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Misleading Measurements


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11 hours ago, Clockmeister said:

From your own point of view and others my suggestion would be do not dismiss any products until you have auditioned them in your own listening environment and lived with it for a good week or so, then only you can make the decision on whether you wish to purchase said device.

 

While this is unquestionably good advice, it is more easily said than done. Finding the DACs you wish to audition locally can present a challenge, as is finding a dealer who will allow a money-back home audition. DACs sold directly by the manufacturer are more likely to be offered for a home trial.

"Relax, it's only hi-fi. There's never been a hi-fi emergency." - Roy Hall

"Not everything that can be counted counts, and not everything that counts can be counted." - William Bruce Cameron

 

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11 hours ago, Clockmeister said:

 

Fas42

 

Pretty basic requirements there, if this is want you wish for then DSP is your friend I believe its been on the market for one form or another for many years in many products, whether it accurately does this is another matter 🤔

 

Ohhh ... I thought the magic occurred in how the raw DAC worked - people mention how particular items did this circus trick, I think MSB DACs were mentioned, and that this is faulty reproduction... a naive chap like me come to the simple conclusion that this in fact is what the recording actually contains, and that units that don't present this information are the ones that are actually faulty ... sorry to misunderstand, 😉.

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21 hours ago, Clockmeister said:

From your own point of view and others my suggestion would be do not dismiss any products until you have auditioned them in your own listening environment and lived with it for a good week or so, then only you can make the decision on whether you wish to purchase said device. My other suggestion would be take your time, do audition a few other devices never make knee jerk reactions and always consider yourself the prime mover in the decision to buy, not five chaps  from a forum who swear its great, only you can  do this within your own time and your own listening space, have faith in your own convictions. But most of all enjoy your music

Thanks for your reply @Clockmeister.  In an ideal world we could audition what we are interested in but this is not always possible, so I consider listening recommendations from multiple vantage points most valuable.

 

In regards to measurements, I presume any commercial component will have been measured by the manufacturer to satisfy the basics - isn't there a common industry bench mark by now? So I'm puzzled by often coming across references to 'it measures well but sounds bad' and vice versa, and wonder what this means in the scheme of things for the average consumer. And why others are so dogmatic about measurements, even seeming to claim a moral high ground.

Topaz 2.5Kva Isolation Transformer > EtherRegen switch powered by Paul Hynes SR4 LPS >MacBook Pro 2013 > EC Designs PowerDac SX > TNT UBYTE-2 Speaker cables > Omega Super Alnico Monitors > 2x Rel T Zero Subwoofers. 

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3 hours ago, bluesman said:

 

I suspect there are measurable parameters (e.g. phase differences?) that would enable separation of IM products into those on the recording and those created by the same instrumental interactions on playback.  This could be a new opportunity to make a truly meaningful measurement

 

I'm probably being naive, but this sounds overcomplicated. Why not create an Audacity file with 4k and 5k tones, and do a mic measurement at speakers and listening position to see if you get 1k products and at what level?

 

You can make this as multi-tonal as you like to simulate the complexity of music.

One never knows, do one? - Fats Waller

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. - Einstein

Computer, Audirvana -> optical Ethernet to Fitlet3 -> Fibbr Alpha Optical USB -> iFi NEO iDSD DAC -> Apollon Audio 1ET400A Mini (Purifi based) -> Vandersteen 3A Signature.

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4 hours ago, bluesman said:

Maybe this will help clear up the mystery.  It's a piece from a researcher at UConn on intermodulation products that's oriented toward music.  And this is a nice little piece on the physics of intermodulation, beat frequencies, and Tartini tones.  The more I read, the more I believe that I'm probably correct.  I know and have shown in the clip I attached to a prior post that natural acoustic intermodulation among instruments is definitely real, audible, and captured by microphones,  And its creation anew on playback could easily have an effect on SQ. For example, the second linked article suggests that intermodulation products in the sub-60 Hz range may cause muddiness.

 

Consider this. The audible beating, to our ears, also impacts other, real materials which are part of the musical instrument. Or the general environment around the music making. And causes that material to vibrate at the beat frequency; because it's reacting to the ebb and flow of transmitted energy. And therefore real, detectable energy at those frequencies will now exist in the air.

 

With regard to Jud's suggestion, consider a thin timber panel being in the region of where the speaker and microphone are set up, versus not being there at all - would readings in the two situations show the same spectrum detail?

 

 

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1 hour ago, fas42 said:

 

Consider this. The audible beating, to our ears, also impacts other, real materials which are part of the musical instrument. Or the general environment around the music making. And causes that material to vibrate at the beat frequency; because it's reacting to the ebb and flow of transmitted energy. And therefore real, detectable energy at those frequencies will now exist in the air.

 

With regard to Jud's suggestion, consider a thin timber panel being in the region of where the speaker and microphone are set up, versus not being there at all - would readings in the two situations show the same spectrum detail?

A solid body guitar is a slab of wood (solid maple in the case of my little experiment) that weighs about 8 pounds and has no resonant cavities in it other than a small opening about 1" deep, 4" long and 2" wide for the controls. The wavelength of a 40 Hz tone is 28 feet.   The heaviest string on a 5 string bass is tuned to a low B, which is 30 Hz at concert pitch.  The lowest string on a 6 string guitar is an 82 Hz E, so that won't resonate at 26 to 30.  The low A on my 7 string guitars is 55 Hz, so that wouldn't do it either.  So you could be correct if I'd done this on a 5 string bass - but I didn't.

 

There is absolutely no way that a 26 to 30 Hz tone is emanating from a normally strung, normally tuned solid body guitar other than as intermodulation between two of the strings.  And there may be information in there that can be used to develop a new and hopefully meaningful measurement to quantify "primary" (i.e. captured from the program) and "secondary" (i.e. generated during playback) intermodulation products.  Interestingly, if this turns out to be doable, the IMD of every component in a system will have to be mighty low if DSP stands a chance of removing the secondary acoustic IM products.

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1 hour ago, bluesman said:

A solid body guitar is a slab of wood (solid maple in the case of my little experiment) that weighs about 8 pounds and has no resonant cavities in it other than a small opening about 1" deep, 4" long and 2" wide for the controls. The wavelength of a 40 Hz tone is 28 feet.   The heaviest string on a 5 string bass is tuned to a low B, which is 30 Hz at concert pitch.  The lowest string on a 6 string guitar is an 82 Hz E, so that won't resonate at 26 to 30.  The low A on my 7 string guitars is 55 Hz, so that wouldn't do it either.  So you could be correct if I'd done this on a 5 string bass - but I didn't.

 

There is absolutely no way that a 26 to 30 Hz tone is emanating from a normally strung, normally tuned solid body guitar other than as intermodulation between two of the strings.  And there may be information in there that can be used to develop a new and hopefully meaningful measurement to quantify "primary" (i.e. captured from the program) and "secondary" (i.e. generated during playback) intermodulation products.  Interestingly, if this turns out to be doable, the IMD of every component in a system will have to be mighty low if DSP stands a chance of removing the secondary acoustic IM products.

 

Look at that spectrum you posted - the intermodulation tone is down about 45dB, from the fundamentals of the strings; that's of the order of 1% of the level of provoking frequencies. You can be 100% sure that there is absolutely nothing in the structure of the guitar, that's going to vibrate from the energy of the strings, and is ever so slightly non-linear in its vibrational behaviour, to the degree that such a frequency node will never emerge; you are completely confident in the perfection of the construction of the guitar, that this can't happen?

 

If I tap the body of the guitar with my finder tip, is that completely inaudible? And if not, what energies occur, at all frequencies in the audible range?

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11 hours ago, bluesman said:

That's what I did.  Listen to the wav file and look at the spectrum to see the results.  There's absolutely no way to explain a peak between ~26 and ~30 Hz except as the IM product of a ~220 Hz tone and a ~246 Hz tone.  There's no structure in a solid body guitar that could possibly resonate anywhere near that low.  And you can clearly hear the beat frequency of the "difference" IM product.

 

The important thing to factorinto this is that the same intermodulation does not seem to occur between digital signals that we find with analog signals.  So if it's done with a digital sine wave generator and the tones are pumped directly into Audacity via digital input, you will not get the same results.

 

I can't understand how this is so controversial - it's intuitively obvious and it's demonstrated very nicely in my simple effort plus hundreds of others you can find on the web.

 

9 hours ago, pkane2001 said:

 

A guitar is a non-linear device. It will generate IM. As will a human voice, as will a piano. All of these are analog devices, and are much, much more non-linear than, say, a microphone, which is also not perfectly linear, being an analog device. Nonlinearities generate IM. There will be IM frequencies generated by a guitar, and that's a given. These will be recorded by any decent microphone if they occur within the range of the microphone. There's no question about that, either: I've recoded and analyzed these many hundreds of times from various analog devices.  And if you record two digitally generated tones purely in the digital domain, there will not be the same IM products... because there is no large non-linearity in the signal path in this case.

 

It's not controversial, and no new explanation required: this is how nonlinearity works in the analog world. 

 

Thanks for the explanations. In this formulation, speakers are analog and non-linear, so the concerns regarding a "second round" of intermodulation, as evidenced by @bluesman's listening/measuring, appear to have some support, though I believe @pkane2001 is attributing most or all of this effect to mics rather than speakers. Of course this narrows down the concern to recordings made with microphones from analog sources, which isn't much narrowing down at all. The same will happen when listening to these sources "live," but whether it's an accurate representation of what one would hear live, who knows? @bluesman is probably as well equipped as anyone to answer that, having played live music professionally for decades.

One never knows, do one? - Fats Waller

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. - Einstein

Computer, Audirvana -> optical Ethernet to Fitlet3 -> Fibbr Alpha Optical USB -> iFi NEO iDSD DAC -> Apollon Audio 1ET400A Mini (Purifi based) -> Vandersteen 3A Signature.

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11 hours ago, bluesman said:

That's what I did.  Listen to the wav file and look at the spectrum to see the results.  There's absolutely no way to explain a peak between ~26 and ~30 Hz except as the IM product of a ~220 Hz tone and a ~246 Hz tone.  There's no structure in a solid body guitar that could possibly resonate anywhere near that low.  And you can clearly hear the beat frequency of the "difference" IM product.

 

The important thing to factorinto this is that the same intermodulation does not seem to occur between digital signals that we find with analog signals.  So if it's done with a digital sine wave generator and the tones are pumped directly into Audacity via digital input, you will not get the same results.

 

I can't understand how this is so controversial - it's intuitively obvious and it's demonstrated very nicely in my simple effort plus hundreds of others you can find on the web.

 

If you can find a recording of two acoustic instruments, one in each channel, and listen via speakers then headphones, it would be interesting to see what the differences are.

One never knows, do one? - Fats Waller

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. - Einstein

Computer, Audirvana -> optical Ethernet to Fitlet3 -> Fibbr Alpha Optical USB -> iFi NEO iDSD DAC -> Apollon Audio 1ET400A Mini (Purifi based) -> Vandersteen 3A Signature.

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8 hours ago, fas42 said:

Look at that spectrum you posted - the intermodulation tone is down about 45dB, from the fundamentals of the strings; that's of the order of 1% of the level of provoking frequencies

That's exactly where it should be.  The intensity of the f2-f1 tone can range from -10dB to -50dB, depending on the absolute and relative intensities of the primary tones, their individual frequencies, and the delta between them.

 

8 hours ago, fas42 said:

If I tap the body of the guitar with my finder tip, is that completely inaudible? And if not, what energies occur, at all frequencies in the audible range?

Of course it's audible.  Many luthiers who build acoustic guitars use a technique called tap tuning to guide the carving and bracing of their tops, which are designed and constructed to have specific resonances.  The artful luthier will gently tap the top at multiple spots as he or she carves and shapes it, using the sound as a guide to further manipulation until satisfied that it will sound as desired.

 

OTOH, tapping the top of a solid body guitar is like tapping a solid block of wood.  The resultant sound is a spectral splash, and resonances are essentially nil, except for an occasional stray midrange vibration.  This has been studied by many researchers like Eric Moon, a physicist at the University of Illinois.  In one really cool experiment, he studied and compared resonances in hollow body acoustic guitars and solid body electric guitars. Here's the bottom line:

 

"Resonances at 90 Hz, 180 Hz, and 786 Hz were discovered on the body of a 1968 Gibson J-45 Acoustic. [A] resonance was discovered at 325 Hz on an Ibanez RG570 solid-body electric guitar. The results suggest that electric guitars do not vibrate in the same way as acoustic guitars. In addition, no harmonic relationships were found among resonance frequencies."

 

The 325 Hz resonance in the solid body guitar is probably from hardware or from interaction between a piece of hardware and the wood.  For example, there's a rod (usually metal and usually adjustable) running through the neck of most electric guitars to stiffen it and keep it in proper orientation and alignment with the body.  A loose or broken truss rod is a relatively common cause of extraneous resonance, as are parts in the pickups, control cavity etc.  But none of these resonances (all of which are undesirable) is below the range of 150-200 Hz and most are above.  These are among the meat and potatoes repair jobs done by luthiers and guitar repair people, because they're audible and amazingly annoying.  They can interact with the note(s) being played to affect timbre, intonation, and the overall SQ of the instrument.

 

Seemingly similar solid body guitars can have very different acoustic and electric sounds.  What seems to determine the sound quality of a solid body guitar is primarily impedance and coupling between parts with different impedance.  The interfaces between guitar parts with different acoustic impedances results in partial reflection of the sound waves. Putting a veneer top over a solid body creates a huge interface that will affect the sound in proportion to the difference in acoustic impedance between the two woods. 

 

Phase shifts and directivity of the reflected waves interact with string vibrations to shape and color the tone and timbre of the guitar. So a saddle bridge will sound different from a stop tail bridge on the same guitar.  Raising the bridge changes the acoustic coupling between strings and body, so it changes the sound.  The angle of the peghead (which varies from maker to maker) affects sound for the same reason.  So do the type of tailpiece used and the hardware fastening it to the body.

 

The wood itself has less impact on the sound than the above - but it definitely has an impact, mostly through frequency specific dissipation of the energy of the vibrating strings.  But there should be no resonances in any solid body guitar below 100 Hz.

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43 minutes ago, Jud said:

 

 

Thanks for the explanations. In this formulation, speakers are analog and non-linear, so the concerns regarding a "second round" of intermodulation, as evidenced by @bluesman's listening/measuring, appear to have some support, though I believe @pkane2001 is attributing most or all of this effect to mics rather than speakers. Of course this narrows down the concern to recordings made with microphones from analog sources, which isn't much narrowing down at all. The same will happen when listening to these sources "live," but whether it's an accurate representation of what one would hear live, who knows? @bluesman is probably as well equipped as anyone to answer that, having played live music professionally for decades.


No, all analog devices are guilty. Speakers too. I didn’t mention speakers because I assumed one was not involved in capturing from a mic in @bluesman’s scenario. 
 

 

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50 minutes ago, Jud said:

 

If you can find a recording of two acoustic instruments, one in each channel, and listen via speakers then headphones, it would be interesting to see what the differences are.

I've done that.  Interestingly, the first order interaction products (sum and difference) are audible from both sources but seem to me to be louder, relative to the fundamentals, through speakers.  And the sound itself is different, although I can't identify a specific difference other than sound pressure levels.  I used to have access to a state of the art acoustics / audiology facility when I was a young faculty member and could have run spectrum analysis on both, to see what's what.  Unfortunately, my current university does not have an audiology research group at that level. As I don't have the equipment to analyze headphone output, I can't compare the spectra.

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Looking up things a bit further, one discover nonlinear acoustics - this relates to the situation, as John mentioned, that when intense pressures are involved that air as a medium for transmitted sound does become non-linear. Which can be exploited for doing interesting things. But otherwise it behaves as a linear system, https://acousticstoday.org/the-world-through-sound-linearity/.

 

I'm still not seeing anything that supports the idea that vibrations in air at normal, everyday amplitudes can create difference tones that will registered by a microphone.

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3 hours ago, fas42 said:

I'm still not seeing anything that supports the idea that vibrations in air at normal, everyday amplitudes can create difference tones that will registered by a microphone.

Maybe this will convince you.  I ran the simultaneous ~220 + ~246 Hz guitar tone wav (for which I posted the spectrogram and wav file in an earlier post) through multiple low pass filters, from a steep shelving drop off to a 6 dB per octave slope.  The intermodulation products remain, despite having removed the fundamentals.  Look at the difference peaks.  Remember that the most prominent tones are at f1-f2, f1+f2, f2-f1, and f2+f1 (which is the same as F1+f2).  The sum tone is being attenuated by the low pass filter.

 

This further convinces me that they're real, are captured by the mic, and are not a psychoacoustic phenomenon.  Here's the spectrum from the 18 dB/octave filter showing how little of the fundamentals remains - yet the difference tone is still there and quite clearly so:

 

IM_minus_fundamentals.jpg.bf65bead52a1d7cc54b44e7080d7f4c6.jpg

 

BTW, I made the recordings with no filters, processing or other manipulation in open air in my library with the A/C running.  The bottom of the spectrum is largely HVAC rumble.  Our condo has through-the-wall units in each room, so the compressor and circulating fan are 10 feet from the mic and currently running on high.  Here's a spec anal of the HVAC :

HVAC.jpg.c3118359c1eedb6700ce7a41a6426417.jpg

 

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6 hours ago, bluesman said:

I've done that.  Interestingly, the first order interaction products (sum and difference) are audible from both sources but seem to me to be louder, relative to the fundamentals, through speakers.  And the sound itself is different, although I can't identify a specific difference other than sound pressure levels.  I used to have access to a state of the art acoustics / audiology facility when I was a young faculty member and could have run spectrum analysis on both, to see what's what.  Unfortunately, my current university does not have an audiology research group at that level. As I don't have the equipment to analyze headphone output, I can't compare the spectra.

 

What speakers did you use for this, BTW?

One never knows, do one? - Fats Waller

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. - Einstein

Computer, Audirvana -> optical Ethernet to Fitlet3 -> Fibbr Alpha Optical USB -> iFi NEO iDSD DAC -> Apollon Audio 1ET400A Mini (Purifi based) -> Vandersteen 3A Signature.

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36 minutes ago, Jud said:

What speakers did you use for this, BTW?

I ran it with LS3/5as, Focal 726 towers, JBL LSR305s and Edifier R1280DBs, each with and without my Yamaha powered sub in the system.  The cans I used were AKG 701QJs, Sony MDRv150s, and JBL wireless ear buds.  Unfortunately, I no longer have an analog recorder, so I couldn't compare analog to digital captures.  But there was little or no difference in playback using an all digital "powerDAC" (Wadia 151), all tube analog electronics (Prima Luna) behind a DAC (iFi, SMSL, and Emotiva), or various combos of analog and digital electronics.

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7 hours ago, bluesman said:

 

This further convinces me that they're real, are captured by the mic, and are not a psychoacoustic phenomenon.  Here's the spectrum from the 18 dB/octave filter showing how little of the fundamentals remains - yet the difference tone is still there and quite clearly so:

 

IM_minus_fundamentals.jpg.bf65bead52a1d7cc54b44e7080d7f4c6.jpg

 

 

 

 

Thanks. Unfortunately, this still does not distinguish whether the difference tones, created as a real phenomenon, is caused by the air, or by some other object in the recording space ... IOW, what is the most likely candidate for reacting in a non-linear way to the vibrations of the strings, and how do we ascertain that in fact this is what happened?

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6 hours ago, Jud said:

 

What speakers did you use for this, BTW?

 

Speaking of speakers 😎 Remember, I claimed that they are non-linear analog devices, right? Here's an example of this non-linearity causing harmonic distortion (which is the same as IMD, except measured with a single tone, as we discussed already). This is from a very detailed and well-done speaker review by Erin from Erin's Corner. This little speaker produces THD exceeding 1% at normal listening levels around 600Hz, for example:

 

image.thumb.png.8590c807b128aa8c9072314a36fced0d.png

 

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