My car's software finally updated last night and I see I got the ANR feature....as a sound guy I had to look into this further. Took a drive and listened. It can be hard to hear if you dont know what to look for, plus it REALLY depends on the road surface. It is the low frequency tire rumble that primarily gets filtered (pretty much what would be expected).
I've read these threads and its pretty clear a lot of people don't really understand how this works....so I am going to do a quick A/B measurement to get a little data out there that can hopefully make things more clear. This is a just a rough frequency level reading, did it quick and dirty with an iphone and the RTA app instead of my laptop and calibrated mic, and there are a ton of variables, but for the purposes of understanding ANR, its good enough.
Turned off ANR. Drove on a consistently blacktop stretch of highway, turned on autopilot at a constant speed, set the RTA app to do a 3 second average (averages help to smooth out the dB readings). Held the phone mic facing up next to my ear for more than 3 seconds, recorded the result.
Turned on ANR. Repeated measurement.
I did this in a small window of time to minimize any variables from changing road surface, other cars, etc...
I then overlaid the two readings, they were close to identical EXCEPT from 70Hz to 250Hz where we see between 1dB to 4dB of reduction. With a maximum difference occurring around 125Hz (in this scenario at least). As a reference point a reduction of 3dB is equal to half as much acoustic energy.....so that's a lot of cancellation going on in that frequency range.
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Sound has a wave length that varies by frequency. The lower you go the longer the length of the sound wave.....the wavelength. In a closed space like the cabin of a car you eventually reach a point where the wavelength is longer than the interior space of the car. It wont fit. At this point it can no longer exist as a wave and just becomes pressure, cycling at the frequency of the sound. To a reference points....the 125Hz sound we are talking about above has a wavelength of around 9'. About the longest dimension of the Model S interior. So its definitely getting into that area where its becoming pressure more than a wave.
To cancel out that sound you play the speakers (probably just the subwoofer in this case if I had to guess) out of phase with that pressure creating destructive interference at that frequency and reducing the sound that gets to the ears of the occupants. Practically, you can only cancel a sound in a car once the frequency gets low enough that it becomes pressure, or perhaps you can stretch it to where the wavelength is long enough that the front/rear, right/left passengers are all existing within the same peaks and troughs of the wave. Trying to cancel frequencies above this point gets problematic as what might have
destructive interference (cancellation) at the front seats may have
constructive interference at the back seats....and you dont want that!
At frequencies above the point where the wavelengths are shorter than the size of the cabin, they can propagate around the car, reflect off hard surfaces, get absorbed by softer surfaces, and interfere with each other constructively and destructively in virtually infinite ways....basically its chaos....and it is not really practical to try to cancel out infinitely variable chaos.
If I had to guess, Tesla will probably refine this some, programming the software to more effectively target some sounds, ignoring others, sussing out mechanical noises like the HVAC or certain specific tires, also while tuning to the auditory perceptions of most people. Plus working out some bugs from what i seen in this thread... Overall I would expect the effect to continue to be subtle though, there are limits to what is possible/practical in this application.
Now I see a lot of people trying to compare this to noise cancelling headphones....you cant do that.
With a noise cancelling headphone the enclosed space and distance from the speaker to your ear is WAY smaller, maybe 1", so you CAN theoretically cancel sounds effectively up to that wave length....a 1" wavelength is equal to about 13kHz for instance....that's in the top octave of human hearing. So noise cancellation in that headphone can exist over a massively larger frequency range, plus WAY less chaos as its just the headphone, the ear, and a couple cubic inches of air space to deal with.