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.
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
- 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.