 Since myself Alia Arsandane, assistant professor from Department of Electronics, Wailchen Institute of Technology, SolarPool. Today, we are going to see the topic acoustics and reverberations from the subject audio-video systems. So, what are the learning outcomes from this topic? At the end of this topic, student is able to define acoustics and reverberation. Also, he can calculate absorption coefficient reverberation time for efficient acoustic effect and design acoustics of auditorium and meeting hall. So, these are the learning outcomes. So, basically what is acoustic? So, acoustic is nothing but it is the basic branch of physics that deals with the study of wave propagations in different types of mediums which include gas, liquid and solids. So, whenever this study is made, the parameters which are considered for the acoustic or the wave propagation is vibrations, sounds, ultrasounds and infrasounds. So, it can also be defined as the qualities that determine the ability of an enclosure such as auditorium. So, here the ability of enclosure basically denotes the reflection of the sound waves in such a way that to produce a distinct hearing or a clear hearing. So, sound suffers reflection, refraction, diffraction and absorption. The reflected sound basically will be heard as a distinct echo. If the time gap between original wave and the reflected wave is greater than 60 milliseconds. So, whenever the echo is heard in some structure, in some building structures then you have to keep in mind that the time gap between the original wave which has been transmitted and the reflected wave from the surface the time gap is greater than 60 milliseconds. So, what is the role of acoustics? So, when we consider the acoustic branch then the role of acoustic branch is very much wider. So, here as you can see the acoustics plays a major role in the medical science. It also plays an important role in the electronics communication system. It also plays an important role in geology. In geology also it plays an important role in terms of architecture and chemistry. So, fundamentals of acoustics basically nowadays we are developing much more content on this. So as you can see here medicine is there, physiology is there, psychology. In communications we are dealing with speech music art and when we consider the geological conditions then we are dealing with the seismic graph of the earth atmospheric physics, seismic waves, sonar waves and ultrasonic waves. So, when we consider them in terms of chemistry then it is used for chemistry, mechanics and architecture. So, these are some of the branches in which acoustics plays a major role. So, why there is a need of acoustics? So, as we know that sound can be classified into three major categories. First one is infrasound where the frequency is less than 20 kilohertz in which or this particular frequency can be heard by the animals. The audible sound basically it has a range of 20 hertz to 20 kilohertz. This can be heard by the human beings and the ultrasound where the frequency range lies much more than 20 kilohertz. So this can be heard by the flying bats and some species of the birds. So here as you can see this can this will give you a basic explanation. So infrasound so when we consider the waves characteristics or the sound characteristics then you can find that infrasound is having less wavelength or it is kind of a lower frequency audible sound. It is having a characteristic which is in proportion that means the crest and trough are in proportion whereas ultrasound the characteristics of the sound or the sound wave which is represented for ultrasound is much more that means these basically denotes low frequency, medium frequency and higher frequencies. So what are reverberations? The prolongation of sound in a hall even though the source of the sound is cut off is called as reverberation. So whenever for this we can take an example of building or construction of a building which is having dome in which when you shout out a name then it is vibrated for seven to eight times. So you are shouting it for once and you are hearing the voice or the repeated name for about seven to eight times. So that particular example is nothing but the reverberation. So what exactly happens in reverberations? So reverberations are gradually fading or the traces of decay are faded that means whenever the reverberations occur there is a decaying of sound. That means at instant whenever we speak the sound gets higher and as it moves out or as the echo goes on increasing then the sound gets decayed and it gets faded and a point comes where it gets totally lost. The reverberations are calculated in terms of time and it is basically given as t is equal to 55.3 V upon C A where C is velocity of sound V is volume of room A is total absorption and reverberation time is defined as time taken for sound energy in a room to drop down for about ten to six times that is 60 dB below. So please understand that 60 dB means maximum so it drops down by 60 decibels. Some examples of the buildings and the reverberation periods. So when these are the constructions of certain buildings and these are having the reverberation time. So when you compare all these buildings and their respective reverberation time then you will find that the lecture hall is having less reverberation time whereas the church building is having the maximum reverberation time which is of about 2.5 seconds. So this will give you an exact idea about the buildings and how reverberation times are there for them. So reverberations can be explained by giving the below example a sound produced in a hall undergoes multiple reflections from the walls floors and ceiling before it becomes audible. So whenever the teacher is speaking the first person or the second person can hear the sound or the voice of the teacher directly but the person who is sitting in third, fourth and fifth row he can hear the sound of that teacher by the multiple reflections which are taking place from the flooring, the ceiling and from the side walls of the classroom and then they are receiving the voice of the teacher. So this is the best example. So it can be also defined reverberation can also be calculated by using Sabin's equation. So Sabin defined the reverberation time as the time taken by sound intensity to fall on one millionth of its original intensity after the source has stopped emitting the sound. So it is given by t is equal to 0.161 v summation 1 to n alpha n into s n where v is nothing but the velocity of the sound alpha and s is nothing but the intensities of the sound and alpha is the Sabin's. So sorry alpha is the total absorption of that particular sound. So further it can be above equation can be written in this particular manner. So Sabin's equation absorption refers to the process by which a material structure or object takes a sound energy when the sounds are encountered and opposed to the reflecting energy. The total absorption is given by alpha is equal to sorry a is equal to summation of alpha into s where alpha is absorption coefficient of the surface area. Factors affecting the acoustics of building first one is reverberation time. If a hall is to be acoustically satisfactory then it should have rewrite reverberation time. Reverberation time should be neither too long nor too short. The total the optimum value for reverberation time basically depends upon the purpose for which the hall is being used. Hence what are the remedies for this particular how we can overcome this particular factors. So reverberation time can be controlled by choosing right building material. So windows can be used or windows which are to be constructed they should be minimum and sometimes they should be kept open or closed to provide the particular or correct reverberation. Similarly the second factor which is affecting acoustic of a building is loudness. For proper hearing of sound sufficient loudness is required more use of absorption surface near the sound of near the source of the sound. Hence due to this less reflection will take place and hence it will decrease the loudness in sound. So how this can be reduced? This can be reduced by choosing reflection surface or the reflection material such that it should be very much nearer to the source of the sound. Over positioning of absorption material can increase the loudness in sound and for larger halls when we consider in these halls if there are more audience then at that particular time we can use two or more speakers which can make a difference in loudness. The third factor is focusing use of concave surfaces can concentrate the sound at a fixed focal point hence concentrations may sometimes lead to deficiency of reflected sound. So such kind of deficiencies are called as dead spots. If there are highly reflecting parallel surfaces in the hall the reflected and the directed sound sometimes get mixed up and it can form a standing wave pattern hence due to this standing wave pattern there will be no uniform distribution of sound in the hall. So how this can be reduced? It can be reduced by proiding sound focal length and removing the dead spots by eliminating curvilinear interiors by using highly absorption materials on these curvilinear or concave surfaces then a parabolic reflecting surface can be used nearer to the speaker which will help in directing a uniform reflected beam of sound in the hall. So these are the factors which are affecting acoustics of the building. So these are the references for you people for the further study. Thank you for watching the video.