Thiele-Small Parameters


Magnetic flux density in gap, in Tesla-meters (TM)


The magnetic strength of the motor structure.

The cross product of the magnetic gauss in the gap ‘B’ with the effective conductor length ‘L’. BL is measured in Tesla meters or Newtons per Ampere (same units)


Most manufactures will not spec this number, however we feel it inappropriate to omit it.

BL is the actual cross product of the magnet field B with the conductor length L. It is not a scalar, but rather a vector. Combine this vector with the current I and you’ll get exactly the force (up or down according to the DC or AC input). But with the way amplifiers are designed, the current I of the speaker is never constant, only the voltage is. It is also not entirely important to know what the force is because is changes so rapidly. In any event, the BL squared divided by the resistance of the voice coil Re in fact gives us a generic force factor in newtons squared per watt. This number is relative for all speakers and the higher the number, the more force a motor can invoke on a cone with the same input power.

Its important to distinguish that BL and Re are related much like inductance and Re are. 32Tm over 8 ohms is identical to 16Tm over 2 ohms, namely 128 N²/W. All other things equal (moving mass and cone size in particualr), A higher BL²/Re will increase the sensitivity of the driver which is a generally a very good thing.


BL Curve
A graph of the magnetic strength of the speaker's motor structure across the frequency spectrum of the speaker's operating range.


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


Acoustical equivalent of Cms


The electrical capacitive equivalent of Mms, in farads


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


Effective diameter of driver, in meters


A driver measurement system that can, among other things, measure how far a driver's cone can travel while still working in a linear fashion.


-3 dB cutoff frequency, in Hz


Enclosure resonance (usually for bass reflex systems), in Hz


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



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


length of wire immersed in magnetic field, in meters


The electrical inductive equivalent of Cms, in henries


This is the voice coil inductance measured in millihenries (mH). The industry standard is to measure inductance at 1,000 Hz. As frequencies get higher there will be a rise in impedance above Re. This is because the voice coil is acting as an inductor. Consequently, the impedance of a speaker is not a fixed resistance, but can be represented as a curve that changes as the input frequency changes. Maximum impedance (Zmax) occurs at Fs.


The total moving mass of the loudspeaker cone.


Diaphram mass, in grams


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


The reference efficiency of the system (eta sub 0) dimensionless, usually expressed as %


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


Acoustical power


Electrical power


The relative damping of a loudspeaker


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


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


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


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


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


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



The system's Q at Fb, due to port losses (turbulence, viscousity, etc.); dimensionless



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


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


Ripple, in dB


Acoustical equivalent of Rms


The electrical resistive equivalent of Rms, in ohms


DC voice coil resistance, in ohms


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


The driver's mechanical losses, in kg/s


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.


Effective piston radiating area of driver, in square meters


Sound Pressure Level, usually measured at 1 watt, at 1 meter in front of the loudspeaker


Term used to define a speaker or system with a balance between focuses on SPL and SQ.


Sound Quality. A term that refers to a focus on accuracy and transparancy as opposed to a focus on how loud the speaker or system can get.


"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. "


Maximum linear volume of displacement of the driver (product of Sd times Xmax), in cubic meters.


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.


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 DUMAX definition is objectively the best one, and it is defined as the shorter of the Xmag and Xsus values, in each direction of cone travel. 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.


Maximum physical excursion capability of the driver. Exceeding Xmech normally results in damage to the driver.Excursion limit due to the driver's suspension.


The point at which the compliance of the suspension has decreased to 25% of the value at the cone's rest position.


This parameter represents the speaker’s impedance at resonance.

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