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Acoustics

  • Sound: energy waves of particle displacement, both compression (more dense) and rarefaction (less dense) within an elastic medium; triggers sensation of hearing by vibrating the tympanic membrane.

  • Amplitude of sound: extent of vibratory movement from rest to farthest point from rest in compression and rarefaction phases of energy waves. In psycho acoustics, sounds with higher amplitudes are perceived as louder than sounds with lower amplitude.

  • Intensity of sound: amount of sound energy through an area per time; refers to sound strength or magnitude.

  • Sound pressure: sound force (related to acceleration) over a surface per unit time.

  • Decibel (dB): unit to express intensity of sound; more specifically the logarithm of the ratio of two sound intensities. One-tenth of a Bel (named for Alexander Graham Bell).

  • Frequency: number of cycles (complete oscillations) of a vibrating medium per unit of time; psychoacoustic correlate is pitch. Time for completion of one cycle is the period.

  • Hertz (Hz): in acoustics, unit to express frequency (formerly cycles per second or cps). Human ear capable of hearing from approximately 20 to 20,000 Hz.

  • Pure tone: single-frequency sound; rarely occurs in nature.

  • Complex sound: sound comprising more than one frequency.

  • Noise: aperiodic complex sound. Types of noise frequently used in clinical audiology are white noise (containing all frequencies in the audible spectrum at average equal amplitudes), narrow band noise (white noise with frequencies above and below a center frequency filtered out or reduced), and speech noise (white noise with frequencies > 3000 and < 300 Hz reduced by a filter). However, the term “noise” can also mean any unwanted sound.

  • Resonant frequency: frequency at which a mass vibrates with the least amount of external force; determined by elasticity, mass, and frictional characteristics of the medium. Natural resonance of external auditory canal is approximately 3000 Hz; of middle ear, 800 to 5000 Hz, mostly 1000 to 2000 Hz; of tympanic membrane, 800 to 1600 Hz; of ossicular chain, 500 to 2000 Hz.

The Decibel

The decibel scale is listed as follows:

  • A logarithmic expression of the ratio of two intensities.

  • Nonlinear (eg, the energy increase from 5-7 dB is far greater than the increase from 1-3 dB because it is a logarithmic scale).

  • A relative measure (ie, 0 dB does not indicate the absence of sound).

  • It is important to state a reference level when speaking of decibels because decibels are expressed with different reference levels, such as sound pressure level (SPL), hearing level (HL), and sensation level (SL).

Sound Pressure Level (SPL)

The referent of SPL is the most common measure of sound strength.

  • Decibels SPL are currently usually referenced to micropascals (but can be referenced to dynes per centimeter squared or microbars).

  • Sound pressure is related to sound intensity.

  • The formula for determining the number of decibels is

    dB Intensity = 10 log Io/Ir

    where

    • Io = intensity of output sound being ...

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