Question

2. 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. (This is a research based critical thinking question and marks will depend upon your creative and innovative

presentation of notes)   

Answer 1

1. INTRODUCTION:-

'Acoustics' is the science of sound, which deals with origin, propogation and auditory sensation of sound, and also with design and construction of different building units to set optimum conditions for producing and listening speech, music, etc. The knowledge of this science is necessary for the proper functional design of theatres, cinema halls, auditoriums, conference halls, hospitals, etc. so that unwanted sound is excluded or insulated.

Sound is generated in the air when a surface is vibrated. The vibrating surface sets up waves of compression and rarefaction in the air and these set the ear drum vibrating. The movements of the ear drum are translated by the brain into sound sensation. When the sound waves are periodic, regular and long continued, they produce a pleasing effect such a sound is known as musical sound. On the contrary, when the sound wave is non-periodic, irregular and of very short duration, it produces displeasing effect such sound is known as noise. A noise is an abrupt sound of complex character with an irregular period and amplitude originating from a source of non-periodic motion.

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.

Characteristics of Audible Sound:-

There are three characteristics of sound (a) Intensity and loudness, (b) Frequency and pitch, and (c) Quality or timbre

l. Intensity and loudness of sound mad

Intensity of sound is defined as the amount or flow of wave energy crossing per unit time through a unit area taken perpendicular to the direction of propagation. mathematically the energy of a wave and hence the intensity at a point is proportional to the square of the amplitude of vibration of the point,

2. Frequency and pitch of sound

Frequency or Pitch is defined as the number of cycles which a sounding body makes in each unit of time. It is a measure of the quality of a sound. It is that characteristic by which a shrill sound can be distinguished from a grave one, even though the two sounds may be of the same intensity. The sensation of pitch depends upon the frequency with which the vibrations succeed one another at the ear the greater the frequency, the higher the pitch and the lesser the frequency the lower the pitch. The frequency scale covers a wide range varying from 20 cycles per second to 1500 cycles per second.

3. Quality or Timber

The quality of a sound is that characteristic enables us to distinguish between two notes of the same pitch and loudness played on two different instruments or produced by two different voices. A study of vibration curves of various musical instruments has shown that the notes emitted by them are seldom pure. They some fundamental tones of frequency n and additional tones (of frequencies 2n, 4n, etc. ) called overtones. The quality of a note is determined by its complex structure and depends upon the presence or absence of a certain number of overtones, on their relative strengths and pitches. It is to be noted that it is the memory of this tonal quality which enables us to recognise a large number of different sounds. Among these are the voices of friends and acquaintances, the various sounds employed in speech and familiar musical instruments and the cries of animals.

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.

Defects due to reflected sound.

The behaviour of reflected sound plays very important role in the acoustical design of an enclosed space. The following are two main defects that may be caused due to reflection of sound waves

(a) Echoes, and (b) Reverberation.

A) Echoes:- An echo is produced when the reflected sound wave reaches the ear just when the original sound from the same source has been already heard. Thus, there is repetition of the sound. The sensation of sound persists for th of a second after the source 10 has ceased. Hence in order that an echo may be distinguished as separate, it must reach the ear th of a second after the direct sound. Taking the velocity of sound 10 as 340 m/sec, it means that sound must come after traversing a distance of 34 m, i.e., the minimum distance of the obstacle from the source must be half of this, i.e., 17 m. If, however, the distance of the reflecting surface is less than this, the sound will appear to be drawn out.

B)Reverberation:- It has been generally noticed that in public halls and auditoriums, the sound persists even after the source of sound has ceased. This persistence of sound is called reverberation. It is due to multiple reflections in an enclosed space. Reverberation is familiar phenomenon in Cathedrals and new halls/rooms without furniture, where, ever after sound source stops the reverberation is heard even upto 10 seconds. A certain amount of reverberation is desirable, specially for giving richness to music, but too much reverberation is undesirable.

Common acoustical defects:-

Perfect acoustical conditions in a big room, hall or auditorium etc. are achieved when there is clarity of sound in every part of the occupied space. For this, the sound should rise to suitable intensity everywhere with no echoes or near echoes or distortion of the original sound with correct reverberation time. Following are the common defects

(1) Reverberation

2) Sound foci

(3) Insufficient loudness

4)Formation of echoes

(5) Dead spots

6)Exterior noise.

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.