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LI0107 - Revision 7 September 2012
Fig 1.1
½″ Low-noise Measurement Microphone
System Type 40HL
Introduction
The G.R.A.S. ½" Low-noise Microphone System Type 40HL (Fig. 1) measures sound pressure
levels well below the threshold of human hearing. It is thus generally suitable for sound-power
measurements on even very quiet products. Its very wide dynamic range permits measurements
down to below – 2 dB re. 20 μPa (in
⅓
-octave bands) from 20 Hz to 20 kHz.
Type 40HL comprises:
•
a special high-sensitive ½" condenser free-field microphone
•
a dedicated ½" low-noise preamplifier.
IMORTANT!
The preamplifier and microphone are an individually matched combination that is assembled and
sealed under clean-room conditions.. For it to retain optimal performance, do not separate the
microphone from the preamplifer .
Type 40HL connects to most standard power modules with LEMO 1B connector (optional accessories
from G.R.A.S. - refer to the section
What to Order
):
The power modules supply voltages for powering preamplifiers and polarizing microphones.
Preamplifier
The preamplifier is a true ½" low-noise amplifier with LEMO 1B connector and has a built-in
compensation filter for free-field microphones.
Microphone
The microphone is a ½" low-noise externally-polarized free-field microphone with a specially reduced
inherent noise floor in order to achieve a high dynamic range and wide frequency range. Its diaphragm
is specially tuned to yield high sensitivity coupled with low internal-noise generation.
Frequency Response and Noise Floor
A typical free-field response of Type 40HL for 0 ° is shown in Fig. 1.2.
Fig. 1.3 shows free-field corrections for various angles of incidence on the ½" microphone.
Operating Type 40HL
For holding the microphone, you can order Tripod AL0006 and Tripod Adapter RA0093
For level calibration of Type 40HL you need:
