Let's make the experiment even simpler. Consider a system which has two signal paths through it: One path is direct, the other path is delayed by 0.2 milliseconds and has a
6 kHz low-pass filter with a slope of 48 dB/octave. The outputs of these two paths are summed linearly into a single output.
Now, consider a second system, which consists of a linear path with a single series circuit consisting of a an inductor and a capacitor whose values are such that the value of 1/sqrt(L*C) is about 16,000 radians per second.
On examining the frequency response of either system, they exhibit a flat response up about 5 kHz, where there is a sharp null, and then they are essentially flat above that frequency.
Question: As they can exhibit essentially identical frequency response, are they both resonant systems?
Depends on how "resonant system" is defined. From what I've seen, it usually means that there is an equilibrium of energy transfer between inductive and capacitive elements (in the case of electronic resonance), so the former system you proposed doesn't quality as it doesn't necessarily have both types of elements.
--
% Randy Yates % "She tells me that she likes me very much,
%% Fuquay-Varina, NC % but when I try to touch, she makes it
%%% 919-577-9882 % all too clear."
%%%% % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr
You've got it (just leave out the word "necessarily"). And of course the transfer doesn't have to be between inductance and capacitance, just two orthogonal energy forms. In the case of a genuine room resonance it is between pressure and kinetic energy. In a mode we just have waves summing and cancelling - so no resonance.
And that is precisely the point of the excercise. Here we have two linear systems which show essentially the same properties in the magnitude frequency response. Are they both resonant systems? To answer that question requires the person answering it to come up with a definition of "resonant" which fits both systems.
Under that definition, I would agree. But a number of people bandy about the term "resonance" without knowing, caring or revealing what that means. The original poster in this thread and the article he is citing are two example of all three: the article cited doesn't have a clue what "resonance" means, doesn't reveal it and doesn't care. It merely sounds cool and sort kinda scientific in a stupid sort of way.
By the way, as I am sure you know, the two systems I describe DO have a fundamental difference that's unambiguous and completely, objectively measurable. That difference may or may not make one resonant and the other not, because, again, that requires the person bandying about the term to define it.
I have no idea what transduction is supposed to mean in this context. But room modes do indeed resonate. :->)
Here's how I distinguish resonance from not resonance: Excite a circuit or room or whatever with a single step impulse. If the result is a sine wave having a non-zero duration, you have a resonance. Therefore...
No. This is an increase in amplitude, but the result signal does not continue after the source signal stops.
Bingo. And that's exactly what I objected to as well. These folks kill me when they at once deride "science" as not having all the answers, then go on to use scientific sounding mumbo jumbo in an attempt to sound legitimate. I hear this all the time on radio ads for bogus alternative medicine products. First they say science doesn't know what they know, then they go on to claim proof of effectiveness via double-blind tests.
Transduction is the change in form of energy from one type to another (hence transducer).
No - any system with a delay in it will do that. Standing waves do it for this reason.Back and forth across a room is a very appreciable delay. And of course at each bounce some of the energy in the wave dissipates, and you see a phenomenon that looks rather like Q in a resonant system.
It does if you bounce it off a couple of walls. This is the problem - the mode is just the limit condition of comb filtering when the wave is at right angles to the wall. Nothing new has happened - the phenomenon is exactly as it was when at an angle; it just gets to do a few more bounces, is all.
I believe you're mistaken here. Any causal, non-ideal system (i.e., a system other than a straight wire) will have a response that continues for some non-zero amount of time after a sine wave with a frequency within its passband has been removed.
This can be seen mathematically as the trailing end of the convolution of the input sine wave with the causal, finite-lengthed impulse response.
If that's what makes a system resonant (and I don't believe it is), then most systems could be classified as resonant.
--
% Randy Yates % "Maybe one day I'll feel her cold embrace,
%% Fuquay-Varina, NC % and kiss her interface,
%%% 919-577-9882 % til then, I'll leave her alone."
%%%% % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr
AFAIK, the system with delay will produce a null every 6 KHz.
We already know this much of the *book* answer - whether one or both are resonant systems depends on the definition of resonance.
Knee-jerk approach, check the Wikipedia. Right away, we find the Wikipedia in error, because their definition (from physics) of resonance is:
"In physics, resonance is the tendency of a system to oscillate at maximum amplitude at a certain frequency. This frequency is known as the system's resonance frequency. When damping is small, the resonance frequency is approximately equal to the natural frequency of the system, which is the frequency of free vibrations."
The obvious flaw in the Wikipedia definition is that oscillation of the system can be either minimum or maximum at a resonant frequency, for series and parallel resonant systems respectively. OTOH, both the series and parallel systems have maximum transfer of energy between the resonant componants at resonance.
But not the identical same amplitude or phase response.
Point of order: If we hope to find that one system is resonant and the other is not, then the definition of resonant must fit one and not the other.
That fits my initial thinking.
If you look at the disambiguation entry in Wikipedia for resonance, one might find people trying to apply one of the other meanings. Again, I'm compelled to point out that the Wikipedia fails because the existing defintion is very physical sciences oriented, and does not contain much from the arts.
For example from the Amercian Heritage Dictionary we have:
"Richness or significance, especially in evoking an association or strong emotion: "It is home and family that give resonance . . . to life" (George Gilder). "Israel, gateway to Mecca, is of course a land of religious resonance and geopolitical significance" (James Wolcott)."
Or, he's being non-technical and perhaps a little poetic.
And that's point of the gedanken, to elicit informed opinions as to which constitutes a "resonant system."
Actually, the definition is not so flawed. In a parallel resonant tank circuit, impedance is at maximum at resonance, while in a series resonant tank circuit, admittance is at a maxximum at resonance. And, respectively, voltage or current is at a maximum at resonance.
But "identity" itself is insufficient a qualifier. Two topologically identical circuits could have non- identical phase and amplitude repsonse: clearly a trivial example. But two radically different implementations could have essentially the same properties in one domain or another and still be radically different of effectively the same, depending upon definitions and requirements.
That, again, is the point of the excercise: to determine what the person means when they use the term "resonance."
What do the arts have to do with it? The current subthread relates to whether room modes are or are not "resonances."
And, to date, no one has pointed out the very fundamental difference between the two models I described, which also have to do fundamentally with the difference between a conventional second-order periodic energy-exchange system such as an LRC tank circuit or a Helmholtz contrivance and a delay system or a room mode, though I know that at least one other contributor does indeed know the difference.
Not so! A reflection off a single boundary, such as one wall of a building outdoors, will not resonate. Nor is there resonance if you sum a signal with a delayed version of itself after passing through a simple DDL or three-head tape recorder etc. (As long as there's no "HELLO...Hello...hello" feedback used within the delay unit of course.)
The key here is back and forth. In your previous post you asked:
"If you plot the frequency response of a speaker close to a wall, positioned as in your comb filtering demo, it exhibits peaks and dips in the frequency response because at some frequencies the waves add, and at others they cancel. Do you regard the high spot between those dips as a resonance, with a Q, and all the usual good resonant stuff?"
You said nothing about repeated reflections between two parallel surfaces! This is the key between resonant peaks and non-resonant peaks.
Again, you can't argue using a moving target. :->)
This brings up an interesting point. I consider comb filtering - more properly, acoustic interference - as the parent property, with modes and resonances and flutter echo et all as subsets. I thought I was alone in this thinking because I've had many self proclaimed "expert" acousticians tell me I'm full of crap and that modes define everything. But modes and resonances are a more complex subset of simple interference, as you correctly stated.
Okay, if you have a REALLY long piece of wire I'm sure the reactive L and C components can become a factor. But those are second-order effects, and in most cases the frequencies involved are well beyond the audible range. I think engineers call that stuff parasitic, yes? In a practical examination of what (I thought) we're discussing, a single reflection combined with the original source is not resonant.
You misunderstood me - I was saying any system OTHER THAN a wire. I mean that if you take a system like an op-amp network with resistors and capacitors and specific bandwidth, or something like a speaker crossover, THOSE types of systems would be resonant according to your definition.
Of course a wire is pretty much a Dirac delta function impulse response (no transients to speak of).
--
% Randy Yates % "Midnight, on the water...
%% Fuquay-Varina, NC % I saw... the ocean's daughter."
%%% 919-577-9882 % 'Can't Get It Out Of My Head'
%%%% % *El Dorado*, Electric Light Orchestra
http://home.earthlink.net/~yatescr
To amplify Randy's point, consider a current state of the art analog-digital-analog conversion system, nothing more complicated than a high-quality sound card in a computer set up in straight pass-through mode. Hit it with an impulse, and it WILL ring for not an inconsiderable time after the impulse is done. Is it resonating? According to ONE of Winer's definitions, it is, but according to another, the existence of some measurable peak or dip in the amplitude vs frequency response, it is not: the response over the bandwidth is dead nuts flat, within a very small fraction of a dB, and the phase response is essentially perfect over the bandwidth.
Take a very high quality professional analog tape recorder (say a Studer) running 30 ips, adjusted for the flattest frequency response over the bandwidth (on such a machine, I'd expect to make to 25 kHz +-1dB at all reasonable recording levels). The output WILL ring for a significant number of cycles. Is it a resonant system?
And all of these system will show a decay tail if hit with a sine wave tone burst.
Build yourself a 4th order butterworth low-pass filter, and hit it with an impulse, and watch the output ring: is it resonant (trick question, because i forgot to tell you you aren't allowed to build it out of components like inductors and capacitors).
Or, one has maximum current amplitude, while the other has maximum voltage amplitude.
One is minumum phase, the other is not.
Thus they are both resonances.
The point being that a person with a stronger background in the arts will define resonance one way, while a person with a stronger background in the sciences will define resonance in another.
Room modes are resonances. They are acoustical, so the acoustical space that they take place in is signficant.
Minimum phase versus non minimum phase comes to my mind first.
I find that after drinking the first glass, the wine all by itself becomes smoother and brighter with longer finish and less burn, astringency, and chalky feeling on tongue.
I expanded this distribution to more of the groups that saw the original.
"Mike Tommasi" >> ...
Sorry to tell you, Mike, but we _all_ harbor some notions contrary to fact. It seems part of how the mind works. (Various wise people have marveled less at how much they do or don't know, than at the things they do know that aren't so.)
I see hocus-pocus among some wine geeks, just like some audio geeks. For instance, mythologies on wine-writing history. Not to mention all that business about magnets (prompting a section in alt.food.wine's FAQ file).
I used to post on audio technologies -- just technical questions, in areas I knew something about. Example from 1991-- this one was popular -- posted after some people were furious and another claimed they'd be damned if something was true (it was):
formatting link
(If that European archive fails, find others by searching word combo oversampling+curious+furious+damned .)
Also no matter how clear-cut the subject matter, hecklers can be relied on, as the night follows the day, to attack any information they don't happen to like. (Again, how the mind works.) I've seen it with postings on consumer technologies, language history, internet history, absinthe, truffles, the AxR-1 vine rootstock debacle, wine literature, even the simple math of multiplying by -1. These hecklers don't always give the impression they are used to dealing much with things like sources and evidence and facts unaffected by what you think. But they don't let such limitations restrain them!
Max (longtime member of Audio Engineering Society)
And it usually gets even better with the next bottle. It would be really interesting to transfer some outrageously expensive wine into screw-top bottles and serve it to a party of wine snobs. Have there ever been any double-blind tests?
You are right of course... but wine geeks who believe in pyramids and magnets tends to be on the fringe and so their problem can be seen as a superstition on a par with astrology, while it seems to me that audio exotica enjoys a higher status more akin to religion. ;-)
I will not go through all the details, but there are people that seriously believe that weighing down a CD will bring less wow and flutter... and there are those who claim to hear the difference that an
8 ohm transmission line makes... and the latest, someone here in France has come up with a filter for those evil AC outlets that brings about a complete transformation of the sound output of CD players and amps, it sells for ?27,000........................................
--
Mike Tommasi - Six Fours, France
email link http://www.tommasi.org/mymail
Not IMO. I would place audio superstition on the same level as astrology, numerology, feng shui, etc etc. Religion is far more widespread and far more insidious.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.