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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 » Mon Dec 20, 2004 5:34 pm

OK, we've started to make progress here. As far as I could tell, there were two sides of an argument:

- Room modes = standing waves.
- Room modes are nothing to do with standing waves.

Without getting into specifics, we seem to have arrived at:

- Room modes are a special case of standing waves.

Agreed on all sides.

Great!

Shall we forget the specifics of who said what and move on from here? If standing waves can occur at any frequency, what does that mean in real terms for the end user?

Thinking about it, as they would only occur when the angle of incidence is 90 degrees to the wall (on both perpendicular planes) is this an argument for positioning monitors in front of and above the monitoring position, angled down, so the listener isn't bisected by either of these planes?

New page, new start. ;) Can we have an interesting, informed and informative discussion without personal attacks please? :lol:
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Re: Standing waves get their own thread - lucky little blighters!

Postby Scott R. Foster » Mon Dec 20, 2004 5:38 pm

Tim Rainey wrote:Ethan's last post:
>I never said room modes and standing waves are unrelated.

A prior post from Ethan:
>In fact, this is further proof that standing waves have nothing at all to do with room modes.

Tim ;o)


Classic Ethan.

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

Postby Tequila Slammer » Mon Dec 20, 2004 5:46 pm

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

Postby Ethan Winer » Mon Dec 20, 2004 5:51 pm

Slammer,

> Room modes are a special case of standing waves. Agreed on all sides. <

Just to clarify, the full statement I made earlier is that standing waves have nothing to do with room modes and everything to do with reflections. So technically, Yes, standing waves are related to room modes, but only at select frequencies and only as a function of reflections. Reflections are the parent principle, not the other way around.

Thanks for summarizing.

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

Postby Tequila Slammer » Mon Dec 20, 2004 6:03 pm

*sigh*

OK, I'm assuming here that room modes are going to exhibit a more marked effect than any other type of standing wave within a room - simply because the room dimentions mean the wave will be amplified in both directions by reflections on either side of the wall.

A standing wave at another frequency would surely be 'broken up' by the reflection at the opposite boundary, reducing the effect?
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Re: Standing waves get their own thread - lucky little blighters!

Postby Ethan Winer » Mon Dec 20, 2004 6:21 pm

TS,

> A standing wave at another frequency would surely be 'broken up' by the reflection at the opposite boundary, reducing the effect? <

Not really. I've measured peaks and nulls at both modal and non-modal frequencies. For any given frequency you'll be able to find a peak and also a null location in the room. Likewise, for any given location you'll be able to find both peak and null frequencies.

This is yet another reason I dispute that a standing wave is the same thing as a room mode. Again, one is a wave and the other is a propensity to vibrate at a particular frequency. Apples and cauliflowers.

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

Postby DoeZer » Mon Dec 20, 2004 6:24 pm

Hi slammer.

funny that.

I've had my stuff in a tiny little room for the past few years, completely UNTREATED acoustically, with two dimensions the same (width = height) and opne pretty close (it was 8 x 8 x 7ft) so it should have been a disaster, and have managed to make music that translates well to the outside world and was of release quality (i.e. a label released it, so it must have been ok...))

and my monitors were placed quite high on the wall (say 6ft up, and were angled down... I always felt that the angling down of the monitors was helping cos it was screwing up the angles of incidence etc. but i am not an acoustics theory guru so this could be rubbish. (i dont think so though ;))

im now moving to a bigger room (8 x 8 x 10.5), and am thinking that I might just place teh monitors in the same way again...
I originally placed them like that, not for of any technical/acoustics reason. simply because the speaker wall brackets I got had an adjustable pivot which allows you to adjust the vertical alignment and my Tannoy Reveals were too heavy so it simply tilts down whether i want it to or not..!

anyone got impressions on that?

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

Postby Paul Woodlock » Mon Dec 20, 2004 7:41 pm

*** Thanks for the support Paul, (I'll wear it always :tongue:) I hope you don't mind me trashing your work but as we're trying the new page, new start approach (courtesy of TS ;)) I'd prefer that we just drop this for now. I'll make my reply to Ethan via PM - as I perhaps should've done initially.

I realise that you also have some valid points to make on this subject but please, for now, limit them to the science. It's not open season on Ethan or anyone else. (Again, I do appreciate the support.) 0VU :) ***
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Re: Standing waves get their own thread - lucky little blighters!

Postby Ethan Winer » Mon Dec 20, 2004 8:35 pm

0,

> I'll make my reply to Ethan via PM <

If you don't mind, it's a lot easier for me to correspond by email. I also check my email much more often than the forums. If you don't mind, please use either the email address listed on my personal site www.ethanwiner.com or the one on the RealTraps site.

Thanks.

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

Postby Anonymous » Mon Dec 20, 2004 8:37 pm

E,

OK, email it is. It's easier for me too in many ways.

0VU.
Anonymous

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

Postby Paul Woodlock » Mon Dec 20, 2004 8:51 pm

Paul Woodlock wrote:*** Thanks for the support Paul, (I'll wear it always :tongue:) I hope you don't mind me trashing your work but as we're trying the new page, new start approach (courtesy of TS ;)) I'd prefer that we just drop this for now. I'll make my reply to Ethan via PM - as I perhaps should've done initially.

I realise that you also have some valid points to make on this subject but please, for now, limit them to the science. It's not open season on Ethan or anyone else. (Again, I do appreciate the support.) 0VU :) ***

Greetings 0VU

I would have preferred to stick to science.

However Ethan's post I was replying to, contained NO science, and was full of personals, and sentances saying nothing. So how could I reply with science to all that?

:)


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

Postby Tequila Slammer » Mon Dec 20, 2004 11:18 pm

DoeZer wrote:Hi slammer.

funny that.

I've had my stuff in a tiny little room for the past few years, completely UNTREATED acoustically, with two dimensions the same (width = height) and opne pretty close (it was 8 x 8 x 7ft) so it should have been a disaster, and have managed to make music that translates well to the outside world and was of release quality (i.e. a label released it, so it must have been ok...))

and my monitors were placed quite high on the wall (say 6ft up, and were angled down... I always felt that the angling down of the monitors was helping cos it was screwing up the angles of incidence etc. but i am not an acoustics theory guru so this could be rubbish. (i dont think so though ;))

im now moving to a bigger room (8 x 8 x 10.5), and am thinking that I might just place teh monitors in the same way again...
I originally placed them like that, not for of any technical/acoustics reason. simply because the speaker wall brackets I got had an adjustable pivot which allows you to adjust the vertical alignment and my Tannoy Reveals were too heavy so it simply tilts down whether i want it to or not..!

anyone got impressions on that?

D

That seems to back up what my A level physics tells me ;)

If the sound is propagating from the speakers spherically (for the sake of simplicity) there will only be a tiny proportion of the sound that ever hits the walls at 90 degrees in both planes. This will be directly in front of, behind, above, below and to either side of the speakers.

A lot of the research I've done so far says a good initial set-up is to put treatment above, in front of, behind and to either side of... The monitoring position!

That always niggled at me. Why would you ever get standing waves there? Any sound crossing that point has to be going at an angle to the wall other than 90 degrees, unless you wear your monitors like headphones. Wide angles won't exhibit standing wave properties at all, and very narrow angles will lose energy bouncing around before they creep over to you.

The mirror trick, which I picked up on the studiotips website after Paul mentioned it on the Mick-Traps thread, sounds like a much more scientifically valid method of treatment placement.

What's everyone's thoughts on having high monitors angled down? It makes complete sense to me, but as I siad, I have all of an A level in physics.

What are the sonic advanages of having your monitors' tweeters level with your ears? Is it worth it to suffer the hideous mangling of audio that comes with standing waves?
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Re: Standing waves get their own thread - lucky little blighters!

Postby Scott R. Foster » Mon Dec 20, 2004 11:42 pm

TS:

Why would a standing wave created by reflections from room boundaries require normal incidence (90 degree) in order to be iterative?
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.......

Postby Paul Woodlock » Tue Dec 21, 2004 2:57 am

Tequila Slammer wrote:......A lot of the research I've done so far says a good initial set-up is to put treatment above, in front of, behind and to either side of... The monitoring position!

That always niggled at me. Why would you ever get standing waves there? Any sound crossing that point has to be going at an angle to the wall other than 90 degrees, unless you wear your monitors like headphones. Wide angles won't exhibit standing wave properties at all, and very narrow angles will lose energy bouncing around before they creep over to you.

The mirror trick, which I picked up on the studiotips website after Paul mentioned it on the Mick-Traps thread, sounds like a much more scientifically valid method of treatment placement.


Greetings Tequila

Acoustic treatment ain't just about standing waves.

1] Treatment for standing waves at Room Mode frequencies is best treated by broadband absorption in corners. Tri-corners being the optimal placement if you can't do ALL corners. Room mode standing waves NEED two or more boundaries, and in the real world they can affect the freqeuncy response of a room from the bass upto around 500Hz.

2] The mirror trick is used to determine placement of broadband absorption for EARLY REFLECTIONS ( i.e those reflections whose path length is less than around 20 feet ( about 20ms ) greater than the direct sound path length. These reflections are heard by the ear as part of the direct sound and cause reduced clarity and detail, innacurate stereo image/source location. And a 'comb filter' type response. All across the audio bandwidth where the sound is directional.

So... "A lot of the research I've done so far says a good initial set-up is to put treatment above, in front of, behind and to either side of... The monitoring position!" is great advice. As it helps to create an RFZ ( Reflection Free Zone ) at the listening position.

And in particular, with free standing non-soffited speakers, placed close to one or more boundaries, there will be a rise in bass response. This is due to the omnidirectional response of a speaker cabinet at lower frequencies, and the wavelengths of the lower frequencies being large compared to the reflection path length resulting from the boundary. As the wavelengths are so long in comparison the reflected sound will pretty much be in phase with the direct sound, causing an additive increase in level. IIRC you get theoretical 6dB increase in level for every boundary. Which means peeps who place their speakers in a reflective tricorner will get a theoretical increase of 18dB. In practise it will less than this due to real world losses, etc.

You can also compensate for this increased bass response by EQ. In fact many active monitors have tilt and shelving controls to do just that.

As you increase the frequency, so the wavelength and path length become a similar size, the reflections cause interference patterns, which results in a 'comb filtered' response.

So with free standing speakers that are close to walls, floors and ceilings ( unavoidable in small rooms ), LOTS of absorption behind, below, above and at the sides of the speakers is well recommended.

The other option is to soffit mount the speakers, which places the sound source at the SAME PLANE as the wall, thus their are NO reflections from behind the speaker to cause any increase in bass levels or comb filtering at higher frequencies. The other advantage of soffit mounting is to stop sound diffracting at the edges of the speaker cabinet.

Another advantage of soffit mounting speakers is the increase in efficiency of lower frequencies ( Sound energy that otherwise would radiate in directions other than forwards, now CANNOT, due to the soffit wall. EQ is used to compensate the response, and thus your monitors have more bass head room. Meaning less speaker distortion.

Genelec have the best FAQs on soffit mounting here..

http://www.genelec.com/support/soffit.php

http://www.genelec.com/support/faq/faq20.php

http://www.genelec.com/support/faq/faq19.php



All this though , in regard to real world treatment, is a seperate issue to Room Mode treatment ( trapping ).



What's everyone's thoughts on having high monitors angled down? It makes complete sense to me, but as I siad, I have all of an A level in physics.

What are the sonic advanages of having your monitors' tweeters level with your ears? Is it worth it to suffer the hideous mangling of audio that comes with standing waves?


The sonic advantages of having the tweeters at ear level is to help the clients sitting on the couch at the back of the studio hear a balanced HF. Angle them down and it's likely the client will hear a dull version ;) Important considerations in the real world.

I've actually designed my monitors to angle downwards in my new studio, as I don't have clients and I angled the speakers ( and front soffit wall ) downwards by 10 degrees to optimise the sound at the listening position in regard for other pieces of equipment like computer monitors obsuring the Audio Monitors.


Sorry if my reply has been too much or too long :) I got carried away.


But it is important, IMO, to show that when treating rooms, the innacuracies we wish to treat, are caused by various situations, and different treatments are used for those different situations.

In short(er).... ;)

Room mode Standing waves need TWO or more Boundaries to exist, and have their maximum pressure at those boundaries. They are pretty much independent of speaker placement, although certain speaker placements can excite them more easily ( such as in the case of soffit mounting ). Corners are where maximum pressure of modes from more than one room dimension add together. So corners are where to maximise the efficiency of room mode absorption treatment.

Early Reflection problems are treated by absorbing ay reflections that will end up at the listening position. The Mirror trick is the easiest way to find the spots, although it can be done pretty well by visualisation for those without mirrors ;)

IMO those are the two types of treatment that shoudl be considered first off. And then once the early reflections and room mode standing waves have been tamed you can then start thinking about diffusion to smooth out the LATER reflections in the case of large rooms, or smoothing out any ambience left over in a small room.

Right! that's enough for now.

back to designing my studio build acoustic treatment.

:)


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

Postby Tequila Slammer » Tue Dec 21, 2004 10:38 am

This is all great stuff! Thanks for taking the time to reply - it's very much appreciated!

:lol:

I understand the difference between simply reflected sound and standing waves. What I still don't understand is why room mode standing waves are independent of monitor position.

As I described in my previous post, the physics of the situation seem to suggest to me that standing waves could only be generated where the speakers are between parallel boundaries.

Assuming the speakers are widely enough spaced apart, that would mean that the only way to create standing waves at the monitoring position would be to be holding a sound source. Or the waves somehow reflecting towards the monitoring position, then changing their angle of incidence suddenly to 90 degrees...

This is assuming that we agree that standing waves are generated by two similar waves travelling in precicely oposite directions - 90 degree reflections at boundaries.

We're also assuming a perfectly cuboid room, I suppose.

So while I understand and/or agree with everything else, I don't see how:

1: Perpendicuar standing waves appear at the monitoring position and
2: Treating above, below and to either side creates a 'reflection free zone'. Reflected sound will be at angles other than 90 degrees, so those placements will surely entirely miss all the reflected sound that would reach the monitoring position!

Treating around the speakers I can see, and treating the diagonals I can also see. Diagonal room modes crossing the monitoring position seems far more likely, and the energy build-up in corners is readily apparent.

I want to stress here, I'm not saying 'you're wrong'. I just want to understand the physics of it... So far the only explanations I've seen have been 'It breaks up standing waves' and 'it works'.

The rest of your post is gold dust :lol:
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