 Hello friends. Myself, Elia, Archanjani, assistant professor from Department of Electronics, Valchen Institute of Technology, Sudapur. Today we are going to see the third topic that is growth and decay of sound from the subject, analog, from the subject audio and video engineering. So what are the learning outcomes from this topic? At the end of this topic, student is able to define acoustic and reverberations. He can calculate absorption coefficient, reverberation time for efficient acoustic effect and he can also design acoustic of auditorium and meeting hall. So what is growth of a sound? How it can be explained? So before starting growth of sound, we need to know what is the basic concept of sound intensity and reverberation. So sound intensity, basically it is related to the growth of the sound and it can be defined as or it can be stated as any piece of machinery that vibrates, it will radiate acoustical energy. The sound power is nothing but it is the rate at which energy is radiated and it is calculated in terms of energy per unit time. Sound intensity basically it can be described as the rate of the rate of energy flow through a unit area and in SI unit or in SI system of units, the unit which is the unit area which is to be considered is of 1 meter square. Thus the unit of sound intensity can be given as watts per meter square. Similarly, the sound intensity also gives us a measure of direction for the energy flow in some direction. So basically by studying sound intensity what we can achieve is we can measure the direction of the energy flow. So we can measure the direction of energy flow in some directions of the atmosphere. Similarly, what is decaying of sound? The growth of sound intensity basically it is given by the equation as 1 is equal to 1, i is equal to i m 1 minus e raise to minus a c t divided by 4 v, where i is intensity of any i is intensity at any instant, i m is maximum intensity and t is the time period of that particular sound intensity, a is nothing but the absorption, v is volume of the room and c is speed of the light. So the decaying of sound basically depends upon the reverberation time period too. As it is called as r t 60, where r t means reverberation time. Basically it was introduced by Valis Sabin, one of the greatest scientist who identified the, who derived the equation Sabin's equation and he defined the reverberation time as measure of acoustic characteristics of a enclosed spaces. Similarly, r t 60 that is reverberation time, what it does? It provides a rough thumb rule figure of the acoustic characteristics of the enclosed space. The decaying characteristics of sound basically depend upon the signature of enclosed spaces. That means here what we can say is that the decaying of sound basically it depends upon the structure of the channel and the type of channel where it, how it is enclosed. So here you can see that there are two basic diagrams, here as you can see there is a person sitting and s is nothing but the source from where the sound gets radiated. So as you can see here the sound when it is radiated or when it is turned on then it travels to this particular person who is sitting on the chair with multiple reflections and also it is traveling to that particular person in a direct way. So here what we can state is that the sound always travels with multiple reflections and in a direct way, directory of the propagation. Hence the direct sound arrives first at t is equal to zero. That means the time taken by the sound to reach to the person from the source directly here the time period considered is zero whereas the reflected components arrive later. That means after the direct sound the reflected components the multiple reflected components which are r1, r2, r3 and r4 they arrive later with a particular time period. The second figure basically it denotes the sound pressure at h how the sound pressure builds up. So as you can see here on y axis we are having sound pressure on x axis we are having time. So as the as the sound pressure increases as the sound intensity goes on increasing so as the pressure goes on increasing. So as you can see here d is the sound intensity and hence the y axis is denoted by the parameter d and as it goes on increasing so the reflected components. So as you can see here d is the direct ray this direct ray when added with the first reflected ray it will give you the ray which is reaching to the person h. So this is nothing but the first ray and its r1 and its sound pressure. Similarly the second ray r2 it is given by d plus r1 plus r2. So here we can find that as the sound intensity goes on increasing as the sound intensity goes on increasing so as the reflection goes on increasing and then by default the sound pressure also gets increased. So here as you can see here if we consider this particular diagram then in this you can see that if d is the direct ray reaching towards the person we have seen d is the direct ray of the sound which is reaching towards the person with t is equal to 0 at that particular point at this particular point the sound pressure is maximum. And as we move further r1, r2, r3 and r4 are the multiple reflections by which the sound reaches towards the person who is stationary at h point. So as you can see here there is the decreasing of sound intensity which you can see at this particular point. So r1, r2 and r3 and r4 basically these are the multiple reflected rays to these rays the sound pressure and the sound intensity goes on decreasing. Hence this explains the decaying of the sound. Therefore what we can say is the decaying of the sound basically depends upon the multiple reflections of the sound. Hence the sound decays exponentially after the source seizes that means after the source seizes means if the source is active for some time and if it gets deactivated after a certain time period then the sound decaying occurs and it is always exponentially. So here you can see the response of growth and decay sound. So this diagram or this response of the sound level and this is the response which is between sound level and the time period. So as you can see here this is the sound level of the source. This is the standard level of the source. So at point A the sound reaches up to a particular level then it grows exponentially. So this growth as you can see here it is in an exponential manner. Then at point C or from point C it remains steady or it remains constant. That means the sound remains constant and at D point it decreases drastically which is nothing but the decaying of the sound. So this response will give you a basic and a good idea about growth and decaying of sound. So as you can see the time periods when we consider the time periods for growth which is from A to C, A, B and B, C these are having the constant time period whereas if you compare the constant state of the sound from C to D it is having maximum time period and when you compare the decaying time period then the decaying time period is dropping down very drastically. Therefore the time period at this particular point is very much less. So this is nothing but the response of growth and decaying of sound. So the response for growth and decay of sound can be explained by splitting the graph as we have seen here. The growth as we have seen it starts so the on time is at this particular point and here it is the off time. So A and B basically explains the growth and decaying of the sound. So B is nothing but the decaying of the sound. This is the constant level of the sound and here there is the drastic drop of the decay. The length of the decay basically depend upon the source and the noise level. So A is the response which replicates the above relation. That means the decaying of the sound basically depends upon the source as well as the noise which gets interfered in the channel. So as here you can see the sound pressure level is in terms of decibel. Here is the time period. In B also we are considering the same time period whereas if you can see at this particular point there is the increase in the sound pressure level and then drastically there is a decrease. But the time period here which we are considering is of 60 minutes or 60 seconds. Hence it is called as RT60 and after this particular time period there is the noise level which is shown at this particular point. So similarly if we compare the B graph with the A you can see here there is a drastic change but the noise gets introduced at 60 decibels itself at the 60 decibels itself. Got it? So this is the noise level which is interfering the channel. So these are the references for you people for the further study. Thank you for watching the video.