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.

Dynamic range of the ear

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 20,000 hertz, or 20 kilohertz. The minimum audible pressure amplitude, at the threshold of hearing, is about 10^-5 pascal, or about 10^-10 standard atmosphere, corresponding to a minimum intensity of about 10^-12 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.A. 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. 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 1,000 hertz—a physically measurable quantity. Fletcher and Munson placed the threshold of hearing at 0 phons, or 0 decibels at 1,000 hertz, but more accurate measurements now indicate that the threshold of hearing is slightly greater than that. For this reason, the curve labeled 0 phons in Figure 10 is slightly lower than the intensity level of the threshold of hearing over the entire frequency range. The curve labeled 120 phons is sometimes called the threshold of pain, or the threshold of feeling.

Several interesting observations can be made regarding Figure 10. The minimum intensity in the threshold of hearing occurs at about 4,000 hertz. This corresponds to the fundamental frequency at which the ear canal, acting as a closed tube about two centimetres long, has a specific resonance. The pressure variation corresponding to the threshold of hearing, roughly equivalent to placing the wing of a fly on the eardrum, causes a vibration of the eardrum of less than the radius of an atom. If the threshold of hearing did not rise for low frequencies, body sounds, such as heartbeat and blood pulsing, would be continually audible. Music is normally played at intensity levels between about 30 and 100 decibels. When it is played more softly, decreasing the sound level of all frequencies by the same amount, bass frequencies fall below the threshold of hearing. This is why the loudness control on an audio system raises the intensity of low frequencies—so that the music will have the same proportion of treble and bass to the ear as when it is played at a higher level.

As stated above, the ear has an enormous dynamic range, the threshold of pain corresponding to an intensity 12 orders of magnitude (10¹² times) greater than the threshold of hearing. This leads to the necessity of a nonlinear intensity response. In order to be sensitive to intense waves and yet remain sensitive to very low intensities, the ear must respond proportionally less to higher intensity than to lower intensity. This response is logarithmic, because the ear responds to ratios rather than absolute pressure or intensity changes. At almost any region of the Fletcher-Munson diagram, the smallest change in intensity of a sinusoidal sound wave that can be observed, called the intensity just noticeable difference, is about one decibel (further reinforcing the value of the decibel intensity scale). One decibel corresponds to an absolute energy variation of a factor of about 1.25. Thus, the minimum observable change in the intensity of a sound wave is greater by a factor of nearly 10¹² at high intensities than it is at low intensities.

The frequency response of the ear is likewise nonlinear. Relating frequency to pitch as perceived by the musician, two notes will “sound” similar if they are spaced apart in frequency by a factor of two, or octave. This means that the frequency interval between 100 and 200 hertz sounds the same as that between 1,000 and 2,000 hertz or between 5,000 and 10,000 hertz. In other words, the tuning of musical scales and musical intervals is associated with frequency ratios rather than absolute frequency differences in hertz. As a result of this empirical observation that all octaves sound the same to the ear, each frequency interval equivalent to an octave on the horizontal axis of the Fletcher-Munson scale is equal in length.

The audio frequency range encompasses nearly nine octaves. Over most of this range, the minimum change in the frequency of a sinusoidal tone that can be detected by the ear, called the frequency just noticeable difference, is about 0.5 percent of the frequency of the tone, or about one-tenth of a musical half-step. The ear is less sensitive near the upper and lower ends of the audible spectrum, so that the just noticeable difference becomes somewhat larger.