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.