be measured in inches or millimeters) The measure of a speaker cone’s maximum excursion (one way) while maintaining a linear behavior.

8.) Power handling (thermal and excursion limited) – (measured in RMS) The continuous sine wave power that can be dissipated by the voice coil and magnet assembly without failure. Most speakers are not harmed by clipping. It is an over thermal issue that will cause the speaker to break down and cause the failure. Therefore, even overpowering a woofer will often not damage the motor assembly. However, it will certainly overheat much faster.

9.) Impedance – (measured in Ohms) total opposition to the flow of A/C current in an electrical circuit.

10.) Nominal Impedance – A simple measurement of a speaker’s voice coil while disconnected from the amplifier. It is important to note that ohm load constantly changes with the frequency of the music. The higher the frequency, the higher the ohm load and the lower the frequency, the lower the ohm load. That is why the reading is easily misunderstood; it is literally a “nominal” reading.

11.) Order – a term that explains the “type” of specific enclosure design that we are constructing or using. An infinite baffle enclosure is considered a “1st Order” design; a sealed enclosure is a 2nd order, and so forth.

Infinite Baffle Enclosures

Infinite baffle (1st order) is a common application when a baffle board separates the trunk (rear wave) from the vehicle’s cab (front wave). This typically requires more power to exhibit adequate control of the driver(s) at higher power levels. This design utilizes a minimum of space, resulting in slightly less accurate sound reproduction and added difficulty in sealing the vehicle to keep the front wave from the rear wave (cancellation). Aperiodic enclosures are a variation of this design.

Sealed Enclosures

Sealed enclosures (2nd order) are the simplest and most common type to design. They provide adequate separation of the front and back waves so that they do not cancel each other out. They exhibit great linearity and have more desirable power handling characteristics. Transient response is an advantage as well.

Vented or Ported Enclosures

Vented or ported enclosures (4th order) are similar to the sealed enclosures with the exception of an added port. The port is “tuned” to use the speaker’s rear wave to strengthen the front wave, thus exhibiting a greater tuned increase in sound (up to 3 db) at the tuned frequency. This can cause the speaker to exhibit greater efficiency when done properly. It can also drastically reduce cone control below the tuned frequency, thus increasing speaker failure potential if not controlled properly.

Isobaric Enclosures

Sealed isobaric (2nd order variation) is when two (alike) woofers are mounted to seal the movements between each other as a group (compound loaded). It is important to note that this is not really an enclosure type, but a variation of the sealed box arrangement. They must be as close together as physically possible, either back-to-back or face-to-face. This design can be used when space is an issue, allowing the use of half of the standard single woofer’s box requirements. This will also increase linearity and power handling.

Bandpass Enclosure

Bandpass enclosure (single/dual reflex) – once again this is the most complex design to use. It has limited frequency range, natural higher and lower frequency filtering, and greater efficiency. There is single reflex, indicating one port in one of the two chambers, and dual reflex, both chambers ported for a 6th order design. It is important to note that bandpass design can have the largest speaker failure rate due to the fact that they do not typically exhibit a distorted sound at higher volume levels, even though the woofer is being severely over driven. Once again, overheating becomes an issue and is the most common cause of failure.

The most common materials used when constructing enclosures are particleboard and medium density fiber board (MDF). Both can be easily obtained at your local hardware store. Plywood is not a recommended choice due to air leakage, lower density, which can cause resonance, and unwanted vibrations that can cause a loss of efficiency in the enclosure. It is important to note that there are several grades of plywood (laminated wood) available. Marine-grade plywood can be a great light wood alternative. Exotic materials are another choice for custom shaping enclosure designs, as long as we keep in mind that there are specific guidelines that must be adhered to when successfully designing speaker enclosures. When they are violated, we will hear the results.

Other common enclosure building materials would include fiberglass, Plexiglass, silicone, various types of sound dampening, flush mounting type screws, several varieties of builder’s glues, liquid nails, air nails (pneumatic air gun applied), yard sticks, measuring tape, circle drawing equipment (compass), drills (cordless and corded types), jig saws, and table saws. Routers may be used for flushing edges and/or shaping edges quickly.

An installer is going to encounter the terms listed above when measuring correct parameters for a specific enclosure design.  There are several companies that have software that will make the calculation job much easier when it comes time to quickly and accurately design any type of enclosure. However, it is important that an installer understand the terms and theory of loudspeaker design before he or she can be expected to excel in the complex world of enclosure design.