Frequency and phase response measuring

Frequency and phase response are dependent on the cabinet where the drivers are fitted. Therefore, measuring should take place after the drivers have been fitted.

The frequency response may be measured in an anechoic chamber. If this is not available modern measuring technology allows the room resonance to be eliminated. In both cases the room size determines the lowest frequency that can still be genuinely measured.

This lowest frequency occurs in a normal room since room resonance is eliminated:
a) distance from loudspeaker to the microphone xa
b) distance from loudspeaker to microphone xb, when the sound is reflected by ceiling, floor or walls

The lower measuring frequency fu = sound velocity / (xb-xa)
An example of a room with a 2.5 metre ceiling height:
xa = 0.60 metre (microphone distance, for smaller speakers)
xb = 2.6 metres (ravelling distance of sound that is reflected)
fu = 340 m/s / (2.6 m - 0.6 m) = 170 Hz

Below that frequency the frequency response needs to be complemented by the Thiele and Small simulation or near field distribution measurement.

Frequency response measuring with Clio, with Dirac XT as an example

What we said above is clearly noticeable when using Clio. First we need to determine a measuring window to be able to eliminate the resonance. Using Clio, a MLS Analysis with Time Domain has to be started (just click on the buttons with the red arrows).

This means: a specific noise signal, containing all desired frequencies, is reproduced by the driver that needs to be measured and the signal recorded by the microphone is analysed using a cross correlation function. Below, we have the microphone signal within the time domain as the result, showing how the impulse response is received by the microphone. If time is multiplied with the sound velocity of 340 m/s (at 20 degrees centigrade), we obtain exactly the distances as shown in our drawing:
the direct sound is received after 1.9 ms (0.0019 s x 340 m/s = 0.65 m) respectively
the first resonance is received after 7.4 ms (0.0074 s x 340 m/s = 2.52 m).

What to do now? That's quite simple: the time window needs to be set (just click the green marked symbols and green marked borders) between the first signal up to just before the first resonance (marked yellow on the time axis). Everything else is now cut out (marked red).
Now measuring takes place without any room resonance. Then click Frequency Domain (when arrow is red)
and please remember that the frequency response is only measured above 1 / Time Domain = 1 / 5.3 ms = 1 / 0.0053 s = 180 Hz.
or click Phase (when arrow is red).
All measuring is done now.

If this was the frequency response of a single speaker then the result could be exported to a crossover simulation programme by clicking File->Export->Data.

If required by our customers we could extend this series with:
- how to dimension a crossover
- what dB / octave should the crossover have; are the Bessel ...filters still current
- how to measure the total frequency (including low frequencies)

Not the easiest way, however, an example of what needs to be taken into consideration when designing a top-notch crossover, using the Lagrange 98 with Scanspeak D2904/9800 as a guide.