The G.R.A.S. ½″ Low-noise Microphone SystemType 40HL measures sound pressure levelswell below the threshold of human hearing. It is thusgenerally suitable for sound-power measurementson even very quiet products. Its very wide dynamicrange permits measurements down to below – 2 dBre. 20 μPa (in ⅓-octave bands) from 20 Hz to 20 kHz