It has been explained to me that microphone feedback is like an endless loop where the microphone picks up ambient noise in the room only to amplify it, broadcast it across the speakers, and the whole mess continues to build from there.
First, is this an accurate explanation for feedback? And, second, if so, why is it ALWAYS the same frequency, a high-pitched squeal? Shouldn’t it vary with ambient noise? (And, shouldn’t it sound more like the avg of all frequencies of the ambient noises in the room?) So, please give me some feedback on feedback!
To piggyback on this post: when I do skype, and the person I am talking to has me on speakers and the mic is pretty far from his mouth, how am I not hearing myself talking on his mic?
It’s not always the same pitch or frequency. It is dependent upon the oscillation loop, that is, how long it takes for the signal to be fed back. Close miking is high-pitched, farther away would be lower. Other factors, like the frequency response of the mic and acoustics in the room, will limit some frequency ranges.
Frequency response: This is simply a term to describe the range over which a device can produce sounds, right? If a sound is outside a device’s frequency response, it cannot produce it…or would it sound distorted?
Feedback occurs when the “loop gain” is greater than 1. In other words, if the sound from the speaker that is picked up my the microphone gets amplified to the point where it is louder than the original input, you will get feedback. Once this occurs, it will tend to oscillate at a frequency determined by the microphone / amplifier / speaker system as a whole. It doesn’t matter what the initial sound is - once the system starts to feed back, it will pretty rapidly settle on a particular frequency. This is one reason why rock performers keep their Mics so close to their lips - they need to be sure that the mics don’t pick up enough ambient sound to create feedback.
I’ve run sound before at many churches. You can spot the inexperienced singer because they hold the microphone way away from the lips, halfway down to their navel. Apparently they think it is going to bite, and it is a complete headache for the soundguy who has to try to capture their voice without getting massive feedback.
I’ve never used Skype, but earlier hands-free telephony used single duplex communication: you could send a message, or receive a message, but unlike regular telephone handsets you couldn’t do both. This was the earliest method of preventing feedback. Full duplex is the ability speak and hear simultaneously. Digital signal processing, and/or echo cancellation technologies are used to resolve this issue.
This. Move the mic closer to the loudspeaker, and it takes less time for the sound to get from the speaker to the mic; this means the pitch of the feedback squeal will be higher. It’s also likely to get louder, since the sound from the speaker that reaches the mic will be louder because of the reduced distance.
Conversely, if you move the microphone farther away, the frequency reduces - and the volume also tends to go down.
Many microphones also have a response that varies with direction, so the feedback may get worse or better depending on how the mic is oriented relative to the speaker.
As noted, the acoustics of the room may select for certain frequencies so that in some cases the volume may go down as you move closer to the speaker - and then if you move even closer it gets louder.
For extreme low frequency feedback, try this. Get two cell phones . Call phone A from phone B; answer the call at phone A, then take the two phones and put them together earpiece-to-mic. Then make some noise. If you don’t get runaway feedback, try putting the phones on external-speaker mode. On cell phones, there’s a pretty long delay between when you speak in one phone and hear your voice on the other phone - like maybe a quarter or half a second - and so the frequency of the feedback you’ll year is extremely low, like 2-4 Hz. Basically it sounds like the phones are having a rapid-fire argument with each other at first, but it quickly turns to mud.
Back when I was doing sound for live bands we would deliberately induce feedback before the show. There might be 15 or 20 microphones on stage, and any one of them might be the source of feedback. So you’d crank up the gain (volume) on one mic at a time and wait for some feedback to start. When it did you could look at an oscilloscope to see what frequency was feeding back, and reduce that frequency for that particular audio channel, which meant you could crank up that microphone louder without getting feedback. There were no guarantees of course – once the performance started the sound would be bouncing everywhere and getting back into numerous mics at once, so feedback was still a possibility.
I’m pretty sure Skype uses some form of this. I’ve noticed that if I speak while the other person is talking, it cuts the volume right down so I can’t hear them talk.
AKA “Ringing out the System”, “Ringing out the Feedback”, “Ringing Out The PA”, etc - key words here are ‘ringing out’. We never used an feedback monitor or any such fancy-pants stuff, just futzed around with the channel equalizers adjusting by ear…
In my experiences with Skype, there were significant delays (> 1 second). I could faintly hear myself in the background of the other person’s mike, but it wasn’t loud enough to feed back. Even if we had wanted feedback (:dubious:), because of the delay, it would have sounded more like Max Headroom than a feedback squeal.
Skype definitely supports full duplex. (Just to be clear, if it were half duplex you wouldn’t be able to hear any sound at all when you speak – just like when using a walkie-talkie where you have to push the button to speak and release it to listen.) However, I have occasionally seen reports like yours that it’s difficult or impossible to hear the other party when you’re speaking simultaneously; perhaps this is something Skype does to compensate for poor network connections, or perhaps it’s even a transient bug (of which Skype certainly has no shortage).
It could also be that the sound card itself doesn’t support full duplex. Especially if you are using USB headphones, which have a small sound card embedded in them.
stuff, just futzed around with the channel equalizers adjusting by ear…