The G.R.A.S. 1-inch Low-noise Microphone System Type 40HF can measure sound pressure levels well below the threshold of human hearing; and is amply suitable for use in sound-power measurements on even very quiet products. Its very wide dynamic range permits measurements down to below – 10 dB re. 20 μPa (in 1/3-octave bands) from 20 Hz to 10 kHz