Principles of sound

ROOM ACOUSTICS

Sources and Sound

· Sound is an energy which is generated by a source and transmitted through a medium.

· Sound travels approximately 340 meters per second.

· Sound in the air can cause by vibration of moving object and moving air.

· The vibrations of sound move forward using wave motion.

· The sound wave can travels through solids, liquids and gases but not in vacuum.

· Sound waves can be specified in terms of frequency, wavelength and velocity.

Frequency

	Frequency (f) is the rate of vibration, determining how high or low the pitch is. Frequency (f) is the number of cycles of vibration per second. Unit: hertz (Hz)

Wavelength () is the distance of sound travels in one cycle. Unit: metre (m) Wave Length = Speed of Sound Frequency = v f Where = wavelength in m v = velocity in m/s f = frequency in Hz  

Wavelength

Velocity

	Velocity (v) is the distance moved per second in a fixed direction. Unit: metres per second (m/s) Therefore the speed at which sound is transmitted, Velocity (Speed of sound) = frequency x wavelength v = f

Cancellation of sound

· If a wave vibration in one direction meets an equal and opposite vibration, the effect is cancelling of sound.

Velocity of sound

· A sound wave travels away from its source with a speed of 344m/s when measured in dry air at 20°C.

· The speed of sound is not affected by the frequency of a sound or atmospheric pressure (weather).

· The velocity of sound is affected by the properties of the material through which it is travelling.

· The velocity of sound in gases decreases with increasing density as the molecules are heavier.

· Moist air contains a greater number of light molecules and therefore sound travels slightly faster in moist humid air.

· In solid and liquid the particles are much closer together and can transmit the energy from one another more rapidly than air/gas. For example, sound travels through steel about 14 times faster than air.

Frequency of sound

· If an object produces sound waves vibrates 100 times in a second then the frequency of that sound wave will be 100Hz.

· The human ear hears this as sound of a certain pitch.

· Pitch is the frequency of a sound as perceived by human hearing.

· Low-pitched notes are caused by low frequency sound waves and high-pitched notes are caused y high-frequency waves.

Resonance

· All objects have a natural frequency at which they like to vibrate. For example, the sound of a metal bar dropped on the floor can be distinguished from a block of wood dropped in the same way.

· The natural frequency depends upon factors such the shape, density and stiffness of the object.

· Resonance occurs when the natural frequency of an object matches with the frequency of any vibrations applied to the object. The result of resonance is extra large vibrations at this frequency.

Sound Intensity

· The amount of sound energy that is transferred in a given time over certain area is called the intensity of sound.

· The energy is measured for 1s through an area of 1m².

· Unit: Watt per square meter (W/m²) or Joule.

· 1 Watt = 1 J/s

Дата добавления: 2015-11-14