BRUEL & KJAER 4204 - Page 25

CHAPTER 3
Characteristics
19
Fig.3.3 Typical directional characteristics (vertical plane) of Type 4204 in free-field conditions
(without reflecting plane)
The Directivity Index, DI, in dB measured vertically above Type 4204 in 1/3-octave bands is
shown in Fig.3.4. These values were calculated from the directional characteristics given in
Fig.3.2 (i.e., Type 4204 radiating in a free field above a reflecting plane) according to the
equation given in ISO/DIS 6926 for reference sound sources, i.e.,
DI = L
pi
– L
p
+ 3 dB
where
L
pi
= sound pressure level in the direction that DI is required, measured in the far field of
the source
L
p
= mean sound pressure level over the reference hemispherical surface
60 dB
70 dB
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
50 dB
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
70 dB
50 dB
60 dB
45 dB
50 dB
60 dB
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
45 dB
60 dB
70 dB
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
50 dB
120551
70 dB
50 40 30 20 10 0 0 10 20 30 40 50
50 40 30 20 10 0 0 10 20 30 40 50
50 40 30 20 10 0 0 10 20 30 40 50
50 40 30 20 10 0 0 10 20 30 40 50
500 Hz
125 Hz
1 kHz
8 kHz
4kHz
10 kHz
2 kHz