Acoustics

Module 1
Introduction to Acoustics
• Nature of Sounds- Propagation of Sound- Velocity,
Frequency, and wavelength of sound-sound intensity-sound
pressure- loudness-Decibel-
• Human ear and hearing characteristics.
Sound effects the well being
Sound Noise
Architectural acoustics
 Architectural acoustics

Architectural acoustics deals with the design of buildings,

such as music halls, auditoriums, theatres etc such that the
sound reproduced doesnot cause any distortion
INTRODUCTION
• Sound is a longitudinal, mechanical wave. Sound is a vibration that
propagates as a mechanical wave through some medium (such as air or
water).

• Sound can travel through any medium (solid, liquid or gas), but it cannot
travel through a vacuum.

• Sound is a variation in pressure. A region of increased pressure on a

sound wave is called a compression (or condensation). A region of
decreased pressure on a sound wave is called a rarefaction (or dilation).

• In the diagram below, the black dots represent air molecules. As the
loudspeaker vibrates, it causes the surrounding molecules to vibrate in a
particular pattern represented by the waveform. The vibrating air then
causes the listener's eardrum to vibrate in the same pattern.
Sound waves exist as variations of pressure in a medium such as air. They are created
by the vibration of an object, which causes the air surrounding it to vibrate. The
vibrating air then causes the human eardrum to vibrate, which the brain interprets as
sound.
 Propagation of sound
Propagation of sound
• Sound propagates through compressible media such as air, water and solids as
longitudinal waves and also as a transverse waves in solids.

• The sound waves are generated by a sound source, such as vocal cords of a person,
the vibrating string and sound board of a guitar or violin, the vibrating tines of a
tuning fork, or the vibrating diaphragm of a stereo speaker.

• The sound source creates vibrations in the surrounding medium. As the source
continues to vibrate, the vibrations propagate away from the source at the speed of
sound, thus forming the sound wave. At a fixed distance from the source, the
pressure, velocity, and displacement of the medium vary in time.

• The particles of the medium do not travel with the sound wave. That is, the
vibrations of particles in the gas or liquid transport the vibrations, while the average
position of the particles does not change. During propagation, waves can be
reflected, refracted, or attenuated by the medium.
PROPERTIES OF SOUND:-
 PROPERTIES OF SOUND:-

1. Velocity or speed of sound

• The speed of sound is the distance travelled during a unit time by a
sound wave propagating through an elastic medium.

• The speed of sound depends on the medium in which it is

propagating.

• Sound generally travels faster in non porous solids and liquids,

than in gases.

• The speed of sound is faster in materials that have some stiffness

like steel and slower in softer materials like rubber.
wave length
3. Wavelength
The wavelength of sound is the distance between
consecutive corresponding points of the same
phase,
such as crests, troughs. It is the distance over which
the wave's shape repeats.
λ=v/f
Where λ = wavelength (m),
v = speed of sound (m/s)
f = frequency (Hz)
1. Frequency
• When a vibrating object is creating a sound wave, the particle of the
medium through which the sound moves is vibrating in a back and forth
motion at a given frequency. The frequency of a wave refers to how often
the particles of the medium vibrate when a wave passes through the
medium.

• The frequency of a wave is measured as the number of complete

back-and-forth vibrations of a particle of the medium per unit of time. If
a particle of air undergoes 1000 longitudinal vibrations in 2 seconds,
then the frequency of the wave would be 500 vibrations per second.

The unit for frequency is the Hertz (abbreviated Hz), where

1 Hertz = 1 vibration/second
2 Pitch
2 Pitch
• The sensation of a frequency is commonly referred to as the pitch
of a sound.

• A high pitch sound corresponds to a high frequency sound wave

and a low pitch sound corresponds to a low frequency sound wave.

• The ability of humans to perceive pitch is associated with the

frequency of the sound wave that impinges upon the ear. Because
sound waves traveling through air are longitudinal waves that
produce high- and low-pressure disturbances of the particles of the
air at a given frequency, the ear has an ability to detect such
frequencies and associate them with the pitch of the sound.
5. Loudness
• The loudness of sound is defined as the degree of sensation of sound
produced in the ear.
• The loudness of sound depends on its intensity. Loudness is a subjective
characteristic of a sound (i.e. it is based on
• personal experience as opposed to the sound-pressure level in decibels,
which is objective and directly measurable).

Units used to measure loudness:

• Sone (loudness N) - Sone is a more linear scale of perception·

• Phon (loudness level L) - Sound pressure measured on a logarithmic (dB)

scale
Intensity

Graphs of the gauge pressures in two sound waves of different intensities. The more intense sound is
produced by a source that has larger-amplitude oscillations and has greater pressure maxima and
minima. Because pressures are higher in the greater-intensity sound, it can exert larger forces on the
objects it encounters.
7. Intensity

• The rate of energy flow across a unit area of the medium per unit time is
known as the intensity of the sound wave.

• The greater the amplitude of vibrations of the particles of the medium,

the greater the rate at which energy is transported through it, and the
more intense the sound wave is.

• Intensity is the energy/time/area; and since the energy/time ratio is

equivalent to the quantity power, intensity is power/area.
• Typical units for expressing the intensity of a sound wave are Watts/meter2.

• As a sound wave carries its energy through a two-dimensional or

three-dimensional medium, the intensity of the sound wave decreases with
increasing distance from the source.

• The decrease in intensity with increasing distance is explained by the fact that the
wave is spreading out over a circular (2 dimensions) or spherical (3 dimensions)
surface and thus the energy of the sound wave is being distributed over a greater
surface area.

• The mathematical relationship between intensity and distance is sometimes

referred to as an inverse square relationship. The intensity varies inversely with
the square of the distance from the source.

• Since the range of intensities that the human ear can detect is so large, the scale
that is frequently used to measure intensity is a scale based on multiples of 10. This
type of scale is sometimes referred to as a logarithmic scale. The scale for measuring
intensity is the decibel scale.
6. Resonance
The natural sympathetic vibration of a volume of air or a structure at a
particular frequency as the result of excitation by sound energy at that
particular frequency.
Sound Pressure
and Sound
pressure level
4. Sound Pressure and Sound pressure level
• The Sound Pressure is the force (N) of sound on a surface area (m2) perpendicular to the
direction of the sound. The SI-units for the Sound Pressure is Pascal (N/m2 or Pa).

• Sound pressure is the local pressure deviation from the average atmospheric pressure,
caused by a sound wave.

• Sound pressure level (SPL) is most often used in measuring the magnitude of sound. It is a
relative quantity and is the logarithmic measure of the ratio between the actual sound m
pressure and a fixed reference pressure.

• This reference pressure is usually that of the Threshold of hearing which has been
internationally agreed upon as having the value 0.0002 dynes/cm2.
• SPL = 10 log (r/rref) 2 = 20 log (r/rref)
• Where r is the given sound pressure and rref is the reference sound pressure.
• · Sound pressure level is measured in decibels (dB)
Human hearing
• Humans are generally capable of hearing sound energy between 20 Hz
and 20 kHz. Sounds with frequencies above the range of human hearing
are called ultrasound.

• Continuous exposure to sound energy above high frequencies can cause

hearing loss. Sounds with frequencies below the range of human hearing
are called infrasound.

• Frequencies below 20 Hz are generally felt rather than heard. The faintest
sound that a human ear can detect is known as the threshold of hearing.

• The threshold of hearing is the minimum sound level that a person can
hear when no other sounds are present. This point will vary from person
to person, but is typically 0 dB.

• The threshold of pain is the point at which pain begins to be felt. It is an

entirely subjective phenomenon and is typically 130 dB.
Sound