BRUEL & KJAER 4204 - Factory Calibration; Directional Characteristics

Reference Sound Source Type 4204 – User Manual18
3.2 Factory Calibration
Each Type 4204 Is individually calibrated before leaving the factory. The reference sound
source is placed on a reflecting plane in an anechoic room. A microphone is connected to a
swing arm of radius 1.5 m. The swing arm is raised by a motor and cord arrangement, so that
equal areas on the surface of the microphone path are swept in equal time. The reference sound
source is placed on a Turntable Type 3922 and rotated during calibration. The signal from the
microphone is integrated over the time of traverse and the sound power output of the source is
calculated in 1/3-octaves, whole octaves and A-weighted. The results are printed out in the
form shown in Fig.2.1. This calibration is performed for both 50 Hz and 60 Hz mains
frequency and conforms with ISO 6926: Acoustics – requirements for the performance and
calibration of reference sound sources used for the determination of sound power levels.
3.3 Directional Characteristics
The directional characteristics of Type 4204 in the vertical plane are shown in Figs. 3.2 and 3.3
with the Type 4204 placed on a reflecting plane and in free-field conditions, respectively. The
curves were drawn for a 50 Hz mains supply to Type 4204, but use of a 60 Hz supply does not
noticeably modify the directional characteristics. It can be seen from the curves that the
Type 4204 lies within the deviation limits specified by ISO 3741, which requires that the
directivity index of the source in any 1/3-octave band, does not exceed 6 dB relative to uniform
hemispherical radiation. The horizontal omnidirectivity is better than 0.6 dB.
Fig.3.2 Typical directional characteristics (vertical plane) of Type 4204 placed on a reflecting plane
in an anechoic room
50 dB
60 dB
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
-25
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
50 dB
60 dB
30°
60°
90°270°
300°
330°
0°
30°
60°
90°270°
300°
330°
65 dB
55 dB
50 dB
60 dB
70 dB
65 dB
55 dB
60 dB
70 dB
75 dB
75 dB
50 dB
50 40 30 20 10 0
0 10 20 30 40 50
0 10 20 30 40 50
50 40 30 20 10 0
0°
50 40 30 20 10 0
0 10 20 30 40 50
75 dB
50 40 30 20 10 0
0 10 20 30 40 50
75 dB
120550
70 dB
70 dB
2 kHz
8 kHz
125 Hz
250 Hz
1 kHz
500 Hz
4 kHz