Resonant Feedback Problem

[20to20K]

Last Saturday my wife took the kids to the movies so I had a rare 4 hour window when I could crank my music as loud as I liked (which is pretty loud). While listening at a pretty high level I started to get severe resonant feedback coming from my left speaker. It would start off quietly (barely noticble) then would gradually build up to loud, booming hum until I turned the volume down slightly. Then it would disappear completely...until I slightly raised the volume again.

I've had this setup in my basement now for about 8 months and never heard this before...but granted I've never played it this loud for this extended period of time before either.

I thought I read something once about how the dimensions of your room can cause feedback. Something like if the width, height, and depth are multiples of each other? My listening area is approxiametely 22'deepx15'widex10'high. The speakers occupy the corners of the width portion(nudged between the sidewalls and the entertainment center) and opposite of the speakers is only about 25% closed off (just the staircase leading back upstairs). I have a rather large circular sofa in the listening room that comes within 5 feet of the left speaker on the long end.

My guess is the sofa causing it...but I guess it could be the room dimensions/acoustics as well. Any suggestions? Can room treatment help me?

 

[soniceuphoria]

Before you go and condem the room you might want to investigate the problem more thoroughly. You might try checking the drivers in your speakers to make shure they are tightened down all the way because this can cause the conditions you are talking about. If that doesn't work you might check your amp/reciever and all components alike by removing them 1 by 1 until you find the source of the problem. I hope these suggestions lead you in the right direction. Happy listening.
Greg

 

[Ethan Winer]

20,

> It would start off quietly (barely noticble) then would gradually build up to loud, booming hum until I turned the volume down slightly. <

That sounds like turntable feedback. Were you listening to an LP? If so, the solution is to mechanically decouple the turntable, or whatever it's resting on, from the rest of the room.

--Ethan

 

[20to20K]

...and the "Winer" is...

20,

> It would start off quietly (barely noticble) then would gradually build up to loud, booming hum until I turned the volume down slightly. <

That sounds like turntable feedback. Were you listening to an LP? If so, the solution is to mechanically decouple the turntable, or whatever it's resting on, from the rest of the room.

--Ethan

You're absolutely correct! I was listening to vinyl and the turntable is relatively close to my left speaker. Now the question is...how the heck do you: "...mechanically decouple the turntable"?

It's sitting atop a glass shelf inside an oak entertainment just next to the speaker. It's got rubber feet on it already...maybe an inch thick.

 

[Ethan Winer]

20,

> It's sitting atop a glass shelf inside an oak entertainment <

Being inside a cabinet doesn't help either.

> It's got rubber feet on it already <

I threw out my last turntable many years ago, so I'm not up on the latest ways to isolate them. The standard isolation material for loudspeakers is two inches of 705 rigid fiberglass, wrapped with fabric to keep the particles from getting into the air. Some types of acoustic foam may work well too, and be easier to find. You can even used dried out kitchen sponges, or an empty cardboard box. But before you buy anything or go to further trouble, there's a simple test you can do to see if coupling between the turntable base and the cabinet is the problem:

Play some music loudly until its on the verge of feedback. Then lift the turntable (carefully!) while it plays and see if the rumble goes away. If so, then you proved that decoupling will work.

--Ethan

 

[bpape]

Your feedback could be either mechanical (via the cabinet) or through the air. Do you have a dust cover on the TT? Is it on when you play it? If so, try it without. Sometimes the cover itself acts like a resonator.

Mechanically, to isolate it, the optimal way is to HANG it from springs. Now granted, that's not the easiset thing to do nor the most asthetically pleasing but it does work.

It also could be that the rubber feet aren't working at the frequencies you're having trouble with. Try cutting 2 tennis balls in half and turn them upside down to use as feet. Then put a piece of 3/4" plywood on top and use that for your TT stand. If you're having problems with the cabinet resonance transferring, this might shift it to a frequency where your feet are better at isolation. I've seen a lot of 'audio cheapskate' types use this type of setup very effectively.

 

[20to20K]

Thanks fellas...

You two have given me plenty of inexpensive experiments to try and I will give them a go this weekend. They all sound plausible so hopfully one of them should help my situation.

I'll let you know...

Thanks again!

 

[sjdgpt]

I forgot about the turntable cover, until I read the previous post.

My first real turntable (cheap technics unit) had a recommendation to remove the cover due to resonance issues.


Back to isolating the turntable.

I had to isolate a computer hardrive from the buildings structure.

(too many freight trains rolling by just inches by the building)


After crashing the hard drive, IBM provided us with a isolation bed that was made with a silicon gel.

The computer chassis was mounted in the isolation gel.

The isolation gel had the consistancy of those Dr. Scholl's shoe inserts.

Just and idea to decouple your turntable from the rack.

Would three or four several small pieces of shoe inserts solve the problem?

By the way, three feet may be more desirable than four feet.

Three feet will cause the turntable to lean (if one of the feet is uneven).

Four feet will cause the turntable to wobble (if one of the feet is uneven).