Acoustics Refresher

Sounds are changes in pressure in an elastic medium (such as the atmosphere) which can be detected by the ear. Normal human ears can hear in the frequency range between approximately 20 and 20,000 Hertz, or cycles per second.

A sound source emits sound waves (pressure disturbances) in all directions. Of course, a source like the human voice will emit (radiate) much more energy in the general direction the person is facing than in other directions. The wave expands, in a spherical shape, losing intensity as it travels away from the source. This energy loss is quite rapid as the distance from the sound source increases, since the total volume through which the sound is being sent grows ever larger. More detail is in the tutorial entitled, Generation & Movement of Sound.

Loudness vs. Power/Intensity

Simply put, the amplitude of a sound is its ‘strength’, or loudness. More technically, the amplitude is the difference between the air pressure levels in the sound waves at their strongest and weakest. Remember that the sound itself is nothing more than a series of compressions and rarefactions in the atmosphere.

Loudness is a perceptual quantity which can only be assessed by an auditory system, including the brain. Our perception of the loudness of a given sound varies according to pitch, because the human ear is not uniformly sensitive to all frequencies. For instance, the ear is most sensitive to pitches in the 1000-3000Hz range. Lower or higher pitches, even if sung/produced at the same volume, will sound softer by comparison.

Acoustic power is a measure of the amount of the total energy produced (regardless of how it is perceived by the human ear) and radiated into the surrounding air, per second, measured in watts.

Intensity is a measure of a sound’s power per unit area, measured in watts/m 2 , or in units called decibels (symbol: dB). The original unit was the bel, but that unit proved to be too large, and so the decibel, which is a tenth of a bel, is used instead. Since the decibel scale is logarithmic, large differences in the power of sounds are reflected as relatively small changes in decibels. A 10x increase in sound power is only a 10dB increase.

Frequency

A sound’s frequency is the number of sound waves produced per second by the sound source. A wave is either a compression or rarefaction.

Pitch vs. Timbre

Pitch is psychological impression of the ‘highness’ or ‘lowness’ of a sound based on the various frequencies it contains. A good example of pitch is the sound associated with the keys on a piano.

Timbre , by contrast, is the ‘quality’ of a sound that we hear, which is determined by the various overtones present in the sound, and their relative strengths. For instance, we might think of a given voice or instrument as sounding ‘rich’, or ‘tinny’, or ‘hollow ‘, or ‘fat’, or ‘thin’, or ‘dry’, or any number of other perceptions. Voice qualities are discussed in detail in the Chapter 10 tutorial.