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Daisy Grace Wakefield 13 April 2015
WHAT IS SOUND?
The Physics Of Sound
What is Sound?
Sounds are vibrations that can travel through water, gas and air
[11].A sound is created through action causing movement such as
speaking. Speaking will cause vocal chords to vibrate and sound
waves to be released from the mouth and the air molecules around
the mouth to vibrate accordingly. This vibration will then cause
the surrounding molecules to also vibrate and those vibrations will
spread from molecule to molecule allowing the sound to travel [8].
The sound waves are compressed, get closer together, and then they
are rarefied, pulled apart. This motion allows the sound to travel
[9].Sound needs matter to travel through. Matter that transports a
sound is called a medium such as a solid, liquid and gasses. Sound
must travel through a medium as the vibrations need to spread
otherwise there is no sound [11]. As sound needs a medium to travel
though, there are no sound waves in space, as
BTEC Music Technology 1
Figure 1.1
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Daisy Grace Wakefield 13 April 2015
there is no air. [8] As they travel through mediums, this makes
sound waves mechanical waves [6][23]. there are two types of
mechanical waves; longitudinal waves and transverse waves[13].
Transverse waves are waves that are perpendicular, ninety degrees,
to the direction where the sound is traveling from. Longitudinal
waves however, as they move through a medium, the particles move
backwards and forwards in the direction the wave is travelling
making sound waves longitudinal waves. Sound is also
omnidirectional meaning that it can travel in every possible
direction[10][14].
Compressions and rarefactions In longitudinal waves, there are
parts where the molecules in the medium, for example air molecules,
are close together, where they are compressed, compressions, and
other regions where the molecules are spaced out, known as
rarefactions.[12]
Wavelength A wavelength is a section of a sound wave.
Wavelengths have high points known as peaks and low points known as
troughs. A wavelength is the distance from one peak to the next or
one trough to the next trough. This is known as a cycle. If the
peaks or troughs are close together then this is a high frequency
but if they are far apart then this is a low frequency
[24][15].
Sound Wave,Sine Wave? The easiest way to demonstrate to sound
wave is to look at a sine wave. This is the simplest type of sound
wave but pure sine waves rarely exist in the natural world. A sine
wave can help show the three main characteristics of a sound wave
which is amplitude, frequency and phase [9].
Frequency and Amplitude Frequency is the rate, or number of
times per second, that a sound wave cycles from positive to
negative to positive again. In other words, how many times a second
does the does the sound wave move from peak (crest) to peak or
trough to trough. Frequencies are measured in cycles
BTEC Music Technology 2
Figure 1.2
Figure 1.3
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Daisy Grace Wakefield 13 April 2015
per second or hertz (Hz). The human ear can hear frequencies
ranging from 20 hertz(20Hz), which is a low, to 20,000 hertz
(20kHz) which is loud. Frequencies do exist above and below 20 and
20,000 hertz but the human ear cannot detect them. As humans get
older their hearing deteriorates [9]. Through age the human ear
deteriorates and becomes a less eective organ or receptor.
Amplitude, or intensity of a sound refers to its strength also
known as volume or loudness. Human ears can pick up a variety of
volumes for example, from a mouse squeezing to an air horn. The
range of human hearing is so vast, there is a scale designed to
measure the intensity of a sound. This logarithmic unit is called
decibels [24][7][9].
Speed of sound The speed of sound refers to how fast the waves
or vibrations pass through a medium. Depending on the medium in
which the vibrations are passing through can aect the speed. For
example, in air, sound travels at 343 meters per second. thats 1
mile every 5 seconds. sound travels 4 times faster in water and 13
times faster through steel [11].
Sonic boom! When something goes faster than the speed of sound,
for example fighter jets, its called breaking the sound barrier.
Breaking this barrier creates a sonic boom. This noise is like an
explosion and it is generated from sound waves that are being
forced together because the fighter jet is traveling faster than
sound [11]. On the decibel scale a sonic boom is around 200dB
(decibels) [11][20][21].
Reverberation VS Echo Reverberation is what happens when sound
bounces o something flat and hard [16]. They are dierentiated by
the length of time between the initial sound and the reflected
repetition [17]. With reverberation, if the distance between where
the sound is released and where that initial sound can bounce o of
is short,
for example a room or your hand in front of your face, the sound
is reflected back to the source less than one-tenth of a second. As
the delay is so short this is reverberation. Reverberations are
added to music as they can give a more natural sound [17]. With an
echo, the sound has a greater distance to travel, for example
calling out over a valley. As it will take longer than one-tenth of
a second for the sound to return, this is classed as an echo.
Unlike reverberation, an echo does not add to the original sound
but is heard
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Figure 1.4
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Daisy Grace Wakefield 13 April 2015
as a distinct repetition of the sound. This repetition is
normally a lot softer or fainter and deteriorates every time it is
repeated until the sound dies out. The sound is fainter due to the
amount of energy lost as the sound waves travel further away from
the source which is known as decay. An echo can be measured by the
strength of the repetitions, time lapse between repetitions and the
decay of time sound [17].
Envelopes Sounds produced by musical instruments have a volume
envelope[9]. This means that all the notes playable on the
instruments have their own recognisable curve where volume rises
and falls over time. Certain instruments like drums start with a
high level of sound but very quickly deteriorate to a lower,
quieter level and then die away to silence [1][9]. Notes on
instruments such as the flute and viola can be held or sustained
for longer periods of time and whilst being sustained, their
dynamics, piano meaning quiet and forte meaning loud, can be
changed. This unique rise and fall to an instrument is called a
sound envelope. There are four main parts to an envelope;
Attack
Decay
Sustain
Release
This can be reduced to an acronym of ADSR [2][9].The attack is
the time taken for the sound to rise in amplitude until it reaches
full amplitude. The
decay is the time taken for the amplitude level to decrease to
the sustained level when the attack is over. Sustain refers to
where the sound level roughly stabilises and the release is the
time taken from releasing the note in the sustained period to fall
into silence. Envelopes can be an issue when recording as depending
on where the sound is being recorded there could be natural
reverberation of the room added to the release tail of the
instruments[3][4]. This means that that envelope of a sound could
continue longer than usual as the reverberation could be added to
the sound. To fix this problem, you could use dierent microphones
or position the microphone closer to the instrument to avoid
unnecessary reverberations. The acronym ADSR are commonly found on
samplers an synthesisers and this function will allow you to alter
the attack, decay, sustain and release of an instruments envelope
[3][5].
Phasing Phase compares the timing between two similar sound
waves[9]. If two sound waves start at the same time, and have the
same frequency, then they are in phase. Phase is measured in
degrees, from 0 degrees to 360 degrees. If both sound waves are in
phase they are measured at being 0 degrees. If they
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Figure 1.5
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Daisy Grace Wakefield 13 April 2015
are measured at being 180 degrees, then the waves are playing at
opposite times, when one wave is at its peak the other is at its
trough, and they are out of phase. When we combine two sound waves
that are in phase, this create a stronger sound wave, meaning it
becomes slightly louder. When the waves are out of phase, the air
pressure cancels the other one out. This means that there is very
little or no sound. This is phase cancellation.
Phasing can be an issue when audio signals are being mixed
because if the original signal and the reflective signal are played
together in a reflective room, this can cancel out the sound
[9].
Standing Waves and Harmonics A standing wave is when a wave is
travelling in one direction and then bounces o something like a
wall and travel back. They can occur when a medium is constrained
at endpoints[39]. These waves are associated with violins, cellos,
guitars and other stringed instruments [38]. One kind of constraint
is a ridged boundary and this is when we pin the medium down, in
this case a guitar string, so it cannot be disturbed but we can
disturb the medium in-between[39]. The wave produced has to be able
to fit in-between the boundaries. This means that the string will
only allow certain wavelengths[39]. When the string vibrates, there
will be parts of the string where the string will not be moving and
these points are called nodes. Parts of the string where there is a
peak or a trough, where the wave is moving, are called
antinodes[43][38].
When plucked, the strings frequency will start to increase each
time it vibrates remembering that Frequency is the rate, or number
of times per second, that a sound wave cycles from positive to
negative to positive again[9]. The lowest frequency this vibration
generates is called the fundamental frequency. The fundamental
frequency depends on two things; the speed and the length of the
string. The longer the string then the lower the frequency and the
shorter the string the higher the frequency. The speed depends on
the medium, in this case a string and this relates to the thickness
of the string and how heavy the string is. If we increase the
tension in the string we create a higher pitch and if we increase
the
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Figure 1.6
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Daisy Grace Wakefield 13 April 2015
thickness or mass of the string this creates a lower and deeper
sound [39]. Remembering that the wave has to be able to fit within
the constrained medium, as the frequency increases, the string
moves more and we get more nodes and this creates a harmonic
series. Its harmonic because the frequencies are all integer
multiples of the same frequency. If we wait a certain amount of
time, all of the waves that fit in-between these two nodes will
repeat the same period over and over and over again[39][24].
Beats In physics, beats are known as an interference in time. It
is when you have two sounds or sound waves that almost have the
same frequency but not quite. For example I could have one sound
wave at 30Hz and the second at 32Hz [40].
There is something called a beat frequency, and this is equal to
the dierence in two frequencies. Back to the example, the dierence
between 30Hz and 32Hz is 2, therefore I would hear 2 beats every
second. The further apart in frequency, the more frequent the beats
are. So if my first wave was still at 30Hx and I increase the
second to 35Hz I would hear 5 beats per second. This beat occurs
due to the interference
between the two waves [41].
An oscilloscope can visually represent a sound wave, and if you
had your two waves at 30 and 35 Hz you would be able to see them
cross over creating this beat. We can change the amplitude of the
waves but the frequency remains the same. When we have two waves
that are close in frequency, they are alternating between periods
of constructive and destructive interference which creates the beat
sound. When the two waves match peak to peak is is constructive
interference but when they match peak to trough, this is
destructive interference. For example, if someone was tuning a
piano, and they pressed down the key for middle C, they would hit
the tuning fork of
middle C to see f they could hear the beats pattern, if they
can, then they are at dierent frequencies, if they cannot, then
they key is in tune. There are also points where the waves are
pulsating so fast that the human ear no longer registers the beats
pattern and interprets the sound as two separate notes [42].
BTEC Music Technology 6
Figure 1.7
Figure 1.8
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Daisy Grace Wakefield 13 April 2015
Decibels Decibels (dB) are what are used to measure sound level
or intensity of sound. The ear can hear everything from a mouse
squeezing to a sonic boom [18]. The quietest sound that humans can
hear is close to 0dB
on the decibel scale. A sound 10 times more powerful is classed
as being 10dB. A sound 100 times more powerful than 10dB is 20dB
and a sound 1000 times more powerful than 20dB is 30dB. Factors
such as distance can aect the intensity of the sound as for
example, if there was a sonic boom created by an aeroplane and you
were right underneath it on the ground, the sound would be louder
to you than someone 100 miles away from you. Any sound above 85
dB can aect a persons hearing. This depends on how long they had
been exposed to the sound and the intensity of the sound. If you
are exposed to 90dB for 8 hours or more the this can over time
cause damage to the ear but if you are subjected to 140dB then the
damage occurs almost instantly [19].
How Instruments Work
What is timbre? The timbre of an instrument refers to the
instruments unique sound, for example if you played middle C on a
flute and piano, it would be the same note but they would sound
dierent as they each have their own unique sound to them [34].
Difference between music and noise Music and noise are both made
up of sound waves of dierent frequencies [25]. Music is the art of
arranging and climbing sounds in order to create a harmonious
melody[36]. Noise is unwanted sound that is usually very loud and
meaningless[36]. Noise has an irregular wave form and length and a
low frequency. The main dierence between music and noise is that
music is a collection of frequencies, usually from music
instruments, that create a harmonious and pleasing sound where as
noise is a collection of frequencies from a variety of sources that
can be seen to be annoying such as screams and
BTEC Music Technology 7
Figure 1.9
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Daisy Grace Wakefield 13 April 2015
loud bangs that is usually background noise [36]. A waveform is
the visual representation of a sound [26]. A musical waveform has a
high frequency with no sudden changes in amplitude or wavelength
and therefore it produces a regular waveform but noise is
unpleasant and has sudden changes in amplitude and has a low
frequency and therefore irregular waveforms [27].
This is subjective. For example, one person might interpret
tapping a pencil on a table as noise but someone else might find
this melodic and therefore music. Again, people who like heavy
metal music will find this musical but other people might interpret
this as just irritating noise [27].
Instruments Acoustic instruments are ones where the should is
generated through a physical object like a cello and electronic
instruments are sounds created electronically though a
computer[28]. Instruments can be placed into dierent
categories;
Percussion
Brass
Woodwind
Strings
Percussion instruments make sounds when they are hit like a
drum, shaken like a maraca or rubbed. Brass instruments are made of
brass or another kind of metal and they create noise by air being
blown inside the mouthpiece. The musician blows their lips to
create a raspberry noise which will create the vibrations. Woodwind
instruments create noise when, like brass instruments, air is blown
inside them. Air may be blown between a reed and a surface like a
clarinet, two reeds like a bassoon or across the edge of a mouth
piece like a flute. Finally stringed instruments make a sound when
their strings are plucked, bowed or sustained like a cello.[37]
The air inside a wind or brass instrument, strings on stringed
instruments and the skin or surface of a percussion instrument all
vibrate when played. This vibration created a sound wave in a
medium and we hear these waves as musical notes [28].
BTEC Music Technology 8
Figure 1.10
Figure 1.11
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Daisy Grace Wakefield 13 April 2015
Sounds created electronically are not the same as acoustic
sounds. Electric sounds are created when an electrical signal is
transmitted to an amplifier which is then played through speakers
[28].
Strings Stringed instruments like a cello create musical notes
by plucking or using a bow against the strings causing them to
vibrate. The pitch is changed due to the tension in the string and
its thickness. A tighter string will create a higher pitch and a
thicker string will create a lower pitch [29]. A piano is said by
some to be a stringed instrument as sound is created by a tiny
hammer hitting the strings to create the sound. If you bowed the
string then this will allow you to sustain the note. String
instruments are tuned by
tightening or loosening the strings [29].
String instruments have something called a soundbox or
resonator. This is usually the body of the instrument and the
loudest part, due to vibrations. They also have a soundboard made
from wood and also vibrates. When the strings vibrate, the
vibration is picked up by the bridge upon which the strings lay.
The bridge then transports these vibrations to the soundboard,
which then vibrates, and uses the soundboard to amplify the sound
making it loud enough for people to hear.
The length of the strings determines how fast they
vibrate as longer strings vibrate slower and create a lower
pitch. The weight of the strings, being heavy or thick, will make a
lower pitch that lighter thinner stings and as said before, the
tighter the string the higher the note will be [30].
Percussion An instrument that is hit, scraped or shaken to
create a sound is a percussion instrument. the action of hitting,
scraping or shaking the instrument will cause the instrument in
question to vibrate and therefore create a sound. Drums for example
are composed of a head, made from a starched synthetic or animal
skin and a cylindrical frame. The drum is played by hitting the
skin of it which will cause vibrations to pulsate and be amplified
by the cylindrical frame. The smaller and tighter the drum head the
higher pitched the
BTEC Music Technology 9
Figure 1.12
Figure 1.13
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Daisy Grace Wakefield 13 April 2015
drum, as the vibrations are faster. Some drums will produce a
single pitch like a snare drum.
A snare drum is usually half as deep as other drums like a floor
tom or rack tom. As it is smaller, this give it a higher pitch. Tom
drums are around 13 to 14 inches in length but a snare drum is only
5 to 6 inches deep. A snare drum also has wires under the bottom
skin of the drum. A lever called the throw o can be used to tighten
and loosen these wires. If they are tightened then the wires will
touch the snare drum. Hitting the head of the drum will cause the
vibrations to travel down to the wires causing them to vibrate,
this created the typical cracking sound associated with the snare
drum. When the wires are not touching the bottom skin, the snare
drum will sound similar to other drums but at a higher pitch due to
its smaller depth. Some drums also have a special coated skin which
is covered in a rough material. When this skin is hit, due to the
material, it will slightly mue the sound. Combining all three
elements, the coated skin, smaller depth and the wires under the
drum, gives the snare drum its unique timbre [31].
Wind There are two main parts to a wind instrument;
Blowing air through it
Creating dierent notes by covering holes with your
fingers[32].
As the musician blows air though the the mouthpiece, this
creates vibrations that move thought the instrument which when
combined with covering the right holes with their fingers creates
dierent notes.
Wind instruments that are made when vibrations, made by air,
travels across a thin piece of wood are known as reed instruments.
Some have one reed like a saxophone and others have two reeds like
an oboe. A flute on the other hand creates sounds when air is blown
across the edge of the mouthpiece. As the air is split by the edge
this causes vibrations [35]. The air hits the side of the flute,
causing the air to move around to create the sound. The pitch is
changed when fingers are lifted up or pressed down to release or
cover holes. This action changes the frequency and therefore pitch.
A higher pitch means that there are less holes covered and a lower
pitch means that there are more holes covered[33].
BTEC Music Technology 10
Figure 1.14