Mic feedback; any thoughts?

[Bernardduur]

Hello all!

OK, I know its not a stompbox pur sang, but I will use is as a stompbox

I build a mic from an old telephone horn; the big problem is that this little guy feedbacks quite easy....... some know a solution to this, DIY?

[Paul Perry (Frostwave)]

One problem with using old telephone parts for a microphone, is that the frequency response is very very uneven. So I'd be trying to tune the peaks out. I'd put a graphic EQ (wiht as many bands as possible) after it & see if it can be tamed that way.
If it can, then you can either  use it with the EQ, or try to make a special fixed filter wiht a response the same as the EQ setting that works.
The other approach, which isn't all that simple, is to do what the professional feedback elimators used to do - build a frequency shifter that moves the signal up 5 Hz or so. The ear doesn't notice, but it is enough to prevent the feedback (because, the signal trying to feedback is now a different frequency fro the original signal). but this takes special precision 90 deg filters ("dome" filters) & multiplier chips...

[Mark Hammer]

The old-fshioned bonehead solution is to use a parametric  EQ.  You set the unit for the narrowest passband (higher Q), set it to maximum boost, and sweep the frequency pot until you get consistent feedback.  THAT's your critical frequency.  Now you simply turn the boost into cut and you have something that approximates relatively flat bandwidth by de-accentuating that bnand that is most sensitive in the mic.

Of course, that's if you're lucky.  If you're unlucky, there might be several peaks to be tuned out, which is, of course the very reason why 31-band graphic EQ units and 4-band parametric units exist.

[zpyder]

The other approach, which isn't all that simple, is to do what the professional feedback elimators used to do - build a frequency shifter that moves the signal up 5 Hz or so. The ear doesn't notice...

Holy shite.  I respect your opinions Paul, and I don't doubt that this has been done if you say so... but that, to me, sounds like a horrible idea!  A 5Hz difference may not be 'discernable' by even most trained ears, and likely not many 'audience' ears, but the beat frequencies!! The beat frequencies!!!  I can just see myself trying to harmonize in the lower register .. ewwwwww

yarrr... cheers,
zpyder

[Sir H C]

Put it on a switch.  Then only when you sing is it on.  Otherwise it is off.  Then baffle on the back to try to cut down the omnidirectionality of the mic.

[Ge_Whiz]

A simple parametric filter will knock out the main peak. It may gain you a few extra dB.

[Mark Hammer]

The other approach, which isn't all that simple, is to do what the professional feedback elimators used to do - build a frequency shifter that moves the signal up 5 Hz or so. The ear doesn't notice...

Holy shite.  I respect your opinions Paul, and I don't doubt that this has been done if you say so... but that, to me, sounds like a horrible idea!  A 5Hz difference may not be 'discernable' by even most trained ears, and likely not many 'audience' ears, but the beat frequencies!! The beat frequencies!!!  I can just see myself trying to harmonize in the lower register .. ewwwwww

yarrr... cheers,
zpyder

It's not how much frequency shift, but where as well.  I'm in your camp when I think of what a global 5hz shift would do to hearing your bass through the PA.  However, a 5hz shift applied in the 600hz range would be considerably less perceptible.  Besides, these days it is entirely possible to have what shows up in in ear monitors be on pitch, and what shows up in the PA pointed at the audience (but sadly at the roof of the club as well) be shifted over just enough to be different than what's going into the mic.

I'm still wondering why nobody has addressed the issue of reflective surfaces in the stage environment.  E.g., does anyone use acoustically absorptive material as a kind of side-wall or overhead canopy to keep stuff from reflecting off nearby surfaces and back to the mic?

[petemoore]

  Heavy handle w/suspension...
  If it's like what I'm thinking..a ~3'' disc from old phone it;s probably somewhat direction in its pickup range, this can probably be improved by 'casing' it like in a regular mic handle.
  The weight/shape and whether What is around the diaphram is 'sealed' like or encloses the diaphram can make a mic less prone to feedback, feedback was not considered in designs of phone mics. try sticking it in a tube, suspending it W/rubber bands or just some kind of light stuffing that won't make scratchy noises.
  By enclosing the sides around the diaphram, then pointing the speaker away from you...Mic<air>speaker, or even Mic< Speaker<, certainly NOT recommended in all decent mic, and pa manual is never Mic><speaker, pointing the mic into the speaker makes for the most effecient airborn waveform transfer between speaker and mic, hence greatest feedback potential. iow how directional the mic is and it's orientation to the speaker and which direction the speaker is pointing.
  weight is nice in a mic handle for reducing handling noise/shunting airborn waveforms attempting to travel from speaker to mic, ...then turned into electric waveforms *amp >speaker>mic>amp>Speaker>Mic'...[helping put a stop in the] feedback *loop'

[John Lyons]

It's unlikely you will get it to sound "good", as in similar to any decent microphone, but the cool thing about phone mic elements is that they sound bad in a good way.
One thing you will want to do is get away from the amp with the mic, any distance you get from the amp will decrease feedback. I've used funky CB, phone, megaphone, raw speakers (no cabinet) and live telephones recorded through another phone in the studio to good effect many times.
As mentioned above. Use a graphic or parametric eq and start sliding and twisting!! There will be one main peak you want to turn down and then other frequencies will creap up and feedback and come into play. I would try getting some distance between the mic and the amp and try using a buffer like the Minibooster for the high input impedance just don't turn it up much. A noise gate or a switch can work well too depending on the vocal style.

John

[Johnny Guitar]

Good approaches to be sure.

Also, if the mic is still in the handset (or an adapted hand set), that might actully be a resonant chamber at the feedback frequency. Changing the handset's resonance (cutting it down) and/or stuffing it with insulationg material (ie, rags) might reduce that kind of resonance.

The thing you might watch out for with EQing is that you might be cutting the interesting frequencies which make a telephone mic have a unique sound. A lot of us have just adjusted the EQ on standard (or sub-standard) mics to get a sound similar to a telephone.

[Bernardduur]

Thanks all!

I will try the suggested ideas!