Tequila Slammer wrote:
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.
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?
Sort of. The room doesn't really generate sound in the way you might think. Although I think pedantically this could be argued.
The room is a giant ( or not so giant ) resonant box yes. The standing waves are actually a storage of sound energy. This also takes time. When you playback a mdee freqeuncy from a speaker, there is a Rise Time before the standing wave reaches it's max level. And when you stop playback ( stop feeding the standing wave with energy )
there is a fall time when the energy stored in the standing wave is dissipated due to friction/absorption, etc. So the standing waves not only distort the frequency response, they cause the sound to 'boom' for want of a better word.
This would lead to the generation of standing waves between every pair of surfaces whose distance matched a room node.
It's room MODE not node. I know it was probably a typo :) , but a node is a place of no disaplcement in a standing wave ( anti-node = point of maximum displacement ) So a dangerous typo :)
You don't really have a mode that doesn't match a distance. Although knowing the complexities of acoustics - phew! - I'm sure Eric will come up with a situation where there isn't a room dimension involved ;). But for the purposes of the present discussion....
It's the sufaces which give rise to modes, the frequency of which is determined by the distances between them. It's not only pairs of surfaces either. You can have modes that are resonances between 4 surfaces ( Tangential Modes ), and modes as a result of 6 surfaces ( Oblique Modes ).
I would imagine that a pentagonal room would also have modes as a result of the 5 walls too. but whether that is true or not, is way beyond me.. ERIC!!!!!! :)
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.
No it's nto strained. Forget about the guitars suond box for a moment. the guitar string analogy I think is good.
nut-string-bridge = Wall-airspring-Wall
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?
Absolutely! If the resonance of the box equals the strings freqeuncy, then yeah, the box would resonanate like a mother!!
Of course the box will have many resonant modes. And I would think a well designed guitar should ideally resonate at every freqeuncy to produce a full bodied tone at every note without any dead spots.
In fact with Mallet instruments, like a mamrimba, each note has it's own resonant box tuned to the note it lies under. Here's a quaint example
:) Giz a tune luv! :)
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.
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.
This is one of the reasons why cars are great places to listen to music, becuase most of the bass simply goes right through the cars body and annoys the outside world - MUWAHAHAHA :)
If you imagine a guitar with a rubber nut and rubber bridge, it might help to visualise this :)
The more the room is soundproofed, the more energy is contained in the room, and the more the excitation of the standing wave.
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.
A standing wave occours across the whole space of the room between the opposing surfaces. The strongest standing waves are the fundermental axial ( 2 surface ) modes. I know this isn't quite what you meant, but reagrding yur question, I'm pretty sure the max pressure of a standing wave in a room is prety much the same at all anti-nodes.
(I am still a bit sloshed from the office Christmas party ;) )
Did ya photcopy your ass? - and NO dont' post any pics :) ( Mind you I have to admit to posting a picture of the concrete burn I painfully got on my ass while pouring the floating floor, in my studio build dairy - hehe :) )
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:
No it's not annoying at all. Apart from being highly interesting anyway, It's also helping me to learn more about it all. I always try to stay within my knowledge bounds, and expect correction if I get things wrong.
But the important thing is raising the subject so the info can filter down through us all.
The Internet Forum IMO is a major leap in the spread of knowledge, but is has it's downsides. And that is the spread of hearsay or false info. In fact that has been one of the main causes of all the upheaval here over the last couple of weeks. And while the methods of debating from myself and others has been a little too uncomfortable for some people. It has certainly been passionate and on a positive note has stimulated some great discussions on the science and application of acoustics.
Anywa...., back to my acoustic treament designs, even though it's gone 4am now :)