Internal bodily noise raises the hearing threshold at low frequencies

Evidence

Threshold in quiet is also interpreted as a masked threshold produced by internal noise. This internal noise is frequency independent at medium and high frequencies, but increases strongly towards low frequencies and is responsible for the rise in threshold in quiet at low frequencies. Heart beats and spontaneous activity of muscles are typical noise sources which produce an acoustical stimulation of our hearing system especially at very low frequencies. The effect of these noises can be increased by closing the outer ear canal. In this case, the sound pressure level can be measured using a probe microphone and appears larger in relation to the values measured with an open ear canal. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 954

Categorization

• XREF: Connects to the reader’s noise/silence theme — the hearing system’s own quiet floor is set by the body’s internal sounds, internalizing the acoustic ecology concept.
• CANDY: Heartbeats and muscle activity physically raise the lowest audible frequency threshold — elegant party fact about the body being its own noise source.