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Natural Science Forum / Physics / Acoustics / March 2004Small room diffusion...fact or fiction?
Hello gentleman, this is my first, and probably last time to post here. Since I've recieved no satisfaction elsewhere, I've come to the conclusion if I don't receive a definitive explanation here, it doesn't exist. And please accept my apology for posting this question here, but it seemed like my last resort, as contridiction on this subject is rampant. I only seek the truth if indeed it exists. At the risk of disobeying this bbs rules, here goes. The subject has plagued me for quite some time now. Being a layman with a casual interest in acoustics, I purchased Alton Everest book in the late 80's. This was the second edition. In the chapter describing "new" diffusion application and test results, a few pages were devoted to analysis and illustration of "small room diffusion" techniques and products. I have spent a great deal of time since then, trying to unravel the mysteries and principles in order to apply this to my situation. However, within the last year, I have become aware of contrary opinions regarding small room diffuse fields, although the word fact eludes me. Here is the point. First, do the diffusion techniques as shown in Mr. Everest book, actually work, in the size rooms as illustrated, or were these simply illustrations of concepts in experimental stages, as I have read many opinions they do not, as diffuse fields do not exist in small rooms? ie..are they figments of imagination? Secondly, in regards to Mr. Campanellas statement on another thread, that "diffusion is the only tool".... at an acousticians command to "eq" a room, what form would this "diffusion" tool take in a small room environment, if indeed diffusion can be achieved? Thirdly.. what are the accepted limits of boundary dimensions that define a "small room". I have read that diffusion is warranted in larger envelopes, but is of little value in "small" ones. Hence my curiosity into what these lables actually mean. Lastly, thank you. Answers to these questions will bring my interest into focus, and allow me to close this chapter in my life for good. As another interested party so aptly stated, what is the point of learning the depth of acoustic principles for one moment of application, only to reside in memory forevermore. I only pursued it as financial constraints would not allow me to hire professionals in the first place. Lord, hindsight is 20/20. Sincerely Rick Fitzpatrick > here, but it seemed like my last resort, as contridiction on this > subject is rampant. I only seek the truth if indeed it exists. Aye! Welcome to acoustics! > First, do the diffusion techniques as shown in Mr. Everest book, > actually work, in the size rooms as illustrated, or were these simply > illustrations of concepts in experimental stages, as I have read many Likely so. > opinions they do not, as diffuse fields do not exist in small rooms? > ie..are they figments of imagination? The term "diffuse field" was born in optics where the wavelength is about 1/2 micrometer (one 20-millionth of an inch). Most optical objects are an inch or more in breadth (except fiber optics sources which are about a cubic millimeter in size, still a plenty big room for light waves). In acoustics, we can think in terms of "diffuse field" only when the room is plenty big compared to a wavelength of sound. To get an idea which frequencies can be treated by diffuse ideas, the larger of acoustic test rooms, aka "reverberation rooms", typically have a volume of 10,00 cubic feet (280 cubic meters). Here, acousticians feel safe in calling them diffuse only for sounds down to about 200 Hz. If they do a lot of things to scramble sound in them, they will feel say they are "diffuse enough" for sound down to 100 Hz. Recently, acousticians have also informally published results for tests in those same rooms with little or no modifications down to frequencies as low as 50 Hz, but they do not claim convincing diffusion. One of the criteria used to determine the likelihood of diffuse field character is the number of modes within the frequency range that you care to average over. I'm sure that most musicians will not allow a very wide averaging interval.. 40 to 50 Hz? Another way of saying the same this in the number of Hz between each of the first few modes. The first mode for a room 22 feet long is 25 Hz, if 11 feet wide, the next mode is 50 Hz, and a second "oblique mode arises at 56Hz. If the room is 8 feet high, its vertical mode is at 69 Hz with two more oblique mode arising at 73 and 89 Hz. And that's all there is!. So how does one hope to play a variety of bass tones in that room of 1900 cubic feet (55 cubic meters)? ? > Secondly, in regards to Mr. Campanellas statement on another thread, > that "diffusion is the only tool".... at an acousticians command to > "eq" a room, what form would this "diffusion" tool take in a small > room environment, if indeed diffusion can be achieved? I'm open to ideas. > Thirdly.. what are the accepted limits of boundary dimensions that > define a "small room". I have read that diffusion is warranted in > larger envelopes, but is of little value in "small" ones. Hence my > curiosity into what these lables actually mean. "Little value" means that not much can be achieved for low frequency sounds such as bass notes, so don't expect big gains from diffusion there (which cannot be achieved). Whatever "diffusion" success that is claimed for small rooms arises NOT from diffusion of low frequency sound, but rather from diffusion of higher frequency sounds, typical from about 500 Hz to about 2kHz or 5 kHz. Here a multitude of "effects" or affections, impressions, etc. can be achieved to dazzle the mind, like the tricks of the toy maker in "Tales of Hoffmann", to invoke pleasure. But I stray from the subject at hand.... > Lastly, thank you. Answers to these questions will bring my interest > into focus, and allow me to close this chapter in my life for good. As [quoted text clipped - 3 lines] > as financial constraints would not allow me to hire professionals in > the first place. Lord, hindsight is 20/20. Welcome again to acoustics! Angelo Campanella > Hello gentleman, this is my first, and probably last time to post > here. Since I've recieved no satisfaction elsewhere, I've come to the > conclusion if I don't receive a definitive explanation here, it > doesn't exist. And please accept my apology for posting this question > here, but it seemed like my last resort, as contridiction on this > subject is rampant. I only seek the truth if indeed it exists. In my experience, the contradictions that you have experienced arise from a lack of thorough investigation. I'm only an amateur acoustician, but what I have found is that there is no substitute for careful and methodical investigation and analysis. Contradictions can usually be sorted out by starting from the fundamental physical principles of sound, and then building on them. > At the risk of disobeying this bbs rules, here goes. The subject has > plagued me for quite some time now. Being a layman with a casual [quoted text clipped - 3 lines] > analysis and illustration of "small room diffusion" techniques and > products. Research into diffusion techniques have moved on a pace since the late 80's. In those days, Schroeder diffusers were the best tool for the job. Since then however, more research has been done into their behaviour, and better diffusive techniques have been developed. > First, do the diffusion techniques as shown in Mr. Everest book, > actually work, in the size rooms as illustrated, or were these simply > illustrations of concepts in experimental stages, as I have read many > opinions they do not, as diffuse fields do not exist in small rooms? > ie..are they figments of imagination? For a sound field to be called truly diffuse, for any given location within that field, there must be an equal flow energy in all directions. Now consider what is happening in a small room. Due to the fact that the dimensions of the room are of the same order of magnitude as the wavelengths of sound at the lower end of the sound field, modal resonance will occur. In the frequency band between the resonant cutoff of the room and Schroeder's critical freqency, modal resonance will be the dominant feature of the sound field. Both the resonant cutoff and Schroeder's critcal frequency are calculated using the room dimensions, therefore if you calculate these boundary frequencies for a range of room sizes, you will see that as the room gets smaller, so both the cutoff frequency and Schroeder's critcal frequency rise. The opposite is true for larger rooms. As the cubic volume rises, so the upper and lower limits of the resonant range get closer and closer. In reality, Schroeder's cutoff frequency identifies the midpoint of the transition range. The is the bandwidth in which a smooth transition occurs between wave-like behaviour at the bottom end, and ray-like behaviour at the top end. For practical purposes, the midpoint of this transition range is taken as the point above which wave-like behaviour (and hence modal resonance) can be ignored. No matter what the absolute values of these boundary frequencies are, the interval is always 1.5 octaves. The larger this room, the lower this 1.5 octave range is. If the room is large enough, then band drops off the botton of the piano keyboard, and becomes much less of a problem. Diffusion is a technique for scattering energy. However, if a certain bandwidth of a sound field is dominated by modal resonance, then resonance is _going_ to occur no matter what direction the sound energy is flowing in. Therefore, Schroeder's critical frequency should be used as the lower limit room for effective diffusion. Below this frequency, lack of diffusion tends not to be a significant problem typically because a loudspeaker's directivity tends to drop as the frequency drops (Sub bass speakers can often be considered as omnidirectional sound sources). > Secondly, in regards to Mr. Campanellas statement on another thread, > that "diffusion is the only tool".... at an acousticians command to > "eq" a room, what form would this "diffusion" tool take in a small > room environment, if indeed diffusion can be achieved? As discussed above, diffusion can be acheived in a small room, but only for frequencies above Schroeder's critical frequency. > Thirdly.. what are the accepted limits of boundary dimensions that > define a "small room". I have read that diffusion is warranted in > larger envelopes, but is of little value in "small" ones. Hence my > curiosity into what these lables actually mean. I can't remember the textbook definition of an acoustically "small" room off the top of my head. But consider this following 3 points: 1) You know that there will be a 1.5 octave range within which modal resonance is dominant. 2) You know that the room dimensions affect the position of this 1.5 octave band. 3) As the room's cubic volume rise, so this 1.5 octave band drops down the scale. Ergo, as the room gets smaller, so this band moves up more and more into the audible bass end of the sound. There will come a point at which this resonant band significantly colours the sound by overlapping a significant part of the bandwidth of the music you are trying to listen to. Hope that helps Chris  SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- Jin Yong Jeon, Sung Chan Lee ,Michael Vorlaländer, Development of scattering surfaces for concert halls. Applied Acoustics 65(2004), 341 -355, might give some insight also to small room phenomena. (This April/May issue is available at least on line). Kari Pesonen Chris, What do you mean with this resonant Cutoff? Do you mean the lowest mode? What do you mean with this 1.5 octave? The lowest mode is defined by the longest measure and is thus room shape dependant. The Schroeder frequency ignores shape, only counts volume, but also relates bandwidth to RT60, meaning dependent on absorption. So if you mean with the cutoff (but not sure what you mean here) the lowest mode versus the Schroeder freq, that can hardly be a constant 1.5 octave since both do cover deviating parameters. But probably I misunderstand you. Eric > > Hello gentleman, this is my first, and probably last time to post > > here. Since I've recieved no satisfaction elsewhere, I've come to the [quoted text clipped - 103 lines] > > Chris Hmmmm, there seems to be a lack of consistent terminology here... > What do you mean with this resonant Cutoff? Do you mean the lowest mode? Anyway, the resonant cutoff is, as you say, the lowest mode as calculated by speed_of_sound / (2 * max(length, width, height)). > What do you mean with this 1.5 octave? The difference between the lowest mode and Schroeder's critical freqency is a pretty constant 1.5 octaves for the absorbency of the room I was working with. Sorry, what I omitted to state earlier was that this value did not change with variation in the room dimensions - only in absorbency. > The lowest mode is defined by the longest measure and is thus room shape > dependant. Yes, this has been accounted for. > The Schroeder frequency ignores shape, only counts volume, but also relates > bandwidth to RT60, meaning dependent on absorption. True. The exact octave spacing varies depending on which equation is used to derive the RT60 value. Sabine always comes out higher, but using Fitzroy for the RT60 comes out at a pretty constant 1.5 octaves for a given average absorbency. > So if you mean with the cutoff (but not sure what you mean here) the lowest > mode versus the Schroeder freq, that can hardly be a constant 1.5 octave > since both do cover deviating parameters. I only partially explained myself... I'll try to be more thorough next time :-) Chris SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- Hi Chris, I saw that you know what you're talking about. Only it's risky to use such generalizations for people not having your background. The relation longest measure versus volume can deviate quite a bit depending on room shape. And the absorption difference between a live and non-environment room can have significant influence. Thanks for responding. Warm regards. Eric > Hmmmm, there seems to be a lack of consistent terminology here... > [quoted text clipped - 34 lines] > > Chris BTW Eric, do you have any experience with hybrid diffusers? I'm going to need to make some soon, and I'd like some info on how others have done it. Chris SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- Hello Chris, I couldn't check the link you referred to. It doesn't work for me. It can also be a problem here, since my CPU is that well protected now (too well) that often my own protection bothers me. But, to be honest, I don't show everything I made (is asset for me). If interested, you made me curious with your 1.5 octave since I never used that as a parameter. So I choose two well-known room ratios one closest to a cube and the other a long room. I further fixed both rooms to a common Volume of 100 m3 = 3531.5 cft. You said 1.5 octaves equaling 4.5 one third octave bands. Look at those results: J.E.Volkmann - 1942 - Room Volume 100 m3: Room Ratio: 2:3:5 1) Low Environment (Industry production hall): 7.3 one third octave bands 2) Control Room/Classroom: 7.6 one third octave bands 3) Pseudo Live Room: 8.3 one third octave bands 4) Standard live Room: 8.6 one third octave bands 5) Very Live Room: 8.9 one third octave bands L.W.Sepmeyer - 1965 - Room Volume 100 m3: Room Ratio: 1:1.14:1.39 1) Low Environment (Industry production hall): 5.7 one third octave bands 2) Control Room/Classroom: 6.0 one third octave bands 3) Pseudo Live Room: 6.7 one third octave bands 4) Standard live Room: 7.0 one third octave bands 5) Very Live Room: 7.3 one third octave bands The minimum range I encountered is still 5.7 rather than 4.5 one third octave bands. This is for a room which has a very low reverb. So I range between 1.9 and 3 octaves. (rounded, based on exact calculations). I think you should check your data once more. Very kind regards Eric > BTW Eric, do you have any experience with hybrid diffusers? I'm going > to need to make some soon, and I'd like some info on how others have > done it. > > Chris > Hello Chris, > > I couldn't check the link you referred to. It doesn't work for me. > It can also be a problem here, since my CPU is that well protected now (too > well) that often my own protection bothers me. Ok, I'll send you the PDF separately. > If interested, you made me curious with your 1.5 octave since I never used > that as a parameter. The reason I chose this was to give a reference for the modal range in terms of a piano keyboard. If I've done my sums wrong, then I'll correct them! But I plan to include a picture of a keyboard in the spreadsheet, and then overlay this with a plot of the modal resonance. This should then connect the somewhat abstract frequencies to notes on the keyboard, thus making the concept easier to visualise. > J.E.Volkmann - 1942 - Room Volume 100 m3: Room Ratio: 2:3:5 > 1) Low Environment (Industry production hall): 7.3 one third octave bands [quoted text clipped - 9 lines] > 4) Standard live Room: 7.0 one third octave bands > 5) Very Live Room: 7.3 one third octave bands I am not really able to give a direct correlation between your results and my results, since your method of calculation applies a predefined average absorption to the room, and mine allows the user to customize the absorption of each room surface. But I will describe my calculations steps: Lowest resonant mode = speed_of_sound/(2*max(L,W,H)) Schroeder's critical freq = 2102*Sqrt(RT60/room_volume) The number octaves between these two frequencies is: Difference (in Octaves) = Log2(Schroeder_Freq) - Log2(lowest_mode) > The minimum range I encountered is still 5.7 rather than 4.5 one third > octave bands. For the control room at my church which is 2.75(H) x 3.13(W) x 3.82(L) and an average absorbency of 0.48, I get the following values: Lowest mode = 44.97Hz Sabine Eyring Fitzroy 1 Fitzroy 2 Arau Schroeder freq 134.1 106.3 108.1 105.8 106.2 Octave range 1.58 1.24 1.27 1.23 1.24 These figures remain constant as the room is scaled up and down in size. At the other end, here are the figures I get for the dimensions of the gym that user Panzzi posted in his question "Sound System for a gym" on rec.audio.pro dated March 1st 2004. Dimensions -> 30'(H) x 70'(W) x 100'(L) He stated that three walls were made of concrete and one was sheet metal. He did not say what the floor and ceiling were made of, so I have assumed a wooden floor and wooden tongue-in-groove ceiling. This gives an average absorbency of 0.23 Lowest mode = 6.44Hz Sabine Eyring Fitzroy 1 Fitzroy 2 Arau Schroeder freq 40.0 37.9 42.6 32.1 40.0 Octave range 2.82 2.74 2.91 2.50 2.82 These figures are very much of the same scale as your calculations. Mine range from 1.23 to 2.91 octaves, which is 3.7 to 8.73 third octave bands. Your calculations probably don't down as low as mine because of the absorbency values used in your calculations (probably slightly higher than mine) > This is for a room which has a very low reverb. > So I range between 1.9 and 3 octaves. (rounded, based on exact > calculations). My range of calculated values extends slightly lower than yours, but I don't think this necessarily a problem. > I think you should check your data once more. I have, and if I have made a mistake in my calculations, I don't know where it is! If you think I have goofed up somewhere, please show me. :-) I think this value will be far more useful if it is shown in relation to a piano keyboard. This ought to make the information immediately comprehensible. You can get the version of my spreadsheet that includes these calculations from http://www.rmmpnet.org/members/ChrisW/index.html Regards Chris SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- Hello Chris, Thanks, but your answers are very extensive. I'll need some time to check. Warm regards Eric > > Hello Chris, > > [quoted text clipped - 104 lines] > > Chris Hello gentleman. Sorry for the delay getting back to you. Thank you so much for you extended replys and Mr. Campanellas welcome. Although the explanations are a bit deep for me, I did grasp enough to answer my questions. May you find the answers to all your lifes questions. Rick Hello Rick, I was off-line in discussion with Chris about some calculation methods. It's complicater than simple rules. Chris is making a very nice spreadsheet, but it's risky from those young experiences to do absolute and general statements. But I admire and respect Chris, for the depth in which he searching the knowledge. In small room acoustics it's often learning what works and what doesn't. That's why there are so many theories (e.g. LEDE, RFZ, non-Environment etc.). If you speak about diffusion in things as Control Rooms coming close to an averaged absorption of 0.3, which is very assymetric distributed, one can hardly speak about a diffuse field whatsoever (with or without diffusers). So one mainly aims to a good soundfield on a relative limited spot. So diffusers there have often the function of scattering early reflections to minimize problems with boundary interference etc. It's also used to improve imaging and it helps to psychologically puch a wall backwards. So it depends, where and why you use diffusion. But you can use it. Large bended panels will diffract and scatter down to roughly 100 Hz (in varying degrees), Shroeder diffusers will start from the low mid, those sculpted panels mostly from the high mid etc. So diffusion seen as creating a nice diffuse field is hardly possible. Diffusion seen as an additional control of early eflections is possible (diffraction, scattering). Note that diffusion only works on traveling (non-modal) frequencies in function of influencing the propagation of sound. Modal frequenties create fixed static patterns in function of the room geometry. Best regards Eric > Hello gentleman. Sorry for the delay getting back to you. Thank you > so much for you extended replys and Mr. Campanellas welcome. Although > the explanations are a bit deep for me, I did grasp enough to answer > my questions. May you find the answers to all your lifes questions. > > Rick Hi Eric > I was off-line in discussion with Chris about some calculation methods. > It's complicater than simple rules. > Chris is making a very nice spreadsheet, :-) > but it's risky from those young experiences to do absolute and general statements. True! > But I admire and respect Chris, for the depth in which he searching the > knowledge. :-) I'm doing my learning rather publicly - ergo, I make my mistakes publicly! But there are always some guys around to correct things. Respect. Chris SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- > :-) I'm doing my learning rather publicly - ergo, I make my mistakes > publicly! But there are always some guys around to correct things. And this calls for courage and honesty and a real scientific attitude. Therefore I repait my respect. I have questions, and any answer generates more questions. That's the case for me and anybody else. The ones telling they know it all, just went crazy somehow. Warm regards Eric Hello again gents. My gratitude cannot be expressed enough Eric. In light of my lack of respect to you on another forum Eric, my only defense is insanity. Yes, thats it. Acoustics drove me completely insane, :) and I blurted out at you for my lack of understanding, and scientific insight. Let me say now though, I feel humiliated at my behavior and it only proves how much of a gentleman you truly are, as you did not allow that to influence your answer to my questions. Thank you sir. You have my complete respect. Although I was totally prepared to exit this rabbit hole(acoustics), I seem to have difficulty leaving it behind. So let me ask ONE LAST question concerning this diffuser issue. On page 314 and 315 of Mr. Everest handbook of acoustics(second edition), there is a picture of a very small control room with extensive use of MDF as a diffuser "shell", up each sidewall, and across the ceiling. My question is, is this complete nonsense, or is this a valid exibition or use of diffusing geometry, and does it really have a valid influence on the acoustics, and if so, what exactly would the benifits be, as I've never seen any other small room with such extensive diffuser use as this. But doggone, sure like the looks and I have a TON of MDF. :) But it would be rediculous to build it for nothing. Although, I own a woodworking shop so it would be nothing to build IF I understood the math involved. Anyway Eric, thankyou again, and same to the rest of the gentleman here. Maybe I will stick around. Might just get something through this thick skull afterall....hehehehe! Rick Hello Rick, I just had a look. At a first glance they are a Hibrid between a Schroeder diffuser and whatever. I don't like to discuss those pictures. Personally I shouldn't think about creating a room like that. I'm in fact classically trained in physics and acoustics. The first time I ever saw Everest books, I thought I had a wrong book in my hands. For me in the studio world, the magic surrounding acoustics is much stronger than I like. What one nowhere describes is that Schroeder diffusers are partly (narrowband) 1/4 wave silencers too. So I leave it like that. (this are endless discussions with subjective arguments) Even with free MDF I shouldn't build such a room as shown on those pictures. Eric > Hello again gents. My gratitude cannot be expressed enough Eric. In > light of my lack of respect to you on another forum Eric, my only [quoted text clipped - 24 lines] > > Rick Hello Eric. Well that about does it. I don't know quite what to say. When finally confronted with truth.... the feeling is quite astonishing. Thank you. Its been a VERY LONG TIME coming. Well now, I feel like having a nice cold beer. And I don't even drink. :) Good day to you sir. Mr. Campanella..I like your style. :) Thanks. Damn, this has been great. Now I know how Alice felt. Rick Hello Rick, It's OK, I felt your frustration back then. Warm regards Eric > Hello again gents. My gratitude cannot be expressed enough Eric. In > light of my lack of respect to you on another forum Eric, my only [quoted text clipped - 24 lines] > > Rick > On page 314 and 315 of Mr. Everest handbook of acoustics(second > edition), there is a picture of a very small control room with [quoted text clipped - 5 lines] > with such extensive diffuser use as this. But doggone, sure like the > looks and I have a TON of MDF. :) But it would be rediculous to build I have no idea what MDF is, but I suspect that it is an absorber. A little bit of my home-spun philosphy on "diffusion": 1- Diffusion, among other things, depends on the variability if acoustic impedace across a surface. 2- An absorber placed on a hard surface changes it to be a place where sound is absorbed instead of reflected. 3- Ergo, places where patches of sound absoption are placed provide a location where the sound impedance has changed and therefore can act as a defacto diffusing element of size equal to its dimensions. > it for nothing. Although, I own a woodworking shop so it would be > nothing to build IF I understood the math involved. 4- Therefore, one need not always build awkward objects of wood, plastic or metal to affect some diffusion: As long as I need or can tolerate a fair degree of absorption in the subject room, I can place absorber patches at a variety of locations, and of variable size, and get a very good start at diffusion. Angelo Campanella. Angelo: MDF = Medium Dense Fibreboard but this is not from this fibrous board. It's rather heavy massive fibreboard roughly 45 to ca 50 pcf depending on thickness. Eric > > On page 314 and 315 of Mr. Everest handbook of acoustics(second > > edition), there is a picture of a very small control room with [quoted text clipped - 29 lines] > > Angelo Campanella. > MDF = Medium Dense Fibreboard but this is not from this fibrous board. > It's rather heavy massive fibreboard roughly 45 to ca 50 pcf depending on > thicknes. That wourl absorb sound only if it is porous (like acoustical tile). If it is not porous, it will either reflect sound, or resonate as a panel with air behind it, to provide some sound absorption near that resonant frequency. Angelo Campanella. |
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