Controlling Mic feedback

[Mosaic]

Hi all:
I have an app with the mic close to the speaker rear and naturally feedback is a prob. I was looking at audio notch filtering to address the problem.

i found this design:
**broken link removed**
and this one:
http://www.geofex.com/article_folders/eqs/paramet.htm#variable_l

Also I am hoping that this s'ware:
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=9270
can allow me to do all the filtering in the PC 'live' b4 developing a h'ware soln.


I could use some advice on this.

 

[Nigel Goodwin]

What exactly are you trying to do? - as that governs what might be possible.

 

[kaleidol]

Have you tried inverting the waveform before sending it to speaker?

 

[Mosaic]

@ Nigel: I have a steel drum instrument amplified but due to the proximity requirements (not under my control) the 18" between spkr and mic I am getting feedback. I was hoping to develop notch filters to control the feedback.
kaleidol: No, but I can give it a try.

 

[Nigel Goodwin]

@ Nigel: I have a steel drum instrument amplified but due to the proximity requirements (not under my control) the 18" between spkr and mic I am getting feedback. I was hoping to develop notch filters to control the feedback.

I'm presuming you need a LOT of volume from the speaker (as otherwise, why mike it?).

I would seriously suggest you try FAR, FAR harder to alter the configuration, and if you absolutely can't don't even consider trying to build a filter - buy something like this:

https://www.amazon.co.uk/gp/product...m=A3P5ROKL5A1OLE&pf_rd_r=1B1TYGFNHM8KT3K1WVDA

Behringer also do larger more specific feedback destroyers as well, but the 'Shark' series have various other uses as well.

kaleidol: No, but I can give it a try.

It won't help anyway.

 

[4pyros]

What frequencies are coming out of the speaker that is causing the feedback?

 

[alec_t]

Rather than use a mic, does your instrument have a line output socket?

 

[audioguru]

A notch filter will not work because the system will simply have feedback at another frequency unless the speaker has a horrible huge peak at one frequency.

If the mic and speaker are fairly close together so that delay, phase shift and echoes caused by distance do not occur then reversing the phase of the speaker might reduce feedback.

 

[Mosaic]

@ Nigel: thanks for the product link...but it is something I have to build so I am looking at these parametric designs:
**broken link removed**
https://damien.douxchamps.net/elec/param_eq/

Perhaps I can build a filter stack to allow 5 or 6 notches in the 150 thru 6000 Hz range. I hope to use NE5532 opamps instead of the TL082/72 and film caps where possible. I am not sure on the LF351 as an option, here.

Can anyone suggest a source for decent stereo pots?

@ Alec_t: I have a KAM RT1 cardioid capsule in spider mount feeding an NE5532 Preamp/tone control into a Class D TDA7492 Amp driving a generic 8" mid woofer housed in a stiff, sealed PVC 1 gallon 'bucket' that was once a chlorine granule bucket, .

The device is a prototype build with the limitations I mentioned in the 4th post. The frequency of the primary feedback is around 200 to 300Hz by ear, but there will be others as amplification is ramped up.
I know this approach seems odd, but I am bound by external design limitations ( mic & speaker location) that I have to work with.

I really appreciate all of your advice and patience!

 

[Nigel Goodwin]

@ Nigel: thanks for the product link...but it is something I have to build so I am looking at these parametric designs:
**broken link removed**
https://damien.douxchamps.net/elec/param_eq/

Perhaps I can build a filter stack to allow 5 or 6 notches in the 150 thru 6000 Hz range. I hope to use NE5532 opamps instead of the TL082/72 and film caps where possible. I am not sure on the LF351 as an option, here.

The problem is that it's a VERY, VERY difficult thing to try and reduce (you can't 'cure' it) - devices like the Behringer work by continually self adjusting, and by doing that give you the most effective reduction whilst maintaining the least possible effect on the sound.

Using fixed filters is going to impact the sound far more, and be far less effective at reducing feedback.

Choosing a more suitable mike could help as well, what type of mike are you using?.

 

[Mosaic]

A KAM RT1 dynamic capsule. I have Panasonic WM61A electrets on hand, but they are more sensitive.

I was wondering if there is a way to tailor the amplitude window of the preamp. The 'sound' is percussion, with a fast attack, whereas feedback has a slow attack.

 

[Nigel Goodwin]

A KAM RT1 dynamic capsule. I have Panasonic WM61A electrets on hand, but they are more sensitive.

That appears to be a cardioid mike, so have you tried positioning it to try and reduce the feedback? - where actually is it fitted?.

Can you get hold of a hypercardioid mike and try that?.

I was wondering if there is a way to tailor the amplitude window of the preamp. The 'sound' is percussion, with a fast attack, whereas feedback has a slow attack.

The Behringer device I mentioned also can be configured as a noise gate.

 

[Mosaic]

Given the advice I have received and the possible risks in reinventing the wheel I am seriously considering this device:
**broken link removed**

It seems small enough and offers noise gating and compression as well which is good for the wide dynamic range percussive sound from the steel drum. The 150Hz low cut is also useful.

What do you guys think, is this a reasonable pick? It discusses fixed percussion mics and feedback in the manual in section 2.2.
**broken link removed**

BTW, since I am using a 'bare' cardioid capsule....its possible that it is behaving like an omni without the mic body. Should i 'create' a PVC body for it?

 

[audioguru]

An 8" speaker in a 1 gallon PVC pail might have strong resonance at a few hundred Hz which makes your feedback worse. A speaker enclosure must be completely rigid and its size selected for the detailed spec's of the speaker.

A cadioid directional mic has tuned air vents in the body at the rear of the diaphragm to allow cancellation of sounds from the rear and from the sides. Without a properly designed body a bare capsule will not be directional.

 

[Mosaic]

I connected a satellite speaker box from an AIWA home stereo and was able to avoid feedback once the box was about 4' away from the drum, got similar feedback when it was close. At full amplification I am getting a bit of AC hum, which I suspect is from the simple 7812/7912 regulator in the preamp. Perhaps it need bigger smoothing caps.
I did some more tests and what seems to be happening is the underside of the 'dome' of the steel pan is reflecting sound back into the mic from the speaker. If I blanket the dome underside with 1/2" felt...much of the feedback is reduced, but that deadens the 'ring' tones from the pan. The mic is only a few inches from the sound reflective base of the pan. Moving the mic around only seems to change the frequency of the feedback, which I guess correlates to the 1/4 wavelength that is being reinforced based upon the distance of the mic from the reflective dome surface.

Some quick calcs here , http://www.sengpielaudio.com/calculator-waves.htm, seems to match the 1/4 wavelength of a few hundred hz to the dimensions I am working with.