note # 1
and Frequency Response measurement of a Loudspeaker
is assumed that a copy of Sample Champion is installed, working
your Loudspeaker on a trestle in the center of the room, away
from obstacles, and a microphone about 2 meters in front of
the output of the sound card to an external amplifier, to feed
the loudspeaker, and the input of the sound card to the microphone
(using, if necessary, a preamplifier).
a cycle of MLS Impulse Response measurements (in the example
following setup has been used: Average 8 cycles,
Sampling Frequency 48 kHz, MLS 64K).
Sample Champion has finished the measure cycle, the impulse
response must be windowed in order to analyze the anechoic part
of the wave: start selecting the impulse (with the "Make
Selection A" command) from its start until the first reflection
(from the walls or from the floor) appears (as a little impulse,
see Application Note #2).
the impulse can be analyzed in the frequency domain by performing
the FFT of the time data included within "Selection A"
cursors. Open the "Measurement type selection" dialog
and select "Selection A in Impulse Response window".
In the example a
FFT length of 64K and a Blackman-Harris Half-Right weighting
window have been used.
for example, the view mode as "Logarithmic Magnitude Power
(dB)" and the graph above will be shown. This is the frequency
response of the loudspeaker. Remember that the lower resolution
limit depends (for every measurement method) on the weighting
window length and type. The rectangular window has a frequency
resolution of 1/T Hz, the Blackman-Harris
could be interesting seeing also the response in 1/3 third octave
bands. This can be obtained by using an MLS coloured by the
internal pink-filter (see also Application
Note #6). The results are the following (the level is different
from the previous measure one because of filtering):
Response topics on this subject on PureBits site.
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