How Active Noise Cancellation Works

about those headphones in a future episode. I'm actually just trying them out now to make sure that I like how they work and all of that. But for now, I just wanted to talk about the science and technology behind active noise canceling or active noise reduction headphones. An R and ANC are the acronyms or initialisms, I should say, not acronyms that are used for those technologies. Now, noise

canceling is more than just muffling sound. You can have passive noise canceling, which really just means things like ear plugs or really well insulated ear muffs. So it's more than that. It's not just soundproofing your ears or something. It's using technology to actively cancel out the sound waves that are heading for your ear drums. But we won't get ahead of ourselves. First, let's talk about what sound is. So, when you get down to it, sound is vibration, and

we can think of vibration as energy. So this energy needs a medium to travel through. Sound travels through matter, and it doesn't matter if the matter is solid, liquid, or gas. It can travel through it, but it definitely needs to have a medium in order to try. This is why out in space there is no sound. You know, in space, no one can hear you scream. Well, that's because in space you have these vast regions where there is so little matter out there that sound cannot pass

through it. There's nothing for the sound to transfer energy to. So it's not just that it's a lack of air, it's that it's a lack of anything, or at least there's not enough significant amount of anything out there that would allow for the transfer of longitudinal waves. By the way, that's how sound travels. Sound travels in longitudinal waves. So broadly speaking, there are two types of waves. There's longitudinal

and there's transverse. Transverse waves are what I think a lot of folks imagine when they think of waves, often it's how we plot waves on a chart. It's a very easy way of doing it. So one way you could actually see transverse waves in action is just with a length of rope, a nice long length of The longer the better, really, because it's easier to see. And if you had a long length of rope and you picked up just one end of it, it's laid out

straight in front of you. Then you started to move the end of the rope from side to side, left to right back again, over and over again. You would see waves traveled down the length of the rope. But this means that the wave is traveling at like a ninety degree angle from the disturbance of the medium, because again you're moving the rope left and right. You're not pushing on the rope. You're moving the rope left and right,

but the waves are traveling down the rope. By the way, the same thing would happen if you were moving the rope up and down right. If you moved them up and down, you would still see the disturbance happening up and down, but the direction of the waves travel would be forward down the length of rope. Now, a longitudinal wave is different. The wave travels in the same direction as the disturbance rather at a ninety degree angle from it.

So in this case, if the rope behaved like a longitudinal wave, if you pushed on the end of the rope, you would see the disturbance and wave traveled down the length. But that doesn't happen with rope. However, it does happen with one of the most important tools in any serious scientist's tool box. I'm talking about a slinky for fun.

It's a wonderful toy. So if you take a slinky and you stretch it out a little bit between two people, and then you ask one of those two people to push the slinky forward really quickly, you would observe that you would see a wave travel forward through the slinky in the direction of the push. Areas of the slinky would compress as this wave travels down, and in fact, in the longitudinal wave business, we would call that compression.

Areas where the slinky links would be further apart as the wave would travel through the medium, we would call that rare faction. So you have rare faction and compression. Now for the purposes of illustration, we often will draw sound waves almost as if they were transverse waves. It's just easier to see the parts of a wave that way.

So if we did that, if we had like your your regular little graph, and we had a sound wave drawn on there, similar to a transverse wave, you would have peaks and you would have valleys, right, because that's the way we envision these, these transverse waves. And it's pretty simple to see a single wavelength, you know. You just go from one peak to the following peak. That's one wavelength. And it's not too hard to understand stuff

like amplitude and frequency. So amplitude describes how high those peaks are from the center line, or how low the troughs are, if you will, And we would say that this correlates to volume with sound waves. Frequency would be how how many wavelengths are passing a given point in a fixed amount of time. This correlates to a sound's pitch. The higher frequencies are higher pitches. But it would be more accurate to illustrate this as a longitudinal wave, because

that's how sound travels. Wavelengths and longitudinal waves refer to the distance between say one point of compression to the next point of compression, or we could actually measure it from one point of rarefaction to the next point of rare affection. That would still be a wavelength. Frequency is fairly simple. It's the number of compressions or rare factions, depending on which one you're starting with. In other words, the number of wavelengths that pass a given point in

a fixed amount of time. Amplitude is a little bit different. It's the distance between points of compression within the sound wave. If a sound wave has many points of compression that are close to one another, it's high amplitude a loud sound. If there's a wave that has fewer points of compression and that maybe they're a little further apart from each other, it's low amplitude. Its lower volume. So with sound, we have something that's causing a vibration, and it could be anything, right.

It could be a tree following in the forest as long as someone's there to hear it. It might be a flower pot falling on the pavement. It could be a fiddler drawing a bow across violin strings. It might be a mine being tortured. It could be anything. The sound travels out in all directions from the source, and it travels through whatever medium it's in. The sound most of us encounter most of the time is traveling through air,

but this also works underwater or even through solid matter. Now, if these vibrations fall within the range of human hearing, then we might very well hear them, as long as we're close enough for those vibrations to get to us before they peter out. That's called attenuation. By the way, the sound attenuates as it travels from its source, That means it gets weaker as it travels out further from the source of sound, which makes sense right. Otherwise we

wouldn't be able to hear anything. Everything would just be equally loud to us all the time, or it would be of the same amplitude as the original sound was and we'd all be deafened by it. So yeah, sound diminishes in strength as it travels. Now, the range of typical human hearing falls between twenty hurts or twenty vibrations per second up to twenty killer hurts or twenty thousand

vibrations per second. Now, as I mentioned earlier, the lower frequencies have lower pitches, So a twenty hurtz sound would be a very deep bass sound. In fact, some of those you might feel more than you hear them, depending upon your hearing, and as we get older, we typically lose some of the ability to hear the higher pitches.

I think the last hearing test I did said my hearing kind of tops out around fifteen or sixteen killer hurtz, So thatre's a lot of space between the upper limits of my hearing and the typical range for human hearing. Part of that is just because I'm old, and part of that is also because I saw a lot of rock concerts or rock shows. I can't call them concerts rock shows when I was in college, thanks to Dick Dale and the Dell Tones and the Hate Bombs and the Woggles and all the other bands I saw that

ruined my hearing. It's really my fault. I should have worn earplugs anyway. Younger people can still hear those higher frequencies typically. I mean we always have to say typically,

because there are obviously exceptions. But you may have heard that some businesses have even installed speakers that would play very, very high pitched sounds in order to discourage youngsters from loitering at that place of business, Like convenience stores and stuff, so the adults could shop in peace, and all those young, unruly hooligans would be chased out by unpleas a high pitched noise. I did a cursory search on this and found devices that are called mosquito alarms. These alarms push

out sound at high amplitude. But if you're like me and you've lost hearing in those frequency ranges, well it don't bug you none. You don't hear it even though it's being played at high volume. But meanwhile, all the little tykes grab their ears and run off, and it leaves me to buy my slim gems in peace. I'm mostly kidding about that. I'm not that grouchy. I'm not really a fan of making spaces inherently unwelcome to slices of demographics. Also, I don't eat slim gems. Apologies to

Randy Savage. Anyway. Let's talk about the perception of sound. So when sound waves enter our ears, the wave travels to the tympanic membrane, which is also known as the ear drum. So the sound wave transfers vibrations to this membrane, and it's actually doing that through very small changes in air pressure in our ear canals, so these small changes in air pressure are essentially pushing and pulling against that membrane. Now, the membrane in turn transfers those vibrations to three very

tiny delicate bones in our middle ears. Those are the hammer, the anvil, and the stirrup, and they're called that because that's kind of what they look like. And these tiny bones kind of act like an amplifier, and they send the vibrations further along into the inner ear, where a snail shaped structure called the cochlea sits. Now, the cochlea contains fluid within it, and the vibrations to the cochlea make this fluid ripple. This in turn creates a wave

on the membrane that lines the cochlea. And there are these little hair like protrusions inside the cochlea. They're called stereocilia. They vibrate from all this rippling, and they transfer those vibrations, this physical vibration into electrical impulses. Those impulse is then travel to our brains and the brain interprets the these a sound. So ultimately the sounds we encounter are in a sense all in our heads. Thus the question if a tree falls in the forest and no one is

there to hear it, does it make a sound? You could argue, really, sound only exists within the mind, and if there is no mind to perceive it, there is no sound. Our brains create the perception of that sound. So this raises a question, is what we hear actually what is happening out in reality? If somehow we were able to step outside of the experience of being human and to perceive things as they truly are, would sounds

appear to be anything like what we perceive. Now that's a question for philosophers, but you know, it sure would be interesting if we found out that every single person's experiences sound differently like. It's just that collectively we all agree that the thing causing the sound is awesome, like maybe a new they might be Giant's song, or that

it's awful like a new kid rock song. Just having fun here, But no, Seriously, because it's impossible for us to step into the experience of someone else, we can't really be sure that the way we experience sound is the same as what other people experience kind of the same way. That there's no way for me to know if the shade of blue I see when I look in the sky is the same experience you have when

you do the same thing. Now, I know I'm getting a little bit whibbly wobbly here, But acoustics, the science of sound involves a lot of psychology. If you listen to my episodes about the MP three format, you know that one way MP three's can serve filespace is the compression algorithm ditches any sounds deemed to be beyond human perception. Like if a very quiet sound follows a very loud sound, typically we can't hear the quiet one. The loud one as almost kind of deadened ust to being able to

hear the quiet one. And I'm talking about like these sounds are like back to back one raft or the other. So the compression algorithm would say, oh, well, no one's going to be able to hear the second sound in the first place, so why would be encode it? Just leave it out, and thus it conserves filespace. Of course, if you start to ditch stuff humans actually can perceive, you begin to affect the quality of sound, and then

you have problems with the decline and quality. All right, I've got tons more to say, and Obviously, we haven't even gotten to active noise cancelation yet, So let's take a quick break and we'll be right back. Okay, we're back, and we're going back to sound waves. So you might recall that the speed of light is constant, which is kind of true, but not all the truth. The speed of light is constant given the medium through which it travels.

When light from the Sun reaches Earth, it actually slows down a little bit as that light hits our atmosphere. And by a little bit, I mean a very small amount, small enough to be deemed insignificant based upon like the speed it travels through a vacuum versus the speed it travels through air, insignificant for most calculations. I mean, it does happen, but it's a very tiny change. The ratio of difference between the faster and slower rates is called

the refractive index. I'm getting a little off topic, although the same thing kind of happens with sound too, But sound, like light, will travel at different speeds depending upon the medium through which it travels. Moreover, the temperature of the medium will affect how fast sound is able to travel through it. Sound travels faster through hot air than it does through cold air. Now, when you think about it,

that makes sense. Sound is vibration. Hot air has molecules and atom They have more energy in them, so they're more inclined to vibrate. They're already moving around. They'll carry movement more easily than cold air does because cold air has less energy in it, so the vibrations come less readily. This gets reflected in the speed of sound. Sound traveling on a day that's around sixty degrees fahrenheit, we'll move

it about two and twenty kilometers per hour. But let's say it's a really chilly night, like negative sixty seven degrees fahrenheit. Then you're talking about sound travel I get around one than fifty six kilometers per hour. Though, you know, if you're out in weather that's that cold, the speed of sound probably isn't is you know, not top of mind to you. You're probably thinking, how do I get

inside before I freeze to death. Now let's get back to sound waves in order to understand how noise cancelation actually works. All right, So I mentioned earlier. Sound waves have wavelengths, and we describe the frequency of sound by the number of wavelengths that pass a fixed point in space within a given amount of time. So if we're using hurts as our measurement, the amount of time is

one second. So let's say that you are able to see sound waves and you're able to count really fast, like time to you doesn't pass the same way it does to everybody else, and you use a stop watch so that you can click down a single second. Meanwhile, you're counting all the sound waves that go past you. Well, if you did that, the number you would come up to that would be the sound's frequency. In hurts, the amount of time it takes just one wavelength to pass

a fixed point is called a period. So for a twenty Hurtz sound, a period is just one twentieth of a second. That's how long it would take a single wavelength to complete one cycle. So if we were to further divide that period into even smaller fractions, we could call those phases. To get into phases in detail would really require visual aid, because otherwise I would just start spouting off formula to you and it would all get very confusing for me. Really. I mean, you might be fine,

but I would inevitably say something wrong. So I'm not even gonna bother doing it because I'll just mess it up. But if you have two or more waves, and let's say all of these waves, these are sound waves, they are longitudinal. So let's say all these waves are aligned in that they have the same areas of compression and rarefaction. As they travel past a fixed point, those sound waves would be said to be in phase with one another. They're all kind of traveling at the same speed in

the same direction. But let's say you have sound wave A and it travels with its various compression and rare faction zones, and then you've got sound wave B. But soundwave b's compression zones are matching with a's rarefaction zones, and b's rarefaction zones are matching with a's compression zones. So they're opposite. They're like the opposite sides of a puzzle fitting in together. And let's say they match each

other in amplitude, so they're both the same volume. Well what happens at that point, Well, what happens is they cancel each other out. It's called destructive interference. Essentially, it all comes down to math. If we were to assign values to these waves, then maybe we say wave A is at value two at a given point, while wave B is at negative two. So then we add these two together two plus negative two equals zero. Now that is an oversimplification of what's going on here, but it's

basically the concept behind noise cancelation. Now, if we think of transverse waves, which I think are easier to imagine, this would be like having two waves where the peaks of wave A are matched up with the values of wave B and vice versa, and that the height the amplitude of those peaks and valleys is exactly the same, and what we get ultimately is cancelation. The two waves sound waves in this case, eliminate each other and we

hear nothing as a result. Or if you prefer those differences in air pressure that happens in our ear canals that ultimately we end up perceiving as sound. Those differences never happen because while one sound wave is pushing one way, the other sound wave is pushing the other way with equal force, and the air particles don't do anything, they

don't go anywhere, So therefore we hear nothing. But getting to that point where we could actually create technology that could detect incoming sound and then produce out of phase equivalence for the purposes of eliminating that sound. That would take a lot of time, the better part of a century actually now. One person who suggested an approach to do this was a doctor of philosophy and medicine in

Germany back in the nineteen thirties. His name was Paul Lug, and he applied for a patent that he titled process of Silencing Sound Oscillations. Now, the patent application begins by explaining that up to that point, the only way to cancel displeasing oscillations was to build in mechanical solutions at the source of the noise itself. His invention would allow for the installation of noise canceling technology that was independent

of the source of the noise. So, instead of trying to figure out a way to mechanically alter a process so that it produced less noise, you could use Lug's invention to eliminate noise no matter where it came from, because this device would be able to take in incoming noise and produce the anti phase version of it. He described the basic components of noise cancelation, and in his description he mentioned microphones or receivers which would detect incoming

sound waves and a quote unquote reproducing apparatus. In other words, essentially a loud speaker of some sort that would produce sounds having an opposite phase to the incoming noise. The opposing phase sound waves would then cancel each other out. Lug also mentioned ways to eliminate only part of a noise or only specific types of noises. So he thought this would be handy if you needed to eliminate noises and loud environments while still allowing people to speak with

one another. So how do you eliminate noise but you allow signal to get through? That was what he was talking about. He was also thinking about ways to eliminate unwanted noises in places like say a theater or a concert hall, where maybe you go to the concert hall and the music is fantastic, but there's some element, some acoustic element in that environment. This is producing something that was not wanted, that is detracting from the experience of

hearing this concert. Luke said, well, we could create this technology that would detect this unwanted frequency, like we would tune it to that and then produce the anti phase version and then you could just enjoy the concert as it was intended, without any of these unpleasant secondary noises

mixed in. Now, Luke's idea was pretty solid. Unfortunately, the technology was nowhere near where it needed to be in order to actually realize his idea was He did get his patent, he got that, granted, but he was getting a lot of resistance in the academic world of Germany because I think a lot of other people realized they just couldn't accomplish what he was suggesting. Not that what he was saying was impossible from a science perspective, but

on a technical level they couldn't figure it out. And moreover, things in Germany were obviously getting rather tense in the nineteen thirties as the world continued to plunge toward total war in the region. Now I tried to find out more information about lug and I did discover a paper that was written in German that was all about him and his life both before and after his proposed invention. Now,

my German is awful. I am not at all fluent, so I depended heavily, really entirely on Google Translate, and was only partially successful in translating the article because the

translation just wasn't fantastic. However, from what I can figure out, Luke discovered that some engineers in England were attempting to build an invention that was really similar to the one he described in his pat and he suspected that someone somewhere along the way had leaked his invention and allow people in England to read about it and try and create the thing that he had proposed, So he wrote a letter to the German Patent Office demanding an explanation.

This turned out to be a bad move because the Nazis were steadily gaining control of all of Germany's political systems and they were not super keen to be called into question, and they in turn decided to turn scrutiny onto Luke, and he was reprimanded for his insolence, and apparently he spent the next decade in fear of what might happen to him and his family. You know, those Nazis held a grudge. And he also couldn't get a job as a physicist. It appeared that he had been blacklisted,

so instead he trained to be a doctor. Now, he survived well after the war, but from what he can tell, he never really got to work on noise cancelation again. Instead, we're going to have to leap on over to the nineteen fifties. An American scientist and engineer named Lawrence J. Fogel would become a key figure in no cancelation, to the point where many will refer to Fogel as the

inventor of active noise cancelation. I'm not sure that's entirely fair, because you know, Lug certainly patented it back in the nineteen thirties, but he never got it to work, Like the technology wasn't there, So I guess you could argue that Fogel is the inventor in the sense that Fogel was able to create an actual working prototype of noise reduction headphones. He had done a great deal of scientific

and practical engineering work in wave dynamics. This guy was like super smart and studied lots of different disciplines, and he knew a lot about wave interactions and wave interference. He had worked in not just sound waves, but like electromagnetic waves VHF and UHF waves, and he understood about waves and how they perform when they are in antiphase with one another, how that can result in destructive interference.

He also knew there was a need for better ear protection for folks who were working in very noisy environments, specifically in the cockpits of aircraft like helicopters and airplanes, and he theorized that he could build a set of headphones that could diminish or even eliminate the noise of propeller driven aircraft so that the pilot could focus their attention on operating the aircraft and also hear communications over their headset without the interference of this incredibly loud noise.

And around the same time, the US Air Force was researching ways to protect hearing while still allowing for communication, and one of the projects involved active noise reduction. I'll talk more about that in just a moment, but first let's take another quick break. Okay. Before the break, I mentioned that the US Air Force was looking for ways to reduce noise in cockpits, and ultimately the Air Force would pursue both passive and active measures to help protect hearing.

So again, passive measures are things like ear plugs or insulated ear muffs that end up sealing the ear away from noise so it's not canceling. So much as just blocking. They also looked at active noise reduction and began to develop their own version, largely based off the same work that Fogel was doing. Now, ultimately the Air Force produced some ear muffs that could reduce noise in the fifty to five hundred hertz band of frequencies, so lower pitched frequencies,

and had an attenuation of twenty decibels. Now, again I mentioned earlier what attenuation is that sound waves decrease in intensity as they travel, and the further they go, the more their intensity decreases, until they are no longer audible to the typical human being. That reduction in sound amplitude again is called attenuation. But then what's a decibel. So decibel is a unit of measurement, but it's one in which you're describing the relative strength of two signals, and

it's also a logarithmic metric as well. So this gets technical. It's the logarithmic thing trips a lot of people up. If you have two sounds and the second sound is twice as loud as your reference sound, you would say that the second sound is a little more than three decibels louder than the first. It gets tricky stuff. Now, generally speaking, zero decibels refers to the least perceptible sound, like you can hear it, but if it were any

more quiet, you wouldn't. A ten decibel sound would be approximately ten times louder than that reference sound, but a twenty decibel sound would be one hundred times louder than the reference sound. So human conversation is generally considered to be around sixty decibels, unless you're talking to me, in which case it'll be much louder because I am obnoxious. A rock concert is regularly in the one hundred twenty

decibel range or even louder. That's also when you're getting into the range of where loudness can cause hearing damage, so where ear plugs when you go to your concerts, folks. And also a minus twenty decibel attenuation could be the difference between hearing loss and keeping safe from hearing loss. So while all this work was going on, noise cancelation technology remained largely in research facilities in various air vehicle

cockpits and military applications. In the late nineteen seventies that would start to change, Doctor Amar Bows of the Bows corporation decided to work on developing a consumer grade noise reduction or cancelation headphone. He had apparently taken one too many noisy airplane trips, so the industry had really taken some steps to increase passenger comfort. But in the old days, my drugies, the headphones and passenger planes worked via pneumatic tubes,

kind of like a stethoscope. So you would wear a pair of hollow, flexible tubes that ended in little ear pieces, and these would carry sound from a small speaker hidden inside the armrest and would carry the sound from the speaker up to your ears through these tubes. So if you actually put your head very close to the armrest, you could listen out of the lead a beaty speaker

in there. But all this changed with the introduction of the Sony Walkman, which introduced inexpensive, mass produced electronic headsets. Before those were just not a thing. You just didn't have the cheap headsets really, But with the Sony Walkman,

that would change. You can actually listen to a recent tech Stuff episode about the Sony Walkman to learn more about that tech Bose would produce headphones that would have a microphone set in each ear muff actually on the outside of each ear muff, so the microphones led to circuitry that could detect the incoming frequencies and amplitudes and direct speakers in the ear muffs to produce an anti

phase signal. This all happened fast enough so that the external sound waves and the internally generated anti phase signals would both hit the wearer's ears at the same time, thus canceling each other out. So you think about that sound is coming at you. Before the sound can get to your ear, it hits the outside of your headphones.

There a microphone picks up that sound, and through this processing is able to figure out how to produce the equal but opposite sound wave, and then both that and the original sound from outside of the headphones combine and you get silence or you get no noise. I guess it depends on how you think about it. Now, this all kind of sounds simple at least in concept, but leaving it making it a reality was far from simple.

Bose and his team would spend a decade and a half and more than fifty million dollars trying to figure out how to make this technology effective enough and reliable enough for the general consumer, how to have it where you know you can actually draw power from something to do this, because obviously that active part of active noise canceling requires power to work. It's not like a microphone and speaker is just going to work without any electricity. So part of this was figuring out a way of

powering these headphones, preferably without requiring a wired connection. Like a wired connection might be needed for you to be able to play audio through the headset, whether that audio is coming from a communications device or the entertainment system of an airplane, but you don't want to have to draw power as well. So the goal was to create battery powered headphones, and it would take a long time

for that to become a consumer product. The earliest mention I can find of a commercially available noise reduction set of headphones was in nineteen eighty nine, when Bose released the Series one Aviation headset, But even then this was a product intended for the aviation industry, as the name suggests, and it was good for reducing noise, but not eliminating it. It would take another decade for the technology to start

appearing in mainstream consumer gear. In nineteen ninety nine, after Bose had been producing active noise reduction and active noise canceling headphones for the aviation industry as well as the military, the company introduced a consumer model specifically for the Hoidy toidies who were traveling first class on American airlines. The movers and shakers appeared to really like the effect, and Bose decided to introduce a consumer available version of the

technology in two thousand. It was called the Quiet Comfort Series. Other companies started doing the same. Sennheiser actually created active noise canceling headphones for pilots at Luftonza in the mid nineteen eighties, so they were producing their headsets the same time BOS was, in fact, at least for the If you're looking at the first series that BOS released, Sennheiser came out with THEIRS a couple of years earlier, but Bose had had prototypes for several years at that point too,

so it's kind of complicated. These days, lots of companies

have noise canceling or noise reduction headphones. Apple released the Airpod's Max headset back in December twenty twenty, with a Bluetooth headset this year, so nos released its wireless noise canceling headphones, the so nos ace so the technology, even though it has its roots in the nineteen thirties, it's only been available for customers like you and me for the last couple of decades, and for those of us who are perhaps not in the tax bracket that travels

first class in a regular base. It's a relatively recent development. They're pretty neat, and if you really like to focus on music or other audio, they are essential. I certainly think of them as being a necessary technology to bring with you when you're on airplane trips. It is a sanity saver, just reducing like the hum of the aircraft and all that kind of stuff. It doesn't eliminate all noise, like you could typically hear certain things outside of bands

of frequencies. A lot of these are designs that you can still hear if someone talks to you. And of course there are also a lot of headsets that have like a pass through feature where instead of blocking noise, it allows external noise to reach your ears more clearly, so that you can stay aware of your environment. That way, if you're in someplace like let's say you're walking along a warehouse floor or something. You're not going to be taken by surprise by like a forklift or something like

that just suddenly appearing behind you. So there are variations this technology as well, but that's how it works. I hope you found that interesting, and I hope you are all doing well, and I'll talk to you again really soon. Tech Stuff is an iHeartRadio production. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.