Speaker Title - The Propagation of Sound
  Project Information Preparation for the Labs Reception of Sound Propagation through various materials Transmission in WaterConclusions

Background to Sound:

A particle produces sound when it vibrates in a matter. This matter could be solid, liquid, or gas. Sound is a longitudinal wave. That is, when something vibrates, it moves and passes on energy to the particles around it. These particles, in turn, move particles and pass on energy to particles that are around it, and so on, carrying the pulse of the vibration through the air as alternating compressions and rarefactions which are effectively changes in pressure. The amount of energy carried by each particle in the wave corresponds directly to the amplitude of the vibration of the particles.

The intensity of a sound reflects the amplitude of the sound wave. It is a measure of the amount of energy transported past a given area of medium per unit of time. The greater the amplitude of vibrations in the particles, the greater the rate at which energy is transported, and therefore the more intense the sound wave is. Therefore, intensity is the energy/time*area, or, since energy/time is equivalent to power, the intensity is power/area. The SI units for expressing the intensity of a sound wave is W/m².

As a sound wave carries its energy through a medium, if there are no reflections or reverberation, the intensity of the sound wave will decrease with increasing distance from the source. This can be explained by the fact that the wave is spreading out over a larger space. Since energy is conserved and the area through which this energy is transported is increasing, the power must decrease. This mathematical relationship between the intensity and the distance is known as the inverse square law.

The unit for the intensity of a sound is the decibel (dB). It is a logarithmic scale. The smallest audible sound to the human ear is assigned the value of 0dB. A sound that is ten times (10^1) as loud is 10dB; a sound that is a hundred times (10^2) as loud is 20dB.

However, sound underwater is significantly different from sound above water, due to the transmissive properties of water. See the comparison chart between airborne and underwater noise sources.

The pitch of a sound is usually a measure of its frequency. However, there are circumstances in which a constant frequency sound can be perceived as changing in pitch. One of these is when a sustained high frequency sound of over 2000Hz is increased steadily in intensity. In this case the sound will be perceived as rising in pitch although it is not. Conversely, if a low frequency sound of below 2000Hz is increased steadily in intensity, it will sound as if it was dropping in pitch.

Another one of these 'psychoacoustic' effects is of the perception of the pitch of short pulses of sound. If a short pulse of a pure tone (i.e. no harmonics) is decreasing in amplitude, it will sound like it is higher in pitch than the same frequency pulse with steady amplitude.

Common terms associated with pitch are treble and bass: in general, a treble is a high-pitched sound or tone, and a bass is a low-pitched sound or tone.

Speakers produce sound by the pushing and pulling of a diaphragm with an electromagnet, producing longitudinal sound waves. There are various types of speakers, such as subwoofers or tweeters, etc. Each of these is designed with a certain range of frequencies in mind. A speaker such as a subwoofer, designed for bass frequencies, is larger and has a fairly heavy diaphragm that the low frequency waves can vibrate more accurately. For a speaker designed for treble frequencies, like a tweeter, the diaphragm is small and light and suited for very fast vibrations created by the high frequency sounds.