How To Select the Right Subwoofer Box

Subwoofer speaker boxes come in three configurations. They are vented, sealed and bandpass. Each variant of the subwoofer speaker box has their own unique properties.

The subwoofer speaker is designed to operate in a particular style of box. Putting the speaker into the correct type of box permits the speaker to work at peak performance. The choice of box is important. A cheap subwoofer loaded in the right enclosure will sound more superior than an expensive one in the wrong box.

Choosing the wrong kind of speaker for your audio system will make the sound worse. You could even experience mechanical damage to the speaker and audio components in your system..

When selecting a cabinets for your subwoofer there are some general guideline you should follow. You can use the Thiele-Small parameters below to help you figure out the best option.

Sealed box woofers need following characteristics:

Qts equal or greater than: 0.40
Fs equal or less than: 35 Hz
Xmax equal or greater than: 4 mm

Vented or ported box woofers need the following characteristics:

Qts equal or less than 0.40
Fs equal or less than 45 Hz
Vas should be relatively low (less than 4 cubic feet unless enclosure size is not a factor).

Also you should calculate the efficiency bandwiith product for the enclosure. To get this you will divide the Fs value by the Qes value. If the result is close to one hundred, a vented enclosure is the right choice. Closer to fifty means this driver will function best in a sealed alignment. The best results (widest passband) for bandpass subwoofers are obtained when the Fs value divided by the Qts parameter is ninety or higher.

Definition of the Thiele-Small parameters


Fs: Driver free air resonance, in Hz. This is the point at which driver impedance is maximum. "This parameter is the free-air resonant frequency of a speaker. It is the point at which the weight of the moving parts of the speaker becomes balanced with the force of the speaker suspension when in motion. As a general rule of thumb, a lower Fs indicates a woofer that would be better for low-frequency reproduction than a woofer with a higher Fs. This is not always the case though, because other parameters affect the ultimate performance as well.
Qes: The driver's Q at resonance (Fs), due to electrical losses; dimensionless. "A measurement of the control coming from the speaker's electrical suspension system (the voice coil and magnet). Opposing forces from the mechanical and electrical suspensions act to absorb shock."
Qts: The driver's Q at resonance (Fs), due to all losses; dimensionless. "The 'Total Q' of the driver and is derived from an equation where Qes is multiplied by Qms and the result is divided by the sum of the same."
Xmax: Maximum peak linear excursion of driver, in meters. "Short for Maximum Linear Excursion. Speaker output becomes non-linear when the voice coil begins to leave the magnetic gap. Although suspensions can create non-linearity in output, the point at which the number of turns in the gap (see BL) begins to decrease is when distortion starts to increase. Eminence has historically been very conservative with this measurement and indicated only the voice coil overhang (Xmax: Voice coil height minus top plate thickness, divided by 2). Xmech is expressed by Eminence as the lowest of four potential failure condition measurements times 2: Spider crashing on top plate; Voice coil bottoming on back plate; Voice coil coming out of gap above core; Physical limitation of cone. Take the lowest of these measurements then multiply it by two. This gives a distance that describes the maximum mechanical movement of the cone."
Vas: "Equivalent volume of compliance", this is a volume of air whose compliance is the same as a driver's acoustical compliance Cms (q.v.), in cubic meters. "Vas represents the volume of air that when compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker. Vas is one of the trickiest parameters to measure because air pressure changes relative to humidity and temperature — a precisely controlled lab environment is essential. Cms is measured in meters per Newton. Cms is the force exerted by the mechanical suspension of the speaker. It is simply a measurement of its stiffness. Considering stiffness (Cms), in conjunction with the Q parameters gives rise to the kind of subjective decisions made by car manufacturers when tuning cars between comfort to carry the president and precision to go racing. Think of the peaks and valleys of audio signals like a road surface then consider that the ideal speaker suspension is like car suspension that can traverse the rockiest terrain with race-car precision and sensitivity at the speed of a fighter plane. It’s quite a challenge because focusing on any one discipline tends to have a detrimental effect on the others.

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