Week One- Lecture

I have revised the first weeks lecture as i have been researching and feel this blog post is far superior to the original attempt.

Sound Waves

Sound is transmitted as wave motion through a medium, such as a solid, liquid or gas.

Waves are divided into classifications according to the direction of the displacement of medium in relation to the motion the wave is travelling.

transverse and longitudinal waves are Two basic classifications of wave.

Transverse Waves

An example of a transverse wave is a ripple on the surface of a pond. The vibrations of the water molecules are at 90 degrees to the direction of motion (in and out of the water).

Longitudinal Waves

An example of a longditudnal wave is a ‘slinky’. Each molecule hits another and  the energy is passed on to the next, and so on, but after the sound wave has passed, each molecule remains in roughly the same location in space.

Compression and Rarefaction

A sound wave is a series of alternate compression (increase in density) and rarefaction (decrease in density) events in a medium, for example air.

Wavelength and Amplitude

For a transverse wave, the wavelength is the distance between two successive crests or troughs. For longitudinal waves, it is the shortest distance between two peak compressions. 

Velocity

Sound in steel moves at a speed of just under 5000 m/sec.

The speed of sound in water is roughly 1500 m/sec. at ordinary temperatures but increases vastly with an increase in the temperature.

The speed of sound in air is roughly  333m/s

Frequency, Velocity and Wavelength

The frequency of a wave is the number of vibrations per second. This is measured in Hertz (Hz).

The wavelength of a wave is the length between a point in the wave repeating itself. E.g. the start of the wave back to the start. This is measured in metres (m)

The velocity of the wave, which is the speed at which it travels, is equal to the wavelength multiplied by the frequency.

Velocity = wavelength x frequency

Standing Waves

Standing waves disturb, but don’t travel through a medium. They are present in such places as a guitar string. The string moves but does not travel.

Harmonics

A harmonic is an integer multiple of the original frequency e.g. 2x original, 3x original and so on.

A sound with a frequency of 1000Hz and another at 3000Hz would have a 3rd harmonic of the 1 kHz original.

Amplitude

The amplitude of a sound wave is the height of the peaks and troughs of a wave, which corresponds to the extent of rarefaction and compression that comes with the wave.

The bigger the amplitude of the wave, the harder the molecules hit the ear drum or microphone and the louder the sound that is percieved.

Ordinary speech, for example, produces sound energy at a power level of about one hundred-thousandth of a watt.

Sound Intensity & Level

These measurements are extremely difficult to make, so the intensity of sounds is generally expressed as an equivalent sound level.

Normal air pressure is 100,000 Pa.

Sound Level and the Decibel

The sounds intensity of normal conversational speech is around 100,000 times of whispered speech.

So that we can talk about and map such a huge range of values, sound intensity level is defined using a logarithm and is measured in decibels (dB).

The Inverse-Square Law

The intensity of the sound received varies inversely as the square of the distance R from the source.

In open air, sound will be around nine times less intense at a distance of 3m from its origin, compared to a distance of 1m.

Echos and Reverberation

An echo is the perceived reflection of sound from a surface. The fraction of sound level reflected is known as the reflection coefficient.

The time difference between the echo and the direct sound depends on the distance traveled and the medium of which the sound is travelling. The difference must be greater than about 100ms to be perceived as an echo.

Spectrum

Since many sounds contain various frequency components it is often useful to display a sound spectrum, which is a graph of sound level against frequency over a short period of time.

Spectogram

The variation of intensity with time and frequency can be displayed as a Spectogram by representing intensity by colour or brightness on a Frequency Time axis.

A really good video I have found on youtube. it simplifies waves and presents them really well.

waves

Week 4- Editing sound files

I have previously altered audio files with acid pro and audacity in high school, but this is my first time using Adobe Soundbooth CS4

Original File (Left) and file after Distortion (Right)

Distortion made the soundtrack sound like it was a heavy metal track.

 Original File (Left) and file after Phaser (Right)

The Phaser seemed to add a strange synthetic sound to the audio, gave it an outer space feel.

Week 4- Lab

After compressing multiple audio samples it seems that the compression lowers the amplitude of the audio wave. The peaks of the wave are affected differently, so that the ‘loudness’ of the audio is level where the human ear will not recognize much change in the amplitude once the compression has been applied.

This also applies to the ‘quiet’ sounds, making the sample louder where we will not hear much variant. This would be used in technology such as CD’s where we have large audio files and a very limited amount of storage.

Compression is also really useful to prevent clipping. Clipping is when an amplifier is pushed too far and attempts to output a voltage out with its capacity and you get an almost buzzing sound.  Compressing the audio file helps reduce the clipping because it reduces the amplitude where clipping may be a problem.

The effects of compression in Soundbooth. The first and last words in the audio file are compressed and the middle wave has not been altered.

Q. Is speech a musical sound?

A. Yes as we all have different bass and treble in our voice, we can alter our voice to sing to where we perceive musical but speech in itself has musical qualities that we use in everyday life but do not notice.

The effects of Reverberation in Soundbooth. The left image is the original, the right has reverberation applied.

Reverberation in enclosed spaces is created by reflections from surrounding walls arriving within short periods of time, for example shouting into a cave, the sound comes back relatively clearly and does not differ in amplitude.

Digital reverb is added by passing the audio signal through a “feedback delay circuit” which creates an echoing effect.

The first test made the signal sound like it had been recorded with a subpar microphone and the second (adding the roller-disco preset) made it sound like it had been recorded underwater.

Week 3- the human ear

The Outer Ear
The outside of the ear, the pinna and the ear canal act as a tunnel, making the vibrations of a sound wave travel to the ear drum which in turn vibrates. this vibration amplifies the sound so that we can process the sound. the pinna also allows us to percieve the direction of the sound source. 

The Middle Ear
The vibration of the eardrum is transferred through the chain of small bones, the ossicles, which then vibrates the oval window of the cochlea.  this chain helps to make up for the lost energy when the vibrations travel through air. this then goes through the fluid in the cochlea.

The Inner Ear
Vibration of the oval window produces pressure waves in the cochlear fluid that stimulate the cochlea. Sensor cells within the cochlea cause neurons connected to our brains percieve this. They transmit timing, amplitude and frequency information to the hearing part of our brain.

Human Hearing and Speech
Human hearing covers a range of frequencies from around 20 Hz to 20 kHz and responds to a huge range of sound levels.

in this range are frequencies and levels generated by speech.
The lowest sound pressure level (SPL) that humans can hear varies with frequency and is called the Hearing Threshold.
the best sensitivity is normally in the frequency range of 1 kHz to 4 kHz.

MPEG/MP3 Audio Coding
The use of MP3 of a lossy compression algorithm is designed to reduce the amount of data required to represent the audio recording and still sound like the original soundtrack. A MP3 file that is created using 128 kbit/s will result in a file that is 11 times smaller than the CD file created from the original audio. A MP3 file can also be constructed at higher or lower bit rates, with higher or lower resulting in a difference in quality.
The compression works by reducing accuracy of waves that are considered to be beyond the hearing ability of most people. This method is referred to as 'perceptual coding'.

Week 2- Getting familiar with waves

Q1 In a recording room an acoustic wave was measured to have a frequency of 1KHz. What would its wavelength in cm be?

λ=v/f
λ= 340/1000
λ= 0.34m
λ=34cm



Q2 If a violinist is tuning to concert pitch in the usual manner to a tuning fork what is the likely wavelength of the sound from the violinist if she is playing an A note along with sound from the pitch fork?

freq. of note at orchestral  A = 440 Hz

T=1/f
T=1/440
T=0.0027

Q3 If an acoustic wave is travelling along a work bench has a wavelength of 3.33m what will its frequency be? Why do you suppose that is it easier for this type if wave to be travel through solid materials?

f=v/λ
f=4000/3.33
f=1201.2 Hz

the molecules in a solid are more tightly compact

Q4 Sketch a sine wave accurately of amplitude 10, frequency 20Hz. Your sketch should show two complete cycles of wave. What is the duration  of one cycle? What is the relationship between the frequency and the duration of one cycle?

Q5 Research the topic “Standing Waves”. Write a detailed note explaining the term and give an example of this that occurs in real life. (Where possible draw diagrams and describe what represent)



A Standing Wave is a wave form that does not seem to travel. They are caused by two waves moving in opposite directions with the same amplitude and frequency, causing interference. The string has two points, a maximum and minimum, the maximum is called an anti-node and the minimum, a node.

When the two waves collide they combine to form a Standing Wave.

An example of a standing wave would be in a Guitar or Violin. The two points of the string are fixed (the nut and bridge) and therefore the waves cannot ‘travel’ and hence a standing wave occurs.

Q6 What is meant by terms constructive and destructive interference?

interference is when two waves meet, constructive interference is when the two waves are 'in phase' (the peaks and troughs are the same). When the interference is constructive, the amplitude increases, making the signal stronger. For Destructive interference is the exact opposite, the waves are not in phase and the amplitude decreases, making the signal weaker.
Q7 What aspect of an acoustic wave determines its loudness?

The amplitude

Q8 Why are decibels used in the measurement of relative loudness of acoustics waves?

Decibels are used because it is a logarithmic measurement that reflects the range of sound intensity our ears can hear and how we perceive loudness

Q9 How long does it take a short 1KHz pulse of sound to travel 20m verses a 10Hz pulse?

It would take the same amount time.

Q10 Does sound travel under water? If so what effect does the water have?

Yes, it speeds the sound up to 1500 m/s