Silent Night--and Silent Days

An Israeli audio-engineering firm wants to block engine noise at its source

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Hi, this is Steven Cherry for IEEE Spectrum's "This Week in Technology."


You know that sound. It's the annoying rush of air coming from a heating, ventilation, and air conditioning system. It's especially annoying on an airplane. Maybe you've thought about getting a pair of noise cancellation headphones; years ago I got a pair when faced with my first 14-hour flight to Asia. They help, but only somewhat. And they're not cheap, and they're yet another thing to carry around along with our cellphones and iPads and laptops and cameras and everything else. The ideal solution would be for Boeing and Airbus to build noise cancellation into the airplane itself.

That might not be impossible, because of the work being done by a handful of audio engineering firms around the world. And the company that might be furthest along in bringing about this auditory nirvana is Silentium, an Israeli firm that already has some products that promise manufacturers up to a 90 percent reduction in noise in their products.

Here's the sound of that HVAC system again, this time before and after Silentium's technology is added.


My guest today is Yossi Barath, Silentium's CEO. Yossi, welcome to the podcast.

Yossi Barath: Thank you, Steven. It's a pleasure.

Steven Cherry: Yossi, do I have the right idea about your company's products? The maker of an airplane would use them to cancel out the background noises of the plane, the maker of a window air conditioner would cancel out the noise the unit makes, and maybe even the manufacturer of a subway car would cancel out the noise of the motor?

Yossi Barath: Well, eventually all those are real targets. Maybe I'll go to the basic solution. The basic solution that everybody is familiar with is putting a barrier. The noise that you hear from the engine in the aircraft—or the noise that you hear when your neighbor is playing his bass guitar—the reason it comes through the walls is because the wall has to be much heavier to block low frequencies. So the existing solutions for blocking noise are passive solutions, that can be a wall or it can be material that absorbs noise like acoustic ceilings that people use in offices, etc. What Silentium does, we use a physics phenomena, a rule of physics that is called destructive interference. If you have waves in the air and you manage to deliver to the air at the same time the opposite phase wave or series of waves, they will cancel each other. This is called destructive interference. We are using this rule to really kill noise or reduce noise significantly. So if you're talking about air conditioners, for instance—noise is generated by the fans or blowers inside the air conditioner, or the compressor—we would try to kill the noise before it even leaves the air conditioner so that you don't have to worry about walls and barriers, etc. Basically, what you want to do if you want to cancel noise is you want to listen to the noise that is generated, take it into a controller—into a brain, analyze the waveforms, generate the counter waveforms, which have to be ideally the same as the noise that is generated but with an opposite phase shifted by 180 degrees. The catch here, or the problem here, is that noise is changing all the time. If noise was the same all the time it was an easy task—everybody could have done it. The problem is that noise is changing from the time that we are measuring the noise; until we manage to deliver to the air the opposite noise, noise has already changed. And if we miss—if we generate not exactly the opposite phase of the noise, not only will we not reduce the noise, but we'll add noise, and this is something we don't want to do.

So what we really have to do is to predict how noise will sound when we hit the air with the opposite noise, and this is really what we have developed. We developed a solution that is based on very smart algorithms that listen to the noise, generate the noise based on some understanding of how noise is changing. So we have in our company both the algorithm know-how, the acoustics know-how, the physics know-how on how noise is traveling in the air. What we are doing is basically active noise cancellation. What we are doing, we are going to the air conditioner for instance, we would put our device in the air conditioner by the manufacturer or as a very small compact silencer that would be replacing very bulky silencers that are being used today in HVAC systems—HVAC is heating, ventilation, air-conditioning systems—and our unit will cancel the noise or the noise coming with the air as it leaves the air conditioner. And what we let our listeners listen to before is the effect of our active noise cancellation solution on this device.

Steven Cherry: Yossi, you sent us three other audio clips. Let's play the first one, but first, why don't you tell us what we're listening to.

Yossi Barath: Yeah. It is what you hear when you are sitting in an aircraft—it's the noise generated from the engine the way it is heard by the passenger inside the aircraft without any treatment at all.


Steven Cherry: Very good. And this second clip—what are we hearing now?

Yossi Barath: Now we—to implement our solution, you're putting this special tile—it's like a tiny wall that we are covering with the internal part of the fuselage of the aircraft. And what we are showing is in two steps: One is just adding this material, which is a honeycomb-type material. It reduces the noise because we are adding another barrier—it's like using double glazing. So there is an additional barrier so noise goes down.


Steven Cherry: And so now in this third clip, we're hearing the passive barrier plus the noise-cancellation chip at work. Is that right?

Yossi Barath: Yeah. Now we are turning the passive barrier into an active barrier by measuring the vibration generated on this barrier and generating the counter vibration to cancel the vibration. So the end result is the panel that we added is not vibrating, and if it's not vibrating, we don't get the noise from the outside.


Steven Cherry: Very good. I understand that there's even a green component to this. Lowering noise somehow makes for energy efficiencies?

Yossi Barath: Today we are investing a lot of money in protecting ourselves from noise. I'll give you an example from data centers: Computers are making a lot of noise. To avoid the problem of noise, we take all the servers and we put them into a special server room. Once we put them there, we have to put an air conditioner because it is becoming very hot in this room, and this is the major driver of your electricity bill.

Steven Cherry: Also, things like blowers sometimes aren't operated at their best speeds because of the noise they produce?

Yossi Barath: Exactly. Before they had to choose a certain size of a blower so that it can generate sufficient airflow and yet not do too much noise. So it ends up being relatively big. Assume that you now can take a blower—don't care about the noise, just make the best design, the smallest blower. What we are doing today for air conditioners—and by the way, next week starting on the 31st of January and for the following three days, there is the biggest event in Las Vegas where all the HVAC, all the air-moving devices, air conditioners, ventilation are presenting their new systems and we're going to present few of our solutions over there. The next generation that we are now working on—it's still in research and development—is to create what we call a "quiet bubble," meaning that you'll be able to take a device, put it on the table, and create what you would call a virtual office. So you'll be able to sit at your desk in a very noisy area, operate our system, and we would control the noise in a given zone, and the zone is going to be like a cube or a ball or a bubble, if you may refer to it this way. But it will control noise coming from all sources surrounding it, so it's like sitting in an office that has virtual walls. And this is the next thing that we're going to introduce, it's a zone-to-zone solution or—our trademark for this is the Quiet Bubble.

Steven Cherry: So let me ask you: In my ideal world, taking the subway to work for me would be as quiet as an elevator ride. One possibility would be for the train manufacturer to be canceling the noise at the motor the same way that the airplane would do it. Another would be for me to create my little bubble of silence around myself with the bubble technology. Is that right?

Yossi Barath: Exactly. You just said what we are doing. Silentium's technology works on the two ends: We go to the source and we try to reduce the noise at the source if it's possible; if the source is too big, then it's very difficult to do so. To control the noise from the engine of the train or the engine of an aircraft is not very easy because of the order of energy that is generated. It would be much easier to reduce by as much as possible the noise coming into the train but to create quiet bubbles that can have many purposes. The quiet bubble can be for you to sit in the train, it can be part of the seat that you are sitting in. You'll just enter into the train—when you walk to your seat, you may still be listening to the usual noise, but you'll take your seat and all of a sudden you'll be sitting in a quiet bubble.

Steven Cherry: Oh, that's fantastic. So I could also have a bubble in my living room chair, and my wife could be on the couch watching TV, and I don't even hear it because I'm in my bubble.

Yossi Barath: Yes. I can tell you one of the things that people came to us for this bubble is somebody came and said, "Look, I have a problem. I am snoring. My wife is complaining all the time. Can you give me something that will create a quiet bubble over her head?" And I said, yes, I hope that next Christmas you will be able to do so.

Steven Cherry: That's fantastic, and my ultimate dream is to sit in a restaurant and actually hear the people that I'm having dinner with…

Yossi Barath: And not hear the table next to you.

Steven Cherry: Exactly. Well, this is wonderful. Very good. Well, thank you very much.

Yossi Barath: Thank you very much.

Steven Cherry: We've been speaking with Yossi Barath, CEO of the Israeli firm Silentium, about an ideal world in which background noise is a thing of the past—and what engineers are quietly doing to make that a reality. For IEEE Spectrum's "This Week in Technology," I'm Steven Cherry.

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