ABSTRACT
The intensity of a sound refers to its physical magnitude, which may be expressed in such terms as its power or pressure. Turning up the “volume” control on a stereo amplifier thus increases the intensity of the music coming out of the loudspeakers. This intensity is easily measured by placing the microphone of a sound-level meter near the loudspeaker. The perception of intensity is called loudness; generally speaking, low intensities are perceived as “soft” and high intensities as “loud.” In other words, intensity is the physical parameter of the stimulus and loudness is the percept associated with that parameter. However, intensity and loudness are not one and the same; although increasing intensity is associated with increasing loudness, there is not a simple one-to-one correspondence between the two. Furthermore, loudness is also affected by factors other than intensity. For example, it is a common experience to find that loudness changes when the “bass” and “treble” controls of a stereo amplifier are adjusted, even though the volume control itself is untouched. (Bass and treble are the relative contributions of the lower and higher frequency ranges, respectively. Thus, raising the bass emphasizes the low frequencies, and raising the treble emphasizes the high.)
We may begin our discussion of loudness by asking whether the same amount of intensity results in the same amount of loudness for tones of different frequencies. For example, does a 100-Hz tone at 40 dB SPL have the same loudness as a 1000-Hz tone also presented at 40 dB? The answer is no. However, a more useful question is to ask how much intensity is needed in order for tones of different frequencies to sound equally loud. These values may be appropriately called equal loudness levels.
