Lecture 1 Sound - Volume

Decibel
A decibel is a measurement of volume. That starts at 0 which is sound that can barely be heard and goes up in levels of 10, so 10 decibels is 10 times the start level, 20 is 10 times 10 decibels so 100 times the start, 30 is 10 times more so is 1000 times the original level. Which caps off for the human ear at about 120 decibels before your ear drum can take no more and can break if pushed high enough.

The Inverse-square law

"The intensity of the sound received varies inversely as the square of the distance R from the source i.e. as 1/R²" 
Traveling through air sound decays, so if the person or microphone receiving the sound was 3 meters away from the source it would be nine times less intense than if  it was 1 meter away from the source.
So if the person was 6 meters away from the source it would be 1/6² - 1/36th of the intensity from 1m.

Though this doesn't always happen in rooms and so on because of surfaces that reflect sound. This is why we have Echos and reverberations. The difference between these is if a sound comes back in under 100ms the brain cant destinguish it as a different sound and it adds on to the sound you've already heard making it a richer sound overall but if the delay is over 100ms the brain hears 2 seperate noises and it is heard as an echo.

Though reflection of sound can be completely removed in an Acoustic Anechoic Chamber.

The chamber is made up of walls normally out of foam and in shaped that deflect the sound instead of sending it back towards the source. These are useful tool for sound recording as it removes all reverberations or echo's.

Lecture 1 Sound - Movement

The velocity of sound changes depending on what the sound is moving through, The standard for sound to move through air is 333m/s which is roughly 700mph but sound moves much faster through water at 1500m/s and even faster through steel 5000m/s. So sound traveling through air down the length of a football pitch (120m) would take : 120/333s - 0.36s. It would take around a third of a second for the sound to make it down the whole pitch. If the pitch was full of water it would take even less time: 120/1500s - 0.08s.

The frequency of a wave is the number of complete cycles per second and this is measures in Hertz (Hz).

The velocity of a wave length can be figured out by multiplying the wavelength by the frequency.

V= λ x f  m/s  , this equation can shuffled round to work out different parts. It can be used to find the wavelength by changing it round to  λ= v/f m . 

So a 1kHz tone in air has a wavelength, 
l = v/f m 
= 333/1000 m 
= 0.333 m 

 Harmonics


Sound is made up of vibrations, there is the main vibration which is the fundamental tone and other vibrations are the harmonic tones. 
" A harmonic is an integer multiple of the fundamental frequency e.g. 2 x fundamental, 3 x fundamental, etc. 
So a sound consisting of components at frequencies of 1000 Hz and 3000 Hz would contain a 3rd harmonic of the 1 kHz fundamental.  "

Lecture 1 Sound - Waves

Sounds exist in this world as a wave, that passes as a motion through different mediums. The waves are split into two different types, Transverse and Longitudinal.

A transverse wave is a wave that moves up and down at a right angle to the direction of the particles, carrying the energy along the path from its source.

(http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html)

A longitudinal wave of the other hand is a vibration that moves parallel to the direction of motion. This kind of wave is the wave that is found in the travel of sound. 

(http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html)

The molecules don't move along the path instead they pass the energy onto a neighbouring molecule and so on, until the energy reaches the end. An example of the transfer of energy is the Newton's cradle.

This means sound waves are made up of a series of alternate compressions and rarefactions which are an increase and decrease in density in the medium the sound is traveling through.

Both the waves can be measured with wavelength and amplitude. The wavelength is, in the case of a transverse wave, the distance between the waves and for longitudinal waves it is the distance between two peak compression. The change in wavelength is what accounts for different pitch. If the sound is of a high pitch the wave length will be shorter between waves and at a lower pitch the wavelength will be longer.