### Click the following links to learn more about sound:

#### Sound:

dosits.org

#### Sound Waves:

macrosonix.com

#### Waveforms:

techtarget.com

#### Auditory Perception:

wisegeek.com

#### Amplitude:

howmusicworks.org

#### Decibels:

earq.com

#### Sound Digitization:

cs.cf.ac.uk

#### Sampling:

clara.net

#### Quantization:

mediacollege.com

#### Sound Compression:

music.tutsplus.com

#### Digital Audio Compression:

ncsu.edu

#### MP3 Compression:

techradar.com

#### Threshold of Hearing:

hyperphysics.phy

#### Masking

vivid-acoustics.com

Did you know that sounds are effectively vibrations? On this page you will learn about what sound actually is, how sound is created and heard, and about sound waves and waveforms.

# What is Sound?

Sound is a unique type of digital media. While all other media is perceived (heard or seen through our senses) visually through our eyes, sound is the only type that is perceived through the sense of hearing using our ears. Sound is created by vibrations, which travel through mediums such as air and water, before it is detected by our ears.

## How Sound is Created and Heard - Tuning Forks

A good example to use for explaining how sound is created and heard would be the striking of a tuning fork (a two-pronged device that vibrates and gives off sound at a specific pitch when stuck).

Once struck, the two tines of the tuning fork thrash back and forth at a very high speed (measured as the frequency, which will be explained later). Whilst the tines move back and forth, they smash into nearby air molecules, compressing (squeezing together) the air so that it moves in time with the vibrations from the fork. The air molecules are then pushed together, creating sound waves (these are explained shortly).

Your browser does not support this video. This video is of a tuning fork colliding with water in slow motion.

The video to the right shows a struck tuning fork hitting water. The video has been slowed down to 1600 frames per second. At normal speed, it is difficult to even spot the movement of a tuning fork, but the vibrations that occur, and their effect on water, are clear in the video.

When sound waves hit our ears, our eardrums vibrate at the same frequency of the speed at which the tines thrash. This this vibration is then transmitted through the inner ear, before being converted into nerve impulses. These nerve impulses are heard by us as a gentle ‘hum’.

Your browser does not support this audio file. This is an audio file with that sound of a tuning fork.

Click the sound icon on the left to hear the sound of a tuning fork, and remember that behind the ‘hum’ that you can hear, there is a long explanation that you now know!

## Other Sounds

Tuning forks vibrate at a single frequency, meaning that there is no variety in the sound produced. Most other sound sources though, do have a wider variety of sound due to their more complex vibrations. For example, a note produced by a guitar string is made up of many components that have multiple frequencies. Like the sound produced by tuning forks, all other sounds come from the conversion of energy into vibrations in the air or through another medium.

# Sound Waves

As mentioned above, sound waves are the product of air molecules that collide. There are two main characteristics of sound waves that we can use to analyse the sound waves, amplitude and frequency.

## Amplitude(Volume)

Amplitude is a measurement of the intensity (strength) of a sound wave and is measured in Decibels (dB). Amplitude effectively describes the size of a vibration, which is why the amplitude of a sound is proportional to how loud (volume) a sound is.

## Frequency(Pitch)

Frequency is a measurement of the speed (number of cycles per second) of a vibration and is measured in Hertz (Hz). Frequency is proportional to the pitch (how high or low a sound is) of a sound.

Below are buttons that play sounds of the same frequency of all of the white keys of the middle (fourth) octave of an 88 key piano (keys C - C). You can see the precise frequencies on the buttons, so play around with them to learn the frequency of different keys on a piano. If there are any musicians out there, feel free to play the good old ‘EastEnders’ tune like we all did back in music class!

Your browser does not support this audio file. These audio files contain frequencies equivalent to the noted of a middle piano octave.

C(261.626) D(293.665) E(329.628) F349.23 | 391.99: G | 440: A | 493.883: B | 523.251: C

## Properties Affecting our Perception

Many properties affect the way that we perceive sound. In terms of sound waves, the two main properties are wavelength and period. Wavelength is the distance that a sound wave travels in one cycle. Period is the time that it takes a wave to complete a full cycle

# Waveforms

Sound can be represented as a waveform by graphically plotting the amplitude of a sound against time. Waveforms show the frequency spectrum (range) of a sound.

The wave form below of a single F piano note. You can see that because it’s a single note, the wave form gets smaller over time (known as decay) before it dies away.

The waveform below is of a more interesting sound than a single note, it is of the song ‘Locked Out of Heaven’ by Bruno Mars, and is of the lyrics ‘I’ve been locked out of heaven’ (click the play button to the left to hear these lyrics). Your browser does not support this audio file. This audio file contains the lyrics 'Locked out of Heaven'. Many different sounds are included in this song including vocals, drums, a guitar and more. Many songs have similar waveforms to this one, due to the variety of sounds that extend over longer periods of time. This constant change results in a constantly changing frequency spectrum.

Consider this: Waveforms can almost be read as a story. For example, imagine a seaside location. If you saw a waveform of this location, you could presume what the different sounds that occur are, and in which order. The waveform might have three or four standout jumps, which you could imagine to be the tide, people shouting or people splashing water. Hopefully you’re now convinced that waveforms are fun to study!