This test will reveal the inherent signal to noise ratio of the wireless system and how well the compandor handles low frequency audio signals. The “inherent signal to noise ratio” is the signal to noise ratio before companding. This test requires listening to the system in a very quiet environment with minimal background noise. Place the transmitter and microphone in a different room from the receiver, or use high isolation headphones to monitor the audio output of the receiver. In either case, there must be minimal background noise near the microphone. Background noise at a high enough level will negate the test.
Set up the system for normal voice levels, then place the transmitter and microphone on a table or counter. Make a fist with your hand and gently bump the table with the meaty part of you hand (not your knuckle). The idea is to generate a low level, low frequency “bump” near the microphone at just enough level to open the compandor on the wireless system. Try varying how hard you bump the table with your fist to find a low level that just opens the compandor and listen to the results. When you“bump” the table, listen for background noise that sounds like a “whoosh” or “swish” that accompanies the sound of the bump.
The idea is to listen to how much background noise is released through the wireless system when the “bump” occurs, and also to whether or not the “bump” heard through the wireless sounds the same as in real life.
This is an excellent test of the difference between a single-band compandor and a dual-band compandor with DNR filtering, as well as a test of the signal to noise ratio of the wireless system. With the transmitter gain set for a normal voice level during this test, the results you hear will be what the system will actually do in real use.
It is also interesting, although not a valid test, to set the transmitter gain at minimum, then turn the receiver output up to maximum, and do the bump test again. The only reason to do this is to help understand just how much noise is actually suppressed by the system in normal use, and to emphasize the importance of proper transmitter gain adjustment.
A wireless mic system design that uses a large amount of preemphasis/ de-emphasis as noise reduction will likely do fairly well in the “bump test,” however, it may also fail miserably in the previous “car key test.” (See Dreaded Key Test)