Audio,Image and Video processing
Overview
1.Sound Waves
2.Wavelength and Amplitude
3.Frequency, Velocity and Wavelength
3.Frequency, Velocity and Wavelength
4.Sound Intensity and Level
5.Echoes and Reverberation
6.Anechoic Chamber
6.Anechoic Chamber
Sound Waves
A sound wave is created when a vibration is transmitted as a wave through either a gas, solid or a liquid. An example of this can be found in musical instruments when a plectrum strums a guitar string and the vibration creates a sound wave. Sound waves are catagorised by their direction of displacement. These two classifications are transverse and longitudinal.
An example of transverse waves would be the ripples that appear on the surface of a liquid. When a transverse wave encounters a material it causes the particles to move back and forth in the direction of the wave. However the particles do not travel with the sound wave they move in and down as the wave passes.
Below is an example of a transverse wave.
source - the lecture slides
An example of longitudinal waves would be someone knocking on a door. With a longitudinal wave it is the individual air molecules that move back and forth as the vibrations radiate from the original disturbance.
Below is an example of a longitudinal wave.
source - the lecture slides
Wavelength and Amplitude
The amplitude for a Transverse wave is the distance between two repeating Crests or Troughs whereas with Longitudinal waves it is the shortest distance between two Peak Compressions.
Here is an example of a Transverse wave
source - the lecture slides
Here is an example of a Longitudinal wave
source - the lecture slides
Frequency, Velocity and Wavelength
Velocity
Sound waves travel at different speeds through different materials, objects, substances etc These are called mediums. Velocity is the rate of change of the objects position equivalent to a specification of its speed and then the direction it is travelling in.
Sound travels at its fastest through a solid.
An example is that through Steel sound moves at just below 5000 m/sec
Sound travels slower when moving through a liquid.
In water sound moves at about 1500 m/sec
Sound travels the slowest when moving through a Gas.
In air sound only travels at about 1/3 of a Kilometer which is roughly 333 m/sec.
So to travel 3m sound will take:
t=3/333s
=0.009 seconds
= 0 milli seconds
Frequency
The Frequency of a wave is the number of vibrations that occur in one second. We measure this in Hertz. The speed which the waves moves at is known as the Velocity of the Wave.
Velocity (metres/seconds) = wavelength (m) x frequency (Hz)
So v = Lambda x f
=333 / 1000m
=0.333m
Wavelength
Standing waves will disturb the solid, liquid or gas but will not travel through them. They vibrate similar to the vibrations found on stringed musical instruments such as a Violin or Guitar. A violin or guitar string will vibrate as one whole object. This creates a node at each end and an anti-node in the middle.
source - the lecture slides
The vibration of a string as a whole produces a fundamental tone. The other vibrations however will produce Harmonic tones. A harmonic is an integer multiple meaning that with each successive harmonic there will be another anti-node per harmonic. So a 2nd harmonic will have two anti-nodes and a 3rd harmonic will have three and so on.
source - the lecture slides
Sound Intensity and Level
Some measurements are very difficult to make so the intensity of a sound is commonly taken to be the sound level. This can be done by comparing a Standard Sound Intensity to any other sound. The Standard Sound Intensity is the quietest sound that human hearing can hear. This is known as the threshold of hearing and it is equivalent to a change in air pressure of 20 micro Pascals. Any sound that is at 10Pa is harmful to human hearing and can cause damage.
A day to day conversation will be about 100000 times the sound intensity of whispering. Sound intensity level can be defined by a logarithm that is measured in decibels (dB).
Sound Intensity level = 10logbase10 (I sound/Istandard) dB
This means that a multiplication of 10 in sound intensity will correspond to an extra 10dB of sound level.
Echoes and Reverberation
Echo
An echo is perceived as reflection of a sound from a surface. This means that the fraction of a sound level reflected can be put as thus 0 is greater or equal to a which is smaller than or equal to 1.
The time difference between the echo and direct sound will depend how far it travels and the speed in which sound travels.
Reverberation
Reverberation is the persistent sound in one space after the original sound has dissipated. When a sound is produced a large number of echoes will build up and after a length of time will begin to fade away.
The diagram below will provide a better explanation. You can see that the original sound still travels to the destination but it also bounces off all the other surfaces before reaching the destination, this is what reverberation is.
source - the lecture slides
Anechoic Chambers
An Anechoic chamber is a room that will completely absorb reflections and waves of sounds. More often than not these rooms are insulated to prevent from outside noises breaching the walls. This duo of absorbing reflections and wall insulation allows the generation of infinite dimensional open space. This is incredibly useful as exterior and outside influences would impede the final results.
There is no definite size of Anechoic Chambers. They can be as small and compact as a microwave found in a house or as big as an aircraft hanger. The size of the chambers will depend on what testing will be taking place and what frequency of signals will be used, however, sometimes a smaller chamber can be used when tested with shorter wavelengths.
The term 'Anechoic' was originally thought of by Leo Beranek. It was used in the context of sound waves minimising the reflections of a room. Now that this idea has been put into practice there are now many types of chambers and each one it built for its own unique purpose.
Wavelength and Amplitude
The amplitude for a Transverse wave is the distance between two repeating Crests or Troughs whereas with Longitudinal waves it is the shortest distance between two Peak Compressions.
Here is an example of a Transverse wave
source - the lecture slides
Here is an example of a Longitudinal wave
source - the lecture slides
Frequency, Velocity and Wavelength
Velocity
Sound waves travel at different speeds through different materials, objects, substances etc These are called mediums. Velocity is the rate of change of the objects position equivalent to a specification of its speed and then the direction it is travelling in.
Sound travels at its fastest through a solid.
An example is that through Steel sound moves at just below 5000 m/sec
Sound travels slower when moving through a liquid.
In water sound moves at about 1500 m/sec
Sound travels the slowest when moving through a Gas.
In air sound only travels at about 1/3 of a Kilometer which is roughly 333 m/sec.
So to travel 3m sound will take:
t=3/333s
=0.009 seconds
= 0 milli seconds
Frequency
The Frequency of a wave is the number of vibrations that occur in one second. We measure this in Hertz. The speed which the waves moves at is known as the Velocity of the Wave.
Velocity (metres/seconds) = wavelength (m) x frequency (Hz)
So v = Lambda x f
=333 / 1000m
=0.333m
Wavelength
Standing waves will disturb the solid, liquid or gas but will not travel through them. They vibrate similar to the vibrations found on stringed musical instruments such as a Violin or Guitar. A violin or guitar string will vibrate as one whole object. This creates a node at each end and an anti-node in the middle.
source - the lecture slides
The vibration of a string as a whole produces a fundamental tone. The other vibrations however will produce Harmonic tones. A harmonic is an integer multiple meaning that with each successive harmonic there will be another anti-node per harmonic. So a 2nd harmonic will have two anti-nodes and a 3rd harmonic will have three and so on.
source - the lecture slides
Sound Intensity and Level
Some measurements are very difficult to make so the intensity of a sound is commonly taken to be the sound level. This can be done by comparing a Standard Sound Intensity to any other sound. The Standard Sound Intensity is the quietest sound that human hearing can hear. This is known as the threshold of hearing and it is equivalent to a change in air pressure of 20 micro Pascals. Any sound that is at 10Pa is harmful to human hearing and can cause damage.
A day to day conversation will be about 100000 times the sound intensity of whispering. Sound intensity level can be defined by a logarithm that is measured in decibels (dB).
Sound Intensity level = 10logbase10 (I sound/Istandard) dB
This means that a multiplication of 10 in sound intensity will correspond to an extra 10dB of sound level.
Echoes and Reverberation
Echo
An echo is perceived as reflection of a sound from a surface. This means that the fraction of a sound level reflected can be put as thus 0 is greater or equal to a which is smaller than or equal to 1.
The time difference between the echo and direct sound will depend how far it travels and the speed in which sound travels.
Reverberation
Reverberation is the persistent sound in one space after the original sound has dissipated. When a sound is produced a large number of echoes will build up and after a length of time will begin to fade away.
The diagram below will provide a better explanation. You can see that the original sound still travels to the destination but it also bounces off all the other surfaces before reaching the destination, this is what reverberation is.
Anechoic Chambers
An Anechoic chamber is a room that will completely absorb reflections and waves of sounds. More often than not these rooms are insulated to prevent from outside noises breaching the walls. This duo of absorbing reflections and wall insulation allows the generation of infinite dimensional open space. This is incredibly useful as exterior and outside influences would impede the final results.
There is no definite size of Anechoic Chambers. They can be as small and compact as a microwave found in a house or as big as an aircraft hanger. The size of the chambers will depend on what testing will be taking place and what frequency of signals will be used, however, sometimes a smaller chamber can be used when tested with shorter wavelengths.
The term 'Anechoic' was originally thought of by Leo Beranek. It was used in the context of sound waves minimising the reflections of a room. Now that this idea has been put into practice there are now many types of chambers and each one it built for its own unique purpose.
No comments:
Post a Comment