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Standing waves get their own thread - lucky little blighters!

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Re: Standing waves get their own thread - lucky little blighters!

Postby Tequila Slammer » Wed Dec 29, 2004 12:57 pm

Paul Woodlock wrote:
No in fact it's the other way around. If the boundaries are allowed to vibrate they will ABSORB sound and take some energy away from the standing wave, thus diminishing it. Although a vibrating wall will re-radiate some sound back into the room. And of course it will radiate some sound on the other side of the wall ( again diminishing the standing wave ), and is thus the cause of many a neighbourly dispute.



Foz wrote:TS:

It takes a fairly large room with fairly stiff walls, floors, and ceilings and some well proportioned dimensions to avoid all those problems.

OK, I'm confused. Are hard walls bad or good for standing waves - or room modes?

I'm beginning to like Ethan's way of thinking on this issue - thinking of room modes as standing waves has caused me a great deal of confusion, while thinking of them as room resonances makes more sense...

OK, To re-iterate, flat 1 had two rooms - one large, one small. The bass was almost non-existant, and I didn't hear the effects of nodes and anti-nodes while walking around the rooms. The walls were very hard indeed.

The house has two rooms, of similar dimentions to the flat. The bass is large ad boomy. When untreated, walking around both rooms gives very obvious node and anti-node effects. The walls are really quite flimsy by the feel of them, and the floorboards are loose enough that walking across the room makes objects vibrate on all flat surfaces. Window sizes and placements are similar.

So what's going on?
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Re: Standing waves get their own thread - lucky little blighters!

Postby thefruitfarmer » Wed Dec 29, 2004 3:35 pm

Tequila Slammer wrote:

The house has two rooms, of similar dimentions to the flat. The bass is large and boomy. When untreated, walking around both rooms gives very obvious node and anti-node effects. The walls are really quite flimsy by the feel of them, and the floorboards are loose enough that walking across the room makes objects vibrate on all flat surfaces. Window sizes and placements are similar.

So what's going on?

The walls and floor could be resonating in sympathy which may then mean they act as a sound box (= problem).

A heavy and solid wall or floor is less likely to vibrate.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Eric Desart » Wed Dec 29, 2004 4:44 pm

Tequila,

It can be good to check that besides the traditional modal issues, objects or panels are resonating.

You can do that by sending a sweep in the room from low to high.
Anything has its natural resonances. It doesn't matter that much if they are damped enough.
With a sweep you can easier localize undamped resonances. Loose, low damped things start clattering/rambling when excited with the corresponding frequency.

Such things should be fixed, since they will screw the sound anyhow.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Scott R. Foster » Wed Dec 29, 2004 5:18 pm

thinking of room modes as standing waves has caused me a great deal of confusion, while thinking of them as room resonances makes more sense


TS:

How does it help to pretend that a "room resonance" is something other than a "standing wave"?

Wait.. don't answer that. I'll agree that as long as the discussion is solely about grasping a rudimentary idea of what changes to a room will help achieve a certain result in modal behavior we don't need physics. In that event, any analogy will suffice [provided it remains valid to real physics]. But let's not let using analogies to physics as a shortcut allow anyone to pretend they have changed the laws of physics... or "discovered" something new. That sort of silliness just breeds misconception and wastes folks time AND money.

It's pointless to argue over how one should look at a physical phenomena if the only goal is to grasp a general rule of cause and effect... it doesn't matter in that context... but it is silly to pretend that looking at something differently changes what it really is.

Room modes result from standing waves... but that don't mean you have to learn to visualize wave behavior to understand how to manipulate room modes. Calling room modes something else besides standing waves [Dragon Farts for example], and picturing the physical elements that give rise to them in some creative way that works for you as you go about trying to control them does nothing to change the real causes or limit your ability to make effective changes [though it may help you get your project completed and move on to other things].

We could forget waves altogether. Maybe we should.. it sounds like that would help you in some way... and I have no objection with that goal.

We could just say the room's resonances are driven by interior dimensions and the reflectiveness of the surfaces [ceiling/floor/walls] and taken together they act like a set of large water jugs spread out on the floor and filled to different levels so that the jugs hum at different notes... and its a good thing if the levels in the jugs vary... so you get a nice even mix of tones... none dominating. Maybe, that is enough for practical purposes of understanding how to address room modes when they cause problems... as long as you don't let the analogy drive you into misconceptions on the real physics behind the problem.

A few points on those humming jugs:

1) The lower toned jugs are the worrisome ones - how high up the band you need to be concerned is a function of modal density. If you have 50 jugs in a given octave, they tend to drown each other out, no particular tone dominates... the more jugs per octave the better. Small rooms have low modal density. The problem is worst low in the band, and the smaller the room the higher on the band the problem will reach. So small rooms are more prone to modal problems, and they are prone to these problems at higher frequencies.

2) Modes arise from a large set of dimensions - not just the distances between parallel surfaces [axial modes] - so there are more than 3 of them [and their harmonics]. Instead there are dozens of modes below critical levels of density for any given room. Choosing to look at only the axials is working in the blind. OTOH the axials are the biggest jugs [the loudest] so problems with this set of jugs can cause the worst problems... but happily axials are the easiest to alter [example if the front to rear wall axial matches the ceiling axial, then a broadband absorptive ceiling and a large diffusion array on the rear wall will go a long way to removing the problem].

3) By shifting just one dimension of the room you alter the level in a large number of jugs - this is trivial if you think about it BEFORE the room is built. Best to do your jug level juggling before you build.

4) Broadband absorption diminishes the loudness of all the tones - and all tones exist in corners - broadband absorption in corners will turn down all of a room's resonances - a good place to start.

The point of the above is to agree that we can talk about room modes without talking about wave behavior... and cite a series of examples... but be careful you don't let the analogy become the topic. The problem for me arises when someone wants to say something incorrect about the physics in the context of an analogy, and then insists the analogy is the proper context for discussion... at some point to get the whole picture, and engage in a real debate of all the elements, you have to drop the analogies and talk about waves.

Does that mean it is necessary to dispense with analogy in order for the average person to wrestle with room modes in their home studio? I don't think so, but if we are going to debate various points on the physics behind the cause and effect of room modes, then I think its time for the analogies to go away. And if you want to participate in that discussion... then you need to grasp that room modes ARE standing waves... anything short of that and you're just talking into your hat.

My $0.02
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Re: Standing waves get their own thread - lucky little blighters!

Postby Tequila Slammer » Thu Dec 30, 2004 10:57 am

thefruitfarmer wrote:
Tequila Slammer wrote:

The house has two rooms, of similar dimentions to the flat. The bass is large and boomy. When untreated, walking around both rooms gives very obvious node and anti-node effects. The walls are really quite flimsy by the feel of them, and the floorboards are loose enough that walking across the room makes objects vibrate on all flat surfaces. Window sizes and placements are similar.

So what's going on?

The walls and floor could be resonating in sympathy which may then mean they act as a sound box (= problem).

A heavy and solid wall or floor is less likely to vibrate.

That was my theory. It seems to be accepted theory that hard walls will reflect more bass however.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Tequila Slammer » Thu Dec 30, 2004 10:58 am

Eric Desart wrote:Tequila,

It can be good to check that besides the traditional modal issues, objects or panels are resonating.

You can do that by sending a sweep in the room from low to high.
Anything has its natural resonances. It doesn't matter that much if they are damped enough.
With a sweep you can easier localize undamped resonances. Loose, low damped things start clattering/rambling when excited with the corresponding frequency.

Such things should be fixed, since they will screw the sound anyhow.

Brilliant idea, thanks! :lol:
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Re: Standing waves get their own thread - lucky little blighters!

Postby Tequila Slammer » Thu Dec 30, 2004 11:11 am

Foz wrote:
thinking of room modes as standing waves has caused me a great deal of confusion, while thinking of them as room resonances makes more sense


TS:

How does it help to pretend that a "room resonance" is something other than a "standing wave"?

Wait.. don't answer that. I'll agree that as long as the discussion is solely about grasping a rudimentary idea of what changes to a room will help achieve a certain result in modal behavior we don't need physics. In that event, any analogy will suffice [provided it remains valid to real physics]. But let's not let using analogies to physics as a shortcut allow anyone to pretend they have changed the laws of physics... or "discovered" something new. That sort of silliness just breeds misconception and wastes folks time AND money.

It's pointless to argue over how one should look at a physical phenomena if the only goal is to grasp a general rule of cause and effect... it doesn't matter in that context... but it is silly to pretend that looking at something differently changes what it really is.

Room modes result from standing waves... but that don't mean you have to learn to visualize wave behavior to understand how to manipulate room modes. Calling room modes something else besides standing waves [Dragon Farts for example], and picturing the physical elements that give rise to them in some creative way that works for you as you go about trying to control them does nothing to change the real causes or limit your ability to make effective changes [though it may help you get your project completed and move on to other things].

We could forget waves altogether. Maybe we should.. it sounds like that would help you in some way... and I have no objection with that goal.

We could just say the room's resonances are driven by interior dimensions and the reflectiveness of the surfaces [ceiling/floor/walls] and taken together they act like a set of large water jugs spread out on the floor and filled to different levels so that the jugs hum at different notes... and its a good thing if the levels in the jugs vary... so you get a nice even mix of tones... none dominating. Maybe, that is enough for practical purposes of understanding how to address room modes when they cause problems... as long as you don't let the analogy drive you into misconceptions on the real physics behind the problem.

A few points on those humming jugs:

1) The lower toned jugs are the worrisome ones - how high up the band you need to be concerned is a function of modal density. If you have 50 jugs in a given octave, they tend to drown each other out, no particular tone dominates... the more jugs per octave the better. Small rooms have low modal density. The problem is worst low in the band, and the smaller the room the higher on the band the problem will reach. So small rooms are more prone to modal problems, and they are prone to these problems at higher frequencies.

2) Modes arise from a large set of dimensions - not just the distances between parallel surfaces [axial modes] - so there are more than 3 of them [and their harmonics]. Instead there are dozens of modes below critical levels of density for any given room. Choosing to look at only the axials is working in the blind. OTOH the axials are the biggest jugs [the loudest] so problems with this set of jugs can cause the worst problems... but happily axials are the easiest to alter [example if the front to rear wall axial matches the ceiling axial, then a broadband absorptive ceiling and a large diffusion array on the rear wall will go a long way to removing the problem].

3) By shifting just one dimension of the room you alter the level in a large number of jugs - this is trivial if you think about it BEFORE the room is built. Best to do your jug level juggling before you build.

4) Broadband absorption diminishes the loudness of all the tones - and all tones exist in corners - broadband absorption in corners will turn down all of a room's resonances - a good place to start.

The point of the above is to agree that we can talk about room modes without talking about wave behavior... and cite a series of examples... but be careful you don't let the analogy become the topic. The problem for me arises when someone wants to say something incorrect about the physics in the context of an analogy, and then insists the analogy is the proper context for discussion... at some point to get the whole picture, and engage in a real debate of all the elements, you have to drop the analogies and talk about waves.

Does that mean it is necessary to dispense with analogy in order for the average person to wrestle with room modes in their home studio? I don't think so, but if we are going to debate various points on the physics behind the cause and effect of room modes, then I think its time for the analogies to go away. And if you want to participate in that discussion... then you need to grasp that room modes ARE standing waves... anything short of that and you're just talking into your hat.

My .02


Well, that didn't work. I was hoping that that comment would prompt a discussion about the physics of room modes, and why they behave the way they do.

Everything you've said is very useful practical stuff, and thanks :) but I'm interested in the physics now. I want to know why thinking about room modes as waves of any kind gets me nowhere.

Talking about jugs and dragon farts isn't helping ;)

Thing is, I thought I knew about waves. So, either I'm missing certain knowledge about wave theory (or misapplying what I do know) or we're dealing with a physical property that is different to or supplemental to wave theory.

I want to know which, and learn about it.

Does anyone know any good books?
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Re: Standing waves get their own thread - lucky little blighters!

Postby thefruitfarmer » Thu Dec 30, 2004 5:39 pm



"Does anyone know any good books?"



Foyles Bookshop in the Charing Cross Road have a science department. I found a physics graduate working there recently who was all happy to chat about maths and help me find a suitable maths text book. I still don't have that much of a scientific mind though..... ;)

Re standing waves try this.

It is the size of the room (the modes) which determines at what frequencies the standing waves will occur at.

I think of it as being when the wavelength of the sound wave is such that it fits neatly in to the size of the room and the reflected wave interacts with the original wave in such a way as to result in the peaks and nulls within the room.

The formation of standing waves is a problem because it means that you may not be hearing the music as it really is. There could be more bass than you hear if you are sitting in a null area or less if you are sitting in a peak area.

So, what people do when building studios is to install absorption to prevent the sound reflecting around the room. If the room cannot reflect the standing waves will not occur. Then you can hear the music without the colouring effect of the room which does make an incredible difference.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Tequila Slammer » Fri Dec 31, 2004 10:40 am

Thanks, Fruit Farmer. Foyles is a probably good place to go - although the temptation to pop over the road to Turnkey may be too great. ;)

I was hoping someone might know a particularly good book on the subject.

As it goes, I'm up on that bit of the theory. I'm just confused about the property of standing waves/room modes that makes them independent of speaker position. The bit that makes treating parallel walls around the monitoring position a good idea. If anyone can explain the physics of that bit, or point me at a thread that does, please do. If anyone can point me at a book that goes into that kind of detail, please do.

I've got as far as 'the air in the room is resonating' but I lack knowledge concerning the difference between the mechanics of a pressure wave in a body of air and the mechanics of a resonating body of air.
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Re: Standing waves get their own thread - lucky little blighters!

Postby Ido » Fri Dec 31, 2004 10:42 am

="Tequila Slammer.

Does anyone know any good books?


I think the best for this (and all around) is the Everest Master Handbook.
His coverage of standing waves to room modes is the most clear and understandable I ever saw [it's also cheap $].
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Re: Standing waves get their own thread - lucky little blighters!

Postby Ethan Winer » Fri Dec 31, 2004 7:21 pm

Slammer,

> I'm just confused about the property of standing waves/room modes that makes them independent of speaker position <

Take an empty soda bottle and strike it gently near the mouth with the blunt end of a drum stick. Now strike the bottle again on the side, and again at a few other places. The basic pitch is always the same, but the tone will change as different partials are excited. The same thing happens when a loudspeaker (the stick) excites a room (the bottle). The basic resonant frequencies of the room stay the same, but depending on where you "strike" it you'll get different proportions of overtones.

--Ethan
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Re: Standing waves get their own thread - lucky little blighters!

Postby Scott R. Foster » Sat Jan 01, 2005 2:22 am

TS:

Here is a resource that will link you to a number of texts and online acoustics journals that might be of interest.

http://forum.studiotips.com/viewforum.php?f=19
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