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Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa). [1]
Make Listening Safe is promoting the development of features in PLS to raise the users' awareness of risky listening practices. In this context, the WHO partnered with the International Telecommunication Union to develop suitable exposure limits for inclusion in the voluntary H.870 safety standards on "Guidelines for safe listening devices/systems."
Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. Although the ear is the primary organ for sensing low sound, at higher intensities it is possible to feel infrasound vibrations in various parts of the body.
The first research on the topic of how the ear hears different frequencies at different levels was conducted by Fletcher and Munson in 1933. Until recently, it was common to see the term Fletcher–Munson used to refer to equal-loudness contours generally, even though a re-determination was carried out by Robinson and Dadson in 1956, which became the basis for an ISO 226 standard.
Infrasonic vocalizations have been recorded from captive elephants in many different situations. The structure of the calls varies greatly but most of them range in frequency from 14 to 24 Hz, with durations of 10–15 seconds. When the nearest elephant is 5 m from the microphone, the recorded sound pressure levels can be 85 to 90 dB SPL. [9]
Although sound pressure and intensity differ, both can describe the level of loudness by comparing the current state to the threshold of hearing; this results in decibel units on the logarithmic scale. [12] [13] The logarithmic scale accommodates the vast range of sound heard by the human ear. Depiction of frequency weighting
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Several countries and states have made it illegal to wear headphones while driving or cycling. [13] It is not uncommon to get a pressure-like feeling when using noise-cancelling headphones initially. This is caused by the lack of low-frequency sounds as being perceived as a pressure differential between the inner and outer ear. [14] [15] [16]