Acoustics

Unit – III

Acoustics and Lighting Designs

 

Acoustics

Methods of sound absorptions absorbing materials noise and its measurements, sound insulation and its measurement, impact of noise in multi-storeyed buildings.

 

Introduction

The branch of physics which deals with generation, reception, propagation and analysis of sound is called acoustics.

The study of sound waves plays an important role in many engineering and non-engineering applications. The areas of acoustical studies and their applications include

Architectural acoustics - Study of sound waves in closed halls and buildings.

Musical acoustics - Physics of musical instruments?

Engineering acoustics - Technology y of sound production production and recording, study of vibrations of solids and their control as well as noise control.

Bio-acoustics / Medical acoustics - Use of sound in medical diagnosis and therapy.

 

Classification of Sound

Sound waves are classified into three types based on their frequencies.

(a) Infrasonics (Inaudible): Sound waves of frequencies below 20 Hz are called infrasonics. They are inaudible.

(b) Audible sound: Sound waves of frequencies between 20 Hz and 20,000 Hz are called audible sound. They are audible.

Audible sound is further classified as musical sound and

(c) Ultrasonics (Inaudible): Sound waves of frequencies above 20,000 Hz or 20 kHz are called ultrasonics. They are inaudible.

 

Musical Sound

Sound which produces a pleasant effect to our  is called musical sound.

Example:

Sounds produced by the musical instruments like sitar, violin and piano are musical sounds.

Properties of musical sound

• Fig. 3.1. shows the wave forms of the different musical sounds.

• The waveform of musical sound has a regular shape.

• Musical sound has definite periodicity.

• There is no sudden change in amplitude.

 

Noise

Any unpleasant sound to our ears is called noise.

Noise causes irritation and strain to our ear. Noise of high intensity may cause permanent or temporary deafness.

Example: Movement of furniture, road traffic, explosion of bombs.

 

Properties of noise

• The wave form of noise is shown in fig. 3.2.

• The waveform of noise is irregular in shape

• It lacks periodicity

• It undergoes sudden changes in amplitude and frequency as shown in fig. 3.2

 

Characteristics of musical sound

The different musical sounds are distinguished from each other by the following three characteristics.

 (i) Pitch or frequency

(ii) Quality or timbre

(iii) Intensity or loudness

 

(i) Pitch (or) frequency

Pitch is the characteristic of a sound which distinguishes between a shrill sound and a grave sound. The pitch of a musical note is the sensation conveyed to our brain by the sound waves falling on our ears.

Pitch depends directly on the frequency of the sound waves.

Example

• The voice of women and children has high pitch because the frequency of sound is high.

• The voice of an old man has low pitch because the frequency of sound is low.

• In guitars, thicker wires give a lower frequency and thinner wires give a length frequency.

 

(ii) Quality (or) Timbre

Quality or timbre of the sound wave is a characteristic which enables us to distinguish between musical notes emitted by different instruments or voices even though they have the same pitch and loudness.

 

 (iii) Intensity of sound (I)

Intensity of a sound wave (I) at a point is defined as the amount of sound energy (Q) flowing per second per unit area held normally at the point to the direction of the propagation of sound wave.

Intensity of sound wave I = Q / tA

where Q Amount of sound energy flowing

t- Time of flow

A - Area normal to the propagation of sound.

Intensity is a measurable physical quantity. It is expressed in joule second^-1 metre ^-2or watt metre (Wm^-2).

 

Loudness

Loudness of the sound is defined as the degree of sensation produced on the ear. It varies from one listener to another. Loudness is different from intensity of sound.

Loudness is a physiological quantity. It is difficult to measure because it depends upon the individual listener. However, it is measured as the logarithmic value of intensity.

 

Weber - Fechner Law

(Relation between loudness and intensity of sound)

According to Weber - Fechner law, the loudness of sound varies with intensity of sound.

 

Statement

The law states that the loudness (L) produced is directly proportional to logarithm of intensity.

The quantity  is called sensitiveness of the ear. It decreases with an increase in the intensity of sound.

 

Sound Intensity Level

Absolute intensity of a sound wave is measured in Wm2. For all our practical purposes, we are interested in the measurement of relative intensity rather than absolute intensity.

Intensity of a sound is measured with reference to some standard intensity. The standard intensity I = 10 Wm 1S chosen for this purpose.

This is the lowest intensity of sound (at 1000 hertz) to which a normal human ear can respond. This standard intensity is known as threshold of audibility.

 

Decibel

Definition

The logarithmic ratio of intensity of a sound to standard intensity is known as sound intensity level or relative intensity of sound.

The, sound intensity level of a sound wave having intensity I is given by

Here, standard intensity of sound is the minimum audible sound intensity heard by our ears.

The unit for sound intensity level is bel named in honour of Alexander Graham Bell, (inventor of telephone).

Sound intensity level, ẞ =  log 1₀ [I/I_O] ……………….....(2)

Consider sound intensity I = 101。

Sound intensity level in bel, ẞ = log ₁₀[10 I _O/I_O]……………..(3)

From equation (3), we get

If I = 100 I_O, then ẞ = 2 bel, and

If I = 1000 I_O, then ẞ = 3 bel.

Bel is the sound intensity level of a sound whose intensity is ten times the standard intensity.

In practice, bel is a larger unit. Hence, another unit known as decibel (dB) is more often used.

1 bel  = 10 decibel

Hence, sound intensity level is measured in decibel

ẞ (in decibel) = 10 log₁₀ [I/I_O] dB

 

Physical significance of a decibel

Let us find the physical significance of a decibel, i.e., what percentage change in intensity represents one decibel.

If sound intensity level increases by '1' dB, (substituting intensity level in dB = 1)

Thus, an increase of sound intensity level by 1 dB would increase the intensity by 26%.

Normally, our ears can hear the sound intensity level between the range 0 dB to 120 dB. The maximum sound intensity level which our ears can tolerate without sensation of pain is 120 dB. This upper limit of hearing is called threshold of feeling or pain threshold.