Lab 1: A full description of purpose and "workings" of an ‘Acoustic Anechoic Chamber’

A Description of an Acoustic Anechoic Chamber

The Anechoic Chamber - The term anechoic means non-echoing or echo-free. An Anechoic chamber is a room that is designed to completely absorb all reflections from sound waves. These rooms are also insulated from outside sources of noise. When both of these combine it simulates an extremely quiet open space. This is very useful for working without the impact of exterior influences which in certain tests or examples could hinder someones progress.

These chambers are commonly used in acoustics to conduct certain experiments in essentially well moderated conditions. This is meaning that there are no reflected signals within and that all sound energy will be moving away from the source with almost none reflected back. Two examples of experiments that have taken place in anechoic chambers are measuring the transfer function of a loudspeaker or the directivity of noise radiation from industrial machinery. The interior of an anechoic chamber is extremely quiet, with noise levels around 10 to 20 dBA. We know that the human ear can pick up sounds above 0 dBA so humans hear the chamber as completely void of sound. This leads to some disorientation. 

RAM or Radiation Absorbent Material is designed and shaped to absorb sound radiation. The most common and effective types of RAM uses an array of pyramid shaped pieces that are made from a specific material. In order for this to work all of the surfaces inside the chamber have to be completely coated in this RAM. A popular material for this is a special rubberized foam material with internal mixtures of carbon and iron. The length from the base to tip of the pyramid is based on the lowest expected frequency that the users will be working with. If the frequency is low then the panels are longer whilst the higher frequencies need shorter panels.

These pyramids have their points pointing towards the centre of the room. Pyramidal RAM atenuates signals by two effects, scattering and absorption. Scattering can occur in two ways, coherently when reflected waves are directed away from the receiver or incoherently where waves are picked up by the receiver. Incoherent scattering also occurs inside the foam pyramids. The pyramid shapes are cut at angles the maximize the bounces a waves make within a structure. With each bounce, the waves loses energy to the foam material and as such exits the material with lower signal strength.

Waves that have higher frequencies have shorter wavelengths and as such are higher in energy. Low frequencies have longer wavelengths and are lower in energy according to the calculation where lambda equals velocity divided by frequency, where lambda represents wavelength. To shield against a specific wavelength the cones are made to a specific size to absorb said wavelength. The quality of a chamber is determined by the lowest frequency operation in which measured reflections from the surfaces inside will be the most significant compared to high frequencies. 

Week 1: Audio, Image and Video Processing - Lecture 1

Audio, Image and Video Processing

Week 1: Waves and More

Lecture:

Sound Waves - Sound is represented as a wave which is created with via vibrations. Various objects that can create these vibrations are guitar strings, tuning forks and human vocal chords. Sound transmits via a physical wave motion through different Mediums such as gases, liquids or solids. The direction of displacement in one of these mediums alongside the direction of the waves motion determines the type of wave being dealt with. Two of the most basic classifications are transverse and longitudinal waves.

As you can see below in a transverse wave as the source moves up and down the coils also move up and down. With a longitudinal wave when the source moves left and right the coins move left and right, compressing and expanding in doing so. In both cases energy is being transported via the wave without transporting matter. Source - http://www.getting-in.com/guide/a-level-physics-longitudinal-and-transverse-waves/


Transverse Waves - A common example of a transverse wave is the ripples that form on the surface of water. A transverse wave causes particles in a material such as metal, water etc to move back and forth at a right from the source or direction the waves are travelling.
Source: - http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

Above is an animation of a transverse wave viewed as a one-dimensional plane. As the wave causes ripples from left to right the particles rather than following this move up and down maintaining a steady position as the wave passes by.

Longitudinal Waves - Longitudinal waves are sound waves. These are formed when the vibration of a wave is parallel to the direction of motion. In a longitudinal wave the wave flows outwards from the centre of the origin. The air molecules caught in this move back and forth parallel to the motion of the wave, compressing and expanding the wave. During this each molecule passes the energy being transported onto the adjacent molecules but once the sound wave has passed the molecules remain in the same location.
http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

Above is an animation of a longitudinal wave as viewed on a one-dimensional plane moving through a tube. Rather than the particles moving along with the waves they instead move back and forth from their initial point. The wave's motion is visible in the compressed regions which move from the left to the right.

Water Waves - Water waves are waves that combine both transverse and longitudinal motions. As the wave travels the particles involved are moving in clockwise circles, the larger the depth the smaller the radius of their movement. This animation below shows an example where the wave is travelling left to right in water that's depth is greater than the wavelength of the waves.
Source: http://www.edu.pe.ca/gray/class_pages/krcutcliffe/physics521/14waves/applets/Longitudinal%20and%20Transverse%20Wave%20Motion_files/water.gif


Mechanical Waves - Mechanical waves are waves that require a material medium in order to move from one point to another. These waves are able to move through a medium because of the interaction forces of the particles(atoms or molecules) in the medium.

Compression and Rarefaction -  A sound wave is a series of alternate increases and decreases in a medium such as a gas, liquid or solid. Source: http://everythingscience.co.za/grade-10/09-longitudinal-waves/09-longitudinal-waves-02.cnxmlplus

As you can see above the decrease is density of the wave is rarefaction and the increase in density is compression.

Wavelength and Amplitude - In terms of a transverse wave the wavelength is the distance between two successive crests or troughs. You can see the wavelength below stretching from one crest to another. The wavelength for a longitudinal wave is the shortest distance between two peak compressions. This means the centre of a compression to the centre of the next compression.

Source: http://www.studyphysics.ca/newnotes/20/unit03_mechanicalwaves/chp141516_waves/images/wavelength02.png - http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCSE/bytesize%20images/amplitude2.jpg

Velocity, Frequency and Wavelength - Sound waves can travel at different speeds or velocities through any different medium. The velocity is the rate at which the position of an object changes equivalent to a specified speed and direction of it's motion. Sound waves travel quickest through a solid (steel = 5,000 m/s) slower through a liquid (water = 1,500 m/s) and slowest through gas (air = 333 m/s).

Because sound travels as one particle hits another it travels fastest through solids due their particles being closer together.

V = Velocity - The wave speed in metres per second, m/s.
f is the frequency in hertz, Hz.
The lambda symbol is the wavelength in metres, m.

f = v over lambda, lambda = v over f and v = f x lambda.

Standing Waves & Harmonics - Standing waves are waves that remain in a constant position and do not travel through the transmission medium. These waves can be found in the strings of a musical instrument. In a violin string when it is either bowed or plucked the string vibrates as a whole between a two nodes at either end and an anti-node in the middle.
Source: http://www.stmary.ws/highschool/physics/home/notes/waves/576px-Overtone.jpg




Above you can see how a violin string also vibrates in halves. With a third node in the middle and an extra node on either side. Likewise this occurs in thirds and other fractions, all at the same time. These vibrational nodes can also occur in certain cavities such as a room. In matter of fact a room can support a standing wave with a node at each opposing wall.

Below is an example of Harmonics. The vibration of a violin string as a whole produces what is called the Fundamental (basic) tone and each of the other vibrations produce varying harmonic tones. Mode 1 is the fundamental tone or 1st harmonic. Mode 2 is the 2nd harmonic and so on and so forth. It's important to note that a harmonic in an integer multiple of the fundamental frequency or 1st harmonic. For example the second harmonic would be 2 * the fundamental. The third harmonic would be 3 * the fundamental.

A sound consisting of components at frequences of 1000hz and 3000hz would contain a 3rd harmonic of the 1khz fundamental.
Source: http://resource.isvr.soton.ac.uk/spcg/tutorial/tutorial/Tutorial_files/standingstring1.gif

Amplitude - Amplitude in terms of sound waves is the degree of motion that the air molecules in the wave take. This corresponds to the extend of both rarefaction and compression that goes along side the wave. The larger the amplitude of a wave then the harder the molecules strike the human eardrum or microphone diaphragm and as such a louder sound is picked up. Amplitude of a sound wave can be measured in units by taking the distance moved by the air molecules or the pressure difference in the compressions and rarefactions or the energy involved. An example follows. Ordinary speech produces sound energy at a power level of around one hundred thousandth of a watt.
Source: http://www.ducksters.com/science/physics/properties_of_waves.php

Sound Intensity and Level - It is very difficult to measure the intensity of a sound. Because of this sound is generally expressed as an equivalent sound level. This is accomplished by comparing any sounds to a global standard intensity that is the quietest 1kHz tone the human ear can pick up. This threshold of hearing is equivalent to an air pressure change of 20 micro pascals. 10 pascals is painful and dangerous to hearing and normal atmospheric pressure is 100,000 pascals.

The intensity level of normal speech is around 100,000 times that of whispered speech. To convieniently graph these values we use a logarithm and measure in decibels.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/intens.html

The Level of Various Sounds - A sound that has an intensity equal to the human threshold of hearing has a level of 0 decibels. Whispering is around 10 and normal speech is around 60.

The Inverse-Square Law - The intensity of a sound received varies inversely as the square of the distance from the source. In open air sound will be around 9 times less intense at a distance of three metres from it's origin as at a distance of one metre. Objects that are in our surroundings produce a reflection or echo which is the absorption and scattering of waves, so that the inverse-square law is often not applicable in direct measurements of the intensity of sound.

An echo is a perceived reflection of a sound from a surface. The fraction of sound level reflected is known as the reflection coefficient. The difference in time between the echo and the direct sound (sound which we receive without any reflection) depends on the distance traveled and the speed of sound. The difference must be greater that 100ms to be perceived as an echo.

Reverberation is the persistence of sound in a particular space following the original sounds dispersal. When sound is produce in a space a large number of echoes build and then fade, this is called damping because the sound is absorbed by the walls, floor and air.

Room reverberation is the accumulated reflections from all the surfaces such as ceiling and walls adding to the direct sound. Multiple level reflections occur as well as single ones.
Sound: http://www.acousticalsurfaces.com/acoustic_IOI/reverberation.htm