Tech: Eliminating feedback, miked guitar

Subject: Eliminating feedback, miked guitar
From: Steve Latimer
Date: 07 Apr 99 - 04:12 PM

My sister is a performer who prefers to mike her guitar than to use the built in pickup. She feels that she has better sound and dynamics this way. Lately she has had some feedback problems. She is well behind the speakers and has experienced this problem with differrent P.A.'s. She has taken to being seated when she plays, I'm wondering if the proximity of her vocal mike to her guitar mike has anything to do with this problem.

I read in another thread (Mandolin I believe) that a foam cover was suggested. Is this to eliminate feedback?

She can't afford to buy an internal mike yet.

Any suggestions would be welcomed.

Steve Latimer

Subject: RE: Eliminating feedback, miked guitar
From: Roger in Baltimore
Date: 07 Apr 99 - 07:31 PM

Feedback occurs when the signal from the mike goes through the sound board and back out amplified and is then picked up by the microphone again causing an audio loop that feeds back on itself, thus called feedback.

As your sister knows, that is clearly a problem if the mike is in front of the PA's speakers.

There are some other possibilities. If your sister is using monitors, it helps to have the microphones "rear end" pointed towards the monitor so the front of the microphone faces away from the monitor. If the mike picks up sound from the monitor, you have another feedback loop.

Nearby flat surfaces are a problem. Therefore it is best if the guitar mike is not pointed at a ninety degree angle to the face of the guitar. This helps eliminate reflective feedback. Other large flat surfaces can also be a problem. The ceiling or the back wall may be too reflective. It takes some doing then to find the right mike placement. In this case, some change in speaker placement may also help, so that the faces of the speakers are not at a ninety degree angle to any of the walls.

I suspect there are other sources of feedback, but you have just reached the end of my knowledge.

Roger in Baltimore

Subject: RE: Eliminating feedback, miked guitar
From: John in Brisbane
Date: 07 Apr 99 - 08:01 PM

One further suggestion that is quite critical. Make sure that the microphone your sister is using is unidirectional. Another name for this is 'cardiod' - there may be other descriptions. These mics pick up sound predominantly directly in front of the mic, with less sound being picked up from the rear.

The other main type of mic is omni-directional. These are great for studio recording of guitar, but are a disaster for live PA use, because they do not reject the sound of the speakers coming from behind.

My apologies if I'm stating something too obvious.

Regards
John

Subject: RE: Eliminating feedback, miked guitar
From: DonMeixner
Date: 07 Apr 99 - 09:51 PM

Another area to look at is mixed impedence, A hi Z vocal mic and a low Z instrument mic can raise hell with equalization. Some people do quite well with it tho'. Rebound off of flat surfaces is an issue regarding feedback. But with Hi Z mics you can get rebound off your face if you are using hotspot monitors like I use.

The electric guitar and PA, for all their faults are still a big improvement of the gas and steam models we had to use a few years back. I keep the old coalburning hi-pressure PA system in the van just in case tho'. Never know when a fuse or something may fail and where would we be?

Don

Subject: RE: Eliminating feedback, miked guitar
From: Alex
Date: 08 Apr 99 - 02:46 AM

Placement of the mic to the front of the soundhole and the top of the fretboard can also eliminate feedback. Use a unidirectional mic (preferably a condenser mic with a flat response i,e all frequencies are reproduced the same way they sound) Then there are some guitars that have a resonating frequency (like singing in the bathroom) where there is a "boom". This is when you need to have a 1/3 octave equilizer (aka a 31-band EQ) so that you can kill that particular frequency without affecting the rest of the sound,

Subject: RE: Eliminating feedback, miked guitar(longish)
From: Bev Lawton
Date: 08 Apr 99 - 09:13 AM

From my experience of sound engineering live PA for acoustic instruments.

1. If the feed back is high pitched it is the monitors. 2. low pitched usually means the front of house speakers. 3. Electric guitarists who use feedback (Carlos Santana) used to due a soundcheck to FIND the spots that caused feed back best and marked them wit tape! You can do the converse here walk around with your guitar with vol turned up and find best/worst spots - good sound man will help here. 4. use a directional mic (cardiod - heart pattern) they tend to be long a thin. The ones with the large round bulbous ends tend to me omni (multi) directional and will pick up sound from behind - ie the monitor sound (with YOUR guitar sound) in them. 5. Most acoustic type feed back accurs at approx 300hz 6. A 31 band equaliser will help (around 300hz) but if you are using a sound person they should be doing this for you. assuming that you are BEHIND the front of house speakers this should be put into the MONITOR circuit, this will cut the offending frequency FROM THE MONITOR and hence cure the feedback but WILL NOT take the frequency from what the audience will hear on the front of house. 7. Feedback never just HAPPENS it builds up ie it feeds on the original signal, amplifies it, transmits it and so on. As a temporary measure as yopu hear/feel the soundboard of the guitar vibrating just press your forearm on the soundboard this will deaden the buildup. 8. BERINGHER have a device called the DSP-1100 Feedback Eliminator cost approx 110 uk sterling. This can be used in a programmed mode - ie you play your insrument into it and when it detects feedback it will down the frequency a knotch and you try again until it is at an acceptable level. It also works in an automatic mode whereby in a live performance it detects feedback automatically and then dampens that frequency to eliminate the feedback. This is a more agressive damping than the pre-programmed type but it stops almost instantly the Wummph or screech of the feedback. One very important difference between this unit and a 31 band aka 1/3 octave graphic equaliser is that it ONLY attenuates the ACTUAL FREQUENCY causing the trouble ie 313 hz and not 280-330 for example. Very clever bit of kit. Can be used on front of house and monitors from either direct instruments or the two outputs from the mixing desk (ie ALL the bands equipment and vocals) highly reccomended. Bev Lawton

Subject: RE: Eliminating feedback, miked guitar
From: Steve Latimer
Date: 08 Apr 99 - 10:03 AM

Thanks to all, I think the first step is to change to a unidirectional mike as she has been using an Omnidirectional. If we can't cure it given all of the above information, then we're in trouble.

Once again, The Mudcat comes through.

Steve Latimer

Subject: RE: Eliminating feedback, miked guitar
From: Alan of Australia
Date: 08 Apr 99 - 11:36 AM

G'day,
You could also try a FeedBackBuster which is a rubber piece that fits in the sound hole. I've only tried it with mikes fitted inside the guitar but it works very well in this case. Mine has stamped on it "Kaman Music Corporation P.O. box 507 Bloomfield CT 06002 USA."

Cheers,
Alan

Subject: RE: Eliminating feedback, miked guitar
From: Bert
Date: 08 Apr 99 - 01:37 PM

My experience is a bit old fashioned, but you could try using a less sensitive mike, such as a moving coil, and move it closer to the sound source.

Bert.

Subject: RE: Eliminating feedback, miked guitar
From: mountain tyme
Date: 09 Apr 99 - 05:33 AM

The foam cover refered to in another thread was to address the max vol from a mandolin. Feedback was not a concern. The purpose of the foam was to allow the mike to be pressed up hard against the mandolin body and disallow the metal end of the mike banging into the body and making a large noise. This is the common way of featuring the mandolin. There is a relation between these two otherwise separate subjects. First a unidirectional cardoid like a Sure 57 with low Z XLR cord/connectors is a must for the purpose here. A sound hole cover is only used and required for a guitar that is internally miked or uses a pickup. The cover prevents the monitor or house sound from entering the chamber and being re-amplified. Feedback of the type mentioned is only experienced when using a preamp with to little gain as found or really not found in low cost mixers. By adding a good quality preamp externally the gain requirement of the amlifier is reduced and feedback is eliminated. The mike as mentioned above with to little preamp will "reach out" only about six inches before beedback begins. With proper preamp level the same mike will "reach out" four feet or more for vocals and guitar (and mandolin) before feedback. The requirement of "contact" miking as with the mandolin above is a statement most often experienced where the preamp is to small to drive the amplifier. This condition can sometimes be helped by turning up the mid range gain or going plus with the equalizer. The addition of an external preamp like an ART 127 at @ $120.00 is the final fix for a system with a weak preamp. I carry my own mike, cords and preamp perchance i encounter this problem when i play out useing someone elses low end system. Hope this helps.

Subject: RE: Eliminating feedback, miked guitar
From: Rick Fielding
Date: 09 Apr 99 - 08:42 AM

Sorry, I overlooked this thread earlier, as I've had mucho experience fighting feedback, and may have a suggestion or two. First, there's no indication of what kind of guitar she's using, and that's a crucial part of the equation. A Gibson J-45 will be much less prone to feedback than a Martin D model. Generally the copies of these styles will react like the originals. My main working guitar is a Lowden copy of a Gibson J 50 (I know, it's a VERY early Lowden) The guitar (unlike it's Gibson counterpart is exceptionally loud and bright, but resists feedback well when miked at the 14th fret, with the mike at a 45 degree angle. It took me a year with a lot of on-the-job experimenting to settle on this placement. The old 0-18 I now use can be miked straight into the hole with no feedback problems. A good approach is to call up an experienced ACOUSTIC music sound person, and ask what they have to say about the general set up they'd use on your make of guitar. I've found they love to talk shop.

Subject: RE: Eliminating feedback, miked guitar
From: Lucius
Date: 09 Apr 99 - 08:59 AM

Yet another sollution: My rig has a "gooseneck" mcrophone inside my guitar--I forget the brand name, something like flexi-tone. I have a humidifier that fits over the entire soundhole, and I get a good miked sound with virtually no feedback.

Hey, any of you smart guys know how to keep my VCR from flashing 12:00?

Subject: RE: Eliminating feedback, miked guitar
From: Bert
Date: 09 Apr 99 - 10:23 AM

Turn it off

Subject: RE: Eliminating feedback, miked guitar
From: Steve Latimer
Date: 09 Apr 99 - 10:25 AM

Rick, she uses a few guitars. Her main one is a Daion, sorry I don't know the model number. It's an odd shaped guitar, an acoustic with a cutaway and an oval soundhole. It's tail is shaped kind of like an acoustic version of the Gibson SG (?) Electrics. If you look closely at the bottom right picture here

www.seen.com/FatCat/jazzday.html

you can just make out what I'm describing. She uses an open tuned Yamaha as well, a very normal acoustic guitar.

She is sometimes accompanied by my other sister who uses a very traditional Goya.

As I said earlier, the first step will be to try an omnidirectional mike, but I have printed all of these posts and given them to her. I'm sure that this shared wisdom will solve the problem.

Thanks again to everyone.

Subject: RE: Eliminating feedback, miked guitar
From: Joe Offer
Date: 10 Apr 99 - 10:04 PM

Click here for a microphone FAQ, and here for a request for information on how to reduce microphone buzzing.
-Joe Offer-

Subject: RE: Eliminating feedback, miked guitar
From: Cap't Bob
Date: 11 Apr 99 - 01:15 AM

We have always used microphones for our instruments and seldom have trouble with feedback. However, on one occasion we had intermittent feedback that seemed to deify our best attempts to eliminate the problem. The room had large flat surfaces which never seem to help. We adjusted the equalizer, volumn controls, monitors, etc. and every time we thought we had the problem solved we would get feedback again. Finally someone noticed that the problem occured every time someone came in through the door and passed in front of the speaker. We elevated the speaker to about six feet and solved the problem. Since that time we always try to elevate the speakers as much as possible and this seems to reduce feedback problems.

GOOD LUCK Cap't Bob

Subject: RE: Eliminating feedback, miked guitar
From: catspaw49
Date: 11 Apr 99 - 01:58 AM

Unfortunately, at a recent performance, someone elevated the speaker with a low flying Cessna and took it to Topeka.

catspaw