 In this class we are going to study about the basics of noise and noise monitoring. As I told you earlier you know in condition based maintenance we do vibration monitoring, we do motor signature analysis, oil analysis etcetera. Noise monitoring per se is not a technique to do CBM, but you know when machinery develops a fault it makes abnormal noise. So, noise becomes an indicator or an alarm to us indicating that something is abnormal with this machine. So, in that sense we will study about what noise is and how noise monitoring is done towards directing such abnormal conditions. Well, to begin with what is noise, now there are very synonymous terms in the sense of noise sound acoustics, well if I was to define sound, sound is something which we hear which our ears receive, we hear we get the sensation of hearing then what is noise, noise you know you can say is very loosely you can say is unwanted sound, they say you know music what is music then, music is a sound which you appreciate, noise is a sound which you do not appreciate ok. So, this is related to the sensation of hearing and this is because of God's gift to us pair of ears. So, we get the sense of hearing, we hear any sound because we have our pair of ears sensation of hearing because of the ears. So, long way between the ears and the machines which we generate we generate noise. So, what is this acoustics, so acoustics is a technical term given to the field of engineering where it deals with the generation, transmission and reception of sound. So, this is the definition of acoustics, it is the science which deals with the generation, transmission and reception of sound. Now, generation means basically talking about the source, transmission means about the media because this sound is basically because of a transfer of energy from a source to a receiver and obviously, this requires a the path which is connecting the source and the receiver and this energy gets transferred and then what is this path as we will recall from your high school physics. The energy is transferred as waves, this waves require a medium or a media, this media can be solid, can be fluid like in air, water or steel structure anything. So, depending on the media, depending on the density of the media, the two parameters which come into play in the propagation of this energy in terms of waves from a source to a receiver, the density of the media and speed of propagation in the media play an important role, okay. Now, this acoustical wave or sound wave, there will be many ways by which it gets transferred. One is because this incidentally the sensation of hearing which you sense is because of a small pressure fluctuation because if you look at our eardrum and I will just draw it as a membrane here, a thin membrane is our eardrum. So, when the pressure waves are incident on this membrane, this membrane is going to have a small deflection, okay and to these membranes are attached many nerve go to the nerve cells, okay and then this goes to the brain. So, because of this pressure fluctuation, small pressure fluctuation, this membrane will undergo a deformation or displacement and this is sensed by the nerve cells attached to the eardrum and then we get a sensation of hearing. Let us look into this pressure wave. For example, right now our eardrum, if you plot the time history of the pressure being incident on the eardrum, this is nothing but the atmospheric pressure and this is about 1.0133 into power 5 pascals. This is incident on our eardrum. So, this is a very high value of pressure or eardrum is being subjected to. On top of this pressure wave, if I do a perturbation of this or this energy is being transmitted because of a somebody speaking because of a machinery making noise. So, there will be small fluctuations in sound pressures and this is I denote by small p t. So, this is the dynamic component. So, the total pressure incident on our eardrum is p atmospheric plus p dynamic I can say and this small pressure is what is known as the acoustic. This small pressure is responsible for generating the sensation of hearing to us. In fact, I will just give you a value here right now in this room when I am speaking. The atmospheric pressure is 1.0133 into the power 5 pascals which is incident on eardrum and because of my voice you may be experiencing order of about 0.521 pascal. This could be your value of p t. So, you see the beauty of ear or God has made our ear so sensitive that over a high pressure of 1.0133 into power 5 pascals even a small fluctuation of 1 pascal can be sensed by our eardrum as disturbance as something which is being incident on our eardrum. You almost have felt you know if you go up a high speed elevator or you know go up a mountain or go under the water while you are diving in the swimming pool. You will get a sensation of as if eardrum is are popping in or popping out that is because of a difference of the pressure around the eardrum. So, your eardrum in some sense is very sensitive to this pressure differences and on top of it a value of 1 pascal is large enough to give you a sensation of hearing. Now this is regarding the amplitude of the sound pressure level. Now if I was coming back to the waves by which the sound gets transmitted from one place to another you would have recalled in your high school physics that there are two types of longitudinal waves and transverse waves. If I was to draw a transverse wave p t so the direction of transmission is along this direction, but the particles have a maxima and a minima perpendicular to the direction of propagation. This is the direction of propagation and this is perpendicular thus it is known as the transverse wave. So, this particle every particle here is oscillating about its mean position. This is the mean position. So, these particles themselves do not move, but they transmit the energy to their successive particles and that you would have studied in the longitudinal waves. So, there is a speed of propagation of the wave front along this direction and that is the speed of sound in the media, but the particles themselves oscillate with a different velocity. For example, if I write down the displacement p t as a sin omega t I can find out or if I write this as the displacement some displacement x t a sin omega t I can find out x dot t is nothing, but a omega cosine omega t. Mind you this is the particle velocity and which is different from the speed of sound in the media. Speed of sound means the speed at which this wave front is moving and this velocity means the velocity at which the particle is oscillating which is perpendicular. Similarly, in the case of the longitudinal waves you will have successive compressions and rarifections. So, what happens because of this successive compressions and rarifections there will be a increase in pressure, decrease in pressure and thus the wave front is going, but here in the case of longitudinal wave the particle motion is along the direction of the propagation of the wave front. Now, coming back to this sound pressure level or pressure which you hear p t. Now, this p t is a very, very small quantity and our human ear can hear as low as 20 micro pascals that is 20 into 10 to the power minus 6 pascals to as high as 1 to 2 pascals 1 to 2 pascals. But, if you see this scale is very large 10 to the power minus 6 to 2 pascals and that is why it is felt or it was felt that this sound pressure level or sound pressure need not be represented in a linear scale, but a logarithmic scale wherein we define something known as sound pressure level as SPL. As 20 log to the base 10 p this is the small p by p reference, where p reference in air is 20 into 10 to the power minus 6 pascals in air, in water it has a different value. So, when p is equal to 20 into 10 to the power minus 6 pascals S 2 is equal to 20 S P L would be 20 log 10 of 1 that will be 0 decibels. So, I have resolved I will 20 into 10 to the power minus 6 as 0 decibels and if you will work out 1 pascal could be as close to about 94 decibels. So, you see by using the log scale all have done is reduce the numbers from 0 to 94 decibels. This is written as d B stands for decibel minus this is capital B. So, small d capital B decibel. So, SPL is the magnitude of the sound pressure level is given in decibel. Now, another important characteristics which you will see when we have this log scale lot of properties of logarithmic addition subtraction etcetera will apply here. For example, suppose I have a machine A which is making a sound pressure level of 90 decibel, I have another machine B which is making 90 decibel. So, in both the machines are there both machines operating the total level will not be an arithmetic average of 90 plus 90 by 2. It will be not 90 decibels, it will be not 94 decibels, it will be close to about 96 decibels. Because you know log of 2 to the best 10 is 0.1 to the power minus 1 3 0 1 0 times 20 will be close to 6 decibels. So, adding 2 sound levels of the same sound pressure levels will be the level will increase by 6 decibels. Now, this sound pressure level is suppose I have a machinery here, this is generating. Now, how is this sound generated? This is machine this is a dynamics. So, these particles about the machines are going to have a vibration. So, the air molecules next to it are going to have a velocity particle velocity which is close to the surface of course, that depends on the radiation efficiency etcetera. So, once these particles are having an energy they are now going to excite or radiate in all direction. But you will see this sound pressure level suppose there was no obstructions they would go down they would go in one particular direction all direction. In fact, and imagine this to be a spherical source. So, the they would radiate sound in all directions, but you will see this SPL is proportional to 1 by distance from the source provided there are no reflections. That means, with every doubling of the distance from the source the SPL will reduce 6 decibels provided this is very important provided there are no reflections and this no reflections means free field reduction. So, if I want to reduce the field reduction find out the sound pressure level radiated by every machine I can always take them to an environment where there are no reflections and then measure at a fixed distance. Usually the standard is in about 1 meter from the source you measure what the SPL is in decibels. The reason you will see later on in the field of acoustics is why they prefer this 1 meter is in SPL in the free field conditions whatever is the SPL in decibels. In fact, that same would be the sound power level you know the radius should be 1. So, I will not go into the details of sound power and sound pressure level, but provided there are no reflections. Now, what do you mean by no reflections? So, how do you ensure that there are no reflections? So, we have to have certain boundary conditions suppose this is a room if this was generating sound in every direction we have to ensure that there are no reflections. So, such rooms or no echo or they are known as the an echoic chambers wherein free field condition exists. There are special chambers wherein we like they have like wedges like this all around them and these are actually acoustical wedges having a very high sound absorbing, absorbing capacities what I will say right now. So, any way which goes in here gets reflected and gets trapped in this material and this. So, this is a free field condition. So, when we do the noise leveling of a product or want to find out the sound radiated by the machine we usually test them in such an echoic chambers. In fact, people test vehicles in such chambers now in fact, they can put a grating here and then they can drive in a vehicle rev it up and measure the sound pressure level radiated by it. In fact, the same phenomena is used in acoustical I mean the electromagnetic absorption of radars and such chambers are also there in the radar and communication centers where then they test antennas as well. And opposite to an echoic chambers is what is known as reverberant chambers wherein the walls are very very hard. So, there are lot of multiple reflections I will not draw all this multiple reflections multiple reflections and because of such multiple reflections anywhere you measure the sound pressure level there will be uniform SPL and such chambers are used for acoustical testing etcetera, but we will not discuss them in detail is just opposite to what an echoic chambers are. Now, coming back to our discussion on sound pressure level. So, we have just talked about the magnitude of the pressure and you know human being or human ears can withstand not more than 90 decibel in fact, the amount is recommended for more 8 hours. In fact, in this room while I am talking in the studio it is a very quiet studio if the studio nobody are speaking this level could be as low as 35 decibel in this recording studio the I mean noise level. Now, once somebody speaking a comfortable classroom this could be about 60 dB in a classroom studio and when an aircraft takes off you know this could be about 130 decibel exhaust. So, you can get a feel of the noise levels which I am talking about. Now, question is you know you would have seen that noise or sound they are waves. So, at any locations the sound waves would be adding up themselves. So, we will have a summation of waves like you saw the example when we have one machine giving 90 decibel another machine giving 90 decibel we have an overall decibel level of 96 decibels. For example, I was to monitor the sound radiated by a machine why need to have another machine next to it running or no. Obviously, your answer would be everything else should be quiet we should be only running the machine whose noise is to be measured. Now, you would have seen from this logarithm scale if one machine machine C generated 80 decibel machine D generated 70 decibel and machine A generated 90 decibel the presence of the machine A is so loud that is 90 decibel the presence of 70 would only raise it by 90 point you know some small fractions same as to about 80 decibels. So, the presence of a machine with a level 10 dB less or 10 dB less or more does not affect the machine. So, for example, this is a 90 dB machine 80 dB presence or a 70 dB presence is not going to affect the overall value of this 90 dB, but if there is 90 and 90, 90 and 85 I need to be get worried. So, anything 10 less than like 90 minus 70, 90 minus 80, 90 minus 60 I need not worry. So, whenever we do noise monitoring of machines we have to ensure that ambient or background noise is less than 10 dB our is less by less by more that is a better way of looking less by more than 10 decibels. So, in a room if I was to measure the noise radiated by a say overhead projector, if the overhead projector makes 40 decibel if the ambient noise level is 20 decibels or 25 decibels I am ok, but then I can say that my the projector made in a 40 decibel of noise, but if the ambient noise is 35 decibel and I would say the projector is making 40 decibel it would be around yes. So, always while you are doing noise monitoring, you have to make sure that the ambient noise level and I would say the projector is making 40 decibel it would be around yes. So, always while you are doing noise monitoring always it is good to measure the background noise level and background noise level should be less than less by more than 10 decibels ok. So, always a check and then there are certain ground rules like when we do a noise monitoring the measurement is done by a sound level meter. Which has a microphone and a built in microprocessor circuitry which gives the SPL in decibel. Now the rule is the convention is that this SLN should be always at a distance of about 1.2 meter from the ground and again ensure that the background will less than 10 10 decibels ok that is only one thing you have to take care. And another important parameter which you have to report is of course, the temperature, humidity and the wind speed wind speed because these parameters may affect the speed of sound because you will see the speed of sound is dependent on the temperature of propagation. In fact, at 20 degree Celsius speed of sound is 341 meters per second in air ok. This speed of sound in water is about 1500 meters per second in air ok. This speed of sound in water is about 1500 meters per second in water in steel it is close to about 5000 with density the speed of sound is also increasing ok. Something you have to keep in mind ok. And density of air is 1.2 kg per meter cube density of water is 1000 kg per meter cube density of steel 7800 ok. So, certain values you have to keep in mind. Now we have just discussed about the sound pressure level in terms of amplitude. But again our ear has been made by God you know this sound is also a wave. So, it has an amplitude and it also has a very important quantity to this frequency. So, the human ear has an audible range which is from 20 hertz to 20 thousand hertz. So, we can hear any sound which is in this band of 20 hertz to 20 thousand hertz or 20 kilo hertz. So, those which are above 20 thousand hertz or 20 kilo hertz are known as ultrasonic waves greater than 20 kilo hertz and intrasonic waves are less than 20 hertz ok. But again like any other linear instrument you would have seen the frequency response of an instrument. So, this is the amplitude ratio output by input output by input will be 1 or 1 in log scale which will be 0 decibel because log of 1 is 1. So, this is typically the response of an instrument and this happens to be its natural frequency ok. If you think of our human ear unfortunately God has made our human ear human ear is highly non-linear. It is not if it did behave like this life would have been very easy for us, but the human's ear sensitivity is somewhat like this. This is about I would say frequency response or sensitivity. Now you may be wondering how this how did I generate such a waveform? Well, these are these are from you know in the in the early 1900s people have done experiments with human subjects and this is what they found out that the subject the human ear response like this. So, we hear poorer at low frequencies and so on ok and this is what is known as the. So, any instrument suppose I use this with them I use measure the sound level with the microphone ok. I will superimpose I will do this witting function on this microphone measurement to give what is known as the human witting ok. So, and these these bands this this could be divided into octave bands ok. You know what octave bands are octave bands are those frequency bands where the f upper is nothing, but twice of f lower and the center frequency is root 2 of ok. So, in every octave band such weightings corresponding to human ear is applied ok. If you go into the hand books you will see for every octave band where the weighting factor is and this is known as the A weighting factor ok. So, 63.5, 125, 250, 500 etcetera ok. For all these octave bands you will see what the weighting factor is, but for 1000 hertz this is about 0 and so on. So, there will be a subtraction. So, this weighting values are applied to the frequency spectrum obtained from the SPL and then we can get what is known as the D B A or D B A. This means the A weighting because of human ear and this is measured at a 40-phone loudness level and that corresponds to normal volume normal pressure. When the levels are high for example, if you are talking about aircraft takeoffs you know very large noise levels you can there are also C level and B level, but the international convention has to be used the A weighting and all if A weighting has been written same 90 decibel could be 87 decibel D B A because this means if you this has a significant low frequency levels ok low frequency signals are there. So, whenever A weighting is given it means that it has been corrected to human hearing because if I give a measured value without the A weighting it may not relate well with our sense of hearing because D B is what a microphone would have given me, but I would be perceiving D B A. So, this weighting factor has to be given so that I get the correct sense of hearing correct sense of the sound level which I have measured ok. Now, let us talk about the acoustical so for example, so if in noise monitoring I have to ensure what of equipment or machine what are the things I have to report one is the SPL level either D B or D B A next is the background level D B or D B A very important is distance from the source number 4 is the ambient conditions that is the temperature pressure humidity wind speed of course, time of the day sketch of the machine or the layout. So, for every machine I could be doing such measurement for every machine and keep it on a record and later on for diagnosis sometimes we may do the sound pressure time as tree recording and store it with us. For example, we can still apply the techniques of F F T to get what is known as the noise spectrum. Now, the spectrum could be a narrow band spectrum or what could be an octave band spectrum or a one third octave band spectrum ok. Now, this helps us diagnose the problem for example, if I if you recall I was talking about a machine if this machine vibrates this will be vibrating at a frequency F and the particles obviously next to it will be also vibrating with the same frequencies. So, sometimes it may not be possible for us to while doing diagnosis to even go near this machine machine may not be accessible. So, but at a remote location I could be just putting a microphone and then doing the narrow band spectrum and then maybe I will find out the running speeds of the machines. So, noise monitoring also gives helps us to access machines remotely. Now, let us talk about the noise fields or sound fields I had talked about a machine which is radiating in all directions. So, I draw a with distance very near the machine the 1 by r law may not be followed and this is about the about half of this distance is what is known as the near field and then the sound level may be a constant. So, I draw the with this red line the sound level and then what happened with distance this is going to come down by 6 decibels and then towards the if this is a boundary. Now, this level has happened. So, there are 3 distinct zones. So, this is the near field and then this is the free field and this is the river, river barren field. Here you will see drop by 6 dB double and double distance and this is the distance r and. So, always a good area to measure as I was telling you is the free field measurements. So, in the noise monitoring you have to also like when you say distance from the source you can also say whether it is in free field or far field. So, always avoid near field and always avoid river barren field. So, in a shop floor you can move your sound level meter or microphone away from the source and see whether this 6 dB decreases there with double in distance and whenever they try to increase you know they are close to the walls in the river barrens field. Like you would have seen if you go to the corner of a classroom and speak because of the reflections in the wall the sound pressure level will increase and this is what we need to measure in. There are few other terms associated with the sound pressure level for a noise monitoring one is the sound power level denoted by SWL sound pressure level. So, this is intensity level SIL and the sound power level is nothing but the sound intensity level times the normal area. There are instruments to measure sound intensity level and then knowing the area you can find out the sound power level. The advantage of using sound power level is sound power level of a machinery is not going to change with environment. SWL will not change with environment or field. So, when I label a product with a certain sound power level it is going to be same whether it is in location A, location B, location C. So, it is very easy to characterize the machinery in terms of the sound power level. If something is wrong with the machine its sound power level will change. Well to measure sound power level it is a little involved process. So, it may not very easy to measure the sound power level in situ at a location, but nevertheless there are methods of the sound intensity level by which we can measure the sound power level. But the sound pressure level if we measure to monitor the health of a machine we have to tell the field conditions the distance from the source and ensure that the background is less than the less by more than 10 decibels. Now, how much sound does human ear perceive? If human ear was to sense 95, 90 decibel and 91 decibel it would not be able to do that. A human ear cannot distinguish less than 3 decibel something you have to keep in mind. You would have done a noise control and brought down the level from 90 to 90 decibels 92 to 90 decibels you will not perceive it. To make a significant difference it has to be more than 5 decibels. So, when we talk about alarms you know sometimes you know a mere difference of 2, 3 decibels will not be noticed by a human ear. So, we have to keep that in mind. So, sometimes during noise monitoring there are machineries or there are online analyzers which will tell you the exact sound pressure level. So, sometimes numbers may guide us to give us the better feel of the machine's condition. So, to summarize in noise, noise as I said in the right in the beginning is not used as a parameter per se to do condition monitoring of machines, but for the fact that they give us an indication that something is abnormal with the machine. We need to study noise and we need to know how the background of the machinery's noise level can be measured and the importance of A waiting, why the numbers of A waiting are there and what is their frequency content. Thank you.