 Namaste. I, Dr. Mrs. Preeti Sunil Joshi, Assistant Professor from Department of Humanities and Sciences, Balchan Institute of Technology, welcome you in this session. The learning outcomes of this session are, by the end of this session, students will be able to state the basics of sound and the terms that are related with architectural acoustics and also can calculate the reverberation time using Sabine's formula. The contents include, first of all let us see what is sound. Sound is a vibration that typically propagates as an audible wave of pressure through a transmission medium such as a gas, liquid or solid. Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gases, liquid and solids. The study of sound plays a very important role in various branches of engineering and has developed to such a level that it has become an independent branch of engineering that is known as acoustic engineering or also called as sound engineering. The area of study of design of musical instruments is known as musical acoustics. The use of sound in medical diagnosis and therapy is known as bio-icostics. The technology of sound production and recording is known as electro-icostics and the design of buildings, auditoriums, musical halls and lecture halls, recording rooms, etc. as architectural acoustics. Architectural acoustics deals in general with the behavior of sound waves in closed spaces and their design to give best sound effects. Though this video is related with architectural acoustics, let us first recall few basics of sound. So, how do we hear sound? A vibrating body excites mechanical waves in the surrounding medium. These mechanical waves propagate as a series of comparations and rarifactions of air molecules. On reaching the ear, they cause the ear drum to vibrate which leads to the sensation of hearing. So, sound is a vibration in an elastic medium with definite frequency and intensity. Sound waves are longitudinal waves. Sound waves are classified based on their frequency into four groups i.e. sonic which is also known as audible, infrasonic, ultrasonic and hypersonic. Audible waves that produce a sense of sound on a human ear and lie in the range of 16 Hz to 20 kHz. Infrasonic waves have frequencies below 16 Hz, ultrasonic waves have frequencies above 20 kHz and hypersonic waves have frequencies of 10-10 Hz and higher and correspond to thermal waves in liquids or solids. Audible sound waves can be further classified according to their frequency spectrum as musical sound and noise. Musical sounds produce a pleasing sensation on the ear. Listen this sound and noise causes irritation and strain to our ears. Musical sound has the properties as such sound has a line spectrum containing multiple frequencies. Musical sounds are periodic vibrations and sudden changes in amplitude do not occur. While noise has the characteristics as it is a jumble of irregularly timed non-periodic vibrations. It consists of a complex spectrum of frequencies and it undergo erratic changes in amplitude and frequency. When sound encounters an obstacle it undergoes reflection as well as diffraction. Now, when the reflection takes place and when diffraction takes place. Sound waves are reflected as the light waves and obey the similar laws. Flat surfaces reflect sound waves in such a way that the angle of incidence equals the angle of reflection. Sound waves are reflected when the dimensions of the obstacle are large in comparison to the wavelength. Sound waves have the ability to go around obstacles in their path penetrating into the area behind them. This figure shows the diffraction of sound passing through the slit. Diffraction occurs whenever the wavelength of the wave is larger than the obstacle or opening. The amount of diffraction increases with increasing wavelength. While studying the acoustics of a hall both reflection and diffraction must be studied simultaneously. Generally in enclosed spaces such as auditoriums the walls and ceiling are quite large which leads to the reflection and diffraction occurs in enclosed spaces because of uneven surfaces, windows, doors etc. The diffraction tends to diffuse the sound uniformly in the enclosed spaces. Therefore sound reflection alone becomes most important in the study of acoustics of halls and buildings. The reflection of sound in an enclosed space leads to two important defects. Namely echo and reverberation. A reflection is called an echo if the time between the original sound and its reflection is long enough that both sound can be heard distinctly. And if a room has lots of echoes and they are very closely spaced in time so that they are not distinct then this large number of echoes is known as reverberation. You can see in this image that with change in distance and time how the generation of echo and reverberation occurs. Now let us see echo in detail. An echo is produced when the sound reflected from an obstacle reaches the ear after the sound from the source has already been heard. Thus there is a repetition of sound in this case. Now please listen this sound. When sound is reflected from a number of reflecting surfaces multiple echoes are heard. Thus the heavy rolling of sound of a thunder is a result of successive reflections of sound from clouds, mountains, rocks etc. Now listen this sound. Now let us see reverberation in detail. Sound produced in an enclosure does not die out immediately after the sound has ceased to produce it. A sound produced in a hall undergoes multiple reflections from the walls, floor and ceiling before it becomes inaudible. A person in the hall continues to receive successive reflections of progressively diminishing intensity. This prolongation of sound before it decays to a negligible intensity is called as reverberation. Reverberation is a familiar phenomena that is experienced in halls without furniture or in caves we hear multiple reflections and persistence of sound. Listen this sound. So we can define reverberation as the persistence of sound due to multiple reflections in a hall with decreasing intensity even though source stops emitting sound. Now how much time the sound lasts is given by reverberation time. The time taken by the sound to fall from its average intensity to inaudibility level is called as reverberation time of the hall. When sound is incident on the surface of any medium it splits into three parts. One part is reflected from the surface, another part gets absorbed in the medium while the remaining part is transmitted through the medium and emerges on the other side. This property of a surface by which the sound energy is converted into other form of energy is known as absorption. In the process of absorption sound energy is converted into heat due to frictional resistance inside the pores of the material. The fibrous and porous materials absorbs sound energy more than the other solid materials. The effectiveness of a surface in absorbing sound energy is expressed with the help of absorption coefficient. It is defined as the ratio of sound energy absorbed by its surface to that of total sound energy incident on the surface. In order to compare the relative efficiency of different absorbing surfaces, it is essential to select a standard in terms of which all the surfaces can be described. A unit area of open window is selected as the standard as the entire sound incident on an open window is fully transmitted and none is reflected. Thus the unit of absorption is open window unit which is named as Sabine after the scientist who established the unit. This table shows some of the materials and their absorption coefficients. Students now pause the video and try to answer these questions. Check for the correct answers. Architectural acoustics deals with the behavior of sound waves in closed spaces. The acoustic properties of buildings were not studied on a scientific basis till about 1900. The Fogg Art Museum Hall in Harvard University, USA turned out to be highly defective when it was built. The lectures given in it were not intelligible to audience. Professor W. C. Sabine, professor of physics in Harvard University, was interested with the responsibility of eliminating the acoustical defects of the hall. Sabine undertook a systematic study of problems and evolved conditions for a satisfactory acoustic quality of a hall. He found that quite often reverberation was the main cause for the defective quality of a hall. Addition to absorbent materials at appropriate surfaces enhances the quality of sound in the halls. Other precautions are to be taken about the shape of the walls, ceiling and the hall in total so that the acoustic defects do not arise. Thus Professor Sabine led the foundations of acoustic engineering. Professor Sabine determined the reverberation times of empty halls and furnished halls of different sizes and arrived at the conclusions as the reverberation time depends on the reflecting properties of the walls, floor and ceiling of the hall. It depends directly upon the physical volume of the hall and it is also dependent on the absorption coefficients of various surfaces present in the hall. And also the reverberation time depends on the frequency of the sound waves. Professor Sabine summarized his results in the form of equation as reverberation time is directly proportional to volume divided by absorption coefficient of the hall. Therefore T is directly proportional to V by A. So the formula for reverberation time is given by T is equal to 0.165 V by A where 0.165 is the value of K that is the proportionality constant. And capital A is given by summation AS which is given by A1S1 plus A2S2 plus A3S3 and so on where A1 and A2 represents the absorption coefficients of the surfaces and S1S2S3 are the surface areas. The Sabine equation works well for large enclosures. Students now please pause the video and calculate the reverberation time. Check for the correct answers. Thank you.