Acoustics Part 1

ACOUSTICS - is a branch of science that deals with the control and propagation of sound.

Objectives of acoustics

1.       to reinforce needed sound

2.       to eliminate unwanted sound

3.       to prevent unwanted sound to be transmitted into an enclosed space

THE NATURE OF SOUND

Sound - originates from a vibrating body. It is pressure have in an elastic medium. It is not restricted, it spreads outward from its source in all directions diminishing in intensity inversely as the square of the distance from the source increases.

These vibrations from the body come outward as compressions and rarefactions in the surrounding air.

Rarefaction - the instantaneous, local reduction in density of a gas resulting from passage of a sound wave, or the region in which the density is reduced at some instant.

Elongation of sound

The vibration from a body are called frequencies

Frequencies - cycles per second (cps) or vibrations per second (vps) and the compressions and rarefactions are called sound waves

SOUND

Is a physical wave or simply a series of pressure vibrations.

For airborne sound, the medium is air.

For structure borne sound, the medium is building materials such as concrete, steel, wood, glass or combinations of all these.

It is always assumed that the hearer has a pair of healthy young ears with the direction range of 20 to 20,000 Hz.

PROPERTIES OF SOUND

Speed of velocity

1125 ft/sec at 20ºF, the velocity increases 1ft/sec for every 10º rise in temperature (in Fahrenheit)

where:

v = velocity

F = frequency

d = distance of the medium through which the sound travels

Frequency

Is the number of cycles of vibrations executed per second. Humans can hear sounds within the 40 vps to 16,000 vps range

Molecular absorption

A sound maybe covered up or absorbed by another sound. This phenomena is called masking of sound and this is very useful in acoustics.

The sound masking is usually of lower frequency than the sound it masks

Soporific effect

That effect brought about by a low-steady sound