Loudspeaker Performance Test & Measurement
We believe that accurate measurements are the key to attaining the optimum performance from any loudspeaker. This true whether we are designing, tuning, voicing, fault-finding or evaluating a loudspeaker. However, it has to be said that there is no single measurement or curve that will tell you how a loudspeaker will sound... but there are a range of measurements that in combination, if done correctly will give a good indication of how a loudspeaker will perform
"Measurement is the first step that leads to control and eventually to improvement.
If you can't measure something, you cannot understand it.
If you can't understand it, you can't control it.
If you can't control it, you can't improve it."
H. James Harrington
We regularly carryout out the following measurements: -
1. Raw Drivers
Impedance curve (to determine Znom; Zmin; Fs; Magnitude & Phase of Impedance by Frequency)
This can only be measured using a swept impedance analyser and should be a 'small signal' test so as not to over-heat the voice-coil during the measurement.
True DC resistance of the voice-coil (Re)
This best measured with a 4-wire DC bridge or DC milli-ohm meter. Any instrument that uses an AC stimulus (includes some multi-meters) will not give a true measure of Re because of the reactive element (voice-coil inductance) that will skew the result.
Thiele-Small parameters - the following parameters can be extracted using either the added mass or standard volume methods
Qes - electrical Q of the motor system (at resonance Fs)
Qms - mechanical Q of the motor system (at resonance Fs)
Qts - total system Q (at resonance Fs)
Vas - the volume of air having the same acoustic compliance as the driver suspension
Mms - mechanical mass of the driver cone assembly
Bl - magnetic motor strength. Flux density x length of the voice coil in the magnetic field gap (B x l)
Sd - effective surface area of the driver cone
Le - inductance of the voice-coil
Cms - mechanical compliance of the driver suspension
Rms - mechanical resistance of a driver suspension
SPL - Sound Pressure Level obtained at a distance of 1m with an input voltage of 2.83 volts
The key 'large signal' parameter is Xmax which is the maximum linear peak (peak-to-peak) excursion of the diaphragm. This is usually taken from the manufacturer's datasheet.
This can be measured near-field to give an indication of performance but correctly the driver should be mounted in a large baffle at measured at a distance of 1m from the driver diaphragm. Frequency response of a driver must be measured under anechoic conditions using a stimulus which sweeps the entire audio band and with a calibrated measurement microphone which flat across the swept band. Presentation of this measurement in typically on a logarithmic frequency by dB SPL scale.
This can be measured near-field using a stimulus of stepped tones and measurement of spectral contaminants such as harmonic or inter-modulation products. Presentation of this measurement in typically on a logarithmic frequency by dB or percentage distortion relative to a reference signal level
2. Drivers in Cabinets
On-axis frequency response
Uniformity over the Listening Window
Uniformity of off-axis response (directivity)