Loudness, Pitch, and Timbre, and Their Relation to Physical Sound Stimulus

Loudness, pitch, and timbre are all involved in the sounds we hear. Most of us can identify these terms, but it is a little harder to pin down their relationship to physical sound stimulus. But before we can understand how loudness, pitch, and timbre are all related to each other, and to physical sound stimulus, we must understand how our auditory system processes sounds to create what we all know as "hearing."

Sound waves are created from the vibration of objects in the outside world, and are then turned into vibrations by the eardrum. These waves travel through the outer ear, to the ear canal, and then to the eardrum, the vibration of the eardrum sets in motion parts of the middle ear which will then amplify the sound by 2.3 decibels (dB) (House Ear Clinic, N.d), which makes then translates into vibrations in the innermost part of the ear. As the inner ear vibrates, sensors within the cochlea oscillate triggering chemicals that will be transmitted through the cochlea's nerve fibers to the brain via the auditory nerve pathway. Lastly, the brain translates the impulses and vibrations into what we know as sounds (Chiras, 2005). Clearly, the auditory system is a very complex system, but it is also extremely sensitive.

Loudness is the range of sound, or how intense the sound is (Heeger, 2006). However, despite this seemingly palpable definition, loudness is not physical, but completely perceptual. Goldstein (2010) describes loudness as the measure of sound pressure, or the amount of air pressure increasing or decreasing the amplitude. This creates sound patterns that can be below the hearing threshold for humans, 0 dB which is the lowest decibel measure a human can hear, and upward of 120 dB which is loud enough to cause a human pain (Heeger, 2006).

Pitch, like loudness is also completely perceptual; and just as loudness is described by the special measurement decibel (dB), pitch is measured in terms of hertz (Hz) Pitch is merely the ear's response to the frequency of a sound wave (Nave, 2005), and this response is generally referred to in terms of "high" and "low." The phenomena of hearing a higher or lower pitch can be explained using a theory called the "Place Theory," which states that higher frequency sounds vibrate the basilar membrane of the cochlea near the entrance, while lower frequency sounds travel further down the cochlea's basilar membrane before causing vibrations that will be translated into the pitch of the sound (Nave, 2005). It is also interesting to note that the perception of pitch changes with the length of the sound wave. For example, short pulses are perceived by the human auditory system as being higher in pitch; while longer, more sustained sound waves are perceived as being lower in pitch. Also noteworthy is the fact that interfering noises can cause differences in perceived pitch (Rossing, Moore, & Wheeler, 2003). Thus, pitch's relation to physical stimulus is the vibration caused in various parts of the cochlea in the inner ear by physical sound waves.

Lastly, timbre seems to tie the other aspects of sound together as it is the distinguishing factor between them. Thus, sounds with the same loudness and pitch can be differentiated by timbre, making what would be two otherwise identical sounds unique (Goldstein, 2010). Timbre can be thought of as sound's character, and is made up of harmonic content, vibrato, and attack-decay. The first of these, harmonic content, is the determinant of the form of the sound wave (Nave, 2005). Vibrato describes a change in pitch evident within the sound wave of one particular tone; and attack and decay describes the intro and outro of a particular sound. Each of these, in combination, allow a sound to develop its own defining characteristics so that we're able to hear a variety of different sounds from the world around us. In fact, timbre is a huge part of music for this very reason! Each of these elements of timbre also describe its relation to physical sound, because each of these are methods of producing and shaping sounds.

References:

Chiras, D. D. (2005). Human Biology, 5^th Edition. Sudbury, Mass: Jones and Bartlett Publishers.

Goldstein, E. B. (2010). Sensation and perception (8th ed.). Belmont, CA: Wadsworth

Heeger, D. (2006). Loudness perception and critical bands. Retrieved November 23, 2009, from http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/loudness/loudness.html

House Ear Institute. (N.d). How we hear. Retrieved November 21, 2009, from http://www.hei.org/education/health/howhear.htm

Rossing, T. D., Moore, R. F., & Wheeler, P. A. (2003). The science of sound, 3^rd Ed. New York: Addison Wesley.