Editing Thiele/Small parameters (section)

==Measurement techniques==
There are numerous methods to measure Thiele-Small parameters, but the simplest use the input impedance of the driver, measured near resonance. The impedance may be measured in free air (with the driver unhoused and either clamped to a fixture or hanging from a wire, or sometimes resting on the magnet on a surface) and/or in test baffles, sealed or vented boxes or with varying amounts of mass added to the diaphragm. Noise in the measurement environment can have an effect on the measurement, so one should measure parameters in a quiet acoustic environment.

The most common ([[DIY]]-friendly) method before the advent of computer-controlled measurement techniques is the classic free air constant current method, described by Thiele in 1961.  This method uses a large resistance (e.g., <math>R_{\rm test}</math> = 500 to 1000&nbsp;[[ohm]]s) in series with the driver and a signal generator is used to vary the excitation frequency. The voltage across the loudspeaker terminals is measured and considered proportional to the impedance. It is assumed that variations in loudspeaker impedance will have little effect on the current through the loudspeaker. This is an approximation, and the method results in <math>Q</math> measurement errors for drivers with a high <math>Z_{\rm max}</math> – the measured value of <math>Z_{\rm max}</math> will always be somewhat low. This measurement can be corrected by measuring the total voltage across the calibration resistor and the driver (call this <math>V</math>) at resonance and calculating the actual test current <math>I = V/(R_{\rm test} + Z_{\rm max})</math>. You may then obtain a corrected <math>Z_{\rm max}</math> = <math>Z_{\rm max(uncorrected)}\times R_{\rm test}/I</math>.

A second method is the constant voltage measurement, where the driver is excited by a constant voltage, and the current passing through the coil is measured. The excitation voltage divided by the measured current equals the impedance.

A common source of error using these first two methods is the use of inexpensive AC voltage meters. Most inexpensive meters are designed to measure residential power frequencies (50–60&nbsp;Hz) and are increasingly inaccurate at other frequencies (e.g., below 40&nbsp;Hz or above a few hundred hertz). In addition, distorted or non–sine wave signals can cause measurement inaccuracies. Inexpensive voltmeters are also not very accurate or precise at measuring current and can introduce appreciable series resistance, which causes measurement errors.

A third method is a response to the deficiencies of the first two methods. It uses a smaller (e.g., 10&nbsp;ohm) series resistor and measurements are made of the voltage across the driver, the signal generator, and/or series resistor for frequencies around resonance. Although tedious, and not often used in manual measurements, simple calculations exist which allow the true impedance magnitude and phase to be determined. This is the method used by many computer loudspeaker measurement systems. When this method is used manually, the result of taking the three measurements is that their ratios are more important than their actual value, removing the effect of poor meter frequency response.