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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 Ethan Winer » Tue Dec 21, 2004 2:18 pm

Slamz,

> what do you think about the idea of downward pointing angled monitors to reduce the effect of standing waves at the monitoring position? <

Angling the speakers downward will not reduce standing waves at low frequencies. Further, studio designer Wes Lachot is adamantly opposed to the practice because it causes the spectral balance to vary as you move forward and back in the room. Although speakers try to radiate omnidirectionally, in practice there are variations. Angling the speakers downward amplifies those variations.

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

Postby Hugh Robjohns » Tue Dec 21, 2004 2:39 pm

Eric,

I have chosen to leave this extraordinary post in its entirely, despite being unrelated to the technical interest of this thread. You have stated your case fully for anyone that has the stamina and interest to read it. It is up to individuals to extract from this what they will.

This is going well outside the bounds of what the SOS forum is about, and enough is enough. Sadly, I fear all this will do is lead to a further post from Ethan re-quoting, arguing, or sidetracking each of your posts. This will serve no useful purpose whatever.

Please, let's move on to less controversial and antagonistic issues.

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

Postby Hugh Robjohns » Tue Dec 21, 2004 3:24 pm

Ethan Winer wrote:Although speakers try to radiate omnidirectionally, in practice there are variations. Angling the speakers downward amplifies those variations.

I would agree that angling speakers down towards the listening position would result in substantial tonal differences if the listener moves further back into the room. I would also agree that positioning speakers in this way is unlikely to have any material benefit as far as standing waves are concerned.

However, the statement that speakers try to radiate omnidirectionally is misleading and I feel the need to comment on this point to avoid any further confusion in the minds of anyone who might venture to read this far into this thread!

There are a few monitoring and domestic loudspeakers that have been deliberately designed to radiate omnidirectionally at all frequencies, but these are rare and generate their own set of specific in-room problems.

The vast majority of loudspeakers are designed to radiate forwards over a relatively narrow angle -- although this often changes dramatically with frequency. Almost all speakers radiate more or less omidirectionally at low frequencies, but not by active design -- a consequence of cabinet size in relation to wavelengths of sound produced.

In the mid and high frequency regions, where the wavelengths of sound produced are similar or smaller than the cabinet and driver sizes, the sound radiates over a reasonably narrow forward angle -- typically much less even than 180 degrees (hemispherical). Indeed, the forward radiation typically narrows more and more as frequency rises, and the radiation pattern is usually manipulated so that the radiation is slightly greater in the horizontal axis than it is in the vertical axis (for practical reasons I wont go into here.)

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

Postby Ethan Winer » Tue Dec 21, 2004 3:31 pm

Hugh,

> I fear all this will do is lead to a further post from Ethan <

I assure you I have no interest in replying to any of that. But I will offer a link to my poll at the alt.sci.physics.acoustics newsgroup so anyone interested in standing waves can benefit from reading the entire thread in context. The discussion is most worthwhile because several professional acousticians offer their views on the semantics of room modes and standing waves. HERE is that thread as archived at Google Groups.

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

Postby Ethan Winer » Tue Dec 21, 2004 4:00 pm

Hugh,

> the statement that speakers try to radiate omnidirectionally is misleading <

Yes, of course, thanks. What I should have said is that loudspeakers aim to send sound equally in all forward directions.

> The vast majority of loudspeakers are designed to radiate forwards over a relatively narrow angle <

I can't agree with that. I believe the goal is to radiate forward and outward without favoring any direction more than others. The point being so the listener can move around in front of the speaker without having the mid/high frequency response constantly change. [edit added:] Note that this applies to studio monitors, not PA speakers where directionality can be used to increase efficiency or direct the sound to selected areas.

> In the mid and high frequency regions, where the wavelengths of sound produced are similar or smaller than the cabinet and driver sizes, the sound radiates over a reasonably narrow forward angle <

Yes, in practice this is what happens, and better speakers disperse all frequencies more uniformly than poorer speakers. This is a big advantage of dome tweeters versus old-fashioned cone tweeters that tend to beam much more. I'm not a speaker expert, but I thought beaming at higher frequencies is more due to the cone rather than the cabinet. I'm sure you'll correct me if I'm wrong! 8-)

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

Postby Hugh Robjohns » Tue Dec 21, 2004 4:18 pm

First of all, thank you most sincerely for your restraint in regard to Eric's earlier post. Long may it continue. :)

Ethan Winer wrote:I believe the goal is to radiate forward and outward without favoring any direction more than others. The point being so the listener can move around in front of the speaker without having the mid/high frequency response constantly change.

Yes of course, but constrained by the inherent radiation patterns of the drivers and the need to match their radiation patterns closely over the crossover region(s), as well as to minimise unwanted reflections from nearby surfaces. In particular, most monitors designed for studio use have far narrower vertical dispersion than horizontal dispersion in order to mimimise reflections from the mixing console. Few monitors can maintain an even frequency response beyond 60 degree off axis.

Yes, in practice this is what happens, and better speakers disperse all frequencies more uniformly than poorer speakers.

Agreed. The off axis contribution of the speaker bounces around the room and plays a very big part in the overall perception of sound quality. Some speakers are designed to optimise the on-axis response at the expense of the off axis response, and these kinds of monitors tend not to sound too good in the more lively listening rooms.

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

Postby Scott R. Foster » Tue Dec 21, 2004 6:33 pm

I'm at a loss as to what is meant by "try to be omni-directional"... but whatever that means, I can't see where it applies to control room monitors. I would be surprised if the control room monitors used by anyone reading this thread are omni-directional.

All of the various products used for this purpose I am aware of are not designed to be omni [if you have links to omni control room monitors, please drop links, I'd like to see em]. Control room monitors I am familiar with beam sound in a carefully controlled manner. Directionality varies [as Hugh alludes there are often shaped horns incorporated to give a larger horizontal splaying versus the vertical splaying to the HF output] but the speakers are meant to deliver the sound in a controlled cone that can be "aimed" at the proper part of the room so as to assist in controlling early reflections. The ability to control the beam diminishes at lower frequencies - but even subs have measurable directionality, and by no means are the designers of control room monitors seeking omni-directional propagation [least wise, no devices I am familiar with].

If the beam of the speaker is canted at the floor then the "site lines" or first reflection rays are spread over a much smaller area of the usable room volume. For a single user control room this is a trivial concern - as the user's head is still much smaller than the "sweet spot, and only one head is of concern. Also, the downward cant can be used to increase the average distance traveled by return echoes from the rear of the room because that path encounters a floor bounce immediately post first listen [the beam hits the ear, then the floor, thus a longer return path than past the ear, straight to the rear wall, and back again].

But, for a multi-user room the reduced usable first reflection volume is a serious design limitation, and since the room size will generally be larger for a multi-user room, the benefits are diminished [there is no need to bounce the speaker beams off the floor to increase average echo path length if you have sufficient distance to the rear wall in the first place... which is quite likely if you have room for a producer's desk and a peanut gallery conversation pit].

On an entirely subjective note.. I hate elevated speakers. You ears are sensitive to changes in all aspects of the direction of a noise sources... both in the horizontal and vertical planes [left to right, and up and down]. I work using a DAW, and when the speakers are mounted above the level of my head, I find myself taking my eye off the screen and looking up at the wall between the speakers during each attempt to focus on critical listening. I stop watching the on screen results of whatever adjusting/mixing/knob twiddling I'm doing every time I want to give complete focus to the acoustic results of the work at hand.

I find it is much more productive to have the sense that my head is precisely in the optimal position for critical listening when working at the desk and looking at the computer monitor ... without the need to look up or lean back. So even in a small room, for a single user, I wouldn't go there. Raised monitors to me would be like setting my chair 2 feet to the left of centerline.. making me leaning over to the right every time I wanted to check the stereo balance... screw that, move the chair.

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

Postby Paul Woodlock » Tue Dec 21, 2004 7:01 pm

Tequila Slammer wrote: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.


The boundaries don't have to be parallel. And of course in a room the speakers will ALWAYS be between two or more opposing 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.


No it really doesn't work like this. A room of any shape will have resonances between opposing surfaces, parallel or not. These are called room modes, and when sound at the freqeuncy of these modes is generated in the room, standing waves will occour due to the room resonating at those frequencies.

Standing waves do NOT have to be at a normal ( 90 deg ) to the surface. For example a Tangential mode is a resonance between 4 opposing surfaces, where a 90 incidence is impossible. Oblique modes occour becuase of 6 opposing surfaces, again 90 incidence is impossible.

Once a room mode resonance has been excited by energy from the speaker, the resultant standing wave is located throughout the room. This is regardless of speaker position. Although speaker position CAN effect the efficiency of excitement, it has NO bearing on the location or position of the standing wave. It is the ROOM which determines the freqeuncy and position of the standing wave.

Using the Guitar string analogy.....

A guitar string will vibrate at it's resonant frequency(s), regardless of the exact point you pluck the string. However the point of plucking will affect how much each harmonic resonance is excited, and this is why of course, different timbres can be heard by varying the point of plucking. But the point of plucking DOES NOT CHANGE the position of the standing waves in the gutiar string, but does change their amplitude.

The Pickup is the 'listening position' in an electric guitar. Move the position of the pick up and the tonal balance changes. Exactly the same as in a room, where moving the listening position will result in a different tonal balance heard by the listener as he/her travels to different pressure points of the standing wave.



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

See above.


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!

I'm not exactly sure what your trying to say here, but sound waves behave in the same way as light waves in this context so the mirror is a great way of finding the areas that will cause reflections to hit the listener.

In most cases the Mirror Trick will define areas that are above, below, and to the sides... somewhere BETWEEN the listener and the speakers. NOT above, below and to the sides of the speakers themselves.

But don't forget the mirror trick will also define areas on the front and back wall too ( and even the surface of any furniture, racks, desks, etc )


:)


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

Postby Hugh Robjohns » Tue Dec 21, 2004 7:04 pm

Foz wrote:I would be surprised if the control room monitors used by anyone reading this thread are omni-directional.

You are almost certainly correct in your supposition. However, Dr John Watkinson (general audio guru and digital audio expert) strongly advocates truly omnidirectional loudspeakers. Indeed, his company (Celtic Audio)manufacturers the same for use in both professional monitoring and domestic environments. He also argues strongly that truly omnidirectional monitors are required for surround sound purposes. A lot of people disagree, but his arguments are interesting and well formed nonetheless.

if you have links to omni control room monitors, please drop links, I'd like to see em.

Try ths: www.celticaudio.com

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Re: .......

Postby Paul Woodlock » Tue Dec 21, 2004 7:39 pm

Hugh Robjohns wrote:
Paul Woodlock wrote: IIRC you get theoretical 6dB increase in level for every boundary.


Just to clarify here, the close wall loading produces a 3dB step increase in sound pressure below about 200Hz, with another 3dB increase in sound pressure for a second wall forming a corner, and a further 3dB increase for a third wall (floor/ceilng).


yes your right, thanks for clarifying that. I was momentarily getting confused with a doubling of signal levels ( 6dB ), rather than power levels. ( 3dB )





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.


This is only true if the soffit mounting enclosure is sufficiently massive to contain the low frequency energy. In other words, the soffit enclosure has to be constructed from bricks/concrete and be very solid indeed. Constructing a false wall from timber is not sufficient as the LF energy will simply be disipaited in causing the structure to vibrate.

hugh



Yes, in practise you are correct. Although concrete/bricks are optimal, timber/plasterboard soffit walls can be used, as long as they are extremely heavy and braced.

I've done this in my own soffit wall in fact. 6 x 2 studs, spaced around 10" apart, with SEVEN LAYERS of plasterboard. Solid as a rock. :)

It's also important to decouple the speakers from the soffits/soffit wall using elastomers/springs ( with a resonant MSM freqeuncy of at least 1.4 times below the lowest reproducible freqeuncy of the speaker), whatever the wall is constructed of, to prevent the wall from vibrating and acting as a secondary sound source.

I'm a big fan of soffit mounted speakers, but as you've confirmed, it has to be done right to be effective.

:)

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

Postby cc. » Tue Dec 21, 2004 9:07 pm

Eric - thanks for your post, now I finally understand what your objection is! If I understand it correctly it is that: although a standing wave can be formed by the interaction of two plane waves traveling in opposite directions, this is not very likely to happen in practice in a room except at the modal frequencies. I think you're probably right.

On a different side of it... one thing I haven't spotted on this thread is anyone pointing out a major characteristic of a standing wave: that there is no movement of energy (that's what is 'standing'). Note this doesn't mean that the air doesn't move, it just means that if you examine any point the sum of the stored energy and the kinetic energy is constant. This is why two traveling waves moving in opposite directions give you a standing wave - because one wave moves energy one way and the other moves the same amount the other way, so there is no net movement of energy.

The 'zero movement of energy' gives you a test to see when you have a standing wave and when you don't. For instance if you have two plane waves (of the same freq and amplitude) - one moving East and the other North then you will get an interference pattern - at some points you will have nulls and at some points you will have peaks. But if you look at the energy flow it is not zero (it's going north-east) - so this is not a standing wave.
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Re: .......

Postby Hugh Robjohns » Wed Dec 22, 2004 12:06 am

Paul Woodlock wrote: Yes, in practise you are correct. Although concrete/bricks are optimal, timber/plasterboard soffit walls can be used, as long as they are extremely heavy and braced.

I've done this in my own soffit wall in fact. 6 x 2 studs, spaced around 10" apart, with SEVEN LAYERS of plasterboard. Solid as a rock. :)

By the time you have put in enough timber, bracing, plasterboard, screws, nails, glue and everything else needed to create sufficient rigidity, you might just as well have knocked up a quick breeze block wall! That really would be solid as a rock, as well as being a lot quicker and cheaper to build than all that timber and plasterboard!

I am reminded of the recently refurbished (and famous) Teldex studio in Berlin. The construction people tried to build a timber soffit wall in the control room to contain three PMC BB5-XBD systems. It didn't work and they had to rebuild with concrete and bricks! Sounds pretty good now though ;)

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Re: .......

Postby Paul Woodlock » Wed Dec 22, 2004 12:28 am

Hugh Robjohns wrote:
Paul Woodlock wrote: Yes, in practise you are correct. Although concrete/bricks are optimal, timber/plasterboard soffit walls can be used, as long as they are extremely heavy and braced.

I've done this in my own soffit wall in fact. 6 x 2 studs, spaced around 10" apart, with SEVEN LAYERS of plasterboard. Solid as a rock. :)

By the time you have put in enough timber, bracing, plasterboard, screws, nails, glue and everything else needed to create sufficient rigidity, you might just as well have knocked up a quick breeze block wall! That really would be solid as a rock, as well as being a lot quicker and cheaper to build than all that timber and plasterboard!
...

Possibly, but I'm a much better carpenter than bricklayer :) Although I did succesfully build two brick walls behind my garage doors, having never laid bricks before. The other situation is the angles of the soffit wall. Quite difficult to do with a brick structure. Especially for me :)

Anyway, I've no concerns about it not working.

Check out my studio build diary. A few pages back from the last page will show piccies of the soffit wall construction. http://forum.studiotips.com/viewtopic.php?t=107 The brickwalls are a few more pages back.

That reminds me, I must index my studio build diary. :)


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

Postby Scott R. Foster » Wed Dec 22, 2004 2:07 am




Interesting...


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

Postby Tequila Slammer » Wed Dec 22, 2004 3:02 am

Paul Woodlock wrote:

No it really doesn't work like this. A room of any shape will have resonances between opposing surfaces, parallel or not. These are called room modes, and when sound at the freqeuncy of these modes is generated in the room, standing waves will occour due to the room resonating at those frequencies.

Standing waves do NOT have to be at a normal ( 90 deg ) to the surface. For example a Tangential mode is a resonance between 4 opposing surfaces, where a 90 incidence is impossible. Oblique modes occour becuase of 6 opposing surfaces, again 90 incidence is impossible.

Once a room mode resonance has been excited by energy from the speaker, the resultant standing wave is located throughout the room. This is regardless of speaker position. Although speaker position CAN effect the efficiency of excitement, it has NO bearing on the location or position of the standing wave. It is the ROOM which determines the freqeuncy and position of the standing wave.

Using the Guitar string analogy.....

A guitar string will vibrate at it's resonant frequency(s), regardless of the exact point you pluck the string. However the point of plucking will affect how much each harmonic resonance is excited, and this is why of course, different timbres can be heard by varying the point of plucking. But the point of plucking DOES NOT CHANGE the position of the standing waves in the gutiar string, but does change their amplitude.

The Pickup is the 'listening position' in an electric guitar. Move the position of the pick up and the tonal balance changes. Exactly the same as in a room, where moving the listening position will result in a different tonal balance heard by the listener as he/her travels to different pressure points of the standing wave.


Paul,

Thanks again! That really helped me out.

I hadn't thought of applying resonance to the situation. This adds a whole new concept to the mix - if the room itself is being excited, it is acting as a giant resonant box. It is therefore generating sound at the resonant frequencies, omnidirectionally from each point of the inside surface, I suppose. Yeah?

This would lead to the generation of standing waves between every pair of surfaces whose distacne matched a room node.

Cool :lol:

(Well, not as far as getting a clean sounding room, but as far as me understanding what the hell is going on here :) )

Now I see the value of treating the spaces forward, back, above, below and either side of the monitoring position.

The mirror trick I fell in love with as soon as I saw it. Thanks again for pointing me at it.

Your guitar string analogy seems a little strained here however. I imagine a guitar string to equate to one 'ray' of sound - a theoretical single wave path between boundaries. The 'room' would be the guitar itself - or the single body attached to the two ends of the string.

How about this - the 'guitar' becomes a simple wooden box and the strings (strung between parallel panels) are all tuned to the same frequency. Plucking one string causes vibrations to travel through the frame and cause sympathetic vibrations in the other strings.

If the frequency of the string is a resonant frequency of the box, the effect will be very marked!

Close or off the mark?

Does this mean room modes are less apparent in more solid structures? A concrete room would vibrate less than a wood and plasterboard structure. Which could help to explain why I'm getting immense amounts of bass at certain frequencies in my current studio (a box room with plaster walls and a wooden floor, where everything on a flat surface jumps around when I walk heavily) and I had almost no bass whatsoever (and quite a flat response) in my last studio - a basement flat with a concrete floor and brick walls.

This could also help explain DoeZer's situation. He was in a concrete structure as well, as I remember.

It still seems to me that the strongest standing waves will be in paths intersecting the speakers between parallel walls - as that is where they're originally being excited and reflections from the walls will enhance the effect.

Second strongest would then be any other paths between parallel walls, as reflections would again enhance.

THEN the standing waves between non-parallel walls would come into play.

Is there just little measured difference between the three?
Or am I missing the point again? (I am still a bit sloshed from the office Christmas party ;) )

Oh, and tell me if this gets annoying ;) I feel like I'm really making progress here however - and I could see this being valuable to other readers. :bouncy:
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