What do you know about Active Noise Cancellation?
September 6, 2016 9:45 AM   Subscribe

The vehicle I'm considering purchasing has Active Noise Cancelation, a feature that apparently can't be turned off. I'm worried that it might be disturbing to my dogs but don't really understand how it works. I have heard people complain about feeling like they need to pop their ears when using the Bose noise canceling headphones and wonder what is at work there. Can anyone shed any light on this? I'll put the manufacturer's description of the system inside.

Active Noise Cancellation technology deflects unwanted sounds with opposite-tuned frequencies and enhances "pleasant" powertrain sounds. It is packaged with the six-speaker sound system. Four microphones in the cabin detect unwanted noises. The system monitors engine rpm to identify harmonic range. The Active Noise Control (ANC) module in the audio system generates a tone, which goes through the adaptive filter to create the exact frequency required to cancel the noise present in the cabin. The system continuously monitors cabin noise to adjust the adaptive filter as necessary.
posted by HotToddy to Technology (14 answers total)
Because physics, full spectrum noise cancellation doesn't really work for zones, only for headphones. It might work at lower frequencies to kill engine noise, road rumble and tire noise, but not higher frequencies. And I believe that's all these new systems are claiming to neutralize -- low and mid frequencies.

If it claims to neutralize higher frequencies, I bet it only does that in the driver's position, and other parts of the cabin get TWICE the noise.

The ear popping effect is because it's so quiet, it "feels" like your ear canals are blocked.
posted by intermod at 9:53 AM on September 6, 2016 [1 favorite]

Active noise reduction works on the principle of destructive interference. If you go back to high school physics, sound travels as a wave - if you have two waves of the same frequency, you can sum the amplitudes at any given instant of time to produce a new wave with the same frequency and a different amplitude.

The way ANR cancels out noise is to use the microphones to shift the cabin noise 180 degrees out of phase, so that the peaks of the original wave coincide with the valleys of the new wave. When you sum them, +A and -A yield zero (ideally), which is what you end up hearing. Your car's system probably also mics the engine and adds in engine noise to the system to get that "pleasant" engine hum.

The system's effectiveness depends on the sensitivity of the microphones it uses and the output of the speakers. For ambient noise, the goal is usually to dampen the low frequencies which can be damaging to your hearing. The dogs should react the same way to the system as you do, but if it's bad at eliminating high frequencies they may notice that more.
posted by backseatpilot at 9:54 AM on September 6, 2016 [1 favorite]

I don't know anything about the system, but I would ask if you could take a test drive with the dogs and see how they react...
posted by neilbert at 9:56 AM on September 6, 2016 [1 favorite]

And what if the system is working imperfectly? I'm wondering if it would be annoying the way that fluorescent lights can have an imperceptible flicker that induces a biological response. Is it a reasonable concern to think that the system can malfunction like anything else, but that they probably wouldn't be able to accurately diagnose it, much less repair it?
posted by HotToddy at 10:21 AM on September 6, 2016

Anything is possible, but it would extremely unlikely that the system would malfunction in such a way that it would keep working but somehow induce some sort of biological response. It is a digital system, not something that can drift out of alignment or something like that.
posted by ssg at 10:32 AM on September 6, 2016 [1 favorite]

Is it a reasonable concern to think that the system can malfunction like anything else

I don't see how. I don't see a way that the microphones would slightly mishear the noise and send it back off set a bit. Even if it did, you'd just end up with more cabin noise.

I think you're worried about nothing and your lack of understanding of the system is creating issues that don't exist. The car just plays anti-noise into the car to cancel out (some of) the noise. Even if there is an issue with the system it's no worse (even if the system fails completely) than being in a car with the radio on. There is MORE noise (total) in that situation, when comparing to a dogs ears, and the worst case in any car of highway speeds with the windows open doesn't seem to bother dogs.
posted by Brockles at 10:40 AM on September 6, 2016

Is it a reasonable concern to think that the system can malfunction like anything else

yes, it's completely reasonable. there's nothing magical about digital systems that makes them immune to failure. i spend most of my days fixing software errors and the naive techno-optimism here from people who should know better is amazing. active feedback systems are notoriously hard to (1) get right and (2) make reliable.
posted by andrewcooke at 10:46 AM on September 6, 2016 [1 favorite]

It is worth saying, probably, that an active noise cancellation system will not produce, say, any ultrasonic sounds that only dogs can hear. The system can't effectively cancel high-frequency sounds to begin with (because physics, as already explained); in fact, it would only make them worse, so they would be blocked entirely by a low-pass filter. The vehicle's audio system probably can't even reproduce such high frequencies anyway.

The system could theoretically get out of whack in some way (though this is not very likely, IMHO), but it is very unlikely that it would bother dogs without also bothering you, and then you'd notice and get it fixed.

Just by the way, this is a feature of the car's audio system. The same DSP, amplifier, speakers, etc. that play back music also are used to cancel some road noise. So you could probably turn it off by pulling the fuse for the stereo, if you wanted to hear how noisy the car really is, just for giggles. (I just Googled it actually, and you can also cover up or unplug the microphones used by the system if you want to disable it.)

Dogs are popular, and ANC systems have been around for years, so you can safely assume assume that you are hardly the first person with dogs who bought a car with an ANC system. The near complete lack of complaints about ANC systems from dog owners should put your mind at ease about their interactions.
posted by kindall at 10:57 AM on September 6, 2016

Just to make sure I understand: I get that the system can't effectively cancel high-frequency sounds or generate ultrasonic sounds, but is my understanding correct that it works to cancel low-frequency engine noise by generating high-frequency tones? My concern isn't necessarily with ultrasonic sounds that only dogs can hear, it's with very high-frequency sounds that might be agitating to them or to us, even if we can't discern what's going on, and especially if the system is not working perfectly, just as fluorescent lights can cause problems with imperceptible flickering. Also, would disconnecting the microphones have any other consequences? Because I would be fine with doing that.
posted by HotToddy at 11:23 AM on September 6, 2016

No, the system generates low-frequency tones itself -- they are just at the opposite phase from the ones the engine produces, so they cancel each other out.
posted by wyzewoman at 11:27 AM on September 6, 2016 [5 favorites]

but is my understanding correct that it works to cancel low-frequency engine noise by generating high-frequency tones?

No, that's not quite right. This is what backseatpilot said, in a slightly different way:

Picture a sine wave. It goes up and down and up and down.

Now, put another wave on top of it that goes down and up and down and up--basically, a mirror image.

When you add those two waves together, they sum to zero: the high bits of the first wave cancel out the low bits of the second one, and vice versa.

Frequency is how fast that wave is going up and down. The sound wave that the noise canceling system generates will be at the same frequency as the one it's canceling out. It's not high frequency cancelling out low frequency.
posted by damayanti at 11:29 AM on September 6, 2016 [1 favorite]

+1 + (-1) = 0
posted by TrinsicWS at 12:48 PM on September 6, 2016

Another thing to consider: this may not be an issue with a car, but some people get dizzy/nauseous when using ANC headphones. I had a pair of (very expensive) earbuds that did this, it was bizarre: click the ANC on, instant nausea, turn it off, feel better. It may be that the car isn't as severe an effect, but after my experience with the earbuds I wouldn't want to try it. Most people do not have this reaction, though, so who knows what your experience would be like. Can you test drive?
posted by epanalepsis at 1:57 PM on September 6, 2016

This video, while directed at recording audio, gives a pretty good visual explanation of phasing. It's cued to start at the relevant piece on how the sine waves cancel each other out.
posted by Deflagro at 6:04 PM on September 6, 2016

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