Question

Given a chance to give a lecture on ‘Acoustics’ in class, what are the ten different important terms / topics you would like to teach in Acoustics? Make a report of your own lecture-notes with those terms giving clear explanations and including pictures /sketches / diagrams /tables.

Answer 1

Acoustis:-

Acoustics is defined as the scientific study of sound which includes the effect of reflection, refraction absorption diffraction and interference. It also deals with the properties of the sound waves, their origin, propagation and their action on obstacles.

sound:- Sound is an alteration of pressure that propagates through an elastic medium such as air which produces and auditory.

Why Do We need Acoustics?

Acoustics are fundamentally important to learning environments. Learning is intrinsically linked with communication, and aural (sound) communication is anoustics. Similarly, learning is about concentration, and external noise is a major distracting factor in education.

The importance of acoustics is not limited to classrooms. Noise in corridors and public spaces can soar if they are too reverberant (too much echo), with voices raised louder and louder to overcome the background echo, just like shouting conversations at a noisy cocktail party or restaurant. So to come over this problems of sounds we need acoustics.

Terminologies related to Acoustics:

Airborne sound - Sound or noise radiated directly from a source, such as a loudspeaker or machine, into the surrounding air.

Ambient noise - Total noise level in a specified environment.

Audible frequency range - The range of the sound frequencies normally heard by the human ear. The audible range spans from 20 Hz to 20,000 Hz, but for most engineering investigations only frequencies between about 40 Hz and 11,000 Hz are considered.

Decibel (dB) - (1) Degree of loudness. (2) A unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average pain level.

Diffraction - The process whereby an acoustic wave is disturbed and its energy redistributed in space as a result of an obstacle in its path.

Direct sound - Sound that reaches a given location by direct, straight-line propagation from the sound source.

Frequency - Repetition rate of a cycle, the number of cycles per second.

Noise - Unwanted sound.

Noise Reduction (NR) - The difference in sound pressure level between any two points along the path of sound propagation.

behaviour of sound in enclosures

When sound is generated in a room, the distance between the source and the walls is so small that there is little or no reduction due to distance. When the sound waves strike the surfaces of a room, three things happen :

(i) Some of the sound is reflected back in the room.

(ii) Some of the sound energy is absorbed by the surfaces and listeners.

(iii) Some of the sound waves set on the walls, floors and ceiling vibrating and are thus transmitted outside the room.

Factors affecting architectural Acoustics:

A)Reverberation time:

When the reverberation time is too high, the sound produced by the speaker will persist for a long period of time.

Similarly , when the reverberation time is low, sound dies quickly and becomes inaudible in a short amount of time. In order to improve the sound, reverberation time of a hall should be increased to an optimum value.

B) Loudness:

Reverberation time of a hall is directly proportional to loudness.

Low loudness results in existence of sound for a shorter period while high loudness results in existence of

C)sound for a longer period.

Therefore sound produced by the speaker should be within audible range.

D)Structure Borne sound:

Sound waves generated inside a hall are known as structure-borne sound. They are produced due to apparent motion of benches & footsteps & propagated through walls and floors.

Acceptable indoor noise levels:-

behaviour of sound in enclosures

When sound is generated in a room, the distance between the source and the walls is so small that there is little or no reduction due to distance. When the sound waves strike the surfaces of a room, three things happen :

(i) Some of the sound is reflected back in the room.

(ii) Some of the sound energy is absorbed by the surfaces and listeners.

(iii) Some of the sound waves set on the walls, floors and ceiling vibrating and are thus transmitted outside the room.

General planning principles and factors which are important for good acoustical conditions in a hall :

1. Site selection and planning.

2. Shape.

3. Treatment of interior surfaces.

4.Dimensions

5. Seats and seating arrangement.

6.Reverberation and sound absorption.

Factors affecting architectural acoustics:

Reverberation time

Loudness

Echelon effect

Structure Borne sound

Echo

Focusing due to Walls and Ceilings

Direct sound

Sound Absorbers:

These sound absorbing acoustical panels and soundproofing materials are used to eliminate sound reflections to improve speech intelligibility, reduce standing waves and prevent comb filtering. A wide variety of materials can be applied to walls and ceilings depending on your application and environment. These materials vary in thickness and in shape to achieve different absorption ratings depending on the specific sound requirements

TYPES :

Acoustical foam panels

White paintable acoustical wall panels

Fabric wrapped panels

Acoustical wall coverings

Ceiling tiles Baffles and banners for ceiling Fiber glass blankets and roll