Active Noise Cancellation

[Music] hi if you've been following this channel for a while you know that one of the projects I'm involved in is building an auxilary power unit for our larger drones and one of the issues is that the gasoline engines produce a lot of noise and in order to try to contain or block that noise from getting to the outside world you need to build an enclosure that will accomplish that and do so efficiently in order to design that you have to understand a little bit about how sound is transmitted and so what I've done on the other end of the room is I've set up a speaker and an amplifier and a signal generator to produce some noise now the speaker on the other end of the room is producing an 1,100 Hertz signal that's passing across the room being detected on this microphone and displayed on the Susilo scope now even though you can see the oscilloscope looks like a series of sinusoidal waves you can even pick up my voice lah but nevertheless these sinusoidal waves are not really the nature of sound waves they're not like surface waves on the surface of a body of water they're compressed shells of air that are moving outward from the speaker and in order to understand how that works the speaker is moving backward and forward under the signal from the amplifier just because this is annoying I'm going to turn this off right now and when the speaker moves forward it impacts the air molecules in front of it and pushes them into their neighbors producing a compressed sheet of air molecules moving away from the speaker at the speed of sound as the speaker moves backward it produces a region of rarefied gas behind the compressed sheet the next cycle then will induce another compression wave and these expanding shells of compression waves and rarefied gas between the compression waves represent sound so sound is really expanding shells concentric shells of pressure waves and verification nodes between them and because they move outward at the speed of sound which at sea level and room temperature is about 1,100 Hertz the distance between the compression waves at from this 1,100 Hertz signal is 12 inches and the nodes between them are 6 inches apart now if you wanted to block that sound by placing something in the pathway of the sound you could use say a heavy metal plate like this this plate is very hard and so therefore when the air molecule sheet or the pressure wave hits the front of this plate what happens is they bounce elastically off the surface of the plate back toward their source because the place it played is so massive compared to the air molecules that strike it most of the energy is returned to the source the plate does move slightly because it's not infinitely massive and so as the plate moves with the progression or the movement of the pressure waves that hit it it induces a secondary set of pressure waves on its back surface but the amplitude of those pressure waves is far less than the amplitude of the pressure waves that hit the front now if you were to build a structure like a box out of simply massive hard flat plates the problem would be that the reflected sound energy from the inside of the the surface that first receives the pressure wave would be reflected to the opposite side of the box it would reflect off of that surface and then what would happen is the pressure waves that bounce back and forth between the two hard plates almost like an acoustic laser and the sound energy within the box becomes much higher than the sound energy would be if the box did not have this reflection and so building a structure out of just hard plates doesn't block sound nearly as well as you might want because the amplitude inside becomes much greater than it would be if the sound wasn't building up inside of the box you could alternatively use a soft foam pad to block the sound waves because the the pad is not elastic when the sound waves the pressure waves hit the front surface it distorts the surface and that distortion causes frictional energy loss energy is transferred from the sound wave to heat energy inside the foam therefore there's a lot less reflection inside of a box that would be built up from say just foam pads but because the foam pads are so light closer to the density of air they move a lot more than the massive plate would move because they don't have a lot of inertia so the secondary sound waves that are set up off the backside of the foam I are going to be much more significant than they would be from the plate alone so building up a structure or a box from foam is also not very effective a much better solution is to combine the two if you take a foam pad and bond it to a metal plate on the back surface what happens is that the metal plate backs up the foam and so when the sound energy hits the foam it forces more of the sound energy to be absorbed because there is more compression of this surface the foam is not moving away from the pressure wave as a result the build-up of energy within the container is much less and therefore the plate has less to resist in terms of inertia to prevent the the flow of sound out of your box there's one other degree of freedom that you have the ability to take the advantage of and that is the fact that when a box is built the sides of the Box support the surfaces so the box itself is much more resistant to the expansion caused by the pressure waves then would just be a plate due to its inertia but those are really the only three degrees of freedom that have you can either use mass absorption or stiffness to try to resist sound transmission outside of a box you don't have any other degrees of freedom you can mix and match the materials you can use a foam that is denser they load some foams with minerals to make them heavier so that they're less likely to move you can make a surface that is stiffer and not as heavy as a plate of aluminum by using say a composite core sandwich more resistance is created by the stiffness of the structure rather than the mass but really you're just trading off the different materials to try to balance these three degrees of freedom there's nothing else you can do unless you introduced the concept of active sound reduction and the way that that works is that what you're hearing on the microphone now and what I hear in my ear is the peaks between the pressure waves and the verification nodes between them if you could fill that that low-pressure zone between the two expanding pressure waves with a pressure wave you would create more of a ripple and a much lower amplitude modulation than if you simply had the original sound with its powerful amplitude modulation that would occur without any kind of active reduction so what I'm going to do is I'm going to demonstrate the property of active sound reduction by demonstrating the effect of phasing or creating anti phase or anti phase noise with a secondary source in order to reduce the sound that's transmitted outside of the box but to do that I'm going to go outside because the sound reflection from the walls interferes with the effectiveness of the demonstration so in just a second we'll go outside and I'll set that up okay so what I've done is I've set up two identical speakers at the same distance from the camera and they're both hooked up to an amplifier and to the signal generator what I'm gonna do is turn on the primary speaker now you can hear the sound coming from just one speaker if I hook up the second speaker in phase with the first you'll hear that the sound level gets louder and that makes sense because you have twice the amount of sound energy hitting the camera if I then take one of the speakers and begin moving it back toward half a wavelength apart you'll hear that the sound progressively gets quieter and quieter and quieter and this is because there are out of phase with spacing if I take the two microphones two speakers and put them at the same distance again but instead of facing them positionally i phased them electronically i simply would reverse the polarity of the two leads into the speaker so that one speaker is moving forward when the other speaker is moving backward you can hear that it becomes much quieter simply because I've phased them electronically and so what we can do is in order to try to actively sound a reduce sound amplitude is we can accomplish it either spatially or we can accomplish it electronically so now I'm going to apply what I've shown you outside to trying to incorporate this into an enclosure to reduce sound transmission outside of an enclosure or box and to do that you can't depend on an arbitrary waveform like produced by a generator you're going to need to be able to sample the sound in order to generate the anti-phase sound that you're going to use to cancel the noise in order to do that what I've done is I've used these small electret microphones each wall of the enclosure has one of these microphones glued to the inside right onto the phone and each microphone feeds a channel of an amplifier and the amplifier then increases the power and sends it to these actuators or shakers that create speakers out of each of the walls of the structure creating essentially anti phase noise out of the structure itself to understand what these things do if you've never seen these before these are essentially the driver system for any kind of a speaker a magnet and a coil and instead of driving a cone they drive whatever you attach them to and right now this is on and you can barely hear it but if you place a surface a large surface on top of it so that there's a larger surface area for it to act on you can hear that it produces a pretty good amount of noise sound and that's what it's doing on the walls of the structure now if I set up this circuit to drive the sound from the speaker inside you'll see that I have a speaker mounted inside of the container it's the same one we've been using all along in the demonstrations and I've enclosed it inside at a metal box with very thin 22 gauge metal and then foam padding like this on the inside to do the dampening and then what I've done is I've hooked up the microphones to a power source and now that they're energized what I'm going to do is I'm going to vary the amplitude or the amplification to make allowances for the fact that the sound pressures are going to be different on the different walls so as they start dialing up the amplitude on these you'll hear that the sound level starts to reduce and as you can tell it's pretty effective I'm going to turn off the microphones as you can tell it's pretty sensitive to the amplitude the amplitude gain in the amplifiers and one of the modifications that we're going to be making is including an active circuit that will adjust the amplification for each one of the walls depending on where the noise is coming from whether it's centered in the box or if it sound pressure like what a microphone tends to be stronger and say one of the walls the nice thing about this though is even without that this doesn't require any programming and it doesn't require any sophisticated structural analysis it simply makes the walls effectively stiffer wherever the actuators are located another limitation is the point support where the actuators are is obviously very flexible and rather small compared to the surface area it may be an advantage to put smaller actuators located in a distributed fashion wouldn't add any more weight they're cheaper and it may give you better sound reduction in addition it may be advantageous to isolate the sound generators from the box itself or essentially from the speaker's more of the microphones more effectively by placing a very thin walled outer structure that creates the anti phase noise therefore the coupling between the shaker and the microphone is less allowing us more amplitude range before we eat feedback those are modifications I'm going to be making and hopefully over the next couple of weeks we'll produce some videos giving some updates as the Box improves but it is effective it does work and it's inexpensive it's relatively easy to create and it would be a lot of fun to play with if you want to get a couple shakers some microphones and play with some amplifiers I'm sure that you could probably improve on what I've done so far and that's what I think makes this is exciting is the possibility of improving on a design that effectively work so this was useful I hope it was enjoyable there's a lot of fun doing this and look forward to seeing you next time you have a good night [Music] [Music]

Channel: Tech Ingredients

Views: 562,775

Rating: 4.9250154 out of 5

Keywords: Noise, Cancellation, Noise Cancellation, Audio exciters, Phasing speakers, Speakers, Sound isolation, Noise reduction, Noise isolation

Id: --c0tiIZG6o

Length: 16min 16sec (976 seconds)

Published: Thu Dec 28 2017

Reddit Comments

[u/serious_cheese]

I'd be interested to see him test it with something other than a pure sine wave, like white/pink noise. I'm not sure it'd work quite as well.