Thiele Parameters

Thiele parameters provide a precise description of driver behavior. They are now routinely provided by speaker manufacturers, and can be measured by a number of systems such as CLIO. There are three cautions:

  1. There are two different categories of parameters, mechanical and acoustic.
  2. Parameters are sometimes given in different units.
  3. Values provided by manufacturers can be significantly in error.

Acoustic vs. mechanical parameters

Acoustic parameters contain an "A" in the subscript, and are normalized by the effective piston area of the driver. The corresponding mechanical parameters, which contain an "M" in the subscript, are obtained by multiplying by the square of this area in the case of mass and resistive loss, or dividing by the square of the area in the case of compliance.

Parameter definitions and units

All equations in the Thiele-Small analysis section use MKS units. Two examples of units as provided by the CLIO system, and by the Solen catalog, are given in the table below. (Solen units are those actually provided for my Vifa driver, which differ from the definitions given in the front of the catalog).

Parameter

Description

MKS units

CLIO units

Solen units

Bl

Force factor

N-amps-1

N-amps-1

N-amps-1

CMS

Suspension compliance

m-N-1

mm-N-1

 

CAS

Acoustic compliance

m5-N-1

m5-N-1

 

RMS

Suspension loss

kg-s-1

 

 

RAS

Acoustic suspension resistance

kg-s-1-m-4

see note

 

MMS

Moving mass

kg

grams

grams

MAS

Acoustic mass

kg-m-4

kg-m-4

 

SD

Effective piston area

m2

mm2

cm2

VAS

Equivalent air volume

m3

liters

liters

RE

DC voice coil resistance

Ohms

Ohms

Ohms

LE

Voice coil inductance

Henries

mH

mH

fS

Resonance frequency in air

Hz

Hz

Hz

QMS

Mechanical Q factor

-

-

-

QES

Electrical Q factor

-

-

-

QTS

Total Q factor

-

-

-

Note: CLIO denotes this as kOhms, but that is incorrect.

Supplementary equations

If the following parameter values are not available, they may be calculated using the following equations. Some of the results are approximate, and measured values are always preferred.

A comparison of measured vs. calculated values shows fairly good agreement.

Parameter

CLIO

Calculated

MMS

0.00642

0.00657

RMS

2.13

2.18

CMS

0.000700

0.000686

Comparison of catalog and measured values

The catalog values are from Solen, for my P13WH-00-08 5" Vifa driver. The measured values are again from my CLIO system. As noted above, there are some significant differences.

Parameter

Catalog

Measured

fS

60

74.99

Qts

0.33

0.4

QES

0.43

0.56

QMS

1.38

1.42

Bl

6

5.57

MMS

0.0075

0.0064

Vas

0.01

0.00725

RE

5.7

6.4

Le

0.7

1.09