Rohde & Schwarz UPD, Application Note

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1GA16_1L.DOC ROHDE & SCHWARZ 24
The measured sensitivity may be used as a reference for measuring the frequency response or the
radiation angle. This is done by pressing the STO SENS softkey (F10).
This measurement can also be used for determining the radiation angle. In this case the standard
microphone (at a constant distance from the loudspeaker) has to be moved away horizontally from the
reference axis in the course of the long-term level measurement until the sound pressure level falls by
10 dB. If the -10 dB should be directly displayed, the sound level unit SENSITIV has to be selected (after
the measured value has been stored at 0°).
5.2.6 Frequency Response of Sound Pressure Level and Transmission Range
Three different test methods can be selected for measuring the sound pressure frequency response:
•
FFT RAND uses a special noise signal for fast measurements of the entire frequency range.
•
FREQ SWP is a frequency sweep with sinewave signals for measurements under open-field conditions
(eg in an anechoic chamber) and near-field measurements.
•
BRST SWP is a frequency sweep using burst signals for far-field measurements (also in not
completely sound-absorbing rooms).
Although the FFT noise measurement ensures a fast display of the transmission range, it can only be
used with restrictions as far as numeric calculations are concerned:
•
If a narrow FFT is selected (1 or 2 k FFT), a coarse frequency resolution is obtained and little
information particularly at low frequencies.
•
With large FFTs , the measurement time itself is still very short but much more samples have to be
measured for determining the transmission range than with a sweep so that the speed advantage of
the FFT noise measurement diminishes with increasing FFT size.
•
When frequency limits are changed, the noise signal has to be recalculated so that a deadtime from
several seconds to one minute may occur, depending on the selected FFT size and frequency
bandwidth.
•
When the FFT size is changed, also the sensitivity has to be recalculated.
FIG 10: Measuring the sound pressure level using FFT noise