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Natural Science Forum / Physics / Acoustics / June 2004Poll: Mode = Standing Wave
Folks, I have a question about acoustics terminology, and I'm hoping the pro acousticians here will comment. I have seen some acoustics people use the words Mode and Standing Wave interchangeably. To me these are completely different things. As I see it, a mode is a propensity to vibrate - sort of like a tuning fork waiting to be struck. Or an equalizer or filter with a reasonably high Q waiting for a signal to be passed through it. On the other hand, it seems to me that a standing wave is, first and foremost, a wave that already exists. So before you could call it a wave something must have occurred to set the wave in motion, whether acoustic interference later caused it to "stand still" or not. What do you folks think? Are they really the same thing, or does my distinction make sense? And for those who do consider a mode the same as a standing wave, where do you feel my distinction strays off the mark? Thanks very much. --Ethan > I have a question about acoustics terminology, and I'm hoping the pro > acousticians here will comment. No, this is not about terminology, but about understanding. > I have seen some acoustics people use the words Mode and Standing Wave > interchangeably. Yes, like Schroeder, Beranek, Kinsler, Frey and Ingard etc. > To me these are completely different things. As I see it, a > mode is a propensity to vibrate - sort of like a tuning fork waiting to be [quoted text clipped - 9 lines] > distinction make sense? And for those who do consider a mode the same as a > standing wave, where do you feel my distinction strays off the mark? The problem seems to have its origin in the fact that you have developed, shall we say, an "intuitive" rather than a scientific understanding of what the words "mode" and "wave" mean. Unfortunately, this has led you way out into left field. You refer to a mode as if it were the "potential" for a vibration to exist, but in reality, the term simply refers to a specific frequency whose wavelength is an integer multiple of one of the dimensions of the room. All waves conduct energy from one place to another, but they do so through a cycle of pressure maxima and minima. If the distance between two pressure maxima (the wavelength) is an integer multiple of the distance between two reflective boundaries, then the energy transfer appears to stop. Hence the term "standing" wave. A standing wave is the consequence of a room being excited at one of its modal frequencies. Turn it around the other way - a room's modal frequencies are the set of frequencies which, when excited, will cause standing waves. If you have not yet understood this concept, then it is very likely that you do not have a clear understanding of the true nature of a diffuse field, or the practical significance of the Schroeder frequency. I find it very worrying that you need to ask for clarification on concepts that are so fundamental to room acoustics. Ethan, a little knowledge is a dangerous thing. Read my sig, then go and study the physics from the ground up without letting yourself jump to irrational conclusions. And don't think you have to reinvent physics in order to make a name for yourself. Chris W  SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- > A standing wave is the consequence of a room being excited at one of its > modal frequencies. Turn it around the other way - a room's modal > frequencies are the set of frequencies which, when excited, will cause > standing waves. > > Chris W Perhaps Ethan did not say it as eloquently, but it seems to me that this is essentially what Ethan was saying. > > A standing wave is the consequence of a room being excited at one of its > > modal frequencies. Turn it around the other way - a room's modal [quoted text clipped - 5 lines] > Perhaps Ethan did not say it as eloquently, but it seems to me that this is > essentially what Ethan was saying. That's what I thought as well. SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. 1000 times sorry Tony: pushed wrongly privat reply button again. This is exactly what Ethan says: Electronic Musician Magazine April 1 2004 Author Ethan Winer A standing wave is a wave that's not moving - it literally stands still. Standing waves occur at node locations in the room, and they result when two equal yet opposite waves arrive from different directions and collide. A few inches away, just outside the node, the waves are traveling toward each other. There's no motion, though, at the one precise location where the wave fronts meet. (This is much like the isometric exercise of pushing your hands together.) Some people wrongly consider modes and standing waves to be the same thing, because standing waves can occur at modal frequencies. But they are not at all the same, because one is a wave and the other, a mode, is merely a propensity to vibrate. Moreover, opposing waves can create nulls at nearly any frequency in any room, not just those frequencies that correspond to the room's dimensions. Ethan relates all static patterns resulting from traveling waves caused by boundary interference to the term Standing waves, so also non-modal interference as e.g. the 1/4 wave dip at the backwall of a control room or tracking room. In fact Ethan calls the static pattern shown by the destructive boundary interference of all frequencies standing waves. They show like that in his EFT program with sinus wave exitation. So the 1/4 wave destructive interference at the backwall (ca plain wave behavior with straight incidence) is somehow a standing wave for Ethan (thus also if non-modal). This is discussed with Ethan numerous times (without success) by many people. This also is at the basis the way Chris formulates things. The question here is to prove the others wrong. Eric > > > A standing wave is the consequence of a room being excited at one of its > > > modal frequencies. Turn it around the other way - a room's modal [quoted text clipped - 8 lines] > > > That's what I thought as well. Another quote from a FAQ referred to 1000 times: SIDEBAR: WHY THEY'RE CALLED STANDING WAVES If you've ever used an ultrasonic cleaner to clean jewelry or small electronic components, you've probably seen standing waves in action. When you drop a pebble into a pond, a series of waves is created that extends outward from the point of impact. Since a pond is large, the waves dissipate before they reach the shore and are reflected back to the place of origin. But in a contained area like the tub of an ultrasonic cleaner, the waves bounce off the surrounding walls and create a pressure front that makes them literally "stand still" within the cleaning solution. The exact same thing happens in your control room when your loudspeakers play a sustained bass tone. Static nodes develop at different places in the room depending on the loudspeaker position, the room's dimensions, and the frequency of the tone. It are those static nodes from traveling waves defined as Standing waves. This does not refer to modal frequencies as is clear from the last sentence. Eric > 1000 times sorry Tony: pushed wrongly privat reply button again. > [quoted text clipped - 48 lines] > > > > > That's what I thought as well. Sorry, Another quote from a FAQ by Ethan Winer referred to 1000 times: SIDEBAR: WHY THEY'RE CALLED STANDING WAVES <snip> > Another quote from a FAQ referred to 1000 times: > SIDEBAR: WHY THEY'RE CALLED STANDING WAVES [quoted text clipped - 79 lines] > > > > > > > That's what I thought as well. >>A standing wave is the consequence of a room being excited at one of its >>modal frequencies. Turn it around the other way - a room's modal [quoted text clipped - 5 lines] > Perhaps Ethan did not say it as eloquently, but it seems to me that this is > essentially what Ethan was saying. He said it colloquially and that's exactly what he said. Bob Signature"Things should be described as simply as possible, but no simpler." A. Einstein > Read my sig, sig = special interest group? > The voice of ignorance speaks loud and long, > but the words of the wise are quiet and few. Eckard > Folks, > [quoted text clipped - 19 lines] > > --Ethan While you may well have encountered some audio people use the terms mode and standing wave interchangeabley, I doubt that you have encountered anyone who has even a limited grasp of acoustics do so. In order to appreciate the distinction, consider a string that is attached rigidly at one end and displaced at the other at some frequency. At certain discrete frequencies, the direct and reflected waves on the string combine to create standing waves. At each of these discrete frequencies, there is a mode shape or mode of vibration. The mode shape consists of nodes and antinodes. At the nodes, the vibration is zero. At the antinodes, the vibration is maximum. In other words, the mode is the spatial pattern of nodes and antinodes which are the result of standing waves. So, in a sense, standing waves are the cause and modes are the effect. >... > In other words, the mode is the spatial pattern of nodes and > antinodes which are the result of standing waves. So, in a sense, > standing waves are the cause and modes are the effect. >... I see what you mean. The modal frequencies are what they are because standing waves can occur at those frequencies. I would put it in another way that does not involve cause and effect:- in order to calculate the modal frequencies and positions of nodes and antinodes, one would consider standing waves. However, standing waves actually occur only when the system is excited, whereas a mode is a property of the system, which is what I think Ethan is saying. We can measure the amplitude of vibration of a standing wave, but we can't do that with a mode. The definitions of mode and standing wave in my dictionary (OED) support that view. As the two meanings are so closely connected, it's not surprising that some people use one word when they should use the other. It can be handy to talk about the amplitude of vibration of a particular mode, when one really means the amplitude of the standing wave corresponding to that mode. Unfortunately (or maybe fortunately), there is no Academy of English that lays down definitions that we have to abide by. I do think it is very useful to try to use precise and correct language as it can help to clarify your thought, or at least make you realise that your thought is not as clear as you thought it was! However I try not to get too excited about other people's use of words, unless there is a real risk of confusion of meaning. SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. Folks, Thanks very much for all the (mostly) great feedback. This is exactly what I was looking for, and is pretty much what I expected: Some acoustics people interchange the terms with no distinction, while others recognize that a mode requires excitation before actual waves can occur. I'll comment on some of the replies. CHRIS: > I find it very worrying that you need to ask for clarification on concepts that are so fundamental to room acoustics ... And don't think you have to reinvent physics in order to make a name for yourself. < There's not need to insult me. As Eric Desart said the other day at RO in the thread Measurements Part 2 (5/29): "Not knowing isn't a problem, searching wondering and learning is honorable. I have millions more questions than answers myself." This is not a beginner question about what modes and standing waves are. Rather, as the subject says, I'm taking a poll to see how terminology is used among professional acousticians. The very fact that seemingly everyone here, other than you and Eric, agree with my distinction shows it was not a stupid question. So lighten up, okay? In fact, by the time you got to the end of your post you said, "A room's modal frequencies are the set of frequencies which, when excited, will cause standing waves." And as Noral pointed out, that's precisely the distinction I was making. GHOST: > While you may well have encountered some audio people use the terms mode and standing wave interchangeabley, I doubt that you have encountered anyone who has even a limited grasp of acoustics do so. < In fact, this poll was prompted after discussions with several professional acousticians who argued that modes and standing waves are exactly the same thing and that the terms can be used interchangeably. Thanks again. Anyone else care to chime in? --Ethan > In fact, this poll was prompted after discussions with several professional > acousticians who argued that modes and standing waves are exactly the same > thing and that the terms can be used interchangeably. > Thanks again. Anyone else care to chime in? I thought you would never ask. Reminds me of the saying" "I know you thought you understood what I said, but what you heard was not what I meant!" Anyway, the term 'modes' might mean either the capability of a room to resonate, or the actual presence of sound at one or mode modal frequencies, aka standing waves. The term 'standing waves', to me, implies only that the sound is there. One would expect the user to elaborate, if asked, as to the cause. This topic has been beat to death here. Can we get onto something else? As to language rigidity, the French are extremely particular about word usage to the extent that, I believe, an association there is dedicated to do exactly that; define the usage and nobody may contradict it. Do we really want that French custom here? My way of coping is to define what I mean when I have the least suspicion of confusion. Angelo Campanella Angelo, > I thought you would never ask. < ROF,L. Thanks for chiming in. > Do we really want that French custom here? < Yikes, no. Anything but the "language police" please! I wasn't at all trying to enforce my view. As the subject says, it's just a poll so I can learn what YOU all think. --Ethan "Ethan Winer" <ethanw at ethanwiner dot com> wrote in message news <SNIP> > In fact, this poll was prompted after discussions with several professional > acousticians who argued that modes and standing waves are exactly the same > thing and that the terms can be used interchangeably. There's been an omission here. I am one of the perpetrators to which Ethan referred originally. My argument was that _in the context of room acoustics_ the terms standing wave and mode (of vibration) are indeed used interchangeably. I clearly cited Beranek in my RO post and I have no doubt I could find other sources to back this up. Incidentally, someone mentioned there being no universal source on such matters of acoustical terminology. Well, there is. (In the US, at least!) The ANSI Standard S1.1-1994 is the "American National Standard for Acoustical Terminology" (http://www.ansi.org) and it provides the following: "5.18 mode of vibration. Characteristic pattern assumed by a system undergoing vibration in which the motion of every particle is simple harmonic with the same frequency. Two or more modes may exist concurrently in a multiple-degree-of-freedom system." AND "6.20 standing wave. Periodic wave having a fixed distribution in space which is the result of interference of progressive waves of the same frequency and kind. Such waves are characterized by the existence of nodes or partial nodes and antinodes that are fixed in space." First, I would point out that "a system undergoing vibration" (mode of vibration def'n above) pretty much solves the mystery of a "propensity to vibrate." Second, I would reiterate that _in the context of room acoustics_ these phenomena are indeed identical. I would summarize this as follows: If you excite a mode in a room, you have a standing wave. If you excite a standing wave in a room, you have a mode. While the possibility exists for a standing wave to exist without the presence of a mode of vibration (note "partial node" reference above), this is _not_ the typical use of the terms in room acoustics. Instead, to describe this "non-modal" behavior, Beranek talks about "traveling" waves. Hope this helps. Best regards, Jeff D. Szymanski Chief Acoustical Engineer Auralex Acoustics, Inc. > Incidentally, someone mentioned there being no universal source on > such matters of acoustical terminology. Well, there is. (In the US, at > least!) The ANSI Standard S1.1-1994 is the "American National Standard > for Acoustical Terminology" (http://www.ansi.org) and it provides the > following: Now, this is a coincidence. It just so happens that S1.1 (1994) is up for its 10-year review! I am on the ANSI S1 committee that is to review said terms and revise as needed. So, if you all have any comments on the contents os S1.1, I'll integrate it into our deliberations (likely to be occurring over the next two years..). Angelo Campanella > Now, this is a coincidence. It just so happens that S1.1 (1994) is up > for its 10-year review! I am on the ANSI S1 committee that is to review [quoted text clipped - 3 lines] > > Angelo Campanella Hi Angelo! I heard your announcement about S1.1 at the TCAA meeting. Then I saw you had replied to this thread right before me. A coincidence indeed! As to how S1.1 fits in here, I would suggest the committee develop a clarification on these two terms as they specifically pertain to room acoustics. I would imagine you could get ample clarification from Dr. Beranek, if he doesn't mind contributing. :-) Let me know if I can help! Best regards, Jeff D. Szymanski Chief Acoustical Engineer Auralex Acoustics, Inc. > As to how S1.1 fits in here, I would suggest the committee develop a > clarification on these two terms as they specifically pertain to room > acoustics. I would imagine you could get ample clarification from Dr. > Beranek, if he doesn't mind contributing. :-) We have not done much yet. I'll be alert to this SW and modes pair... Angelo Campanella Jeff, Thanks for chiming in. It's never the same without you! :->) > While the possibility exists for a standing wave to exist without the presence of a mode of vibration ... < Yes, this is a key factor, and it's another item we've disagreed about in the past. To me, a standing wave is a pattern of nodes and anti-nodes caused by a wave bouncing off a room boundary and interfering with waves still emanating from the sound source, and with other reflected waves. I find no reason to distinguish a wave that's standing still based on whether or not it happens to exactly fit between two parallel boundaries. It's either standing still or it's not. Therefore, all standing waves are not modal, which further distinguishes them from modes. --Ethan > ..........I find no > reason to distinguish a wave that's standing still based on whether or not > it happens to exactly fit between two parallel boundaries. It's either > standing still or it's not. Therefore, all standing waves are not modal, > which further distinguishes them from modes. Who says (air) modes occur only between parallel boundaries? SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. Tony, > Who says (air) modes occur only between parallel boundaries? < A LOT of acousticians say that standing waves occur only at modal frequencies. To me this is part of the issue I raised initially. As I see it, simply boundary interference - comb filtering - is the primary mechanism that defines wave behavior in a room, and modes and modal standing waves are a subset of that. --Ethan > A LOT of acousticians say that standing waves occur only at modal > frequencies. To me this is part of the issue I raised initially. As I see > it, simply boundary interference - comb filtering - is the primary mechanism > that defines wave behavior in a room, and modes and modal standing waves are > a subset of that. Comb filtering; true. Also, A plane wave of a single frequency, on reflecting from a wall, also creates "standing waves" in the wall vicinity. But in that case the colloquial expression is "interference pattern. Can we distinguish standing waves as being being only in a closed room, or at least between parallel walls? Angelo Campanella Angelo, > Can we distinguish standing waves as being only in a closed room, or at least between parallel walls? < That's a great question. For me the answer is No - even with a single wall you can have standing waves. Please tell me where my logic is wrong: Let's say we have a single large brick wall outdoors, place a loudspeaker 20 feet away pointing at the wall, and start a 100 Hz sine wave playing. The wave leaves the speaker heading toward the wall, at which point it's traveling. It then hits the wall at a 90 degree angle and bounces straight back into the path of the waves still coming from the speaker. Since the wall is extremely rigid at 100 Hz, nearly all of the energy is reflected back. At 1/4 wavelength from the wall (about 3 feet for 100 Hz) the waves are opposite and very nearly equal in level, so a deep null occurs. At that exact spot, 1/4 wavelength from the wall, the waves are no longer traveling. I often equate this to the isometric exercise where you press your hands together very hard in front of your chest. Each arm is exerting real energy, and just before your hands came together your arms were traveling toward each other. But right at the exact spot where your hands meet there's no motion and so your arms are standing still - even in the presence of considerable energy. By extension, it seems to me that ANY case where acoustic interference causes a deep null (we can define "deep" separately) has by definition also created a standing wave. What do you think? --Ethan > Angelo, > [quoted text clipped - 13 lines] > > --Ethan OK, acoustics isn't my primary field but as I see it, a standing wave is ANY fixed spatial distribution, whereas a mode is a relative maxima of peak to node (standing wave) ratios. Put another way, any quiescent sound field will have a fixed spatial distribution with spatially fixed standing wave ratios, and these ratios will exhibit relative maxima at modal frequencies. A modal, or more specifically a modal frequency would then be a frequency about which the standing wave ratio will decrease [ dF/d(SWR)=0 ] ....or some such. I tend to view "room" modes as a mulit dimensional realizations of comb filtering plots. Yes, brick walls in space would exhibit standing waves as well as modes... Later... Ron Capik -- > ..... but as I see it, a standing wave > is ANY fixed spatial distribution, whereas a mode is a relative maxima > of peak to node (standing wave) ratios. That's YOUR extemporaneous definition for the moment. > Put another way, any quiescent sound field will have a fixed > spatial distribution with spatially fixed standing wave ratios, and > these ratios will exhibit relative maxima at modal frequencies. > > A modal, or more specifically a modal frequency would then be a frequency > about which the standing wave ratio will decrease [ dF/d(SWR)=0 ] A wall reflecting a wave can, according to your definition, create a standing wave. But then I ask you, does that constitute a 'mode', as known to be the case in a closed room with a standing wave at the frequency of that mode? I think that a mode is not existing when only a wave reflects from a wall (and creates a standing pattern thereby). Angelo Campanella > > ..... but as I see it, a standing wave > > is ANY fixed spatial distribution, whereas a mode is a relative maxima > > of peak to node (standing wave) ratios. > > That's YOUR extemporaneous definition for the moment. ...and I make no claim other than that. > > Put another way, any quiescent sound field will have a fixed > > spatial distribution with spatially fixed standing wave ratios, and > > these ratios will exhibit relative maxima at modal frequencies. > > > > A modal, or more specifically a modal frequency would then be a frequency [ Note: should read: "A mode, or more specifically... " ] > > about which the standing wave ratio will decrease [ dF/d(SWR)=0 ] > [quoted text clipped - 5 lines] > > Angelo Campanella As I said, " ...not my primary field." and I'm just putting my 2 cents into Ethan's pole. Thus as I see it Ethan's wall example seems to fit the previously quoted definition: "5.18 mode of vibration. Characteristic pattern assumed by a system undergoing vibration in which the motion of every particle is simple harmonic with the same frequency. Two or more modes may exist concurrently in a multiple-degree-of-freedom system." I believe the sound source and the wall constitute system and they do form a characteristic pattern. Now, in this case the solution to the mode equation would be peak and node surfaces and these surfaces would have some gradient. The example seems to fit the above definition. What am I missing? Perhaps my understanding of acoustic standing waves is flawed, or... ? Later... Ron Capik -- > I believe the sound source and the wall constitute system and they > do form a characteristic pattern. Now, in this case the solution to the > mode equation would be peak and node surfaces and these surfaces > would have some gradient. The example seems to fit the above definition. > What am I missing? Perhaps my understanding of acoustic standing > waves is flawed, or... ? After a few decades of participation in writing standards, I have come to realize that there are two domains or phases of terminology details. The first phase is the employment of terms to organize and hopefully simplify the task of teaching and learning, sometimes created ad hoc in a single technical paper, but certainly practiced in the classroom, etc. Thus a teacher would soon use the word 'mode' to characterize an eigenfuntion or characteristic pattern of a certain motion. The second phase occurs when the terminology of a more widely used practice finds redundant or conflicting uses and meanings. At that point (often faced in applying old science to solve new problems, or in the midst of writing a new standard), the conflicts are bared for everyone to see. Like the "well tempered klavier", compromises have to be struck, and some meanings warped and refitted into a pattern that is logical and suits the purpose intended for that standard. I have seen the definitions of a decibel, equivalent noise level, barrier and others undergo this sometimes painful process. This 2-step process is now being accelerated by the emerging and widespread use of "expert software" where it is striven to be that, and claimed that, this software properly place the knowledge of many preceding experts into the hands of newly minted practitioners, and by golly, "we had better be right". In the case of modes and standing waves, it is apparent that each may imply the other, but that implication alone soon will not satisfy the users (us). At the moment, I am favoring the concepts that: Modes mean the 2D or 3D pattern of the motion, especially at system resonance. Standing waves mean the array of waves within a bounded space. At least two bounds are needed (e.g. two parallel walls; one is not enough). Angelo Campanella Signature--------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover "> Ron Capik wrote: > > I believe the sound source and the wall constitute system and they > > do form a characteristic pattern. Now, in this case the solution to the > > mode equation would be peak and node surfaces and these surfaces > > would have some gradient. The example seems to fit the above definition. > > What am I missing? Perhaps my understanding of acoustic standing > > waves is flawed, or... ? Have a look, for example, at Koopman G H, Pollard H F, A joint acceptance function for enclosed spaces, J. Sound Vibr. 73(1980)3, 429-446. Subject: quantifying the geometric coupling between the acoustic modes of an enclosure and the vibratory motion of the enclosing surfaces. Kari Pesonen Angelo, > A wall reflecting a wave can, according to your definition, create a standing wave. But then I ask you, does that constitute a 'mode' < This is the crux of it. A mode definitely requires at least two boundaries in order to foster a resonance or "mode of vibration" as Jeff Szymanski so nicely put it. > I think that a mode is not existing when only a wave reflects from a wall (and creates a standing pattern thereby). < I agree with that too. A single wall can reflect a wave such that it stands still at the exact null point some distance away. But there's no mode. To me, a mode is a *subset* of the more general case of acoustic interference, and has the additional property of resonance. At non-modal frequencies in an enclosed space standing waves still develop. But in the special case of modal frequencies, the only distinction is that the waves happen to fit exactly between the boundaries. This is why I think of modal standing waves as being a subset of acoustic interference, rather than the other way around. --Ethan > To me, a mode is a *subset* of the more general case of acoustic > interference, and has the additional property of resonance. At non-modal [quoted text clipped - 3 lines] > standing waves as being a subset of acoustic interference, rather than the > other way around. As others have pointed out, modes are much more general than sound in air, and do not have to involve waves at all, therefore they may not have anything to do with interference. A mode is normally taken to mean a resonant condition*. For example, a vibrating machine supported on springs has a number of modes (vertical and horizontal translation, rocking and twisting) none of which involve waves. In rooms we're usually interested mainly in the modes for two reasons. Firstly they are resonant, therefore they give rise to higher sound levels than other standing waves. So the fact that they "happen" to fit exactly between boundaries is not just of minor significance. Secondly they are stable features of the room, although the levels of their associated standing waves vary according to how they are excited. By contrast non-modal standing waves will be different for every position of a sound source. However we may have to consider these for fixed sound sources such as loudspeakers. *For example, I pulled a book from my shelf that I knew would refer to modes, Craik's "Sound transmission through buildings using statistical energy analysis". Craik wrote "Modes (or resonances) occur when the path travelled by a wave is such that after travelling around the subsystem it arrives back at its starting place travelling the in the same direction and in phase with itself so that constructive interference occurs." This seems to me a pretty good definition for those modes to do with waves (the only sort relevant to SEA) - it takes care to exclude reflections from a single boundary. SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. > As others have pointed out, modes are much more general than sound in air, agreed > and do not have to involve waves at all, therefore they may not have > anything to do with interference. A mode is normally taken to mean a agreed > resonant condition*. For example, a vibrating machine supported on springs Not agreed. In general, a mode can be defined as any depicted or established condition. A room or building will have sound or vibration distributed in an organized fashion even if the lotion is not resonant. > has a number of modes (vertical and horizontal translation, rocking and > twisting) none of which involve waves. agreed. what is at stake here is the refined definition that uses the noun 'mode'. There are three refining adjectives; 'resonance' (or 'resonant'), 'normal', and 'characteristic' ('eigen-'). The 'resonance' mode is as used here, and may also be termed 'eigen-'. The 'normal' mode, I am not so sure of the difference between it and 'eigen;'. Perhaps someone here can elaborate. > In rooms we're usually interested mainly in the modes for two reasons. I believe that you really mean 'resonant mode' (one needs the adjective, 'resonant'). > Firstly they are resonant, therefore they give rise to higher sound levels > than other standing waves. So the fact that they "happen" to fit exactly > between boundaries is not just of minor significance. Secondly they Here, we see revealed that the common use of 'mode' has carried within it the term 'resonant'; perhaps it is time to always state it so.... > By contrast non-modal standing waves will be different for every position of Here we see it obligatory to specify that a standing wave includes different entities than just the modal property. > a sound source. However we may have to consider these for fixed sound > sources such as loudspeakers. 3D affairs naturally require more entities than implied in 'mode'. > *For example, I pulled a book from my shelf that I knew would refer to > modes, Craik's "Sound transmission through buildings using statistical > energy analysis". Craik wrote "Modes (or resonances) occur when the path Here again, it is reluctantly included that the word 'mode' is insufficient to the task. > travelled by a wave is such that after travelling around the subsystem it > arrives back at its starting place travelling the in the same direction and > in phase with itself so that constructive interference occurs." This That's one good definition of a resonant mode. > seems to me a pretty good definition for those modes to do with waves (the > only sort relevant to SEA) - it takes care to exclude reflections from a > single boundary. In treating such complex systems as done by SEA, the user is obliged to become more specific.... I suppose we are getting somewhere in this discussion.. Angelo Campanella > > A mode is normally taken to mean a > > resonant condition > Not agreed. In general, a mode can be defined as any depicted or > established condition. A room or building will have sound or vibration > distributed in an organized fashion even if the lotion is not resonant. Angelo, (or anyone else) can you quote a use of "mode", in the context of physics and vibration, in an established book or by an established expert, which does not mean resonance? From what I see, resonance seems to be the normal meaning. Another example, my old Bruel & Kjaer booklet "Measurements in building acoustics", un-dated but must be a good 20 years old:- "One can consider a room acting as a complex resonator with a certain number of natural resonances or modes ..." SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. Tony, I agree with all you and Angelo have stated. This is great stuff, and it helps a lot by forcing all of us (okay, me) to examine these specific terms very closely. > a vibrating machine supported on springs has a number of modes (vertical and horizontal translation, rocking and twisting) none of which involve waves. < Yes, but this is true only when the system is at rest. Give it a little push and the waves will appear. > it arrives back at its starting place ... in phase with itself < Yes, excellent. --Ethan > > a vibrating machine supported on springs has a number of modes (vertical > and horizontal translation, rocking and twisting) none of which involve > waves. < > > Yes, but this is true only when the system is at rest. Give it a little > push and the waves will appear. Sorry, don't agree, Oscillatory motion does not equal waves. There will always be waves when something moves because no body is completely rigid, but waves within the machine or within the springs are not relevant to the basic modes of the mass on the springs. SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. >> > a vibrating machine supported on springs has a number of modes (vertical >> and horizontal translation, rocking and twisting) none of which involve [quoted text clipped - 7 lines] >but waves within the machine or within the springs are not relevant to the >basic modes of the mass on the springs. Acoustics is only the oscillatory motion of the media (air, etc.). In a linear system no net displacement of the media occurs. Ethan's point is especially relevant because it introduces the idea of a finite Q, or non-zero damping. Any excitation of a resonant system of finite Q will cause wave motion at all resonant modes. Discussion so far has been confined to the dimensional component of resonant modes and has ignored the dissipative. Chris Hornbeck > > Oscillatory motion does not equal waves. There will > >always be waves when something moves because no body is completely rigid, [quoted text clipped - 3 lines] > Acoustics is only the oscillatory motion of the media (air, etc.). In > a linear system no net displacement of the media occurs. Not sure what net displacement has to do with it, but if you are disagreeing with me I will stick to my point which is quite fundamental. Waves move through a medium or system. They have velocity and wavelength as well as frequency. Not all oscillations do this. SignatureTony W My e-mail address has no hypen - but please don't use it, reply to the group. > Acoustics is only the oscillatory motion of the media (air, etc.). In > a linear system no net displacement of the media occurs. > > Ethan's point is especially relevant because it introduces the idea > of a finite Q, or non-zero damping. Any excitation of a resonant > system of finite Q will cause wave motion at all resonant modes. Don't know if I buy the " ...all resonant modes" part. Care to elaborate? > Discussion so far has been confined to the dimensional component of > resonant modes and has ignored the dissipative. > > Chris Hornbeck I find the question interesting but I don't see how system Q fits in with the basic definitions under discussion. Best match I can see at the moment is that acoustic systems may perhaps be modeled in the s-plane like a control system. Maybe an active acoustic system, like a PA system, could be represented by root locus plots etc.... is that the kind of thing you had in mind, like maybe modes being expressed in the s-plane, or some such? Later... Ron Capik -- >I find the question interesting but I don't see how system Q fits in >with the basic definitions under discussion. [quoted text clipped - 3 lines] >root locus plots etc.... is that the kind of thing you had in mind, like >maybe modes being expressed in the s-plane, or some such? Sorry, but you're way over my head. Let me try this way: All resonant systems contain "lossless" elements: mass/spring, inductance/capacitance, room dimensions/wave velocity, etc. and "lossy" elements: viscosity, resistance, acoustical damping, etc. The lossy elements "drag" any excitation to excite *all* resonant modes. Discussion limited to the lossless elements gives an incorrect picture, even assuming that the losses are linear, which they ain't. For example, one incorrect prediction would be that a resonant mode must be excited by energy at *its* wavelength/frequency to produce its associated standing waves. Not true. Chris Hornbeck > Sorry, but you're way over my head. Let me try this way: > All resonant systems contain "lossless" elements: mass/spring, [quoted text clipped - 12 lines] > > Chris Hornbeck Sorry back at you. Guess I read more in to the question than I needed to. To back off a bit, basically the resonant frequency is the frequency of peak response. Excitation at ANY frequency will couple into the system and how well that energy couples depends on the Q of the system. If the Q is high the rate at which coupling efficiency falls off as one moves away from resonance is very rapid and vice versa. High Q systems will also have a larger enhancement of the energy peak because more energy is stored in the cavity; a wave bounces more times before it is dissipated and will have deeper nulls because, umm, less out of sync energy is bouncing about. At an excitation frequency to either side of resonance the cavity will be excited at the excitation frequency but at a somewhat lower level due to cancellations; the push of the next wave is a little out of sync with the waves in the cavity. Maybe a bit simplified, but that's about it in a nut shell. Hope that helps. Later... Ron Capik -- <immaculate conception clipped for bandwidth> >Maybe a bit simplified, but that's about it in a nut shell. Hope that >helps. Dude, you should be writing some books. If you are, I should be reading them. Chris Hornbeck Chris, > All resonant systems contain "lossless" elements: mass/spring, inductance/capacitance < Capacitors and inductors have losses too. This shows up as resistance in series and in parallel with the device. --Ethan >Capacitors and inductors have losses too. This shows up as resistance in >series and in parallel with the device. Yeah, the real world is messy. Air is lossy too, and sometimes we can ignore that and sometimes not. My caveat about the path of the discussion was that it seemed to ignore the real world's lossiness. Chris Hornbeck Hi all, I can not attempt to resolve this discussion as it now seems to be about taxonomy as much as anything. Just for clarity though, the statement "At that exact spot, 1/4 wavelength from the wall, the waves are no longer traveling" seems odd to me. If they are not, how does the standing wave exist? Surely the waves still travel but as well as this a null is created at a certain distance from the wall??? Andrew > Angelo, > [quoted text clipped - 28 lines] > > --Ethan > Folks, > [quoted text clipped - 5 lines] > > CHRIS: Is a drum head vibrating vs a guitar string vibrating, both at a constant frequency and not dampening (physists love to ignore reality) a good example of the two "distinctions"? An unhit drum head is not vibrating in any mode, it's just sitting there. In the RF world, a wave cruising down a waveguide does so at different modes (TE and TM). For that matter, so does light traveling down an fiber optic cable (think elliptical/spherical coordinate transformations for that one - yuck!). I'm not an expert either, but I once slept at a Holiday Inn Express. -Eric [snip] >I'm not an expert either, but I once slept at a Holiday Inn Express. Funny thing about that. The Holiday Inn Express I once slept in had a chandelier hanging from the ceiling. I bumped into it, and saw that it moved in two ways. One was the head and the chain moving together, like a simple pendulum. The other was the head moving one way, the chain moving the other, so it bent where they were connected. Both of those were modes. I really cannot picture either being considered to be a wave motion. Normal modes are a general concept. Standing waves are one kind of mode. The world is wider than just acoustic waves. Ken Plotkin I feel obliged to give some support to an old man: The Ghost. Only female chicken may lay an egg. So the cause (wave, egg) precedes. > Normal modes are a general concept. Standing waves are one kind of > mode. The world is wider than just acoustic waves. Yes. But standing waves are not necessarily acoustic ones. A concept is something artificial like software while e. g. electromagnetic waves were reality long before someone was even able to count. Eckard Blumschein > I feel obliged to give some support to an old man: The Ghost. > Only female chicken may lay an egg. So the cause (wave, egg) precedes. [quoted text clipped - 7 lines] > > Eckard Blumschein Firstly, I am not as old as you might think. In fact, you are probably a good decade or two my senior. Secondly, Ken Plotkin makes an excellent point about which I was aware but failed to include in my previous post. The concept of modes of vibration is a general concept that pertains to mechanical, electrical, and many other categories of physical systems, and does not necessarily require standing waves. An example is the lumped-element system consisting of a plate supported its corners by springs. Such a system will have several modes of vibration at low frequenies, none of which are the result of standing waves. Lastly, despite the well-intentioned motives of the original poster, it is a good thing that truth in science/engineering is not determined by or established by polls and/or surveys. [snip] >... point about which I was aware >but failed to include in my previous post. The concept of modes of >vibration is a general concept that pertains to mechanical, >electrical, and many other categories of physical systems, and does >not necessarily require standing waves. An example is the [snip] You can't spoon feed the whole answer to everyone. >Lastly, despite the well-intentioned motives of the original poster, >it is a good thing that truth in science/engineering is not determined >by or established by polls and/or surveys. You're correct about truth. Unfortunately, beliefs are something else. Ethan's original post sounded like he knew the truth, but was heavily distracted by the beliefs of others. Ken Plotkin > Folks, > [quoted text clipped - 19 lines] > > --Ethan To me a standing wave can exist without a mode or resonance. The classical example would be a plane wave getting reflected perpendicularly against a rigid wall, so that there are two waves traveling in opposite directions. There is no energy transportation in this wave field, thus the term "standing" wave, as opposed to the propagating wave. But I also sometimes use the term standing wave in a sloppy manner when I mean "excited room resonances" or similar. I don't like it, but I do. Svante, > To me a standing wave can exist without a mode or resonance. The classical example would be a plane wave getting reflected perpendicularly against a rigid wall < Thanks, yes, that's how I see it too. --Ethan |
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