BRUEL & KJAER 4220 User Manual

Introduction
The Pistonphone Type 4220 is designed for the absolute calibration of pre-
cision sound measuring systems.
Sound level measuring equipment consists basically of a microphone, followed
by suitable amplifier stages and an indicating instrument. The amplitude
and frequency characteristics of the electronic components are generally
accurately defined, but the exact determination of the sensitivity of the
equipment in relation to sound pressure is less easy to obtain.
The recommendations of the IEC for Precision Sound Level Meters (Technical
Commission no. 29) requires: "The complete apparatus shall be calibrated
in absolute values at any specified frequency between 200 and looo Hz. The
accuracy of the calibration at this frequency, including errors due to the
free field measurement, and those due to the actual eleclroacoustic measure-
ments, shall always be better than ± 1 dB."
Different methods ' of sound level calibration have been adapted by Briiel &
Kjrer, i.e.: the Recipro~ity Calibration, the Electrostatic Actuator, the Noise
Source,
and the Pistonphone. Each method has a particular field of applica-
tion. They are all carried out ~s pressure calibrations (fictitiously in the case
of the actuator), the calibrating sound source being tightly coupled to the
microphone by a coupler of small dimensions. For small condenser micro-
phones, such as the B & K microphones, there is no appreciable difference
under looo Hz, between the pressure and the free field sensitivities.
The Reciprocity technique*), which is the method retained in the American
Standard ASA Z 24-4 (1949), is based upon the law of reciprocity of reversible
transducers. It is carried out by permutation between three microphones and
the measurement of voltage ratios only, with the knowledge of a few physical
constants. This laboratory method enables absolute pressure calibration of
microphones to be made with excellent accuracy. On the other hand, it is a
relatively time-consuming operation.
The Electrostatic Actuator*) produces a fictitious acoustic pressure on the
diaphragm of a condenser microphone by electrostatic attraction. It is parti-
cularly useful and very reliable as a variable frequency exciter for the
automatic recording of frequency response curves. However, the inaccuracy
in the determination of the equivalent sound pressure prevents the electro-
static actuator from bein g used as an absolute calibration means.
*) See the Instruction Manual for the B & K Microphone Calibration Apparatu~ Type 4142.
3

BRUEL & KJAER 4220 Specifications

General

Brand	BRUEL & KJAER
Model	4220
Category	Measuring Instruments
Language	English