The Subwoofer DIY Page
02 May 2019

The diagram below shows the various components of a typical driver:

DUAL-VOICE-COIL (DVC) DRIVER: a loudspeaker driver in which two separate voice coils are wound on the same former. Each coil can be connected to a separate channel on a stereo amplifier, or they can be wired in series or parallel and powered by one amplifier. A DVC driver can be used in place of two separate drivers when space is at a premium.

FILTER: an electronic network that allows certain frequencies to pass while blocking others. Active filters contain powered components such as operational amplifiers (op-amps) and are normally inserted before the main amplifier. Passive filters do not contain any powered components and are normally inserted between the amplifier and the loudspeaker.

The filters normally used in loudspeaker design are as follows:

  • Low Pass: passes lower frequencies, attenuates higher frequencies
  • High Pass: passes higher frequencies, attenuates lower frequencies
  • Band Pass: frequencies outside of a specific range are attenuated

ISOBARIC: (isobarik, compound loading) - a method of using two drivers working in tandem in order to achieve a smaller box size for a given design. Theoretically, the Vas of the complete system will be half that of a single driver, which results in a net box size that's also reduced by half. The sensitivity of the isobaric system will be the same as that of a single driver, but you've got to spring for that extra driver! "Clamshell" mounting, where the two drivers are mounted face to face and one driver is wired out of phase, seems to the most popular isobaric system used today, as it is the easiest to build.

SAF: Spouse Acceptance Factor. Basically, a rough measurement of what you can get away with in the house before your better half (who usually controls the finances as well) raises a fuss.

SUBWOOFER: A speaker designed to produce the lowest audio frequencies at an adequate volume. Most subwoofers, or "subs" as they're commonly called, are designed to operate from 80 Hz downwards, as the ear can usually pinpoint the source of any higher frequencies. The bass units of small three-piece systems are commonly referred to as subwoofers, but they often have limited output below 50 Hz or so.

T/S (THIELE/SMALL) PARAMETERS: terms used to describe the characteristics of a given driver. The most common T/S parameters encountered are:

Fs = resonance frequency of the driver. In free-air, the driver's impedance will peak at this frequency.
Pe = Thermal capacity of the driver, in Watts.  If continuously driven above its rated Pe, the driver may prematurely burn out and fail.
Qes = Electrical Q of the driver at Fs.  Qes is a measure of the driver's tendency to resonate at Fs, based on its electrical characteristics, e.g. magnet strength, magnetic circuit characteristics, etc.).  The driver's overall resonance characteristics are usually dominated by Qes.
Qms = Mechanical Q of the driver at Fs. Qms is a measure of the driver's tendency to resonate at Fs, based on its mechanical characteristics, e.g. surround compliance, the compliance of the spider, weight of the cone, etc.
Qts = Total Q of the driver at Fs.  Qts is a measure of the driver's tendency to resonate at Fs, based on its overall characteristics.   Qts can be calculated using the equation: 

Qts= Qms*Qes/(Qms+Qes))

Re = DC resistance of the driver's voice coil. Re is less than the driver's rated impedance (normally 4 or 8 ohms).
Sd = Effective surface area of the driver.  Roughly equal to the area of the cone plus 1/3rd of the surround. 
Xmag = Excursion limit due to the magnetic limitations of the driver's motor.  Xmag is defined as the displacement at which the BL product has fallen to 70% of its value at the cone's rest position.
Xmech = Maximum physical excursion capability of the driver.  Exceeding Xmech normally results in damage to the driver.
Xsus = Excursion limit due to the driver's suspension.  Xsus is defined as the point at which the compliance of the suspension has decreased to 25% of the value at the cone's rest position.
Xmax = Linear (one-way) travel of the cone.  Xmax is used to determine the maximum linear SPL capability of the driver, and can be defined in a number of ways.  The most useful way to define Xmax is to state it in relation to a specific level of distortion, i.e. "Xmax=13mm (10%THD)", indicating that at 13mm, THD will have risen to 10%.This definition is more useful than the older definition of Xmax, which was solely dependent on the length of the voice-coil vs. the length of the gap, and did not take into consideration the impact of the geometry of the motor or the characteristics of the driver's suspension.
Vas = Equivalent air compliance.  The volume of air that has the same compliance ("springiness") as the driver's suspension. Because less air is more "springy" than more air, a large Vas represents a "loose" suspension
Vd = Peak displacement volume.  Vd = Sd*Xmax