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john k...
diyAudio Member

Join Date: Aug 2004
Location: US
Quote:
 Originally Posted by JohnPM Nice to see where your misunderstanding has come from Elias, but unfortunately you have things back to front. The response to a sinusoid, just like the response to any input, is determined by the frequency response (including the transient behaviour). The frequency response is not determined by hanging around for the transients of applying an infinity of sinusoids to decay.
Actually it is. That is exactly what an FT does for you given the impulse. Or, for example, given the system transfer function, T(s) =A(s)/B(s) how do you find the frequency response? We let s = jw and calculate the amplitude and phase of T for each value of frequency of interest. The result is not a function of time, just of w. Thus, it is the steady state result.

It's really a chicken - egg argument.
__________________
John k.... Music and Design NaO dsp Dipole Loudspeakers.

Last edited by john k...; 11th August 2010 at 10:13 PM.

JohnPM
diyAudio Member

Join Date: Oct 2008
John, that was not the context of my objection to the terminology:
Quote:
 Indeed "frequency response" is a steady state measure, and not very suitable to analyse temporal phenomena. Why limit yourself to steady state?? I'm yet to hear music signal which is steady state! Music is not steady state!
Quote:
 The more 'steady stateiness' in the log sweep the more accurate is the measured impulse response.
Quote:
 "frequency response" indicates system's response at steady state
In all the posts there is the statement or implication that the system's frequency response is only useful for "steady state" behaviour. That is simply not true. The frequency response of a system gives all the information needed to determine its temporal behaviour, it is a complete characterisation of how the system will react to any input in time or frequency domains and we can skip back and forth between the domains via the FT. It is in no way a "limitation" nor does it require "steady stateiness" to be determined - there is an infinity of transient signals from which we can determine a system's frequency response, from impulses to sweeps, the only restriction is that the signal spectrum is not zero anywhere.

john k...
diyAudio Member

Join Date: Aug 2004
Location: US
Quote:
 Originally Posted by JohnPM John, that was not the context of my objection to the terminology: The frequency response of a system gives all the information needed to determine its temporal behaviour, it is a complete characterisation of how the system will react to any input in time or frequency domains and we can skip back and forth between the domains via the FT. It is in no way a "limitation" nor does it require "steady stateiness" to be determined - there is an infinity of transient signals from which we can determine a system's frequency response, from impulses to sweeps, the only restriction is that the signal spectrum is not zero anywhere.
I agree. FR and impulse response for an LTI system are one in the same. Given one or the other the system response to any input is obtainable. I was just saying that how SS is determined really isn't the issue. That FR is a SS result is. I didn't interpret you comments in the correct context.
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John k.... Music and Design NaO dsp Dipole Loudspeakers.

mikey_audiogeek
diyAudio Member

Join Date: Sep 2004
Location: Northland, New Zealand
Quote:
 Originally Posted by JohnPM John, that was not the context of my objection to the terminology: In all the posts there is the statement or implication that the system's frequency response is only useful for "steady state" behaviour. That is simply not true. The frequency response of a system gives all the information needed to determine its temporal behaviour, it is a complete characterisation of how the system will react to any input in time or frequency domains and we can skip back and forth between the domains via the FT. It is in no way a "limitation" nor does it require "steady stateiness" to be determined - there is an infinity of transient signals from which we can determine a system's frequency response, from impulses to sweeps, the only restriction is that the signal spectrum is not zero anywhere.
Hi John,

Isn't that like saying "the engine speed and throttle position gives all the information needed to determine fuel injection quantity on an engine"?

The analogy between speakers and engines may be useful; engines exhibit some dependence on transient inputs. For example, this is why acceleration enrichment is still required on fuel injected engines.

I would think that any system that exhibits energy storage and release cannot be completely understood from its "steady state" behaviour.

mige0
diyAudio Member

Join Date: Jan 2005
Location: Austria, at a beautiful place right in the heart of the Alps.
John and John

my guesstiimation of Elias line of arguments is that he is fully aware of what you outline - though he is emphasizing on the point that you need quite some "reading the tea leaves" experience to predict whats going on from IR and FR only.

As was made pretty clear - both IR and FR are just two sides of the same coin - *but* you always have to (in the sense of : easier you do so) look at both sides to get the rather complete pix and even then its way less (intuitively) clear how to "read" the IR/FR plots compared to a time-frequency plot.

So the difference in comparing "wavelet versus IR/FR" is more in the user friendliness and accessibility as well as in the scalability of wavelet versus IR/FR.

Sure the system is determined also by FR/IR but it simply is a PITA to distinguish reflections from normal resonance from looking at FR/IR only.
I guess this is why CMP behaviour hasn't made it into consciousness of audio geeks until now.

The second point in Elias line of arguments - as I see it - is valid as well.
The more "steadyness" we look for in measurement (making it a looooong process) the better accuracy - possibly not so much because we could not gain the exact same information by a very short pulse measurement - but simply from mere practical circumstances (background noise compromising accuracy).

Quote:
 Originally Posted by mikey_audiogeek I would think that any system that exhibits energy storage and release cannot be completely understood from its "steady state" behaviour.
Not in an "easy and intuitively" way IMO - but actually that it *can* be done for usual speaker systems is pretty much out of question.

I'm though still awaiting an outspoken answer if a CMP system is in any case determined by IR/FR just the same.

To me, a discontinuity in temporal behaviour - as happening in CMP systems - questions that (most obviously seen in the - theoretically - 100% correctability of deep nulls) - but as said - thats better be answered by the math magicians.

Michael

Last edited by mige0; 12th August 2010 at 04:55 AM.

JohnPM
diyAudio Member

Join Date: Oct 2008
Firstly, let me apologise to Elias as my posts probably come across as being directed personally at him. That is not my intent, what I am against is the common treatment of time and frequency domains as being somehow unconnected, which can creep into our thinking in many subtle and not so subtle ways and lead to erroneous conclusions. I'll illustrate using the post on sweeps, as it simultaneously contains much well considered truth and yet much fallacy.
Quote:
 To be accurate, log sweep cannot measure system's impulse response. Not unless you start the sweep from DC. But one cannot do that because it would take infinite time to play the sweep. Anything else yields only approximation of the impulse response. In practise approximations are often sufficient, but it does not remove the (approximate) steady state condition requirement. The more 'steady stateiness' in the log sweep the more accurate is the measured impulse response. That is, the longer the sweep the more accurate is the impulse response. To understand this try to think what will happen if you make your log sweep infinitesimally short in duration.
In my view the basic misconceptions underpinning the above are:

- To measure a frequency response we should really use a frequency, and the more like a frequency (sine wave) the test signal is the better the result would be

- A sweep is a frequency signal and the shorter you make it the less like a frequency it becomes and so the less suited it is to measuring frequency response.

All signals are time signals, a sweep is no less a time signal than an impulse or a step. It is easy to start thinking about a sweep as if it lived in the frequency domain, we talk about its start and stop frequencies after all, but that is already the start of misunderstanding it. All time signals have spectra in the frequency domain, and those spectra are nearly always far richer than we imagine. A sweep that starts at 20Hz and ends at 200Hz has a spectrum that extends from DC to Nyquist, reducing the content outside the notional range of the sweep requires considerable care in tapering the onset and decay of the signal, and even then the levels outside the range we may be most interested in are only reduced, they are not zeroed. To address the DC comment, a single half cycle of a 1kHz sine wave has DC content. So does a single half cycle of a 1MHz wave or a 1GHz wave. An impulse has DC content! The slightest imbalance in a time signal yields DC content, constructing a sweep without DC is quite difficult (and of course unnecessary).

A system's frequency response is just a way of describing its behaviour (all its behaviour, not just "steady state"), that description is valid in the time and frequency domains. To determine it all we need to do is pass a signal through the system and divide the spectrum of the output by the spectrum of the input (that division is the reason the spectrum of the signal used must not be zero anywhere). The quality of the result is determined by how much energy the test signal contains across the whole spectrum -within the linear range of a system, a sweep that is twice as loud improves the accuracy of the derived frequency response by the same amount as a sweep that lasts twice as long. That frequency response can then be used to generate the time or frequency responses of the system to any stimulus by simply multiplying it by the spectrum of the stimulus. To see that as a time response, just inverse FT the product.

None of this is an argument against the utlility of different ways of presenting the information contained in a signal, whether they look purely at time, frequency or a combination of the two, but it is important not to treat frequency as being somehow independent of time or frequency responses as not containing full information about time domain behaviour.

Elias
diyAudio Member

Join Date: May 2003
Location: Suomi
Hello,

Is there any change to access these open baffle impulse responses? (I didn't read the whole thread ) It would be interesting to compare them to the one posted measured in the box.

- Elias

Quote:
 Originally Posted by augerpro Elias my tests of the AE drivers were on an open baffle, so there was no rear wall reflection. From John J's comments I take it that this is a suspension or cone issue. It is not as noticeable with the fabric surrounds, although it is still there. It is also most noticeable on the 15" drivers. My guess is it is cone/surround interface issue. Similar to the dip you see around 800-1000hz, on smaller drivers, especially 8" woofers for whatever reason. Or cone flex issue that is mitigated by fabric surrounds but not helped by foam surrounds. Anyway I haven't seen any big woofers with better IR than the AE drivers so not sure what the fuss is about.
Quote:
 Originally Posted by g3dahl Same here. I tested without the cabinet, and the 2.5 ms wrinkle was still there. My feelings exactly! Judging by the results, I'm pretty sure that Nick McKinney worked hard to chase out every problem he could while developing this driver. All of the 15" drivers I have used have brought a certain quality (solidity, authority) to the presentation that I really like. As with everything else in speaker design, there are compromises as well. The TD15M doesn't measure as cleanly as the little Vifa drivers in the Ariel, but its strengths are obviously different. As much as I have liked the other 15" drivers I have used (Altec 416, 515, 604, Klipsch K-48), I don't expect the TD15M will be a disappointment. Of course I am curious about the cause of the 2.5 ms ripple, but I have less than two weeks remaining before starting back to work, and need to focus on completing the crossovers. Gary Dahl

Elias
diyAudio Member

Join Date: May 2003
Location: Suomi
Hi JohnPM,

Quote:
 Originally Posted by Elias FFT calculates the steady state response.
Quote:
 Originally Posted by JohnPM That is another incorrect statement.

In order to get over this issue you may like to review the properties of the Fourier transform, but you must know it already which makes it strange why someone would argue over such a basic fundamental definition. FT transforms time domain to frequency domain where time information is lost, thus frequency domain is steady state domain. FT does not care what you transform, anything in time domain signal will end up into steady state frequency domain signal even your time domain signal is not steady state. Frequency response is a frequency domain concept, thus frequency response is a steady state response.

This is what frequency response IS. One can use it freely to suit his purposes.

It's not clear to me if you agree or not about your quote below, but if you don't mind would you explain how to measure the frequency response? Yes, I emphasize to measure to differentiate it from to calculate, that is to determine frequency response without FT.

Anyway, I gave the answer earlier in my post which is also the definition of the frequency response, but it does not exclude you to give a different answer.

- Elias

Quote:
 Originally Posted by JohnPM - To measure a frequency response we should really use a frequency, and the more like a frequency (sine wave) the test signal is the better the result would be - A sweep is a frequency signal and the shorter you make it the less like a frequency it becomes and so the less suited it is to measuring frequency response.

Last edited by Elias; 12th August 2010 at 11:16 AM.

JohnPM
diyAudio Member

Join Date: Oct 2008
Quote:
 Originally Posted by Elias FT transforms time domain to frequency domain where time information is lost, thus frequency domain is steady state domain. FT does not care what you transform, anything in time domain signal will end up into steady state frequency domain signal even your time domain signal is not steady state. Frequency response is a frequency domain concept, thus frequency response is a steady state response.
There are a few things to correct here, but let's start with the most important:

No information is lost when FT is performed

None at all. To understand why, it may help to consider what the FT is, but from a different perspective. There are many ways to represent a time signal. The most familiar are some equation in which time is a variable, or for our purposes a sequence of the amplitudes of the signal sampled at uniform time intervals. There are circumstances, however, where it would be useful to be able to represent that time signal in other ways that are more amenable to certain kinds of processing. One way to do this is to take some other set of time functions and see if we can find a way to create the original time signal through a combination of those functions. The functions are referred to in mathematical jargon as basis functions. In the case of FT (and here we are considering the Discrete Fourier Transfrom, since our signal is defined at the discrete time intervals at which we sampled it) we choose sine waves as the time functions. When we calculate the DFT of our signal we are finding the answer to the question "what are the amplitudes and phases of a set of sine waves that when added together will give the original time signal". (Strictly speaking, we calculate the real and imaginary parts of a complex number whose amplitude and phase are the answers we seek). We call that set of sine wave amplitudes and phases the frequency response. After we have done this, we can perform processing that alters the amplitudes and phases of those sine waves rather than altering the samples of the time signal directly, but we are still dealing with the original time signal, just represented in a different way. If we want to get back to the time series we just perform an inverse FT, which simply adds up the sine waves for us to get back to the time signal.

In the context of a system and its transfer function, the time signal we are representing is the impulse response. The system frequency response is just a different way of representing the impulse response, it still contains all the information the impulse response contains.

Quote:
 if you don't mind would you explain how to measure the frequency response? Yes, I emphasize to measure to differentiate it from to calculate, that is to determine frequency response without FT.
In the general case what we are trying to measure is the system's transfer function, either its impulse response or its frequency response (as we have seen above, they are same signal in two representations). If we determine one we have determined the other, since they are the same signal, but I'm sure you want more than methods of determining the impulse response, especially as you wish the FT to be banished

Fortunately, we can go directly from time signals to the real and imaginary parts of the frequency response amplitudes and phases. To do this we need a signal generator that will produce a sine and a cosine signal. We use our generator to produce two sweep signals, the sweep from the sine wave output goes to the system we want to measure. That sine sweep and the cosine sweep are both fed through time delays that match the time delay through the system we are measuring, in the acoustic case that would typically be the time it takes sound to travel from the speaker to the mic - we need to determine the delay in advance, but that is not difficult to do. The signal captured from the mic goes to two multipliers, in one of them it is multiplied by the sine signal, in the other it is multiplied by the cosine signal. Each multiplier has a low pass filter on its output. The output of the sine wave multiplier (after the low pass filter) is the real part of the frequency response, the output of the cosine multiplier is the imaginary part. Since we are controlling the generator we know the frequency the generator was at for each moment during our sweep, so we know what frequency the real and imaginary signals correspond to. So, in a matter of a few seconds of a sine sweep, we have captured the frequency response of the system. And the FT was left in the toolbox, unwanted.

That process is called Time Delay Spectrometry, it is typically used with linear sweeps as that makes life a bit easier in terms of the effects of the low pass filters. It is the measuring principle used in the TEF system. TDS has some advantages (it is quite good at rejecting noise, distortion and room reflections) but also some disadvantages. Log swept sine is overall a better method of transfer function measurement, but of course that uses the dreaded FT

I hope that helps.

Elias
diyAudio Member

Join Date: May 2003
Location: Suomi
Hi,

Quote:
 Originally Posted by JohnPM No information is lost when FT is performed

I should have been more clear in my previous statement, but actually there IS information loss in the process of transforming the time domain to the frequency domain. What is lost is the information if the input signal was at the steady state or not before the FT was performed. There is no means in FT to express that information. FT allways outputs steady state freq response regardless of input, but when calculating IFT no information is available if the time domain signal should be at steady state or not. Usually one assumes it is, but generally it is unknown.

If one inputs arbitrary signal into the system and calculates FT at the output, the result will vary until the steady state is reached. In steady state input and output signals must also be steady state signals. This will take us back into measuring the frequency response and the importance of reaching the steady state before a freguency measurement achievement is declared. And of course since we are measuring at the steady state the frequency response must be a steady state measurement. LOL did I say it again Never mind

Quote:
 Originally Posted by JohnPM Fortunately, we can go directly from time signals to the real and imaginary parts of the frequency response amplitudes and phases. To do this we need a signal generator that will produce a sine and a cosine signal. We use our generator to produce two sweep signals, the sweep from the sine wave output goes to the system we want to measure. That sine sweep and the cosine sweep are both fed through time delays that match the time delay through the system we are measuring, in the acoustic case that would typically be the time it takes sound to travel from the speaker to the mic - we need to determine the delay in advance, but that is not difficult to do. The signal captured from the mic goes to two multipliers, in one of them it is multiplied by the sine signal, in the other it is multiplied by the cosine signal. Each multiplier has a low pass filter on its output. The output of the sine wave multiplier (after the low pass filter) is the real part of the frequency response, the output of the cosine multiplier is the imaginary part. Since we are controlling the generator we know the frequency the generator was at for each moment during our sweep, so we know what frequency the real and imaginary signals correspond to. So, in a matter of a few seconds of a sine sweep, we have captured the frequency response of the system. And the FT was left in the toolbox, unwanted. That process is called Time Delay Spectrometry, it is typically used with linear sweeps as that makes life a bit easier in terms of the effects of the low pass filters. It is the measuring principle used in the TEF system. TDS has some advantages (it is quite good at rejecting noise, distortion and room reflections) but also some disadvantages. Log swept sine is overall a better method of transfer function measurement, but of course that uses the dreaded FT
So, you are proposing using sweep. Are you sure it is steady state to be able to determine the frequency response accurately?

- Elias

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