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What is the cheapest/convenient way to isolate a mobile phone from recording with microphone?

My constraints:

  • The cellphone has to stay in the room, I can't put it outside
  • Maximum 100$ solution
  • Must not take more than 30 seconds to put in and out of the container
  • Three cellphones have to fit in
  • Tiniest as possible
  • Can't dismantle the device

I've thought about putting the telephone in a container full of sand, sand is use in South America by narcotic gangs to acoustic proof a room occupied by a prisoner. But it sounds inconvenient to put the phone in a plastic bag to protect it from the sand and dig a bit each time you want to isolate it.

UPDATE: Some ways to solve this problem:

  • Interference: Generating white noise through an application run by a phone in the box can work. The problem is that white noise can be removed quite easily. What might work is generating random noises inside the container.
  • Saturation: There's probably a sound level for the microphone that makes any sound recovery impossible, saturating the input.

For both of those techniques, a closed box is needed (at least to prevent sound recording in the room).

  • Also a phone usually disables the internal mic if an external (broken) one is plugged in. I'm not sure if this is hardware of software or spook proof. – daniel Aug 4 '17 at 11:39
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    Related question: Surveillance: blocking laptop's microphone from spying on you? While the title is not focused on cellphones, the answers usually apply to both equally. – WhiteWinterWolf Aug 4 '17 at 11:42
  • The dummy plug seems promising but I am not sure this is done automatically on the hardware level in a way that the microphone is unable to be activated at the software level. – Totem Aug 4 '17 at 13:54
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    Even if you can completely disable the microphone, it's possible to use the gyroscopic motion sensor as a crude microphone on some phones. On some computer hardware, it's even possible to turn the speaker into a microphone. So if you think your phone is listening to you, it's still best to isolate the device physically rather than trying to disable the microphone. – Johnny Aug 4 '17 at 15:31
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As well as using a source of noise for drowning out the sound of talking in the room you should also use an insulated box. The insulation partially blocks the noise of the talking getting to the phone, and partially blocks the annoying white noise of your device.

Maybe open the walls of the cooler and fill the insulating cavities with sand, if you never need to move it or explain to anyone what it is.

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    Yes I just made some tests in a fridge and it works well. Fridge also generate a background noise on it own ! – Totem Aug 4 '17 at 12:37
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    @Totem you could use the man in the fridge that turns the light off to also turn the noise generator on if you wanted to be fancy – daniel Aug 4 '17 at 12:38
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    I don't trust that guy. That's why he's in the fridge. – Totem Aug 4 '17 at 12:41
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    +1 You can also use this to motivate your guests to drop their phone in there - phones in exchange for cooled drinks. – AnoE Aug 4 '17 at 17:01
  • Or you can put a smaller cooler inside a big one and fill the big one with sand :) – IAmJulianAcosta Aug 4 '17 at 18:45
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White noise is actually not as easy to deal with as the article you've found claims. Filtering only works well so long as the signal to noise ratio remains relatively high. By putting the phones in an enclosed box together with a noise generator you can easily make sure that phones record more noise than signal, at which point conventional filtering methods become ineffective.

Check out Shannon-Hartley theorem: by making sure the noise is +20dB stronger than the voice, you're reducing the theoretical information rate of an acoustic signal to below 0.15Kb/s. The lowest bitrate for an intelligible human speech is around 0.3Kb/s, with compression. This means someone could be recording the voices, compressing them to MP3 or similar and retransmitting that data as audio using an error-correcting code, and the phones would still be unable to record that compressed signal intellegibly.

  • +1 for the +20dB bit. just attach a strip to a little motor and make it go round. what an easy cantrip. – user2497 Aug 4 '17 at 16:35
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    But beware of spatially diverse microphone arrays (e.g. stereo or better) and directional separation techniques. – Ben Voigt Aug 4 '17 at 22:26
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    @DmitryGrigoryev: According to pocketnow.com/2016/02/04/smartphone-noise-reduction, array of 3 in recent LG flagships, 4 in Nokia. HTC lists "3 microphones with noise cancellation" I don't think you can find a flagship phone with just one microphone anymore, two is pretty standard, more than 2 is considered a competitive advantage in clarity and noise cancellation. And devices designed for far field audio pickup, such as Amazon Echo or conference table microphones, have even more (Echo has 7) – Ben Voigt Aug 6 '17 at 16:43
  • @BenVoigt Still, 7 microphones can be easily defeated by making the noise twice as loud. – Dmitry Grigoryev Jan 15 '19 at 20:40
  • @DmitryGrigoryev: A microphone array provides far more directional gain than a mere 3dB ("twice as loud"). Unless you either saturate the pickups, or cause a reflection from the same direction as the signal, source separation will be able to discard your interference. – Ben Voigt Jan 15 '19 at 21:39
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Tinfoil-time! The most effective way to prevent a microphone from picking up sound is to present it with a random noise pattern that touches on the entire spectrum of human voice and hearing. While sand is probably great, you can't trust mexican low-tech solutions. Sand... no wonder El Chapo got caught. Sand and Sean Penn; a fatal combination!

Use splashing water. A small waterpump (5V, can run off a USB battery), a 2-3 cm drop into a container that refeeds the pump is adequate. It will be impossible for any sound filter to pick up anything else. Look at this pump. You can make a pretty fountain with real fish in it, the pump will ensure they have oxygen. You don't even need to settle for goldfish, you could have some cool ones.

However, the best way is not water. Simply disassemble your phone, disconnect the microphone, insulate the jack with electrician's tape, and close it up. From then on, use a headset. Some phones have several mics for better sound quality.

If you also want to shield it from the network, you can line a box with ... alu-foil. Since it reflects radiowaves, there should not be any gaps.

Caveat: I have not done this, because if I had a stalker, I'd love making farting sounds and just being myself until he went braindead 👍🏿

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    @Totem If you record water splashing in e.g. Audacity, and compare it to a sample of a motor running, you will see that a fan does not-so-much for your application. Water falling into water generates noise all over the spectrum, while a motor can be filtered easily. – user2497 Aug 4 '17 at 9:11
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    @Totem Oh, yes very clever actually. I misunderstood. But it will not be as wide in the audible spectrum as water. – user2497 Aug 4 '17 at 9:16
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    Using water to stop leaks makes me smile. – James Bradbury Aug 4 '17 at 10:58
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    @Totem only if the white noise is not random. A motor/fridge will run at a constant frequency and will make a pattern that you can isolate and remove from a recording (which leaves you with the recording sans motor). Stuff like water is plenty random and non-periodic, so it's very difficult to isolate and eliminate. – Delioth Aug 4 '17 at 15:13
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    @Totem Playing any recording is not going to work since it is non-random and periodic, even if it is a recording of a random noise source. It needs to actually be securely random to prevent filtering. – trognanders Aug 4 '17 at 20:10
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Build or buy a vacuum chamber. A common science experiment is putting a bell or buzzer in a bell jar and pumping the air out. The first video I found on youtube seemed to come pretty close to the 30 second mark. The process would be sped up by making a custom chamber with a capacity just larger than your typical three cell phones. For added security put the phones in a container you levitate with magnets, within the vacuum chamber.

  • Additional seismic isolation is a good idea, but levitating by magnets is hard and it is indeed not more effective than just using soft enough rubber bands or springs. – Emil Aug 5 '17 at 9:49
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This answer has a really good solution, but makes it an aside.

All you have to do is make the microphone not work at all! Either disassemble the phone and remove it, or jab at it with a needle. Then whenever you actually need a microphone for your phone, plug one in/connect one with bluetooth.

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Isolate

First of all, you want your phones acoustically isolated as much as possible. Other users have suggested a cooler; I think that would work well. Put a bag filled with sand on the bottom, put the phones on it, then cover with another bag.

Test it by making recordings with your own phone.

Mask

Now you want to add other sounds. This is called sound masking. White noise is one way of doing that, but there are others.

If you want to mask conversation, the best thing to use is more conversation. Have some pre-recorded conversation of sufficient length. Preferably several conversations, overlapping. Add some white noise as an extra layer, since white noise generators are easy to find — just get an old transistor radio and tune it off channel.
As a matter of fact, several transistor radios would be a good idea. Tune some to a news station, some to different music stations, and some off channel. They can run on batteries and can all go in the isolation box with the telephones.

Again, test the setup with your own phone.

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