Lownoise measuring systems

Normal measurement microphones have a very wide dynamic range and cover most practical applications. There are however special situations where special microphones are required. One such application is in the measurements of very low sound pressure levels. Normal measurement microphones have a noise floor around 10 dBA re. 20 μPa in 1/3 octave bands, while the human ear is able to detect levels down to around 0 dB. In fact the 0 dB level was originally defined as the threshold of the human hearing ability at 1 kHz.

In some applications it is required to measure down to and below the threshold of the human ear. This is possible by using special high sensitive microphones combined with special low noise preamplifiers.

One of the applications of such microphones may be the measurement of the sound power of high-end personal computers. These are not only used in noisy office environments, but tend to move into living rooms, meeting rooms and hotel rooms. In some hotel rooms the traditional TV set has been replaced by a computer, delivering not only all TV channels, but also pay channels, account status, wake up calls and other services. This requires the computer to be turned on all the time, and to avoid disturbances during sleep the noise level has to be below the threshold of hearing. In turn, this requires that component manufacturers of hard drives, fans, etc. also deliver very low noise devices.

In order to achieve the very low noise floor of the microphone and preamplifier, these have been specially matched and adjusted together. This further enables the microphone/preamplifier combination to be switched to be used for free-field measurements or for pressure measurements.

The special preamplifier and matching circuit require a higher supply current than can be obtained from traditional microphone preamplifier supplies, the Type 40HH and Type 40HF must be used together with the Type 12HF low noise system power supply. To avoid damaging traditional microphone preamplifier supplies, the 7-pin LEMO on the low noise preamplifier is different from the 7-pin LEMO normally used for microphone preamplifers.

The very high sensitivity of low noise microphones means that the sound pressure level used for calibration should be limited to 94 dB to avoid overloading.A special coupler, RA0090, for the pistonphones Type 42AA or Type 42AP is available to reduce the level from 114 dB to 94 dB.

GRAS lownoise measuring systems are among the range of special microphones that furthermore includes 

• Array microphones
• Flush-mount microphones
• Infra-Sound Microphones
• Intensity Microphones
• Intensity Probes
• Outdoor microphones
• Plate microphone kits
• Probe microphones
• QC microphones
• Surface microphones
• Turbulence Screen Microphones