when should one perform fft based filtering?
in [DSP]
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From: Rune Allnor on 28 Jul 2010 06:15 On 28 Jul, 07:34, spop...(a)speedymail.org (Steve Pope) wrote: > robert bristow-johnson <r...(a)audioimagination.com> wrote: > > >i would > >expect a plate reverb to be describable as an LTI system. > >i know a plate 'verb can sorta "resonate" on some frequencies, > >depending on its dimensions, but does a plate reverb create new > >frequency components? if so, is that not just due to the transducers > >going nonlinear? > > If different frequencies travel at different velocities through > the plate medium, is that not a nonlinearity (at least in terms > of behavior of the medium)? Nope. It's a dispersive medium. Not necessarily nonlinear, as linearity might hold at any one frequency (think waveguides). That said, analyzing broadband stuff is a mess in such a scenario. Rune
From: Rune Allnor on 28 Jul 2010 06:20 On 28 Jul, 08:26, "Nasser M. Abbasi" <n...(a)12000.org> wrote: > On 7/27/2010 10:34 PM, Steve Pope wrote: > > > > > If different frequencies travel at different velocities through > > the plate medium, is that not a nonlinearity (at least in terms > > of behavior of the medium)? > > > Steve > > How can a frequency travel at some "velocity"? Semantically the statement is nonsense. As an everyday-conversation- simplification-because-one-doesn't-want-to-be/isn't-an-autistic-jerk it can reasonably be interpreted as a simplification of "if different monochromatic waves with different frequency propagate in a spatio- temporal environment, more specifically a plate-shaped solid, is that not a ..." Rune
From: Rune Allnor on 28 Jul 2010 06:39 On 28 Jul, 11:15, Richard Dobson <richarddob...(a)blueyonder.co.uk> wrote: > On 28/07/2010 07:26, Nasser M. Abbasi wrote: > > > > > > > On 7/27/2010 10:34 PM, Steve Pope wrote: > > >> If different frequencies travel at different velocities through > >> the plate medium, is that not a nonlinearity (at least in terms > >> of behavior of the medium)? > > >> Steve > > > How can a frequency travel at some "velocity"? > > > Frequency is not a material object that can "travel" at any velocity? It > > is a property of an object, not the object itself? > > > Do you mean 2 waves or signals traveling with different frequencies? > > > The term "frequency" and "travel" combined as you have it, is something > > new to me, and never heard any of my teachers at school mention this > > term before. > > > Is this something that engineers use out in the real world and that is > > why I never heard it before? > > > When I leave school one day, and go to the "real world", will this term > > be a common term used by DSP engineers? > > > --Nasser > > I am not an acoustician, so caveat lector: the speed of sound in metal > is (apart from being much faster than in air) dependent on (among other > things) density and stiffness. While we make the generally reasonable > assumption that air is at equal density within any contained space, this > cannot be assumed for metal bars and plates. A plate reverb is a species > of metallophone (which as a type ranges from the piano string to a > vibraphone bar and a tam-tam), with some sometimes very dominant > vibrational modes determined by the shape; all together they contribute > to the special character of a plate reverb. Whether in a signal > processing sense it can be called non-linear I am not sure, but we > certainly get the sense that a stimulus at one frequency may activate a > dominant mode at a different one. And or course for a hyper-complex > material such as a hand-hammered cymbal or bell, where both density and > stiffness may vary from one square mm to another, the modal behaviour is > so complex that it can scarcely be modelled at all. My undestanding is > that this definitely comes within the domain of "nonlinear acoustics", > and in that sense at least, one can reasonably talk about locally > varying speeds of propagation with respect to frequency. > > This may all be rubbish of course; one of the many subjects I have yet > to get around to studying properly. You are mixing several different issues here. 1) Sound speed vs material properties: In *fluids* you are right. The sound speed varies, to first order, mainly with density and compressibility. These parameters might be influenced by other factors, like pressure, flows rates, temperature, chemical compositions, etc. For solids you need to include shear stiffness; I never remember if that's teh bulk modulus or Young's modulus. 2) For geometrically constrained environments (fluids in enclosures, solid objects), internal reflections can interact to produce 'weird' results. Sound propagation in infinitely long corridors can interact to produce wave composites (normal modes) that behave very differently from what free-field-based intuition would suggest. In solids, like plates, the combination between bending and compressing forces might interact to produce separate wave phenomena. 3) Linear wave phenomena in simple geometres can be studied rather easily, using well-established analytic theory, more precisely the study of separable PDEs. For not-so-simple-geometries-but- still-linear waves the technicalities of such a study requires a numerical software, but is otherwise not particularly difficult. An object with shape and material compositions like the cymbal could easily be studied in such a context. 4) The non-linear problems with e.g. cymbals are caused by material deformations. The problem is not the shape or the material cymbals are made of, but that they deform significantly when hit hard. One significant division (it's not the only one) between 'linear' and 'nonlinear' acoustics goes at 'material deformation'. If the acoustic field somehow alters the shape or other properties of the medium enough that the change influences wave popagation properties, the problem is nonlinear. Rune
From: Jerry Avins on 28 Jul 2010 11:11 On 7/27/2010 6:16 PM, robert bristow-johnson wrote: > On Jul 27, 4:45 pm, Jerry Avins<j...(a)ieee.org> wrote: >> On 7/27/2010 10:24 AM, Rune Allnor wrote: >> >> ... >> >>> But the computational algorithms - and thus the computational >>> workloads - are different. >> >> The algorithms, workloads, and latencies differ, but the results can be >> bit exact. > > "bit exact" means that the round-off error would be identical in both > cases. ... I was careful to write "can be". That is, of course, given enough bits to calculate with. My aim was to challenge Rune's mere approximation. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Jerry Avins on 28 Jul 2010 11:20
On 7/28/2010 2:26 AM, Nasser M. Abbasi wrote: > On 7/27/2010 10:34 PM, Steve Pope wrote: > >> >> If different frequencies travel at different velocities through >> the plate medium, is that not a nonlinearity (at least in terms >> of behavior of the medium)? >> >> Steve > > How can a frequency travel at some "velocity"? > > Frequency is not a material object that can "travel" at any velocity? It > is a property of an object, not the object itself? > > Do you mean 2 waves or signals traveling with different frequencies? > > The term "frequency" and "travel" combined as you have it, is something > new to me, and never heard any of my teachers at school mention this > term before. > > Is this something that engineers use out in the real world and that is > why I never heard it before? > > When I leave school one day, and go to the "real world", will this term > be a common term used by DSP engineers? Frequencies moving at different speeds is semantic shorthand for energy at different frequencies moving at different speeds. In optics and transmission lines, the phenomenon is called "dispersion". Jerry -- Engineering is the art of making what you want from things you can get. ����������������������������������������������������������������������� |