Thiele/Small Specs - What They Mean

The Thiele/Small specs that are constantly being referred to throughout these help files are the product of two Australians...namely A.N. Thiele and Richard Small, who, back in the late '60's and early '70's, applied filter synthesis methods to box design. Before that, it was process of trial and error and some rules of thumb!

These two gentlemen deserve some more publicity, so I'll include photographs of theirs into this help file. Here are the personal photographs of Nevielle Thiele and Richard Small:


Neville Thiele	Richard Small

They accomplished this by equating a woofer in a box to an electronic low or high pass filter. While they weren't the first to discover the aforementioned analogy, they were indeed the most prolific at it. So much so, that even today, speaker designs are almost exclusively based on their research, math, and formulas...ie, the Thiele/Small Specifications that you hear so much about.

Here is explanation to various driver parameter encountered in driver editor (from upper left to bottom right):

Thiele/Small-Parameters section

Qes: Electrical Q (=damping), lower value means higher damping. It describes a drivers ability to resonate at fs based on electrical means.
Qms: Mechanical Q (=damping, lower value means higher damping). It describes a drivers ability to resonate at fs based on mechanical means.
Qts: Total damping (parallel coupling of Qms and Qes).
Vas: Equivalent Volume of air to Cms.
Fs: Free air resonance frequency of driver.

Electro-Mechanical parameter section

Mms: Mechanical Mass of the vibrating part of the driver including air load.
Cms: Compliance of driver (inverse of spring stiffness).
Rms: Mechanical damping, gives you the mechanical damping of the diaphragm arising from mechanical friction, including the resistive part of the radiation load. Rms can be compared to Rme, and Rms is similarly related to Qms. Larger Qms gives smaller Rms. For woofers this is normally desired because the suspension then operates closer to a perfect spring.
Re: DC-resistance of voice coil.
BL: Magnetic Induction crossed with wire length in the airgap. (crossing = cross product, a mathematical vector-operation)
Dd: Diameter of Diaphragm.
Le: Voice coil inductance.
Sd: Surface-area of Diaphragm.
fLe: The frequency at which Le and KLe is to be determined.
KLe: Voice coil semi-inductance in [H·sqrt(Hz)], after Vanderkooy

Large-Signal Parameter section

Xmax: Max linear excursion, usually calculated as abs(Hc-Hg)/2, and sometimes multiplied by a factor (1.15 or 0.87, depending on how much distortion is accepted). WinISD does not multiply with any of these factors. Some manufacturers erroneously gives you Xmax as the damage limit, see Xlim. WinISD needs this as peak value.
Xlim: Damage limit excursion, also a peak value.
Hc: Height of coil.
Hg: Height of airgap.
Vd: Volume Displacement, how much air the driver can move in its linear range.
Pe: Thermal limited max. continuous electric power handling. If a driver is driven continuously above Pe, then it will eventually fail.

Miscellaneous parameters section

no: Efficiency (n should be the greek letter "eta") in percent [%].
Znom: Nominal impedance of the driver (not used in simulation).
USPL: efficiency in deciBell (SPL = sound pressure level) in [dB/2.83V] dB per 2.83 Volt (similar to 1W into an 8 ohm load). This SPL-measurement is similar to SPL (see above), but gives different values. This shows you the difficulties about matching drivers. With 8 ohm drivers 2.83 Volt gives you 1 Watt and the two figures (SPL and USPL) will be similar, but at lower impedance levels the USPL level will increase. USPL is the socalled voltage sensitivity and is closer to application with voltage amplifiers. To a limited extent you could match drivers for a loudspeaker system with this factor.
SPL: Efficiency in deciBell (SPL = sound pressure level) in [dB] per Watt directly related to no, but definately not an "accurate" figure in applications. In other words, if a speaker driver is specified by the manufacturer to some other value, do not use that value for WinISD unless you need it to calculate some Thiele-Small parameters and approximate values are better than no values at all. WinISD assumes distance 1 meter, radiation into halfspace (2*pi), and voltage driv. SPL is the so-called power-sensitivity, not really related to application, normally voltage amplifiers are used, but can become relevant if you want to compare two similar drivers with different nominal impedance levels.
Voicecoils: Voicecoils is a descriptive parameter. It just tells how many voicecoils there are. So ordinary drivers have 1, DVC drivers have 2 etc.

Thermal Parameters Section

AlfaVC: AlfaVC is resistance temperature coefficient of voice coil material. It tells what is relative change of voice coil resistance per unit of temperature. Expressed in 1/K. Normally, copper has AlfaVC of about 0.0039 1/K at +20 °C.
R(t): R(t) is thermal resistance of driver from voice coil to ambient box air in Kelvins/Watt (K/W). This is not used yet in simulations.
C(t): C(t) is thermal capacity of driver voice coil assembly in Joules/Kelvin. This is not used yet in simulations.

"figure of merits" parameters section

SPLmaxLF: SPLmaxLF gives how loud driver can play in closed box or infinite baffle into half-space at maximum excursion at 20 Hz. Distance from this imaginary baffle is 1 meter. It gives "feeling" on Vd. Note also, that it doesn't apply to vented or any other assisted enclosure.
SPLmax: Maximum thermal limited SPL in [dB] (at maximum Pe, assuming power compression = 3 dB) playing into 2pi space
Rme: Electromagnetic Damping Factor in [N·s/m] (the unit for viscosity), gives you the mechanical control/damping of the diaphragm arising from the electro-magnetic motor system. Rme is related to Qes in a way similar to how Rms is related to Qms. Rme is often used as a measure for power of the magnetic motor system, see Mpow and Mcost.
Gamma: The acceleration factor (acceleration per ampere) in [m/(s²·A)].
Mpow: Motor power-factor in Newton per square-root Watt [N/sqrt(W)]. Similar to Mcost. I have seen Rme as a measure of motor power, but this is simply the square-root of Rme, and it provides a simple measure in Newton, which I (Claus Futtrup) prefer, and which seems to relate the actual (subjectively perceived) power in a linear way. The square-root Watt unit can be difficult to understand, but should be interpreted as square-root of Volt * Ampere. In this respect it becomes clear that Mpow is independent of the drivers impedance level, and therefore does not prioritize high or low impedance drivers. Mpow is purely a motor system power-factor.
Mcost: Motor cost-factor in [N·s/m] (or [kg/s]). Mcost expresses how powerful the motor system is (based on Rme, Xmax and either Hc or Hg depending on whether the voice coil is overhung or underhung), and the Xmax value includes an indicator of how much efficiency is "lost" in the design. This factor is therefore a description of how expensive the motor system is. This is an indicator on the price of the driver, but please forget about the unit. Other factors comes in, like diaphragm material, manufacturing tolerances etc. This version of Mcost (instead of using Rme) is based on an extension suggested by T. L. Clarke, where the cost of getting a high Bl at low impedance must be even higher when the driver is significantly overhung or underhung.
EBP: Efficiency-Bandwidth-Product in [Hz]
Gloss: Gloss tells how many percent of Xmax driver cone will sag, if driver is mounted horizontally. This figure is only approximate, because surrounding air shifts resonance frequency to somewhat lower value. So result is somewhat over-pessimistic by some amount. Generally, figures of more than five percent tell you that driver in question shouldn't be mounted horizontally. Value of gravity acceleration used when calculating sag is 9.80665 m/s².

Dimension parameters section

Thick: The thickness of the basket plate.
Depth: Depth of the driver.
Magnet Depth: Magnet depth/height/thickness (cylinder height).
Magnet: Magnet diameter.
Basket: Basket diameter (the hole to cut in the baffle).
Outer: Outer diameter (the space to make room for on the baffle for the driver).
VCd: Voice coil diameter.
Dvol: Driver displacement volume. Approx. the box volume occupied by the driver, when mounted with magnet pointing into the box.

Ultimately, speaker design is more of an art than a science, and as such you could dig up, process, and ponder a lifetime of math, specs, and equations when designing a speaker. In fact, you could spend 1000 years designing, calculating, and crunching numbers in an attempt to create the "perfect" speaker. However, even after spending that 1000 years of planning, somebody would still find something "that could be better", about your box!

Therefore, I will not even make an attempt to cover "all" of the speaker design technology here (not only am I under-qualified for such a task, the very field itself lends itself to constant controversy which I refuse to lose sleep over).

WinISD was not written/developed as a "this is it, no matter what you say" program. Instead, it is intended to give even the casual audiophile a means to build a GREAT sounding box (using the available technology).

Just use the program as it is intended to be used, and enjoy the results, because you WILL end up with one Helluva great sounding box. Leave the constant bickering and "one upsmanship" alone!