Ch-3 Acoustics_KB

L. J. Institute of Engineering and Technology Subject: Physics (Group-1) Subject code: 3110011 F.Y. - B.E. Kamaldeep Bhatia

Architectural Acoustics Classification of sound waves: 1) Infrasound (f < 20Hz) 2) Audible Sound (20Hz < f < 20kHz) 3) Ultrasound (f > 20kHz) CLASSIFICATION OF AUDIBLE SOUND (Difference between Musical sound and Noise) Musical Sound Noise 01 Sound which produces pleasing Sound which produces jarring effect on ear. effect on ear. 02 Musical sound waveforms are:- Noise waveforms are:- -Regular in shape -irregular in shape -Have definite periodicity -don’t have definite periodicity -Do not undergo sudden change in -undergo sudden change in amplitude. amplitude 03 Examples: Guitar, Violin, Birds Examples: Sound of airplane ,road chirping etc traffic ,crackers etc. CHARACTERISTICS OF MUSICAL SOUND Pitch: related to frequency of sound Loudness: related to intensity of sound Timbre: related to quality of sound Pitch: It is a sensation which helps in distinguishing between a note of high frequency and low frequency of same intensity produced by some musical instrument. The sound produces by ladies and children is of high pitch due to high frequency. Greater the frequency of sound, higher is the pitch and vice-versa. Loudness: It is a degree of sensation produced on ear. It varies from one listener to another. It depends upon intensity and sensitiveness on ear. It is a physiological quantity. 2

������ ∝ log10 ������ ������ = ������ log10 ������ Loudness is directly proportional to the logarithmic function of intensity. This is known as Weber-Fechner law. From above equation, ������������ ������ ������������ = ������ Where, ������������ = sensitiveness of ear ������������ Sensitiveness decreases with increase in intensity. Timbre: it is the quality of sound which enables us to distinguish between two sounds having some loudness and pitch. It depends on the presence of overtones. INTENSITY AND INTENSITY LEVEL The amount of sound energy flowing per unit area in unit time when the surface is held normal to the direction of propagation of wave is called Intensity. ������ = ������ . ������������ Also, ������ = 2������2������2������2������������ It is a physical quantity which depends upon frequency, amplitude, velocity and density of medium. Unit - ������/������2 The logarithmic ratio of intensity I of a sound to the standard intensity (I0) defines Intensity level (������������) or relative intensity. ������ ������������ = ������ log10 (������0) Let I and ������0 be intensities of two sounds and ������1 and ������0 be corresponding loudness then, ������1 = ������ log10 ������ and ������0 = log10 ������0 Intensity level, ������������ = ������1 − ������0 3

= ������ log10 ������ − ������ log10 ������0 ������������ = ������ log10 ( ������ ) bel ������0 If k=1, ������������ = log10 ( ������ ) dB (∵ 1bel=10dB) ������0 Standard or Reference or Zero Intensity: The minimum intensity which a human ear can sense is called threshold intensity. Its value is 10−12������/������2 (0������������). Threshold of feeling: The sound of intensity of level 120dB produces a feeling of pain in the ear. DISTINGUISH BETWEEN LOUDNESS AND INTENSITY Sr Loudness Intensity 01 It is a degree of sensation produced It is the amount of sound energy per on ear. unit area in unit time 02 It varies from listener to listener. It is independent of listener 03 It is a physiological quantity. It is a physical quantity. 04 It unit is sone. Its unit is ������/������2 TYPES OF SOUND ABSORBING MATERIAL 1) Porous absorbents: Whenever sound waves strike the surface of a porous material, a part of waves get reflected while the other part enter the porous material. The part of reflected waves energy is reduced while, part of waves entered into the material is converted into heat. Ex:- rock, wood, wood wool, glass, silk etc. 2) Cavity resonators: A cavity resonator is a chamber having small opening for sound waves to enter when sound waves enter the resonator, due to multiple reflections inside, sound waves are absorbed. Ex: air conditioner plant, machine etc. 3) Resonant absorbents or Panel absorbents: The absorbent material is fixed on a framing with an air space between the framing and the wall. When sound waves strike the panel, due to flexural vibration of panel, certain amount of sound energy is absorbed by panel which is converted into heat energy. Ex: windows, doors, gypsum boards etc. 4

4) Composite Absorbers: A composite absorber is a single unit which does the function of other three absorbers. It consists of a perforated panel fixed over an air space containing porous absorbents. When sound waves strike the panel, they pass through it and damped by resonance of air in cavity. ABSORPTION COEFFICIENT AND MEASUREMENT OF ABSORPTION COEFFICIENT Sound energy absorbed by surface Absorption coefficient = Total sound energy incident on it Unit is Sabine or O.W.U (Open window Unit). Let ������1 be the reverberation time without placing any sound absorbing material in a hall, ������1 = 0.167������ …(1) ∑ ������������ If the sound absorbing material of absorption coefficient a, and surface area ������1 is placed in the hall then, ������2 = 0.167 …(2) ∑ ������������+������1������1 From eq (1) and (2), 1 − 1 = ∑ ������������ + ������1������1 − ∑ ������������ ������2 ������1 0.167������ 0.167������ 1 − 1 = ������1������1 ������2 ������1 0.167������ 0.167������ 1 1 ������1 = ������1 [������2 − ������1] FACTORS AFFECTING ACOUSTICS WITH REMEDIES 1) Reverberation Time: The time taken by the sound to fall below minimum audibility level is called reverberation time. Persistence or prolongation of sound after source cuts off defines reverberation. If reverberation time is too long the loudness becomes inadequate if it is too long, it will lead to confusion. 5

Remedies:  By providing windows and openings  By covering floor with carpets. 2) Loudness: Due to excess sound absorbing material loudness may get reduced. Uniform distribution is important in a hall. Remedies:  By constructing low ceilings  By using loudspeakers 3) Focusing and Interference: The pressure of concave surface in room may make sound to be concentrated at focus region. In addition to focussing, there should not be interference effects due to reflected waves. As constructive interference produces sound of maximum intensity and destructive interference produces sound of minimum intensity at some places. Remedies:  Curved surfaces should be avoided.  If curved surfaces are present, they should be covered with suitable sound absorbing materials. 4) Echo: If the reflected sound comes back in less than 1 ������ℎ of a sound, it helps in 15 increasing the loudness otherwise it will create confusion. Echo is heard due to reflection from a distant object. Remedy:  By covering long-distance walls and high ceilings with sound absorbing material. 5) Echelon effect: It refers to the generation of separation due to multiple echoes. Remedies:  Cover reflecting surfaces with sound absorbing material.  Cover staircase with carpet 6

6) Resonance: If the window panel and sections are not fitted properly, they may start vibrating, thereby creating extra sound, due to matching of frequency. Remedy:  Windows and doors should be fitted properly. 7) Noise: a) Air borne Noise Extraneous noises which are coming from outside through open windows, doors and ventilators. Remedy:  Room can be made air conditioned. b) Structure borne Noise: The noise which is conveyed through the structure is called structure- borne. Remedy:  By using double walls with air space  By using anti-vibration mounts this type of noise can be reduced. c) Inside Noise The noises which are produced inside the hall is called inside noise. Ex. A.C, fans, typewriters etc. Remedy:  Such machineries should be placed with sound absorbing materials. 7