Thiele-Small Parameters

Spl (dB) – sound pressure level.

Measured under specific conditions. Usually this is specified at an input of 1 watt or 2.83 volts (2.83 volts = 1 watt into an 8-ohm load) at a distance of one metre.

Fs (Hz) - resonance frequency of loudspeaker moving mass (in free air).

The frequency at which the combination of the energy stored in the moving mass and suspension compliance is maximum, and results in maximum cone velocity, at this point, impedance of the loudspeaker is maximum. A more compliant suspension or a larger moving mass will cause a lower resonance frequency, and vice versa. Usually it is less efficient to produce output at frequencies below Fs, and input signals significantly below Fs can cause large excursions, mechanically endangering the woofer. To find find speaker resonant frequency is usually used tone generator.

Fb (Hz) - bass reflex resonance frequency.

Vas (L) - equivalent volume. It is a loudspeaker driven closed volume of air that has a flexibility equal to that of the Cms (Moving System).

Sd (cm²) - effective projected area of the cone.

Usually measure from junction of the surround with the cone, this is approximately 50-60% of the structural area. Subwoofers with wide roll surround can have significantly less effective area than conventional types with the same frame diameter.

Xmax (mm) - maximum displacement of the diffuser in one direction.

Xmech (mm) - maximum displacement of the diffuser in both directions.

Vd (L) - The volume displaced by the cone, equal to the cone area (Sd) multiplied by Xmax.

A particular value may be achieved in any of several ways. For instance, by having a small cone with a large Xmax, or a large cone with a small Xmax. Comparing Vd values will give an indication of the maximum output of a driver at low frequencies. High Xmax, small cone diameter drivers are likely to be inefficient, since much of the voice coil winding will be outside the magnetic gap at any one time and will therefore contribute little or nothing to cone motion. Likewise, large cone diameter, small Xmax drivers are likely to be more efficient as they will not need, and so may not have, long voice coils.

Re (Ohm) - DC resistance (DCR) of the voice coil.

Re should not be confused with the rated driver impedance, Re can be tightly controlled by the manufacturer, while rated impedance values are often approximate at best. American EIA standard RS-299A specifies that Re (or DCR) should be at least 80% of the rated driver impedance, so an 8-ohm rated driver should have a DC resistance of at least 6.4 ohms, and a 4-ohm unit should measure 3.2 ohms minimum. This standard is voluntary, and many 8-ohm drivers have resistances of ≈5.5 ohms, and proportionally lower for lower rated impedances.

Qms (unitless value) - mechanical Q-factor of the loudspeaker at the resonant frequency (Fs), takes into account mechanical losses.

Mechanical damping of the driver, that is, the losses in the suspension (surround and spider.) High Qms indicates lower mechanical losses, and low Qms indicates higher losses. The main effect of Qms is on the impedance of the driver, with high Qms drivers displaying a higher impedance peak. One predictor for low Qms is a metallic voice coil former. These act as eddy-current brakes and increase damping, reducing Qms. They must be designed with an electrical break in the cylinder (so no conducting loop). Some speaker manufacturers have placed shorted turns at the top and bottom of the voice coil to prevent it leaving the gap, but the sharp noise created by this device when the driver is overdriven is alarming and was perceived as a problem by owners. High Qms drivers are often built with nonconductive formers made from paper or various plastics.

Qes (unitless value) - electrical Q-factor of the loudspeaker at the resonant frequency (Fs), takes into account electrical losses.

Electrical damping of the loudspeaker. As the coil of wire moves through the magnetic field, it generates a current which opposes the motion of the coil. This so-called "Back-EMF" (proportional to Bl × velocity) decreases the total current through the coil near the resonance frequency, reducing cone movement and increasing impedance. In most drivers, Qes is the dominant factor in the voice coil damping.

Qts (unitless value) - total Q-factor of the loudspeaker at the resonant frequency (Fs), takes into account all losses.

Combined electric and mechanical damping of the driver. The value of Qts is proportional to the energy stored, divided by the energy dissipated, and is defined at resonance (Fs).

Cms (m/N) - compliance of the suspension system (displacement under the influence of mechanical load).

The reciprocal of its 'stiffness', the more compliant a suspension system is, the lower its stiffness, so the higher the Vas will be. Cms is proportional to Vas and thus has the same tolerance ranges.

Mms (g) - mass of the suspension system (including acoustic load).

This is the mass of the cone, coil and other moving parts of a driver, including the acoustic load imposed by the air in contact with the driver cone.

Mmd (g) mass of the suspension system (without the acoustic load).

This is the mass of the cone, coil and other moving parts of a driver, without the acoustic load imposed by the air in contact with the driver cone.

Rms (N·s/m) - mechanical resistance of a loudspeaker suspension (i.e., 'lossiness').

Units are not usually given for this parameter, but it is in mechanical 'ohms'. Rms is a measurement of the losses, or damping, in a driver's suspension and moving system. It is the main factor in determining Qms. Rms is influenced by suspension topology, materials, and by the voice coil former material.

Bl (T·m) - magnetic induction coefficient.

The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field. Technically this is B×l or B×l sin(θ) (a vector cross product), but the standard geometry of a circular coil in an annular voice coil gap gives sin(θ)=1. B×l is also known as the 'force factor' because the force on the coil imposed by the magnet is B×l multiplied by the current through the coil. The higher the B×l product, the larger the force that is generated by a given current flowing through the voice coil. B×l has a very strong effect on Qes.

Le (mH) - Voice coil inductance (measured at 1 kHz).

The coil is a lossy inductor, in part due to losses in the pole piece, so the apparent inductance changes with frequency. Large Le values limit the high frequency output of the driver and cause response changes near cutoff. Inductance varies with excursion because the voice coil moves relative to the polepiece, which acts as a sliding inductor core, increasing inductance on the inward stroke and decreasing it on the outward stroke in typical overhung coil arrangements. This inductance modulation is an important source of nonlinearity (distortion) in loudspeakers.