Topic 5. Sound Basics

Katrine Tranegaard Sigmer

Human Senses and Perception

Sound Basics

The topic discussed in this Miniproject report
is the topic of the basics of sound. Various terms related to the basic of
sound will be explained, and the relation between specific terms will be
explained as well. The aim of this Miniproject report is to gather and explain
knowledge concerning various terms within the topic of basic sound.

First some basic terminology relating to the basics
of sound will be explained, such that these concepts can be applied in the
context of the specific terms that will be explained and related to each other.
The specific terms in question will then be explained. The Miniproject report will
end with a discussion about the gathered information, and how it can be applied
in digital media.

Some basic sound terminology

Sounds can occur from vibrations from objects.
When a Sound is produced it spreads out into the surroundings in a sound wave
which changes the pressure of molecules in the medium the sound is traveling
through. When the wave passes through the medium it first compresses the
molecules of the medium this is called condensation, and it then, as the wave
progress, pull the molecules further apart than their normal state, this is
called rarefaction (Moore, B. C. J., 2003) When the entire sound wave has
passed through the medium the molecules will have returned to their normal
state.

Sound waves can be both periodic and
non-periodic. When a sound wave is periodic it means that the wave has a
distinct pattern it cycles through. A non-periodic sound wave won’t have such a
cycle. When a wave is periodic it will have a specific frequency. The frequency
of a wave is a measure of how many times the wave repeats its cycle per second.
The frequency of a sound wave is measured in hertz which is cycles per second. Similarly,
the period of a sound wave is the time it takes for the wave to complete one
cycle.

The phase of a wave is a measure of how far the
wave has gone through its cycle at a specific time from a specific place. I.e.
if two points on a waveform graph are given, the distance between the points is
the phase difference between the points. This could for example be half of a
cycle, which is equal to pi, or 180 degrees, if the phase started at zero.

The simples form of a periodic wave is the sine
wave. The sine wave, also called a sinusoid have one distinct shape that is in
theory, repeated indefinitely. The sinusoid is also called a simple tone. Other
forms of tones are complex tones, which are tones that aren’t simple tones e.g.
notes from a piano.

Another aspect of sound waves is their amplitude.
The amplitude of a sound wave describes how much the pressure, in the medium
the wave is traveling through, is altered around the mean pressure as the wave
passes through.

When measuring how powerful a sound wave is,
the amount of energy transmitted through the wave per second or other unit of
time, is called the power of the wave.

Periodicity and Pitch

Periodicity

As mentioned earlier a periodic sound have a
waveform that repeats itself indefinitely at regular intervals and the number
of cycles per second is called the frequency of the wave. This is true for
simple tones, however if the sound wave is periodic but isn’t a simple tone e.g.
sounds from a musical instrument, the number of cycles per second is instead called
the periodicity of the wave.

Pitch

The pitch of a sound wave is a perceived aspect
of sound. Where frequency or periodicity is a physical aspect of sound, the pitch
of a sound is something psychological that can be perceived and as such can’t
be measured directly. According to the book American
Standard Acoustical Terminology the pitch of a sound can be described as, “Pitch
is that attribute of auditory sensation in terms of which sounds may be ordered
on a scale extending from low to high, such as a musical scale.” (ANSI, 1951,
p. 22)

A sound wave with a frequency or periodicity
between 20Hz and 5000Hz will give the listener a sense of a pitch in the sound.
Sounds that have a pitch and such can evoke a sense of melody, are called
tones. In simple tones the pitch can be related to the frequency of the tone,
generally the higher the frequency of a tone the higher the perceived pitch
will be. In complex tones the pitch can be indicated by the frequency of a
simple tone with the same pitch as the complex tone.

Loudness and Intensity

Loudness

The loudness of a sound wave is again an
attribute of sound that can’t be measured directly. Moreover, how loud a sound
is perceived is individual, and depends on multiple factors. One key factor
that loudness depends on is the intensity of a sound wave.

Intensity

The intensity of a sound wave is a measure of
how much power is transmitted through the wave front of a sound wave. It can be
calculated by taking the power of a sound wave and dividing it by the area of
the wave front of the sound wave. So, the intensity (I) is equal to the power
(P) divided by the area (A) I = P/A. the intensity of sound is measured in
decibel (dB).

Loudness and intensity is then related in such
a way that, the further a sound wave have travelled the quieter the loudness of
the soundwave will be. That is because of the nature of a sound wave the area
of the wave front will increase as the wave travel away from the source of the
sound, and so the intensity will decrease as time goes by.

Logically this also means that the closer
someone is to the source of a sound the louder it will be. E.g. if one were to
be standing right next to a speaker playing music, the sound is much louder
than if one is far away from the speaker.

Timbre and Fourier Spectrums

Timbre

The timbre of a sound is another aspect of
which various sounds can be distinguished from each other. With the timbre of a
sound it is possible to tell two different tones with the same pitch and
intensity apart, assuming that the timbre of the sounds differ.

Fourier Spectrum

Fourier’s theorem states that any periodic
complex tone can be expressed as the sum of various sine and cosine waves,
which have specific amplitudes, frequencies and phases (Colman, A. M., 2008).
This means that any periodic tone can be deconstructed in such a way that the
sum of various sine and or cosine waves will give the waveform of the complex
tone. When a periodic complex tone is deconstructed into the various simple
tones that make up the complex tone, these simple tones can be expressed in a
chart that shows the frequency, and amplitude of each tone, such a chart is
called the Fourier spectrum.

The timbre of a sound can’t be measured or
ordered the same way e.g. the frequency of a sound can. As with the pitch of a
sound the timbre is something than can be perceived rather than measured. However
according to (Deutsch, D., 1984) it is possible to assume various qualities
related to the timbre of a sound by studying that sounds Fourier spectrum.

Timbres Examples

A C note played on a keyboard will sound
different than a C note played on a guitar even if they have the same pitch and
loudness, this is due to the different timbres of the different instruments. However
two identical instruments playing the same note, with the same pitch and loudness,
can and will most likely have different timbres as well. This is due to the
nature of sound and various differences in the two instruments, i.e. even if
two musical instruments are produced the exact same way, they won’t be exact
replicas of each other.

Discussion

Summary

Conclusion

References

ANSI (1951). American Standard Acoustical Terminology (pp. 22) New York, NY:
American Standards Association.

Colman, A. M. (2008). A Dictionary of
Psychology (3rd ed.) Oxford, England: Oxford University Press

Deutsch, D. (Ed). (1984). The Psychology of Music. New York, NY: Academic Press.

Moore, B. C. J. (2003). An Introduction to the Psychology
of Hearing (5th ed.). San Diego, CA: Academic Press.

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