The measurement of loudspeaker impedance has traditionally been accomplished through the use of constant current source (CCS) techniques. This method supplies a constant drive current (typically 10mA) to the speaker, and produces a variable voltage across the speaker which is proportional to the speaker’s impedance. While this method is convenient to use, it has the following well known problems:

• Extremely nonlinear drive level across frequency.
• No resonance damping provided by the current source.
• Setup and connections must be changed to measure SPL.
• Measurements taken at extremely low fixed power level (typically 1mW).

The operating conditions imposed by the CCS method are in reality the exact opposite of normal operation. In normal operation a loudspeaker is driven directly from a power amplifier at virtually zero source impedance. The drive method is by constant voltage source (CVS). In CVS operation the speaker is driven with a linear voltage vs. frequency characteristic, with virtually zero source impedance, and at a much higher power level.

In order to determine impedance using the CVS method, a current shunt is employed to measure the current flow through the device. Knowing the current flow through the device, and the voltage across the device, the true impedance can then be calculated using basic Ohm’s law as Z=V/I, and the true power determined as P=V*I.

All loudspeakers, to varying degrees, are nonlinear resonant devices. Measuring the impedance for these devices presents the following difficulties:

(a) When a resonant electroacoustic device is driven from a current source of high impedance, all damping of the resonance behavior is determined solely by the mechanical (Qms) losses in the device. Under these conditions dynamically measured impedance can be greatly effected by the type of stimulus and/or the rate at which a sine wave is swept. Under CVS conditions, the damping is determined largely by the electrical system (Qes), and offers dramatically higher damping than the CCS method. This results in substantially greater precision and consistency for dynamic measurements.

(b) The nonlinear characteristics of electroacoustic devices imply that all measurements be taken under identical conditions as to which the device will operate. For CCS methods, measuring the device at a drive level of 1mW is 1000 times lower than the power level employed when commonly measuring SPL response at 1W. Moreover, the voltage produced from the CCS method follows the impedance curve itself, and changes radically near the resonance frequency. This can exaggerate and intensify nonlinear effects in the magnetic and compliance systems of the device. Using the CVS method produces measurements based on device operation identical to normal operation. The measurement level can be 1W, or at any other desired power. The nonlinear effects of the device are measured accurately under identical drive conditions to, and are representative of, normal operation.

Determination of electrical and mechanical loudspeaker parameters is commonly performed from measured impedance data. The quality and accuracy of the parameters produced is therefore a direct function of the quality and accuracy of the original impedance data. In order for the parameters to accurately represent the device under normal operating conditions, the impedance data must also accurately represent the same characteristics. The CVS method should always be used where accurate, repeatable, and high precision parameters are required.