Typical applications and use

• Very Low Sound pressure measurements
• Lownoise product measurements
• Lowlevel Sound power measurements
• Measurements on hard-disk drives, computer products, quiet rooms etc.

Design

The GRAS 40HF comprises:

• a special high-sensitive 1-inch (23.77 mm) Condenser Microphone Type 40EH
• a special 1-inch (23.77 mm) Low-noise Preamplifier Type 26HF

The preamplifier and microphone are an individually-matched combination. To complete the measuring system, a special power module is required and is available from GRAS, i.e. GRAS 12HF for single-channel measurements or GRAS 12HM for multi-channel (1 to 10) measurements. The chosen power module provides all necessary voltages for powering the preamplifier(s) as well as polarizing the microphone(s).

Microphone

The GRAS 40EH is an externally polarized 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.

Preamplifier

The GRAS 26HF Preamplifier has a built-in overload indicator (which is repeated on the chosen power supply) and a frequency correction facility for both pressure-microphone operation as well as free-field microphone operation.

Dehumidifier RA0095 (optional)

For use when humidity is high and only with a rear-vented version of the GRAS 40EH Microphone. Order only at the time of ordering a Type 40HF.

Frequency response and noise floor

The chosen power supply has a two-position switch for selecting which microphone operation to use, i.e. pressure or free-field. A typical free-field response for an angle of incidence of 0 ° is shown in the data sheet when the GRAS 40HF is switched to free-field operation. Also shown in the data sheet is a complete low-noise measuring system, a typical noise floor in 1/3-octave bands for both the linear and A-weighted cases.