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Headphones can be used as microphones. Why?

  1. Phorphora
    I have been arguing with some friends about the way my Urbanears Zinken could be used as a pair of microphones.
    After I have inserted my amplifier aux-in cable into one of the headphone holes (another input) and after I have powered the amp, I was able to hear on the speakers what I was saying through the two earpads.
    My voice had a very low-frequencies tone, and even with the graphic equalizer of the amp I couldn't adjust it remarkably.
    My friends think it's the noise cancelling microphone, but I don't agree with them.
    Zero, Urbanears doesn't mention any noise-cancelling technology apart from the thick soft pads.
    First, shouldn't a noise-cancelling mic be placed externally? I suppose it wouldn't be useful if placed inside, because most of the sound inside the earpads is made by the song (heavily bass boosted by my headphone drivers).
    Second, if it really is a noise cancelling mic, why does it carry the received sound through the jack cable? Isn't it necessary to carry the sound to the driver to obtain a destructive interference? Why would my iPhone need to know what the external sound sounds like?
    Third, if I talk while I'm listening with an ear to the other earpad, I can hear nothing. I can only hear my voice because it goes through the amp and reaches the speakers (and because the amp volume is high).
    Instead, I think it's about the headphone drivers that work as a low quality mic diaphragms and convert the air vibrations into electromagnetic signals, that continue the circuit (in this case, the jack aux-in cable).
    These are my toughts about the situations, let me know your opionions. I haven't got other headphones with detachable cable, so I'm unable to do further tests.
    Also, I've inserted my earbuds into the mic-in input on my pc, and recorded my voice with Audacity. They recorded very quietly but hey, I think this confirm my hypothesis.
  2. Dulalala
    0. Yes that is correct, the Urbanears don't have a built in noise cancelling mic on the headphone itself but from what I can see there is a mic on the cable provided.
    1. Well the mic is usually placed where it can have a connection to the PCB which are within the headphones so if you mean that the mic shouldn't be on the headphone itself, that (from my experience) is incorrect.
    2. Most noise cancelling headphones use the built in mic for the DSP (the noise cancelling) and to make calls so the signal from the mic needs to be able to be sent to the phone too.
    3. I'm not quite sure what you mean by this ...
    But you are correct that a headphone driver can work as a "low quality" mic. In fact, dynamic drivers work exactly the same as dynamic microphones but in reverse. Inside a dynamic driver there is a magnet, and when you plug in the headphones, you are sending a AC current through it which will turn the voice coil into a magnet. Depending on the direction of the current, the polarity of the magnet will change. Now since we know for a magnet, same charges repels and different charges attract, we know that the voice coil will move back and forth depending on it's polarity at the time. The voice coil is attached to the diaphragm which will then move back and forth creating a sound wave. The same occurs in reverse, if you move a coil through a magnet you produce a current instead. Since your voice is pushing the diaphragm, it is moving the voice coil back and forth generating the current you have observed.
    Last edited: Jun 10, 2017
    Phorphora and Surf Monkey like this.
  3. Surf Monkey
    Exactly. The technology that makes a mic work is the same as the tech that makes a speaker work. Diaphragm and magnet. A speaker plugged into a microphone jack can pick up sound and deliver it to the amp.
    Phorphora likes this.
  4. Phorphora
    Thank you both for your exaustive answers, my friends will appreciate your easy explanations
    But If you don't mind I would like to understand better two points:
    I tought that the mic for the noise-cancelling technology could catch ambient sound only if it listened outside the housing. I'm sure that if it have listened between the headphone driver and the human ear (as one of my friend stated) it would have recognized as "noise" the sound made for the 99% of music, resulting in a counterproductive feature.
    I meant to say that if I connected the headphones to the amp and spoke to the R (right) earpad, I couldn't hear nothing from the L earpad (I could only hear my voice from the amp speakers).
    I tought that if I had talked to a real noise-cancelling mic inside the R earpad, I would have been able to hear something from the L headphone speaker, because of the noise-cancelling sound produced and sent to the speakers
  5. buke9
    I wound not think a anc mic would be in the signal path at all only to the processor that is putting out the noise to counter act the outside noise. Could be wrong but doesn't make sense to put it in that path. Yes mostly all speakers including headphone drivers can be microphones .
    Surf Monkey likes this.
  6. Dulalala
    I'm not sure about this, but I'm going to assume that the mic in the headphone is probably cardioid microphone so it doesn't exactly pick up sounds behind it that well. Besides, ANC isn't very good at pick up different quick changes in frequency, which is what music is.

    I think you are misunderstanding how noise cancelling works. From what you have wrote, it seems like you are going by the assumption that there is another sound wave that comes and "cancels" it out. ANC works in a circuit. The circuit is either analog or digital (DSP is used) and within that circuit, the noise is removed. If there is no signal being sent to headphone itself, there's nothing to add the changes to.

    Some headphones with noise cancelling can be used as a mic to call so there must be some sort of other pathway to the receiving end I'd assume.
    Last edited: Jun 11, 2017
    Surf Monkey likes this.
  7. buke9
    Yes that might be true but it only shares a ground as the signal is thru the bottom ring and sleeve of the connector and the left+ is on the top and right + is on the first ring and the mic is on the second ring and sleeve with the ground so still shouldn't be in the path .
    Surf Monkey likes this.
  8. Dulalala
    Ah, I misunderstood what you meant earlier. Yes you are right.

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