BU3: Acoustics Module 5 - Sound Waves

sound wave

a PRESSURE DISTURBANCE that travels through a MEDIUM by means of particle-to-particle interaction

Velocity

- Speed in a given direction

- rate of change of displacement

Speed

- rate of change of position of an object in any direction

- rate of change of distance

Frequency

the number of complete wavelengths that pass a point in a given time

wavelength and frequency

The velocity of a wave is equal to the product of its __________ and _____________ (number of vibrations per second) and is independent of its intensity.

0.33 km/s or 0.2 mile/s

Sound has a velocity of about ____________ in air

1.5 km/s

Sound has a velocity of ______________ in water

5 km/s

Sound has a velocity of _________________ in steel

Air

All soundwave travel with the same speed in ______ regardless of their frequency

False

True or False

When an object moves outward, it pushes air molecules which create an area of low pressure. When the object moves inward, it creates an area of high pressure.

speed of sound

the distance travelled per unit time by a sound wave

1. Elasticity of Solid

2. Density of the Medium

3. Temperature

Factors Affecting Speed of Sound Wave

Faster

Faster or Slower?

The more elastic the medium, the ________ is the speed of sound.

Clue: Rigid materials such as steel are considered to have a high elasticity.

Slower

Faster or Slower?

The greater the density of the medium, the less responsive they will be to the interactions between particles and the ________ that the wave will be.

Helium

A sound wave will travel nearly three times faster in _______ than it will in air.

Sound propagation

travel of sound from the sound source to the surrounding medium.

343m/s

average speed of sound

Wavelength

The distance travelled by the pressure wave during one cycle

Simply the distance from the crest of one wave to the next nearest crest

Crest

Highest point of a wave

Trough

Lowest point of a wave

Amplitude

Height of a wave

Sound Intensity

Also known as Acoustic Intensity

Defined as the MEAN value of the acoustic energy which crosses a unit area perpendicular to the direction of propagation in unit time

Sound wave intensity

The AMOUNT OF ENERGY that is transported past a given area of the medium per unit of time.

watts/m^2

Typical units for expressing the intensity of a sound wave are

Sound intensity level (SIL)

the level of the intensity of a sound relative to a reference value

Inverse Square Law

States that a specified physical quantity is inversely proportional to the square of that physical quantity.

In acoustics the sound pressure of a spherical wave front radiating from point sources decreases by 50% as the distance r is doubled.

Intensity Decrease = Density Increase

Sound Field

- any area where sound waves are present

- within given boundaries (walls, floors, ceiling)

1. Free Field

2. Diffuse Field

3. Near Field

4. Far Field

Types of Sound Field

Free Field

a sound field without any obstructions (reflections)

anechoic chamber

a sealed room in which all the surfaces are designed to completely absorb all sound produced in the room

Example of Free Field

Diffuse Field

Sound is reflected from many surfaces

Near Field

the part of a sound field that is close to a sound source

evanescent waves.

Very close to the source, the sound energy circulates back and forth with the vibrating surface of the source, never escaping or propagating away.

Far Field

the part of the sound field that is far from a sound source

The wavefront is deemed as a plane wave and will decay at a rate of 6 dB per distance doubled from the source.

begins approximately at a distance of 1 wavelength, and extends outward to infinity

Sound Pressure

- the pressure caused by the vibration of the sound source

- the force of sound (N) on a surface perpendicular to the direction of sound

- ultimately what our ears hear (loudness)

- the most commonly used indicator of the acoustic wave strength

Pa or N/m2

The SI-unit for sound pressure

Sound Power

ENERGY of sound per unit of time (J/s or W)