Modifying Resonant Frequencies

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thecommabandit
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Modifying Resonant Frequencies

Postby thecommabandit » Thu Feb 28, 2008 2:05 pm UTC

Ok, so recently (i.e. for the past three months or so...) we've been doing a lot of stuff about SHM in physics, and resonant frequencies have come up. Now, in the room where my computer is (and hence where the best sound system in the house is) there's this door. This door likes to vibrate and rattle rather annoyingly at a certain bassy frequency of sound - it's rather low on the scale of the kind of things you'll generally hear in music. In fact, I think there might be two resonant frequencies for the door - one for the door or hinge and one for the glass in the frame above the door. Now, to the actual question at hand: is there any way I can easily and cheaply modify the resonant frequency of a door/its hinges/a pane of glass in a frame? Preferably I want to lower the frequency so its either out of my hearing range or the resonant frequency is lower than the bass most songs will use.

Bear in mind my kitchen's being done up at the moment, so I don't have a huge access to tools (and no access at all to power tools) or materials beyond a few screwdrivers and spanners and some duct tape and though I'm sure it's possible to remedy the solution with duct tape it just won't be a particularly elegant solution.
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SpitValve
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Re: Modifying Resonant Frequencies

Postby SpitValve » Thu Feb 28, 2008 2:30 pm UTC

Alternately, you could put padding or something on the door to damp the sound?

I know what you're talking about though: I've had doors and things "ring" annoyingly with certain music (and once a room actually was resonating with someone's voice). Sometimes just opening the door helps.

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Re: Modifying Resonant Frequencies

Postby fjman » Thu Feb 28, 2008 2:53 pm UTC

Start with the basics.

Make sure the screws that hold the hinges to the wall, and the ones holding the hinges to the door are all tight. Same goes for the strikeplate (the thingy in the jamb that the knob's sticky-outy-thingy pokes into).

Walk up to the (closed) door and tap your finger on the glass. Does the glass rattle/click when you touch it? Depending on the style, some doors you could remove one piece of the trim holding the window in place and put a little paper/cloth between the trim and the window to tighten it (I'd go the whole way around, one section at a time).

With the door closed, can you push/pull on it any? If it moves slightly, it's possible to wiggle the strikeplate a bit to make it close more securely, so that it takes a bit of a push to latch closed. Another option is felt-type weatherstripping all around the jamb so there isn't wood/wood contact.

Or just turn down the !#@$!# music. :P

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Re: Modifying Resonant Frequencies

Postby Mathmagic » Thu Feb 28, 2008 4:25 pm UTC

fjman wrote:Or just turn down the !#@$!# music. :P

Changing amplitude != Changing the frequency
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Re: Modifying Resonant Frequencies

Postby olcaddy » Thu Feb 28, 2008 4:50 pm UTC

Changing amplitude != Changing the frequency


But would obviously change the amplitude the door is resonating. Just glue some additional masses to anything resonating I believe the frequency response is proportional to (k/m)1/2 more mass lower frequency. Or weatherstripping and caulk

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Re: Modifying Resonant Frequencies

Postby thecommabandit » Thu Feb 28, 2008 5:16 pm UTC

I shall find my trusty screwdriver now and have a go at the screws! In the meantime, would the fact that the door frame is part of a house make any difference to the effectiveness of adding mass? And how much mass would I need to add to bring the frequency down a few hertz? I would do some number crunching but it'd make me feel like a huge geek (also, I don't know the stiffness constant for a door frame :P).
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Re: Modifying Resonant Frequencies

Postby olcaddy » Thu Feb 28, 2008 5:40 pm UTC

Well you could use your favorite tone generation software (Audacity is free) and see what frequency(s) the buzzing occurs then doubling the mass should cut the frequency by a factor of 4. Keep in mind that this is a very mad-scientistesque solution caulk, a screwdriver and weatherstripping makes much more sense. Tho it would be fun to run the worst note and tape a quarter onto the glass pane and see if it shuts up.
Also if I'm actually interpreting the relationship correct there are still lots of assumptions made. Like the door has to dampening and others I'm sure.

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Re: Modifying Resonant Frequencies

Postby Velifer » Thu Feb 28, 2008 6:17 pm UTC

Whip up a suction cup inertial dampener, and stick it to the window at an antinode?

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Re: Modifying Resonant Frequencies

Postby fjman » Fri Feb 29, 2008 4:01 am UTC

mathmagic wrote:
fjman wrote:Or just turn down the !#@$!# music. :P

Changing amplitude != Changing the frequency



Correct, however solving the practical problem is still fixing the rattle. :lol:

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Re: Modifying Resonant Frequencies

Postby OneFish » Fri Feb 29, 2008 5:50 am UTC

fjman wrote:
mathmagic wrote:
fjman wrote:Or just turn down the !#@$!# music. :P

Changing amplitude != Changing the frequency



Correct, however solving the practical problem is still fixing the rattle. :lol:


Exactly. Tighten everything up, put a little super glue into cracks if that's what's rattling, put some little felt dots on contact points that might be rattling. That's easier than trying to alter the door's physics.

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Re: Modifying Resonant Frequencies

Postby mrbaggins » Fri Feb 29, 2008 6:26 am UTC

Yeah, don';t mess around trying to change the doors mass.... That's just crazy talk.

By some felt squares and place them in the door jam, or move the strike plate back a bit. Or solidify the glass into its mounting.

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Re: Modifying Resonant Frequencies

Postby The Reaper » Fri Feb 29, 2008 6:52 am UTC

If its one of those hollow doors, add 2 hinges, drill a hole in the top, and fill with lead dust. ;)

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Re: Modifying Resonant Frequencies

Postby Bobber » Fri Feb 29, 2008 7:08 am UTC

mrbaggins wrote:There are two tools in life, duct tape and WD40. If it moves and shouldn't, use the tape. If it doesn't move and should, use the WD40.


So true - so sigged.
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mrbaggins wrote:There are two tools in life, duct tape and WD40. If it moves and shouldn't, use the tape. If it doesn't move and should, use the WD40.

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thecommabandit
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Re: Modifying Resonant Frequencies

Postby thecommabandit » Fri Feb 29, 2008 3:25 pm UTC

mrbaggins wrote:There are two tools in life, duct tape and WD40. It it moves and shouldn't, use the tape. If it doesn't move and should, use the WD40.

I am crying with laughter at the moment. Epic. Prodigious. Genius.
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Re: Modifying Resonant Frequencies

Postby gmalivuk » Sat Mar 01, 2008 5:46 pm UTC

For the record, the genius is not mrbaggins's, as I've seen that quote around for ages.
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thecommabandit
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Re: Modifying Resonant Frequencies

Postby thecommabandit » Sat Mar 01, 2008 9:26 pm UTC

I echo my previous statement to whoever it was that thought it up through the magic of the internet and/or tachyons.
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Re: Modifying Resonant Frequencies

Postby mrbaggins » Sun Mar 02, 2008 1:13 pm UTC

Yeah, I wasn't being insightful. I think I got it from bash actually. Which, knowing bash, someone there took it from someone else.
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