There are two important characteristics of sound or noise - frequency and loudness.
What is the sound level in dB for a sound whose intensity is 5.0 x 10-6 watts/m2?
Frequency of Sound. Sound is the quickly varying pressure wave travelling through a medium.
When sound travels through air, the atmospheric pressure varies periodically. The number of pressure variations per second is called the frequency of sound, and is measured in Hertz Hz which is defined as cycles per second. Loudness and the Decibel Scale. Another property of sound or noise is its loudness. A loud noise usually has a larger pressure variation and a weak one has smaller pressure variation. Softest Noise just Heard by a Human Ear.
Launching of the Space Shuttle. Full Symphony Orchestra. Diesel Freight Train at High Speed at 25 m.
Soft Whispering at 2 m in Library. Unoccupied Broadcast Studio. Softest Sound Human can Hear.
Adding Sounds or Noises together on the Decibel Scale. In real life, several sources of sounds often occur at the same time. One may be interested to know what results when one sound is combined with another, i.mail.mccurdycandler.com/258.php
Intensity – The Physics Hypertextbook
The "A-weighting". The following diagram shows the "A-weighting" scale: In the "A-weighting" scale, the sound pressure levels for the lower frequency bands and high frequency bands are reduced by certain amounts before they are being combined together to give one single sound pressure level value. Index Page. Back to Top. The idea of noise is fundamental to the sound of many vibrating systems, and it is useful in describing the spectra of vocal sibilants as well. Just as white light is the combination of all the colours of the rainbow, so white noise can be defined as a combination of equally intense sound waves at all frequencies of the audio spectrum.
A characteristic of noise is that it has no periodicity, and so it creates no recognizable musical pitch or tone quality, sounding rather like the static that is heard between stations of an FM radio. Another type of noise, called pink noise , is a spectrum of frequencies that decrease in intensity at a rate of three decibels per octave. Pink noise is useful for applications of sound and audio systems because many musical and natural sounds have spectra that decrease in intensity at high frequencies by about three decibels per octave.
Other forms of coloured noise occur when there is a wide noise spectrum but with an emphasis on some narrow band of frequencies—as in the case of wind whistling through trees or over wires. In another example, as water is poured into a tall cylinder, certain frequencies of the noise created by the gurgling water are resonated by the length of the tube, so that pitch rises as the tube is effectively shortened by the rising water.
The ear has an enormous range of response, both in frequency and in intensity. The frequency range of human hearing extends over three orders of magnitude, from about 20 hertz to about 20, hertz, or 20 kilohertz.
The minimum audible pressure amplitude , at the threshold of hearing, is about 10 -5 pascal , or about 10 standard atmosphere , corresponding to a minimum intensity of about 10 watt per square metre. The pressure fluctuation associated with the threshold of pain , meanwhile, is over 10 pascals—one million times the pressure or one trillion times the intensity of the threshold of hearing.
In both cases, the enormous dynamic range of the ear dictates that its response to changes in frequency and intensity must be nonlinear. Shown in Figure 10 is a set of equal-loudness curves, sometimes called Fletcher-Munson curves after the investigators, the Americans Harvey Fletcher and W. Munson, who first measured them. The curves show the varying absolute intensities of a pure tone that has the same loudness to the ear at various frequencies.
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The determination of each curve, labeled by its loudness level in phons , involves the subjective judgment of a large number of people and is therefore an average statistical result. However, the curves are given a partially objective basis by defining the number of phons for each curve to be the same as the sound intensity level in decibels at 1, hertz—a physically measurable quantity.
Sound Intensity and Sound Level
Fletcher and Munson placed the threshold of hearing at 0 phons, or 0 decibels at 1, hertz, but more accurate measurements now indicate that the threshold of hearing is slightly greater than that. Brainstem auditory evoked potentials BAEPs recorded at different sound stimulation intensities after Legent. This type of recording is a type of objective audiogram. Note the presence of 5 obvious waves at 70 dB.
Related Sound Minimum Intensities
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