Lecture 28B MLS-based measurements

Lecture 28B MLS-based measurements - Liberty Instruments,...

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Liberty Instruments, Inc. Home Products Technical Articles LAUD User info PRAXIS FAQ Contact Us Maximum Length Sequence (MLS) based measurements with LAUD by Bill Waslo (portions of this text were previously published in "The IMP Goes MLS" in Speaker Builder Magazine, 6/1993) T here is a one-to-one relationship between an impulse response (the waveform with which a device responds to a sharp impulse at its input) and that device's frequency response (the magnitude and delay changes that the device imparts to sinewaves of different frequencies applied to its input). If you are able to determine the impulse response, you can transform this information into the frequency response. Similarly, if you can determine the frequency response you can derive from this the impulse response, (but you must have both magnitude and true phase to do such a transformation). The mathematical operations that do these conversions are the Fourier Transform and the Inverse Fourier Transform . A measurement of a system such as a loudspeaker can start from either point of to obtain these equivalent types of measurement data: it can apply sinewaves to find the frequency response, or in can apply impulsive stimuli to obtain the impulse response. Once either type data is obtained, transformation from one to the other is a simple matter of using computer postprocessing. A narrow pulse is attractive as a measurement stimulus for several reasons. It is easy to generate using inexpensive circuitry. It contains a wide frequency spectrum allowing simultaneous measurement of most or all of a speaker's or amplifier's range. Both the phase and magnitude of a narrow pulse's spectrum are essentially uniform over a wide range of frequencies. In the time domain, echoes in a device's pulse response are easily identified and removed, to that mesurements equivalent to those from an anechoic chamber can be obtained NOISE INTRUDES . A weakness of pulse testing, however, is the rather low energy content of the test signal [2]. The pulse stimulus is very brief while the response data collection time must be many times longer in order to provide low frequency information. This allows a long opportunity for noise to intrude into the measurement. The test signal energy cannot be arbitrarily increased by "turning up the volume" into the unit under test. The crest factor, defined as the ratio of the signal's peak power to its average power, is very high for a pulse stimulus. You may clip your amplifier or drive your speaker into nonlinear operation with many watts of peak power, while only applying microwatts of average power to do battle with the noise. Due to the nature
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Lecture 28B MLS-based measurements - Liberty Instruments,...

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