Explain it like I'm five: high-pitched sounds and playback
July 8, 2021 9:39 AM   Subscribe

My husband is older and has experienced some hearing loss. We have bats in the wall of our house (that will hopefully be tended to soon.) I could hear a bat squeaking in the wall and brought my husband over and he couldn't hear it. But when I held my phone up and recorded the bat squeaking, and then played the recording for him, he could hear it. Why is this the case?
posted by PussKillian to Science & Nature (8 answers total) 2 users marked this as a favorite
Best answer: One possibility is that the phone is creating lower frequency sounds than the input. The two likely culprits are aliasing when recording the sound or exciting lower frequency vibrations in the mechanical speaker when playing it. Aliasing happens when you digitize a sound that's more than twice your sampling frequency - it incorrectly presents power from higher frequencies into lower ones. Most analog-to-digital converters in phones have pretty good filters to prevent it, but there may still be some signal left after filtering. If it's loud and there's no other signal, it might be audible. Playing very high frequencies into speakers that aren't meant for it can lead to lower frequency vibrations.

If you have a second phone or computer with a microphone, using an audio spectrometer program to see if there are lower frequency components might be fun. Spectroid is a good choice in Android. Audacity has a power spectrum plugin on any windows/mac/linux computer. (Both will also include the filter associated with the microphone, which may be significant.)
posted by eotvos at 9:53 AM on July 8, 2021 [7 favorites]

Your phone compressed the sound and probably also altered the frequency.
posted by 0bvious at 9:55 AM on July 8, 2021 [9 favorites]

Could it just be the case that he was able to get closer to the speaker of the phone than he was to the bats in the wall?
posted by fight or flight at 9:55 AM on July 8, 2021 [3 favorites]

Did you turn the volume all the way up on your phone? Maybe you amplified the sound so it would be loud enough that he could hear it.

The other possibility is that the bat's squeaks are outside the frequency range supported by your phone's audio recording system. The range of frequencies that can be captured in a recording is not infinite. Your phone's recording hardware measures audio signals at a certain rate, typically 44.1 kHz—44,100 times a second. This limits the high frequencies that can be recorded to 22 kHz.

Any sounds at frequencies above that range can get mirrored back down into a lower frequency, meaning they appear in the recording as a lower-frequency sound, which may be audible to your husband.

Usually there is something called a low-pass filter which restricts upper frequencies so that they can't be "aliased" into a lower-frequency sound. I'm not sure if a typical phone has such a filter, but if it doesn't, this could explain why your husband was able to hear it in the recording—the bat's squeaking was artificially lowered in frequency by your phone to something your husband could hear.
posted by vitout at 9:58 AM on July 8, 2021 [3 favorites]

You can download an app that will sweep frequency and test how high you can hear vs your husband. One is called: tone gen.
posted by St. Peepsburg at 11:28 AM on July 8, 2021

eotvos' explanation sounds like the right one to me, and if you'd like a visual demonstration of a similar phenomenon, try pointing a regular infrared television remote at your phone's camera. On the screen you should see a red light, while your eyes will not be able to perceive that on the remote itself.

The phone's camera can pick up frequencies outside the range of normal human sight and translates them into the closest color it can display on the screen. That seems to be what may be happening audio-wise with the microphone and speaker, translating a sound frequency into a range that the speaker can reproduce.
posted by subocoyne at 12:01 PM on July 8, 2021

Best answer: I can take a stab at the 5 year old part, mostly about the input, compression and then amplification.

First up, the input. Both the microphone you used and way the sound recording was saved to your device only capture a small part of the original noise. The point is to reduce the amount of data that needs to be saved, and this has been optimized for human voice. The phone microphone only picks up some parts of the bat noise - do you remember what an old time analog phone sounded like? That was because only a very narrow range of sound was picked up by those old microphones. You can get the same effect using an equalizer and filtering out everything but the "mids" and keep only the noise between about 400 Hz and 4 KHz. That's old time phone voice. So your cell phone microphone is 'better' than that but it's fundamentally the same and it's to avoid your device from stuff like picking up wind noises. So first thing - lots of the bat noises aren't captured, especially those sounds that humans can't make, but you can hear and your partner can't.

Once your phone has this filtered noise from the microphone it is going to smash it down to make a much smaller file size and it just throws out a whole lot of information using fancy math and a general idea of what the average human won't notice is missing. Upwards of 80% of the noise is actually tossed straight out during compression, and I expect quite a bit of the bat sounds get binned at this stage. This is now a digital representation of the bat noises.

Finally playback, which is this whole process in reverse - the recording is de-compressed and then played through small speaker(s) that can't accurately reproduce all noises, but can be made louder. The result is only the part of the bats noise that the microphone picked up and made it through compression remain, and when you replay it only this portion is getting artificially made louder. That is what why your partner can hear this - the recording is several steps away from the original sound and quite a bit different, and louder.
posted by zenon at 1:53 PM on July 8, 2021 [1 favorite]

Response by poster: Thanks, all. That was very interesting.
posted by PussKillian at 3:07 PM on July 8, 2021

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