Solved Example Problems on Acoustics (intensity of sound, Decibel)

ANNA UNIVERSITY SOLVED PROBLEMS

Problem 3.1

The intensity of sound produced by thunder is 0.1 Wm2. Calculate the intensity level in decibel.

Given data

Intensity of sound produced by thunder,

I = 0.1 Wm^{- 2}.

Standard intensity I_O = 10^-12 Wm^-2

Solution

Intensity level of sound (B) = 10 log₁₀ [I/I_O]

Substituting the given values, we have

Hence, sound intensity level produced by thunder = 110 dB

Problem 3.2

The intensity of sound in a street during heavy traffic is 10^-4 Wm^-2. Calculate intensity level in decibel.

Given data

Intensity of sound in the street, I = 10^-4 Wm^-2

Standard intensity of sound I_O = 10^-12 Wm^-2

Solution

Sound intensity level (ẞ) = 10 log₁₀ [I/I_O]

Substituting the given values, we have

TABLE 3.1

Acoustics of buildings

The subject of physics which deals with the design and construction of rooms or halls so as to give the best sound effects is called acoustics of buildings or architectural acoustics.

The acoustical properties of a room or halbi have considerable effect on the clarity and intelligibility of speech or music produced in the hall.

The acoustic properties quofis buildings were studied systematically by Prof. Wallace C. Sabine, Professor of Physics in Harvard University, U.S.A in the year 1900. ads audT

Now, the science of acoustics of buildings has a prominent role in the design of modern buildings.

Sound Absorption Coefficient

When a sound wave strikes a surface, a part of its energy is absorbed, a part of it is transmitted and the remaining part is reflected.

The property of the surface to convert sound energy into other forms of energy is known as absorption.

The effectiveness of absorption of sound energy by the surface is expressed as absorption coefficient.

Absorption coefficient (a) is defined as sound energy absorbed by its surface to that of total sound energy incident on the surface.

α = Sound energy absorbed by the surfaces./ Total sound energy incident on the surface

Practical definition of absorption coefficient

In order to compare the relative sound absorption of different materials the open window is taken as standard reference since it is a perfect sound absorber.

It is because the whole of the sound energy passes through the open window and none is reflected.

Absorption coefficient of a surface is the ratio of sound energy absorbed by 1m² of the surface to that absorbed by 1m² of an open window.

α = Sound energy absorbed by 1m² of the surface / Sound energy absorbed by 1m² of open window

Clearly, the value of absorption coefficient varies between 0 and 1. (1 is being the maximum value of absorption coefficient).

Thus, absorption coefficient is measured in Open Window Unit and is written as O.W.U. or sabine/m m².

For example, absorption coefficient of plastered wall is 0.03 O.W.U. It means that sound energy absorbed by 1 m² of plastered wall is 0.03 times that absorbed by 1 m²of an open window.

TABLE 3.2

Absorption coefficients of some common sound You absorbing materials