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Vibrating shelves

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Vibrating shelves

Postby RichardT » Fri Dec 07, 2018 6:38 pm

Dear all,

I have a vibration problem with the shelving unit my monitors are sitting on. It's made of hard material, possibly concrete, and has three shelves each about 3cm thick which are cemented into the wall at one side. The shelves vibrate at upper bass frequencies causing smearing to the sound, which I can feel when I rest my hand on the top of it, as well as hear! I can't easily remove it or replace it.

The speakers are on iso-l8r stands standing on top of this unit.

Is there anything I can do that will reduce this vibration effectively?

Many thanks

Rich
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Re: Vibrating shelves

Postby Wonks » Fri Dec 07, 2018 6:56 pm

So each 3cm thick shelf is just cantilevered out from the wall, with no other support?

A photo might help.
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Re: Vibrating shelves

Postby Funkyflash5 » Fri Dec 07, 2018 7:20 pm

If the shelf itself is strong enough, you might be able to add mass to the shelf to shift the resonance lower? Not sure it that would be an improvement, but should be easy to try. or else wedging a couple of boards between the bottom of the shelf and the floor to stiffen and damp it?
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Re: Vibrating shelves

Postby RichardT » Fri Dec 07, 2018 7:25 pm

Wonks wrote:So each 3cm thick shelf is just cantilevered out from the wall, with no other support?

A photo might help.

Thanks Wonks

here are a couple of photos:

https://drive.google.com/file/d/1MSB9WkEfuOeaYb362DofQY1JLW1YM8OM/view?usp=sharing

https://drive.google.com/file/d/1WqHLRsuxI9ibScllzFc4unQrCW7pQ15r/view?usp=sharing

The structure on the end away from the wall continues to the ground, though you can't see that in the pics.

Many thanks

Rich
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Re: Vibrating shelves

Postby Mike Stranks » Sun Dec 09, 2018 5:58 pm

Hmmm... I have some of those stands, although I use mine on the long connecting rods. They've been very good at tightening up the sound for me.

It's possible that the vibration/resonance is not being transmitted through the stands, but is rather caused by the bass in the room causing a resonance. For instance, in my room, at very high levels my ordinary desk surface vibrates more the farther away from the stands I test with finger touch. To me that suggests airborne, not structure borne, resonance, but I could be totally wrong.


Suggestions:

I'm a bit anxious about that cable-reel resting on the shelf. That could be resonating sympathetically. If it's needed, could it be placed somewhere else?

Those semi-open cavities around the shelves could be contributing. Could you get some mass and volume onto them? Lots of heavy books would be good.

Can you try using a different sort of isolation for the speakers. Some here use the purpose-built composite pads, but remember to get ones appropriate to the weight of your speaker - they're not one-size-fits-all. Others will use stacked telephone directories with success.

... and what sort of music are you listening to... and at what levels?
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Re: Vibrating shelves

Postby Wonks » Sun Dec 09, 2018 6:18 pm

Are those stands a good match for the size/wight of the speakers? I'm not sure what the speakers are (KEFs of some sort?), so can't look up the data directly. It may be that the stands are too small for the weight/size of the speakers so are overloaded and aren't isolating the speakers properly.

If they are something like the KEF LS50, then IsoAcoustics recommend the ISO155 (or Aperta if you want a smarter finish) stand, though it may depend on the version and weight of your actual speakers.

I'd suggest removing them and mounting the speakers directly on to the top with a blob of Blu-tac at each corner to see if that makes any difference at all. If the shelves are concrete or something similar, I'd be surprised that they'd be shaking that much, especially when supported at the ends. However, they may be wood that's been plastered over (or similar) to give a concrete effect, so may be more prone to vibration. But even so, the speakers are located at the corners, where the shelf has lots of support and will vibrate less there anyway.

As Mike says, it may be down to room modes and volume levels. Can you sketch out the rough room layout with dimensions and the respective shelf and speaker positions in relation?
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Re: Vibrating shelves

Postby ef37a » Mon Dec 10, 2018 12:30 pm

Woah! You have three resonant cavities there. I would first of all fit uprights to break each panel into a higher, more easily damped frequency but make each "hole" a non-integer lengh from the others.
Next fill the spaces. Books would work or stacks of SoS mags! (call round, got loads) .

Don't know how good those isolators are? I would use a small paving slab sitting on 25mm of foam then Blu-Tak the speakers to it.

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Re: Vibrating shelves

Postby Wonks » Mon Dec 10, 2018 1:42 pm

Dave, if you assume that the shelves are about 5 feet long, then the lowest resonant frequency is around 225Hz, which isn't really 'upper bass'.

It is probably a good idea to fill the spaces with books or magazines as that would help damp and mid and bottom shelf resonances.

Those IsoAcoustics supports are supposed to be very good when used correctly, (see SoS reviews of them) but you do need to have the correct one to match the weight of the speakers. Too small a unit and the compliant supports get fully compressed and stop isolation and start transferring vibrations. The physical small size of the supports shown in the photos also means that the rear support legs are located near the middle of the underside of the speaker, where the base will be vibrating most. So a larger support will help support the speaker base much nearer the edges, where vibration will be less, and the speaker weight should then be within the range of the supports.

The position of the right-hand speaker against the wall will also increase the amount of bass put out by around 3dB compared to the left-hand speaker. It doesn't look like the speakers have any LF filtering to reduce the bass frequencies, so the RH speaker really needs to be moved away from the wall as much as possible, or else the DAW output to that speaker filtered to reduce the bass level.

If the OP has a DAW feeding the speakers, then using a sine tone, it may be worth seeing what frequencies actually set the shelves moving. Is it a narrow band of frequencies in the 225Hz or 450Hz area - indicating shelf resonance, or is it a wide band - indicating probable transference from the speakers to the shelf top?

If it is true bass resonance, then it could be a room mode causing the vibration, with the shelves located in an area of maximum bass wave amplitude.
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Re: Vibrating shelves

Postby RichardT » Mon Dec 10, 2018 4:14 pm

Thanks very much to everyone for their replies.
I have lots of things to try!

I did some measurements on the vibration frequencies of the shelves as Wonks suggested. I got the frequency zone wrong. In fact the top shelf vibrates at 70 Hz and the lower two at 95Hz. The vibrations are strongly peaked at these frequencies, so that I hear musical notes when I tap the shelves. The vibration I feel from the test tones is strongest in the middle of the shelves, not by the speakers.

I don’t have any spare books to hand but I have tried some blankets fitted tightly between the shelves - this does make a difference.
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Re: Vibrating shelves

Postby RichardT » Mon Dec 10, 2018 8:37 pm

RichardT wrote:Thanks very much to everyone for their replies.
I have lots of things to try!

I did some measurements on the vibration frequencies of the shelves as Wonks suggested. I got the frequency zone wrong. In fact the top shelf vibrates at 70 Hz and the lower two at 95Hz. The vibrations are strongly peaked at these frequencies, so that I hear musical notes when I tap the shelves. The vibration I feel from the test tones is strongest in the middle of the shelves, not by the speakers.

I don’t have any spare books to hand but I have tried some blankets fitted tightly between the shelves - this does make a difference.

I just tried loading the shelves with some spare wall tiles, about 10-15 kilos on each. That’s brought a big reduction to the vibration, and I can hear more detail when I listen to the speakers. A great result, thanks!
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