This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.

These are a group of parameters outlined by A. N. Thiele, and later R.

H. Small, which can completely describe the electrical and mechanical

characteristics of a mid and low frequency driver operating in its

pistonic region. These parameters are crucial for designing a quality

subwoofer enclosure, be it for reference quality reproduction or for

booming.

`Fs'

Driver free air resonance, in Hz. This is the point at which

driver impedance is maximum.

`Fc'

System resonance (usually for sealed box systems), in Hz

`Fb'

Enclosure resonance (usually for reflex systems), in Hz

`F3'

-3 dB cutoff frequency, in Hz

`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

`D'

Effective diameter of driver, in meters

`Sd'

Effective piston radiating area of driver in square meters

`Xmax'

Maximum peak linear excursion of driver, in meters

`Vd'

Maximum linear volume of displacement of the driver (product of Sd

times Xmax), in cubic meters.

`Re'

Driver DC resistance (voice coil, mainly), in ohms

`Rg'

Amplifier source resistance (includes leads, crossover, etc.), in

ohms

`Qms'

The driver's Q at resonance (Fs), due to mechanical losses;

dimensionless

`Qes'

The driver's Q at resonance (Fs), due to electrical losses;

dimensionless

`Qts'

The driver's Q at resonance (Fs), due to all losses; dimensionless

`Qmc'

The system's Q at resonance (Fc), due to mechanical losses;

dimensionless

`Qec'

The system's Q at resonance (Fc), due to electrical losses;

dimensionless

`Qtc'

The system's Q at resonance (Fc), due to all losses; dimensionless

`Ql'

The system's Q at Fb, due to leakage losses; dimensionless

`Qa'

The system's Q at Fb, due to absorption losses; dimensionless

`Qp'

The system's Q at Fb, due to port losses (turbulence, viscosity,

etc.); dimensionless

`n0'

The reference efficiency of the system (eta sub 0) dimensionless,

usually expressed as a percentage

`Cms'

The driver's mechanical compliance (reciprocal of stiffness), in

m/N

`Mms'

The driver's effective mechanical mass (including air load), in kg

`Rms'

The driver's mechanical losses, in kg/s

`Cas'

Acoustical equivalent of Cms

`Mas'

Acoustical equivalent of Mms

`Ras'

Acoustical equivalent of Rms

`Cmes'

The electrical capacitive equivalent of Mms, in farads

`Lces'

The electrical inductive equivalent of Cms, in henries

`Res'

The electrical resistive equivalent of Rms, in ohms

`B'

Magnetic flux density in gap, in Tesla

`l'

Length of wire immersed in magnetic field, in meters

`Bl'

Electro-magnetic force factor, can be expressed in Tesla-meters or,

preferably, in meters/Newton

`Pa'

Acoustical power

`Pe'

Electrical power

`c'

Propagation velocity of sound at STP, approx. 342 m/s

`p'

Density of air at STP 1.18 kg/m^3 (rho)

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