Instruction Manual

Sound Level Meter Types 2245 – Instruction Manual Page 24 of 110
Fig. 2.1 Error from self-generated noise
Fig. 2.1 shows the error on the measured sound levels from the presence of self-
generated noise. The curve can also be used for compensation by subtracting the error
from the measured sound levels. This is equivalent to using the formula.
2.11 Measuring at Low Static Pressure
The frequency response of the microphone depends on the static pressure. Using a
sound level calibrator to adjust the sensitivity of a sound level meter at the Calibration
Check Frequency (1kHz) provides no information on the influence of static pressure on
frequency response. Fig. 2.2 shows the changes in frequency response with changes in
static pressure.
The microphone’s sensitivity and frequency response are affected by variations in the
ambient pressure. This is due to changes in air stiffness in the cavity behind the
diaphragm, and changes in air mass in the small gap between the diaphragm
and the back plate. The effects are shown in Fig. 2.2 and Fig. 2.3. The typical pressure
coefficient at 250 Hz for Prepolarized Free-field 1/2" Microphone Type 4966 is –
0.012 dB/kPa, well within the ±0.025 dB/kPa limits required for Class 1 sound level
meters by IEC 61672.
Fig. 2.2 Typical variation in frequency response (normalized at 250 Hz) from
that at 101.3 kPa as a function of change in ambient pressure