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Understanding of what is meant by transmission of data
Sound
Sound is an oscillation of pressure transmitted through a solid, liquid, or gas (there is no sound in outer space as space
is a vacuum and there is no solid, liquid or gas to transmit sound through!). A speaker works by moving its centre cone
in and out, this causes the air particles to bunch together forming waves. These waves spread out from the speaker
travelling at 340 m / s. If your ear is in the way, then the waves of sound particles will collide with your ear drum,
vibrating it and sending a message to your brain. This is how you hear:
When you hear different volumes and pitches of sound all that is happening is that each sound wave varies in energy
for the volume (larger energy waves, the louder the sound), or distance between sound waves which adjusts the pitch,
(smaller distances between waves leads to higher pitched sound).
1 - base volume and frequency
2 - double volume and frequency
3 - same volume treble the frequency
Sound is often recorded for two channels, stereo, feeding a left and right speaker whose outputs may differ massively.
Where one channel is used, this is called mono. 5.1 surround sound used in cinemas and home media set ups use 6
channels.
A computer representation of a stereo
song, if you look carefully you'll see the
volume of the song varying as you go
through it
This section of the book will cover how we record, store and transmit sound using computers. Sound waves in nature are
continuous, this means they have an almost infinite amount of detail that you
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could store for even the shortest sound. This makes them very difficult to record perfectly, as computers can only store
discrete data, data that has a limited number of data points.
The discrete approximations (in red) can be used to Sound is a
continuous set of data points recreate the original sound (grey). However, due to formed by a wave. Computers
sample limitations in the number of samples we take we are this sound at discrete points to store a often unable to
truly represent a sound wave, though
digital approximation we can get close enough for the human ear not to
notice the difference.
Analog and digital
For a computer to store sound files we need to get the continuous analogue sound waves into discrete binary values:
An analogue sound wave is picked up by a microphone and sent to an Analogue to Digital (ADC) converter in
the form of analogue electrical signals. The ADC converts the electrical
signals into digitalvalues which can be storedona computer.
Once in a digital format you can edit sounds with programs such as audacity.
To play digital audio you convert the sound from digital values into analogue electrical signals using the DAC,
these signals are then passed to a speaker that vibrating the speaker cone, moving the air to create sound waves
and analogue noise.
nalogue to Digital Converter (ADC) - Converts analogue sound into digital signals that can be stored on a computer
Digital to Analogue Converter (DAC) - Converts digital signals stored on a computer into analogue sound that can be
played through devices such as speakers
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fig 1. The original analogue sound wave is a
continuous set of points
fig 2. A C converts sound into digital data
fig 3. DAC converts digital data into analogue
sound, the analogue wave produced may differ
significantly from the original sound wave
Sampled sound
Sound waves are continuous and computers can only store discrete data. How exactly does an Analogue to Digital
Convert convert a continuous sound wave into discrete digital data? To do this we need to look at how computers
sample sound.
Sampling is amplitude of sound wave taken at different points in time and measurement of value of analogue signal at
regular time intervals/a point in time.
Sampling Rate - The number of samples taken per second
Hertz (Hz) - the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon
To create digital music that sounds close to the real thing you need to look at the analogue sound waves and try to represent
them digitally. This requires you to try to replicate the analogue (and continuous) waves as discrete values. The first step in
doing this is deciding how often you should sample the sound wave, if you do it too little, the sample stored on a computer
will sound very distant from the one being recorded. Sample too often and sound stored will resemble that being recorded but
having to store each of the samples means you'll get very large file sizes. To decide how often
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you are going to sample the analogue signal is called the sampling rate. Take a look at the following example:
Original Sound High sample rate
original continuous sound wave digital looks like original
1/2 high sample rate 1/3 high sample rate 1/4 high sample rate
digital loses sharpness loss of peaks poor resemblance to original!
To create digital sound as close to the real thing as possible you need to take as many samples per second as you can.
When recording MP3s you'll normally use a sampling rate between 32,000, 44,100 and 48,000Hz (samples per second).
That means that for a sampling rate of 44,100, sound waves will have been sampled 44,100 times per second!
Recording the human voice requires a lower sampling rate, around 8,000Hz. If you speak to someone on the phone it
may sound perfectly acceptable, but try playing music down a telephone wire and see how bad it sounds.
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Comparison of the same sound sample recorded at 8k z, 22k z and 44kHz sample rate. Note the spacing of the
data points for each sample. he higher the sample rate the more data points we'll need to store
Sampling resolution
Sampling resolution - the number of bits assigned to each sample
As you saw earlier, different sounds can have different volumes. The sampling resolution allows you to set the range of
volumes storable for each sample. If you have a low sampling resolution then the range of volumes will be very
limited, if you have a high sampling resolution then the file size may become unfeasible. The sampling resolution for a
CD is 16 bits used per sample.
Amplitude
Amplitude is the fluctuation or displacement of a wave from its mean value. With sound waves, it is the extent to which
air particles are displaced, and this amplitude of sound or sound amplitude is experienced as the loudness of sound.
Dynamic range
Abbreviated DR or DNR, is the ratio between the largest and smallest values of a changeable quantity, such as in
signals like sound and light.
The 16-bit compact disc has a theoretical dynamic range of about 96 dB for a triangle wave or 98 dB for
sinusoidal signals.The perceived dynamic range of 16-bit audio can be as high as 120 dB with noise-shaped dither,
taking advantage of the frequency response of the human ear. Digital audio with undithered 20-bit digitization is also
theoretically capable of 120 dB dynamic range. Similarly, 24-bit digital audio calculates to 144 dB dynamic range. All
digital audio recording and playback chains include input and output converters and associated analog circuitry,
significantly limiting practical dynamic range. Observed 16-bit digital audio dynamic range is about 90 dB.
Dynamic range in analog audio is the difference between low-level thermal noise in the electronic circuitry and high-
level signal saturation resulting in increased distortion and, if pushed higher,
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clipping. Multiple noise processes determine the noise floor of a system. Noise can be picked up from microphone self-
noise, preamp noise, wiring and interconnection noise, media noise, etc.
File sizes
Bit rate - the number of bits required to store 1 second of sound
To work out the size of a sound sample requires the following equation:
File Size = Sample Rate * Sample Resolution * Length of sound
This is the same as saying:
File Size = Bit Rate * Length of sound
Let's look at an example:
To work out the size of a sound sample requires the following equation:
File Size = Sample Rate * Sample Resolution * Length of sound
This is the same as saying:
File Size = Bit Rate * Length of sound
Let's look at an example:
Example: Sound File Sizes
If you wanted to record a 30 second voice message on your mobile phone you would use the following:
Sample Rate = 8,000Hz
Sample Resolution = 16 bit
Length of Sound = 30 seconds
Therefore the total file size would be:
8,000 * 16 * 30 = 3 840 000 Bits = 480 000 Bytes
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Extension: Sound Editing
If you are interested in sound editing you can start editing your own music using a program called Audacity. Using
Audacity you can create your own sound samples with different sample rates and sample resolutions, listening to the
difference between them and noting the
different file sizes.
Additional Info
Sound
In physics, sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement,
through a medium such as air or water. In physiology and psychology, sound is the reception of such waves and their
perception by the brain.
OR
Sound is defined by ANSI/ASA S1.1-2013 as "(a) Oscillation in pressure, stress, particle displacement, particle
velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such
propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a).
Sound wave properties and characteristics
Sinusoidal waves of various frequencies; the bottom waves have higher frequencies than those above. The horizontal
axis represents time.
Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these
generic properties:
Frequency, or its inverse, the period
Wavelength
Wave number
Amplitude
Sound pressure
Sound intensity
Speed of sound
Direction
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal
frequency, which emphasizes the contrast to spatial frequency and angular frequency. The period is the duration of
time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example, if a newborn
baby's heart beats at a frequency of 120 times a minute, its period – the interval between beats – is half a second
(60 seconds (i.e., a minute) divided by 120 beats). Frequency is an important parameter used in science and
engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio
(sound) signals, radio waves, and light.
https://en.wikipedia.org/wiki/Frequency
The SI unit for period is the second. For counts per unit of time, the SI unit for frequency is hertz (Hz), named
after the German physicist Heinrich Hertz; 1 Hz means that an event repeats once per second. A previous name for
this unit was cycles per second (cps).
A traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or
rpm. 60 r/min equals one hertz
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Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave— the distance over which the
wave's shape repeats. and the inverse of the spatial frequency. It is usually determined by considering the distance
between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a
characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is
commonly designated by the Greek letter lambda (λ). The concept can also be applied to periodic waves of non-
sinusoidal shape. The term wavelength is also sometimes applied to modulated waves, and to the sinusoidal
envelopes of modulated waves or waves formed by interference of several sinusoid
In the physical sciences, the wavenumber (also wave number) is the spatial frequency of a wave, either in cycles
per unit distance or radians per unit distance. It can be envisaged as the number of waves that exist over a specified
distance (analogous to frequency being the number of cycles or radians per unit time).
Amplitude
The amplitude of a periodic variable is a measure of its change over a single period (such as time or spatial
period). There are various definitions of amplitude (see below), which are all functions of the magnitude of the
difference between the variable's extreme values. In older texts the phase is sometimes called the amplitude
Sound pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average, or equilibrium)
atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in
water with a hydrophone. The SI unit of sound pressure is the pascal (Pa)
Sound Intensity
Sound intensity or acoustic intensity is defined as the sound power per unit area. The SI unit of sound intensity is
the watt per square metre (W/m2). The usual context is the noise measurement of sound intensity in the air at a
listener's location as a sound energy quantity.
Sound intensity is not the same physical quantity as sound pressure. Hearing is directly sensitive to sound pressure
which is related to sound intensity. In consumer audio electronics, the level differences are called "intensity"
differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone.
Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound
waves is called intensity of sound.
Sound intensity, denoted I, is defined by
where
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p is the sound pressure;
v is the particle velocity.
where
P is the sound power;
A is the area.
Both I and v are vectors, which means that both have a direction as well as a magnitude. The direction of sound
intensity is the average direction in which energy is flowing. The average sound intensity during time T is given by
Speed of Sound
The speed of sound is the distance travelled per unit time by a sound wave propagating through an elastic medium.
The SI unit of the speed of sound is the metre per second (m/s). In dry air at 20 °C, the speed of sound is 343.2
metres per second (1,126 ft/s). This is 1,236 kilometres per hour (768 mph; 667 kn), or a kilometre in 2.914 s or a
mile in 4.689 s.
Direction
Direction is the information contained in the relative position of one point with respect to another point without the
distance information. Directions may be either relative to some indicated reference (the violins in a full orchestra
are typically seated to the left of the conductor), or absolute according to some previously agreed upon frame of
reference (New York City lies due west of Madrid). Direction is often indicated manually by an extended index
finger or written as an arrow. On a vertically oriented sign representing a horizontal plane, such as a road sign,
"forward" is usually indicated by an upward arrow. Mathematically, direction may be uniquely specified by a unit
vector, or equivalently by the angles made by the most direct path with respect to a specified set of axes.
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Decibel
The decibel (dB) is a logarithmic unit that expresses the ratio of two values of a physical quantity, often power or
intensity. One of these quantities is often a reference value and in this case the decibel expresses the absolute level of
the physical quantity. The number of decibels is ten times the logarithm to base 10 of the ratio of two power quantities,
or of the ratio of the squares of two field amplitude quantities. One decibel is one tenth of one bel, named in honor of
Alexander Graham Bell; however, the bel is seldom used.
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