Revision as of 14:58, 16 October 2011

what is sound?

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard.

sound vs noise

Sound is a form of energy that is transmitted by pressure variations which the human ear can detect. When one plays a musical instrument, say a guitar, the vibrating chords set air particles into vibration and generate pressure waves in the air. A person nearby may then hear the sound of the guitar when the pressure waves are perceived by the ear. Sound can also travel through other media, such as water or steel.Apart from musical instruments, sound can be produced by many other sources - man's vocal cord, a running engine, a vibrating loudspeaker diaphragm, an operating machine tool, and so on. Noise is unwanted sound. Usually the sound of a violin is referred to as music - is something pleasing. Depending on other factors, the sound may be perceived as noise.Noise perception is subjective. Factors such as the magnitude, characteristics, duration, and time of occurrence may affect one's subjective impression of the noise.

sound wave terminology

Sound waves are often described in terms of sinusoidal (smooth repetitive oscillation) two-dimensional waves, and are characterized by the following properties:

Frequency the number of occurrences of a repeating event per unit time. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example, if a newborn baby's heart beats at a frequency of 120 times a minute, its period (the interval between beats) is half a second.

Wavelength the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase.

Amplitude the magnitude of change in the oscillating variable - It translated into the strength of sound waves and determines loudness.

Sound pressure level The sound pressure level is used to describe the amplitude of the sound wave. However the sound intensity is also a way to describe the amplitude of a sound wave. The difference between the two is the following; the sound intensity level (SIL) is the power density from a sound source at the measurement point and the sound pressure level (SPL) is the sum of the sound pressure waves at the measurement point. The SIL and the SPL are equivalent if there is only one pressure wave from the sound source. When extra pressure waves due to reflection occur the SIL and SPL differ. It's confusing that both the SIL and SPL are able to describe sound amplitudes, however a general reference to sound level implies that the SPL is being use. The pressure component corresponds most closely to what we hear.

Sound intensity level The sound intensity represents the watt (energy) per unit area from a sound source. This formula describes the sound intensity level: 10log10(Iactual/Iref)

Speed of sound distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s). This is 1,236 kilometres per hour (768 mph), or about one kilometer in three seconds or approximately one mile in five seconds.

Direction Information contained in the relative position of one point with respect to another point without the distance information

human perception and reaction to sound

text / diagrams / annoyance - related to noisiness (absolute /relative)

physical noise characterization

geometries / ground attenuation / fixed sources....

general sound glossary

Absorption The tendency of sound waves to be soaked up by soft surfaces.

Amplifier Increases the amplitude of a signal.

Attenuate To make weaker.

Conductor element that allows the free transmission of sound.

dB (Decibel) A relative unit of measure between two sound or audio signal levels. A difference 1 dB is considered to be the smallest that can be detected by the human ear.

Dispersion The area throughout which the sound produced is distributed.

Distortion The alteration of the original shape (or other characteristic) of a sound waveform.

Dynamic Range The difference between the softest and loudest extremes within an audio signal.

Efficiency The ratio of a device's energy output to its energy intake.

Filter A device that removes unwanted frequencies or noise from a signal.

Frequency The number of sound waves that pass a given point in one second. The determiner of pitch.

Inductance A circuit's opposition to a change in current flow.

Omnidirectional Capable of picking-up sound or radiating sound equally from all directions

Phase The relationship of an audio signal or sound wave to a specific time reference.

Pitch The degree of highness or lowness of a tone.

Sound Level Meter A device that measures, in dB, the amplitude of sound waves.

Sound Reinforcement The use of electronic devices to reinforce, alter or increase the level of sound.

Tone Sound that can recognized by its regularity of vibration; The overall quality of sound.

Unidirectional Picking up sound primarily from one direction.

Vibration The analogous motion of particles of mass the analogous motion of the particles of a mass of air, whose state of equilibrium has been disturbed, causing the transmission of sound.

sources: environmental protection department - goverment of Hong Kong / Long, Architectural Accoustics. Elsevier Academic Press, London 2006.