Calibration Responses.

The Liberty Instruments IMP/MLS is a Fast Fourier Transform based audio frequency measurement device. It is a standalone box which interfaces with a PC-type computer through a parallel port. It has two sampling rates, 61200Hz and 1920Hz. It is a '4-wire' system in which variations from non-linearity in the microphone and electrical system are compensated for mathematically in post-processing. Additionally, it can perform many other functions such as impedance measurement, and derivation of T/S parameters.

Below you see the plotted frequency response of the IMP when it's input is connected directly to it's output. Note the falloff in the high frequencies due to the action of it's anti-aliasing filter. This is compensated for during data processing.

Below is the actual system electrical frequency response at the speaker input terminals. Note the approximately 2dB further attenuation at 19995Hz. This is because of the use of maximum length cables, i.e. 25 feet length, with the device. Cable capacitance results in additional high frequency attenuation. Fortunately the IMP is able to account for these changes.

One further note on the matter of calibration. The IMP and the LAUD system require the operator to manually apply the calibration to compenstate for the system's electrical response. If one 'forgets to CAL' or does not consider it necessary, then the frequency response one derives from impulse data is in error by an amount similar to that shown! It is understood that the LAUD device suffers the same problems with high frequency attenuation due to cable capacitance (for extended lengths).

Below is the calibration plot for my Mitey Mic I microphone (Old Colony Sound) supplied by Joe D'Appolito. Mr. D'Appolito (at one time) provided fee-based calibration service for the Mitey Mic microphone. The approximately +3dB peak in the response of the microphone at about 15KHz is part of the impulse responses shown in this section; and, is not removed until post-processing calibration is performed.

CARVER60 vs RD75 UNDERDAMPING COMPARISON

NO BAFFLE Impulse Response Frequency Response Waterfall FR
BAFFLED Impulse Response Frequency Response Waterfall FR
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The RD75 Dipole Baffle Study - Table of Contents.
Acoustic Line Source Research - Table of Contents.