Lecture 2 Acoustics (PDF)

Basics of sound and Behaviour of sound Lecture-01 Topic - Acoustics  Wave (transfer of energy from one point to another) Non mechanical Mechanical wave (electromagne...

Basics of sound and Behaviour of sound Lecture-01 Topic - Acoustics  Wave (transfer of energy from one point to another) Non mechanical Mechanical wave (electromagnetic) wave Required medium to travel Does not required any Example : ? medium Example : ? Sound: The sound is a mechanical wave generated by disturbance or turbulence which passes through a physical medium in the form of waves from a source to a receiver causing a sensation of hearing. Acoustics: The branch of physics concerned with the properties of sound. It is the science of sound, including its production, transmission, and effects, including biological and psychological effects. The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations. Compression: The crowding of molecules in a given space Rarefaction: The thinning out of molecules in a given space Compression and rarefaction make-up a sound wave  Objective of acoustics Noise Sound Privacy reduction intelligibility Behaviour of sound: Speed of sound: Depends upon the medium which it travels. Travels four times faster in water, and fifteen times faster in iron or steel than in air As a general rule, the denser the material, the faster it can conduct sound. vs > vl >vg On encountering barriers posed by the enclosure, sound waves are likely to behave in the following ways: Reflection Absorption Refraction Diffusion Diffraction Transmission Reflection Reflection is the occurrence of sound striking one or more surfaces before reaching its receiver In the case of an enclosed space, the sound waves hit every side of the enclosure continuously until the sound energy reduces to zero. The amount of waves reflected depends on the smoothness, size, and softness of the materials of enclosure. The angle of incidence of sound rays is equal to that of the reflected rays only if the surface of the reflector is flat. But when it is curved, the angles are different. Reflection of sound waves. Sound waves reflected at a convex surface are magnified WAVE FRONT CONVEX and are considered bigger. REFLECTOR They are attenuated and therefore weaker. SOUND SOURCE So convex surface may be used with advantage to spread the sound waves throughout the room. Reflection of sound waves. CONCAVE REFLECTOR The sound waves reflected at a concave surface are considered smaller. The waves are most condensed WAVE FRONT and therefore amplified. The concave surface may be provided for concentration of SOUND reflected waves at certain SOURCE points. Absorption When sound waves hit the surface of an obstacle, some of its energy is reflected while some are lost through its transfer to the molecules of the barrier. The lost sound energy is said to have been absorbed by the barrier. The thickness and nature of the material as regards its softness and hardness influences the amount of sound energy absorbed. Refraction This is the bending of sound when it travels from one medium into another medium. The difference in the composition of the two different media bends the sound i.e. the angle of incidence changes into an angle of refraction as it travels into the new medium. Diffraction Diffraction describes how waves bend, or change direction, as they travel around the edges of obstacles. Diffraction occurs in water waves, sound waves, and light waves, but the amount of diffraction depends on the size of the obstacle or opening in relation to the wavelength of the wave. Transmission In this phenomenon, sound wave is carried by molecules of the obstacle through vibration and re-emitted at the other side irrespective of the medium. It can be structure borne, air borne or impact sound. Figure: Phenomena occurring when a sound wave strikes a rigid wall of some material having a velocity of transmission greater than air. The sound waves are shown as "rays" which represent the directions of travel of the wave fronts.  Interference Whenever waves interact, interference occurs. the phenomenon is best understood by thinking in terms of the compressions and rarefactions of the two waves as they arrive at some point. When the waves are in phase so that their compressions and rarefactions coincide, they reinforce each other (constructive interference). When they are out of phase, so that the compressions of one coincide with the rarefactions of the other, they tend to weaken or even cancel each other Constructive Destructive (destructive interference). The interaction between the two waves produces a resultant wave. Such interference can be reduced by use of sound-absorbing materials on reflecting surfaces. Characteristics of Sound Pitch of a Sound Wave  Pitch is a term used to describe how high or low a note is.  The pitch of a note depends on the frequency of the source of the sound.  A high frequency produces a high pitched note and a low frequency produces a low pitched note. Loudness  Loudness depends on the amplitude of the sound wave.  The larger the amplitude the more energy the sound wave contains therefore the louder the sound.