Noise Cancelling Shnozphones

Yesterday a friend sent me a link to this crowdfunder on Indiegogo, with the appended question: “This has to be ludicrous BS, right?”

The alleged ludicrous BS is a device called Silent Partner which claims to ‘quiet snoring noise’ (we’ll examine those exact words in a bit) through the mechanism of Active Noise Cancelling (or the popular acronym ANC).

You will be familiar with ANC if you’ve ever worn a pair of noise-cancelling headphones on a plane. Pop those suckers on, and the sound of the plane vanishes almost magically. You can actually hear the in-flight movie soundtrack.

The technology behind those headphones is really quite simple, and has been known for over half a century. In this post we’ll delve a little into how noise cancellation works, what it’s used for, and why Silent Partner is almost certainly BS. Not as ludicrous as it first appears perhaps, but let me just say that I’m not pitching in to the Indiegogo campaign any time soon.

The Acoustic Principle Behind Noise Cancellation

As you probably know, sound propagates through a medium via waves. Most people have seen graphic representations of soundwaves these days. They look something like this:

Screen Shot 2016-03-10 at 11.11.53 AM

That’s a fairly complex soundwave. The simplest manifestation of a sound is a sine wave. This one represents a pure tone, like you might hear if you strike a tuning fork:

sineposgraph

The red bar shows you the frequency (pitch) of the sound and the green bar marks out the amplitude (loudness). I’ve marked the highest and lowest amplitude with an arbitrary value to aid explanation. One thing to understand – and is easy to see in this simple sine wave – is that as a wave propagates, it changes its amplitude value across the zero axis; there is a time when the amplitude has a positive value, and a time when it has a negative value. This is called the phase of the wave, and this is true of all waves, not just sound waves. So in this case, the frequency of the sound stays exactly the same (a constant pitch), but the amplitude is wavering between positive and negative, as it does in all sound waves.

You can’t hear this constant amplitude change. You are only aware of the overall amplitude between the peak and the trough of the sound – this is the volume, or loudness, of the sound.

Somewhere, some time ago – no-one really knows when, but we can guess it was likely to around the beginning of electronic sound recording – someone discovered that it is possible to artificially ‘flip’ the phase of a recorded sound wave.*

sineneg2

Here’s the thing: this flipped sound wave will sound to your ear exactly the same as the one we depicted above. EXACTLY. If I played them to you one after the other, you would not be able to tell which one was which. However, if we artificially combine both these waves so that they are coherent – that is, they start at precisely the same time, and stay in step with one another – then the positive value of the first peak of the sine wave is completely cancelled out by the negative value of the first trough of its phase-reversed counterpart. It’s just simple mathematics: you add +1 to -1 and you get zero. The result is total silence. The positive ‘loud’ bit of one wave totally cancels the negative ‘loud’ bit of the other.

The first time this was demonstrated to me as a sound student, I was astonished. It seems like a weird kind of audio magic. Nevertheless, it’s just a property of wave mechanics, and we use this phase cancellation trick in numerous different ways in the pro audio business.

How Does Phase Cancellation Work in ANC Headphones?

Even though we’ve been looking at an example of a pure sine wave, phase cancellation works on any complex audio wave you care to submit to it. You take a recording of Kanye West, flip it out of phase and recombine it with the original and zap, perfect silence. An exemplary case of nature balancing itself for the better good.

The only condition is that the two waveforms must be completely in step with one another – coherent.

One day it occurred to someone that it might just be possible to accomplish this trick in real time, with live sound. This is how it would work in theory: you’d aim a 180º out-of-phase microphone at a sound and pump it through a speaker in the vicinity of the actual sound. The amplified phase-inverted sound would interact with the real sound and cancel it out. And indeed, it does work. Sort of. It’s very dependant upon where the listener is positioned in respect of both the phase-inverted source and the real sound. Too close to one or the other and the necessary coherence between the two sounds gets out of step. The effect completely vanishes. It works, then, as long as you tell your listener to sit exactly there, and don’t move your head even the tiniest bit. Not particularly satisfactory for most practical purposes.**

If only you could stick the inverted phase sound right at the listener’s ear drum, where it would arrive at precisely the same time as the real life sound… and of course, you can do almost exactly that, providing that the listener is wearing headphones.

So ANC headphones have a little microphone on each earphone, which captures almost the identical sound that is heading toward your eardrums. That sound is then amplified to the exact same amplitude as the original, and inverted in phase. It arrives at your eardrum at the same time as the real sound, effectively cancelling it out. It’s a very cool trick, but it’s highly reliant on the microphone and the speaker diaphragm being very close together, and both those things being as close to your eardrum as is technically – and biologically – feasible.

That All Sounds Plausible! What’s the Problem with Silent Partner?

This is where things get a little more complex than they have been so far, but you already know most of what you need to know to follow me.

I’m speculating that this is how Silent Partner is intended to work: the sound of the sleeper’s snoring is picked up by some microphones – I assume this is what is designated as the sensor in the rather lite ‘technical specifications’. The captured audio is then phase-inverted, amplified and played back through the speakers and ‘resonance chamber’, where it presumably combines with the sound of the actual snoring with the intention of cancelling it out.

You can see one big problem already: as I’ve mentioned above, the most effective sound cancelling happens if the coherent waveforms arrive as close to the listener’s ear as is achievable. In this case, we have something of a distance between the snoring source and the beleaguered sleeping partner – plenty of distance for the waveforms to dis-cohere, and also plenty of distance for another wave-related phenomenon to come into play. I’m talking here about polarisation. Without getting too technical about it, waves are not just two-dimensional like the representations in the graphs above. They are fully 3D, and so for exact noise cancellation to happen effectively, the waves and their anti-phase cohorts must be fully coherent in those three dimensions. This is really hard to achieve in a real-world environment, even with precise, point-source sound. It’s especially hard to achieve when you consider the nature of snoring, which doesn’t actually come from just the nose, as anyone who’s slept with a snorer knows. So, even in the best possible technical case, with the Silent Partner reproducing the snoring sound in every tiny detail of frequency and amplitude, it’s not going to be aligned in proper polarity and coherence with the sounds coming from the nose, throat and chest cavity of the snorer. It just can’t be.

The first clue we have that the makers of Silent Partner are either unaware of this issue (odd, since they supposedly trialled a proof of concept in May 2015), or worse, are being disingenuous about it, is in the video clip. It pertains to their demonstration of the ‘Silent Zone’ around the snorer. In the clip you see a ‘scientist’ holding a decibel meter close to the nose of the snoring person, and showing how the sound level drops as you get further away. Well, yes, this is exactly what you’d expect with normal snoring – sound falls off with distance according to the well known properties of the Inverse Square Law. But this is not what you’d expect to see if the Silent Partner was working as claimed. What you’d actually observe is no change in the sound level. The noise-cancellation effects should be in play – and in fact optimal – right at the source of the snoring. The sound level test in the video is showing us nothing at all but what you’d expect to see with natural snoring.

In fact, the whole meter thing is quite amusing if you know anything about sound. If the meter is measuring decibels (and I’ve no reason to think that it’s measuring anything else – that’s what sound level meters usually use) it’s showing that the snoring volume is dropping from about 84dB to 60dB. To give you a sense of that in terms that might be more familiar, that’s about the level of noise in a moderate factory or a busy road, down to the level of a normal conversation. I’d expect average snoring to have less effect on a dB meter! It’s certainly not an endorsement of a Silent Partner working as claimed.

And all this assumes, as I said, a best-possible case: that the Silent Partner is exactly reproducing the snoring sound. I’m skeptical that it could do even that. Snoring is made up of highly complex human sounds, and is not just one rhythm or frequency. And, as I’ve also pointed out, it doesn’t just come from the nose and mouth, so the entire upper body can contribute to the frequency characteristics. I would be very surprised if tiny components such as those in the Silent Partner – even with resonance chambers – could deliver an acceptable frequency response to enable the kind of phase and frequency matching required for this technique to work.

Aside from all the sound stuff, something just feels really fishy about this whole thing. First of all, the snoring-relief market is chock full of pseudoscience, which indicates an area of high subjectivity and therefore exploitable credulity. You can get anti-snoring pills, anti-snoring rings, anti-snoring pillows, anti-snoring homeopathy, anti-snoring acupuncture and so on and so on, none of them based on even the most delicate whiff of science. So it’s a field rich for the pickings, as they say.

Furthermore, Silent Partner deploys a technique beloved of purveyors of said pseudoscience: it piggybacks on something that to the layperson sounds credible and possible, but brushes a lot of actual dirt under the Carpet of Mystery: “It’s like magic!” boasts the hyperbole – a claim which is actually probably quite true, because it would be magic if it worked, since it defies the laws of physics. Like many pseudoscientific gadgets and gew-gaws, it fails to provide any real science to substantiate its claims, just a lot of hints at how it might work based on other things that do work. ***

There are many other red flags:

• The Silent Partner website is full of exhortations to Order Now! I don’t know about you, but for me this may as well say Give Us Your Money Quick, Before The Truthful Reviews Start Rolling In! Although the Indiegogo timeline includes a proof-of-concept as of May 2015, there is no indication that this showed any kind of convincing result. If it was my product, I’d really want to demonstrate that the thing worked. Not that it seems to matter, evidently, since the campaign is already way over-funded.

•There is a vagueness about the whole affair that is offputting. As we saw at the outset, the gadget claims to ‘quiet’ snoring – but that’s a very subjective thing. Nowhere do we find any definitive terms about how much it will quiet the snoring. The one piece of solid empirical data comes from the decibel meter in the videop clip, and I can tell you, that’s not showing any ‘quieting’.****

•The ‘team members’ for the campaign include far too many marketing and advertising people for my liking. Because that’s what the whole thing smacks mostly of – marketing.

•There are some uncomfortable legal problems. The Indiegogo campaign for Silent Partner holds a disclaimer that says

“Participation in this campaign is in accordance with the Terms and Conditions listed on the Silent Partner website.”

As of this writing, there are no Terms and Conditions listed on the Silent Partner website. There is a link there, alright, or at least words saying ‘Terms and Conditions’, but clicking on them does nothing. I could not find any such terms elsewhere on the very spare site. I think you can see that this is a legal disaster. People who have contributed to the Indiegogo campaign have agreed to do so under non-existent terms. Or terms that don’t exist yet. The makers of Silent Partner could, at any time, write a contract that simply says “I agree that the product might not work as stated and I understand that I will not get my money back under any circumstances”. And then make the link active.*****

In this legal respect, I also feel that Silent Partner might be wandering into the dubious area of being a medical device. This is a highly-controlled and legally rigid field. I’m sure they will vehemently deny it is such a device, but snoring can be a sign of underlying medical problems, and it’s an issue that you should take up with your doctor if it’s a significant detriment to your sleep [to be fair, the FAQ on the Indiegogo campaign site does indicate that you should consult your doctor if you have sleep apnea, but sleep apnea is only one of quite a few serious snoring-related problems].

To Wrap Up

My professional audio experience and technical knowledge tells me that Silent Partner is quite unlikely to show results in the manner claimed. Indiegogo provides little redress for failed campaigns, and especially for investors getting their money back. This, and the fact that there are no Terms and Conditions listed anywhere on the Indiegogo page or on the Silent Partner official site should make someone think twice before spending their money on this product.

But hey, you did learn a thing or two about noise cancellation and acoustic science, right, so your time here has not been entirely without benefit…


Footnotes:

*It is possible to hear naturally-occurring phase related phenomena. For this to happen, a sound needs to be combined with a copy of itself in such a way that it has some semblance of coherence and phase alignment. The most common manifestation of this is with reverberation or echo. You may have experienced this as a kind of ‘dead’ spot with your tv sound, perhaps; when you sit in a particular spot in the room, the sound seems to be muted and quiet, compared to most other places in the room. This is because the phase-changed reflections off the wall are combining with the actual tv sound to effect a phase-cancellation situation. It will never be as perfect as an electronically contrived version, but it can be quite profound given the right circumstances.

**Car manufacturers in concert with electronics firms like Harman and Siemens – have been playing with ANC for a fair few years now. Because the concern here is mostly with inhibiting road rumble, it’s actually not out of the question that it can work; longer, lower waveforms are less directional for the human ear, and so the necessity for the listener to be in a ‘sweet spot’ is not nearly so critical. Nevertheless, the technical requirements are still complex, and it hasn’t so far caught on in a big way.

***It also uses technical waffle: “subwavelength active noise cancellation” is an entirely meaningless term. Sub what wavelength? The clip with the decibel meter is technical waffle too, which as we’ve seen means precisely nothing.

****An expert audio colleague of mine points out that the sound meter display in the video clip is actually an added visual effect. It’s hard to know what to make of this – the inference is of course that the creators of Silent Partner consciously chose the numbers that are being displayed – numbers that are, as we discussed previously, really only indicative of rather loud normal snoring. We could speculate that this a a kind of ‘cover your ass’ move, inasmuch as they can always point to it and say “This is all we promised it would do…”

*****There are a few non-active links on the site. It opens up the possibility of jiggery-pokery. In the past I’ve seen this problem and commented on it and then, mysteriously, the links work – and the content under them is as outrageous as you expect. I’m not saying the Silent Partner people are doing this, just that it does happen with rather alarming frequency with these kinds of products.

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