BRUEL & KJAER 4220 - Page 7

pressure level limit the use of this type of pistonphone to laboratory work
and demonstrations.
The Pistonphoue Type 4220 is a mechanical device of a new design which.
by using two symmetrically mounted pistons, with a floating retaining spring
arrangement, eliminates shaft eccentricity, and back - lash of bearings from
any influence on the stroke of the pistons, and thus avoids the drawbacks of
the old pistonphone. This small unit produces a pure tone of 250 Hz, with
a sound pressure level of 124 dB (RMS) re. 2 X 10-4 .ubar, allowing correct
calibration to be made even in very noisy surroundings. The Pistonphone 4220
features outstanding simplicity and rapidity of operation and has a calibration
accuracy of ± o.2 dB, which is of the same order as the precision usually
obtained by the reciprocity method. It can be used anywhere where a well-
defined sound source is required, and is especially suited for the direct
calibration of precision sound measuring equipment in the field.
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