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actonern

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if the violin is a linear device, it is a minimum phase device, as typified by loudspeaker drivers. For such devices, there is no phase shift from the device in excess of that shown in the power spectrum amplitude. The phase shift from the spectrum is the derivative of the power spectrum amplitude. The phase may be obtained if only the magnitude is known and vice versa. Spend some time with the link for some more detailed info. Whether the violin is in fact, a linear device I will leave to the experts, although I am under the impression that Woodhouse thinks it is. The FFT does indeed calculate phase simultaneously with the amplitude of the response.

http://forums.klipsch.com/forums/storage/3/1027017/6901heyser.pdf

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Try it for yourself (those who have the equipment), with an interval which generates a strong difference tone, like 1.25. The difference tone is quite audible, and pulses at the difference tone frequency show on the recording program. There is nothing that I can see on the FFT at that frequency, beyond usual background noise.

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Try it for yourself (those who have the equipment), with an interval which generates a strong difference tone, like 1.25. The difference tone is quite audible, and pulses at the difference tone frequency show on the recording program. There is nothing that I can see on the FFT at that frequency, beyond usual background noise.

I just did. I played a 3rd on the piano and I can see the combination tones. Please give me some ideea of your set up.

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Please do not modify my posts. Entertain us with something you are actualy competent in. If anything.

Read his posts and answer the question yourself.

This type of spikiness is new on MN and seems to have become rather prevalent in the theoretical lutherie camp.

But hey, what do I know - I'm just the Janitor.

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Please give me some ideea of your set up.

Sine waves were generated using Adobe Audition. The subtractive pulses can be counted both in the generated sound file, and when this file is played through speakers on one computer, and recorded on another.

The FFT is SpectraPlus, using the infinite averaging function; tried both "uniform" and "blackman" processing; 48000 sample rate; tried decimation ratios 1 and 3.

Input on the desktop computer is an external USB analog-digital converter. When switching directions to verify, I used the internal mic and audio chip set in the laptop.

One difference between your experiment and mine is that I was using single frequency sine waves. Using a piano, you would have also had string harmonics, perhaps some phase stuff going on between multiple strings tuned to the same pitch, some resonances from part of the piano like the soundboard, and maybe a tempered interval rather than true. Don't know if those things would make a difference or not.

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Sine waves were generated using Adobe Audition. The subtractive pulses can be counted both in the generated sound file, and when this file is played through speakers on one computer, and recorded on another.

The FFT is SpectraPlus, using the infinite averaging function; tried both "uniform" and "blackman" processing; 48000 sample rate; tried decimation ratios 1 and 3.

Input on the desktop computer is an external USB analog-digital converter. When switching directions to verify, I used the internal mic and audio chip set in the laptop.

I'll try get Adobe Audition in a couple of days. I tried again by generating 2 tones with a signal generator into a speaker and listening with the computer's microphone. Worked fine.

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I'll try get Adobe Audition in a couple of days.

SpectraPlus will generate tones too. So will Audacity (free), but you may need to put each tone in a separate track.

Edit: Just tried it again, generating the tones with a different program, and using a different FFT. Same result.. there is an audible and countable difference tone, but no difference tone that I can find on the FFT.

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SpectraPlus will generate tones too. So will Audacity (free), but you may need to put each tone in a separate track.

Thank you, David. Give me a few days - I have no computer available at the moment to install Spectra.

If you made a wave file and fed it to Spectra then you would not see the combination tones because no "mixing" happened. If you played them through the speakers of a 2nd computer then they should be there. Maybe the amplitude is too small and they're burried into the noise floor.

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Thank you, David. Give me a few days - I have no computer available at the moment to install Spectra.

If you made a wave file and fed it to Spectra then you would not see the combination tones because no "mixing" happened. If you played them through the speakers of a 2nd computer then they should be there.

I tried it both ways, meaning that I also played them through speakers.

Maybe the amplitude is too small and they're burried into the noise floor.

Possibly. SpectraPlus is pretty good at letting you change the scaling though, and find things which are difficult to detect on other FFT programs.

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Boy, I gotta' admit that after trying to read this thread thru from beginning to end (well, I got to about page 9...),I've discovered there are a few really sarcastic muthas on this board, smart though some of you undoubtedly are!

I've tried many, many instruments over 4 decades, and if it helps this discussion along at all, I'll say that I decide if I generally like the sound of an instrument within the first 30 seconds (think of the women in a fern bar...), I'm looking for a certain "sizzle" (if you will...I don't know what else to call it). Unfortunately, the acoustics in the typical violin showroom can be wildly mis-leading. If I understand the conversation so far, I have to say that it is this "sizzle" we all look for, because it indicates our sound will project. As always, it's hard to describe sound in words, but like the Supremes on discussing pornography, "I know it when I see [hear] it."

(Besides, we have to take the fiddle home to make sure every single friend agrees with our highly-attuned ears, and to offer their opinion as to why we shouldn't buy the thing anyway. BTW, the very same thing happens re bows...)

But I digress...

It should be noted that this projecting quality has nothing to do with price. I'm sure a modern-day luthier, with all his training, research, expertise, talent etc., can make an excellent, projecting instrument, while I've played (monstrously) expensive instruments that were suitable only for planting (possibly).

As with cars, as you go up the ladder in rarity it's natural to become more and more critical. Without having done the research, I'd go out on a limb and say that there were several luthiers who equalled Stradivari, but then few people outside the profession have heard of Peter of Mantua (of course I know that isn't true here...). Francesco Ruggeri comes to mind, as do (of course) Montagnana, Gofriller and Tecchler, and while they may not have been exquisite knife men, the sound of their instruments is unsurpassed, and of course that's what most of us are willing to pay for (assuming daddy was a successful cardiologist...) Just look at the difference between a Strad and a del Gesu, and you'll get my meaning.

One last thing to keep in mind: while we look at prices of the old stuff, it is much easier to buy than sell, and you better hope for a degree of inflation as the dealer takes his 20% off the top (no offense to anyone here...)

Well, now I've forgotten the point I was trying to make, but that's characteristic of what happens at my age, and if y'all are lucky, you'll get to experience it in due time.

Now back to your ring announcer.

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I believe they are, if I'm understanding you correctly. I think that's what creates Tartini tones (even though it's often called a phychoacoustic effect), and the same type of pressure pulses would show. Again, I'd need to check this. It was quite a while ago that I experimented with all this.

John, FFT only recognizes sounds which meet a certain set of definitions, such as a certain kind of wave shape?

An fft will describe sound as a bunch of sine waves of different frequencies. If you add up the right frequency sine waves in the right proportions you can create sawtooth waves and square waves and any other wave. Even the beat frequencies are included in the fft, they just aren't obvious. The beat frequencies (and Tartini tones) come about from adding sine waves of different frequencies, so they are included on the fft but not as a spike at the beat frequency or pitch of the Tartini tone. The beat frequencies can be determined by calculating the difference between any two peaks on the fft. Like so many other things, ffts require interpretation in order to get all of the information out of them. We have a habit of just focusing on the frequencies of the peaks on an fft display but really we should be looking at the differences between each of the peaks too since that is something that human ears pick up on.

There are tricks that you can play to get beat frequencies and Tartini tones to show up in an fft-like plot but I haven't played with it much.

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Same result.. there is an audible and countable difference tone, but no difference tone that I can find on the FFT.

Is there any way you can show us what you are counting... i.e. what the pressure trace looks like?

I tried the sound samples on the site that Anders posted (thanks... great site), and they mentioned that the difference tones were best perceptible at higher volume. I tried it loud, and the difference tone was perceptible. Lower volume, and it wasn't. To me, that says something in the ear/brain is creating the effect. Also, just from knowing what it is (two sine waves played simultaneously), there would be no FFT signal at the difference tone, and the difference tone is really just a high-speed beat. When they add, there will be pressure peaks, and when they subtract, the pressure will go to zero. If you count only the pressure peaks, it would space out at the difference tone.

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Is there any way you can show us what you are counting... i.e. what the pressure trace looks like?

It looks like the picture Johnston put up in post #261. You can see that there are many individual lines representing the higher frequencies, but they combine in a way to produce periodic pressure pulses at a lower frequency. If the window is long enough to count many of them, and the window includes a time scale, you can count how many of them there are in one second, and determine the frequency of the "tartini" or "heterodyne" or "combination" note, which will be the same as what you hear.

If the interval between the notes is larger, there will be fewer individual spikes composing the pulse, and the combination note will be higher. (That's a trend, I haven't checked it for all scenarios)

I tried it loud, and the difference tone was perceptible. Lower volume, and it wasn't. To me, that says something in the ear/brain is creating the effect.

To me, since there is evidence for actual periodic pressure pulses conveyed through the air at those frequencies, it's more than just an ear/brain thing. It fits the definition of real sound.

Right now, I think there is a lot of information which can be extracted from simple graphic representations of sound files, which is more difficult to extract from FFTs.

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So is it fair to say that not all cyclic air compression and rarefaction, conveyed through the air (which I think is what both microphones and our ears mechanically respond to), shows up directly on typically configured FFTs, without further interpretation required?

Wikipedia would have a decent article on Fourier Transforms. I don't know that the extra "F" adds beyond frequent sampling.

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To me, since there is evidence for actual periodic pressure pulses conveyed through the air at those frequencies, it's more than just an ear/brain thing. It fits the definition of real sound.

The pressure variations that you are talking about are real but they are just the result of adding other waves together, all that is needed to describe them is knowledge of the two original waves. I don't know if this was linked to earlier but it should be linked to at some point because it is what you are observing, http://en.wikipedia.org/wiki/Interference_(wave_propagation) This is a fairly broad subject. You can add waves of different frequencies together and get beats and Tartini tones. If you have two waves created at two different points in a room, but same frequency, then as you walk around the room there will be places where the waves add together into a loud sound and other places where the two waves cancel and produce no sound. This is essentially why the sound of a violin is very directional at high frequencies, of course I have left many details out. Lots of fun things happen when you add waves together.

Beat frequencies, http://en.wikipedia.org/wiki/Beat_(acoustics)

This one is more interesting. This is a way to hear beat frequencies when the two tones are not even added together indicating that some component, not all, of beat tones really are in our heads, http://en.wikipedia.org/wiki/Binaural_beat

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It looks like the picture Johnston put up in post #261. You can see that there are many individual lines representing the higher frequencies, but they combine in a way to produce periodic pressure pulses at a lower frequency. If the window is long enough to count many of them, and the window includes a time scale, you can count how many of them there are in one second, and determine the frequency of the "tartini" or "heterodyne" or "combination" note, which will be the same as what you hear.

To me, since there is evidence for actual periodic pressure pulses conveyed through the air at those frequencies, it's more than just an ear/brain thing. It fits the definition of real sound.

I think we have too many names for the same thing. Tartini=heterodyne=combination=difference... they are all technically just a beat phenomenon. You will see areas of high amplitude and low amplitude, at the perod equal to the difference of the sine waves that create it. And it can sound like a tone, but technically it isn't actually a "frequency" of any sine wave component... it's the frequency of the beat. So FFT's won't show it, 'cuz they're showing the pure sine signal components of the sound (in this case).

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You will see areas of high amplitude and low amplitude, at the perod equal to the difference of the sine waves that create it. And it can sound like a tone, but technically it isn't actually a "frequency" of any sine wave component... it's the frequency of the beat. So FFT's won't show it, 'cuz they're showing the pure sine signal components of the sound (in this case).

Frequencies are communicated to our ears as pressure variations. High and low amplitude are pressure variations too, so they also can communicate frequency information. Both represent real sound.

If FFTs aren't reliable for easily demonstrating all forms of real sound, and this isn't widely known, is there too much reliance on them?

Edit: I should have added that there are some frequency processing components in the ear that work differently than a microphone, but they still function on the delivery of timed pressure pulses.

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If FFTs aren't reliable for easily demonstrating all forms of real sound, and this isn't widely known, is there too much reliance on them?

Like any tool, you have to know what it does and doesn't do, in order to use it properly. It is an incredibly useful tool, at least for me. I don't pretend to know all the details of how FFT's work, but the general idea is good enough for what I do with it... basic impact spectra and bowed note spectra. Trying to hunt for difference tones with an FFT is like using a Sawzall to sew on a button... it might sortof move like a button-sewing machine, but it ain't gonna work. (I just KNOW you're going to try to prove me wrong!)

How the heck did we get here? This sure has nothing whatsoever to do with soloist instruments, does it?

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Boy, I gotta' admit that after trying to read this thread thru from beginning to end (well, I got to about page 9...),I've discovered there are a few really sarcastic muthas on this board, smart though some of you undoubtedly are!

I've tried many, many instruments over 4 decades, and if it helps this discussion along at all, I'll say that I decide if I generally like the sound of an instrument within the first 30 seconds (think of the women in a fern bar...), I'm looking for a certain "sizzle" (if you will...I don't know what else to call it). Unfortunately, the acoustics in the typical violin showroom can be wildly mis-leading. If I understand the conversation so far, I have to say that it is this "sizzle" we all look for, because it indicates our sound will project. As always, it's hard to describe sound in words, but like the Supremes on discussing pornography, "I know it when I see [hear] it."

(Besides, we have to take the fiddle home to make sure every single friend agrees with our highly-attuned ears, and to offer their opinion as to why we shouldn't buy the thing anyway. BTW, the very same thing happens re bows...)

But I digress...

It should be noted that this projecting quality has nothing to do with price. I'm sure a modern-day luthier, with all his training, research, expertise, talent etc., can make an excellent, projecting instrument, while I've played (monstrously) expensive instruments that were suitable only for planting (possibly).

As with cars, as you go up the ladder in rarity it's natural to become more and more critical. Without having done the research, I'd go out on a limb and say that there were several luthiers who equalled Stradivari, but then few people outside the profession have heard of Peter of Mantua (of course I know that isn't true here...). Francesco Ruggeri comes to mind, as do (of course) Montagnana, Gofriller and Tecchler, and while they may not have been exquisite knife men, the sound of their instruments is unsurpassed, and of course that's what most of us are willing to pay for (assuming daddy was a successful cardiologist...) Just look at the difference between a Strad and a del Gesu, and you'll get my meaning.

One last thing to keep in mind: while we look at prices of the old stuff, it is much easier to buy than sell, and you better hope for a degree of inflation as the dealer takes his 20% off the top (no offense to anyone here...)

Well, now I've forgotten the point I was trying to make, but that's characteristic of what happens at my age, and if y'all are lucky, you'll get to experience it in due time.

Now back to your ring announcer.

Right, violins 30 sec., candy that hits the floor 5, got it :lol:

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