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# Learning more about the science of sound - Page 15

Quote:
Originally Posted by RevMen

Very small correction that doesn't change the meaning of your post in any way.... sound pressure level (SPL) is a ratio of the sound pressure squared to the reference sound pressure squared.  Also "sound pressure level" specifically refers to the dB representation of sound pressure.  In acoustics, when you say "level" it's implied that you're referring a dB representation.  Lp (SPL), Ldn, Lmax, Lmin, Ln, Leq, Lw are all "level" and they're all in dB.

Hm, "measure" is probably not really the right word to begin with.

You know, I don't think I've seen it notated with squares, at least in a while.  I usually just see 20 log10(A / B) when looking at ratios of field quantities like voltage (like pressure), rather than 10 log10(A^2 / B^2), which is of course mathematically the same.  On another note, seems like convention is on the side of sloppy shorthand notation with regards to implicit reference levels and so on.  Can't blame them; I'm guilty of that all the time.

Right, the shortcut for SPL is to just say 20*log(P/Pref).  But a decibel is 10*log(X/Xref), and that's what we use for Sound Power (where the reference quantity is 10^-12 W), so there is a difference there that does sometimes come into play.

Really, though, it ends up not being a very important distinction since the vast majority of sound pressure calculations we do in pressure squared anyway.  In 10 years in acoustics I've probably used naked sound pressure 2 times.  Maybe 1.

Quote:
Originally Posted by RevMen

Right, the shortcut for SPL is to just say 20*log(P/Pref).  But a decibel is 10*log(X/Xref), and that's what we use for Sound Power (where the reference quantity is 10^-12 W), so there is a difference there that does sometimes come into play.

Really, though, it ends up not being a very important distinction since the vast majority of sound pressure calculations we do in pressure squared anyway.  In 10 years in acoustics I've probably used naked sound pressure 2 times.  Maybe 1.

That makes sense. Cheers!

I was looking into purchasing the O2 and ODAC. Do you guys agree with NWAVGUY's analysis of the two products that they are completely audibly transparent (on par with the benchmark DAC Pre 1). I am hearing a lot of anti ODAC and O2 comments when users compare them to their more expensive gear. I am putting them down to the usual placebo effects but am still slightly worried :S? Or am I succumbing the usual BS head-fi trend where expensive is better?

Edited by uchihaitachi - 5/26/13 at 10:41am

Sorry for the double post, another query.

I was using the built in iTunes function and converted a 128kb mp3 to a 320kb mp3. How is this possible. Itunes managed the conversion and the newly converted 320kb mp3 seems to be a larger file size than the 128kb as if extra data appeared out of nowhere?!

It's decoding back into 1411.2 kbps PCM (as it would for playback; all the info still gone and artifacts still there) and re-encoding that version to 320 kbps mp3.

Quote:
Originally Posted by mikeaj

It's decoding back into 1411.2 kbps PCM (as it would for playback; all the info still gone and artifacts still there) and re-encoding that version to 320 kbps mp3.

SO I assume the extra data is just noise... :(

I'd suggest not to think of it that way, as CD -> 128 kbps encode -> 128 kbps transcode again would produce different (and worse) results than only the first 128 kbps encode.

In your situation it's more like you have a 320 kbps encode that approximates the 128 kbps version.  It's not like there is 192 kbps of data / second representing noise or distortion that is appended to the original 128 kbps.

Quote:
Originally Posted by uchihaitachi

So the stereo crosstalk of 30 db would be the ratio of desired to undesired signals?

Yes.

30 dB represents a ratio of about 31.62 to 1. So if your desired signal is 1 volt, then your undesired signal would be 1/31.62 or 0.03162 volts.

To figure out the ratio from a decibel figure, take the decibel figure and divide it by 20. Then take 10 to the power of that result. That gives you the ratio. So for 30 dB, that would be 30/20, or 1.5. 10^1.5 is 31.62. To check your result, take the log of that and multiply by 20. So log(10)31.62 is about 1.5. 1.5 times 20 is 30.

se

Out of curiosity, when engineers test their audio gear, why do they only use square waves. Can't they use actual music?

I hear a lot of people complaining measurements are not legit as they are too simplistic. You can test square waves but that doesn't tell you how the DAP will handle actual music. Is there any validity to this statement or can one draw many conclusion from just a single square wave test?

Quote:
Originally Posted by uchihaitachi

Out of curiosity, when engineers test their audio gear, why do they only use square waves. Can't they use actual music?

We don't just use square waves, that's only one possible test waveform, and though popular, it is difficult to correlate the image of a distorted square wave with an audible effect.  It's useful for testing some specific aspects of performance, though.  We also use sine waves, because it's easy to determine and quantify the changes a device makes to a pure sine wave.  We also use various other signals, broad-spectrum noise, tone bursts, impulses, swept sine waves, mixes of more than one sine wave, or sine and square, the list is long.  All of these signals have something in common, though: the original is known, so any changes a device makes on them will be measurable or observable.  Most are actually quantifiable.

We don't usually use music because the original is not well known, or predictable, and music is non-cyclic, at least in the sense of a measurable signal.  It can be used in some cases, and has been, but it's very difficult to use and expect quantifiable results. However, nobody measuring audio gear should or would not listen to it with a variety of music types.

Quote:

Originally Posted by uchihaitachi
I hear a lot of people complaining measurements are not legit as they are too simplistic.

The key to understanding the complaint is the understanding of the position of the one making it.  Often the two sides posture, and refuse to even listen to the opposite opinion.  However, many of the tests we do today are a result of hearing something we didn't properly measure.

Quote:
Originally Posted by uchihaitachi

You can test square waves but that doesn't tell you how the DAP will handle actual music. Is there any validity to this statement or can one draw many conclusion from just a single square wave test?

Yes, there's a lot of validity to that.  A square wave test is hyper-sensitive to a lot of mechanisms that result in square wave deformity, but are absolutely not audible.  It's not a good test by itself, it's really only helpful when taken in the context of other types of tests.

There are many tests that do represent a devices ability to reproduce actual music.  Test wave-forms may be repetitive and non-musical, but they do reveal very audible distortions.

Quote:

The key to understanding the complaint is the understanding of the position of the one making it.  Often the two sides posture, and refuse to even listen to the opposite opinion.  However, many of the tests we do today are a result of hearing something we didn't properly measure.

Yes, there's a lot of validity to that.  A square wave test is hyper-sensitive to a lot of mechanisms that result in square wave deformity, but are absolutely not audible.  It's not a good test by itself, it's really only helpful when taken in the context of other types of tests.

There are many tests that do represent a devices ability to reproduce actual music.  Test wave-forms may be repetitive and non-musical, but they do reveal very audible distortions.

So Jaddie, would you say the entire mainstream range of tests an engineer uses to check audio gear pretty much covers every nook and cranny? Ie to an extent that the 'tests are not enough when actual music is not used' party really don't have any validity to their claims?

Quote:
Originally Posted by uchihaitachi

So Jaddie, would you say the entire mainstream range of tests an engineer uses to check audio gear pretty much covers every nook and cranny? Ie to an extent that the 'tests are not enough when actual music is not used' party really don't have any validity to their claims?

Every nook and cranny?  Hard to say. Most of the tests we now use are 30 years old or older, and outside of using computers and software, we aren't really doing much testing that's different today.  There've been no significant new tests added since TIM surfaced, became popular to test for, and then later was summarily dismissed.    Remember, FFT analysis was around in the late 1970s, and I was driving an old Crown (Techron) TEF analyzer in the 1980s.

But ask yourself this: if we test with music as the test signal, what is the analysis mechanism?  All we can really do with software (or hardware for that matter) is analyze the total spectrum over time, and look for a difference from the original, which may or may not tell the whole story.  But that's not what you're getting at, is it?

The claim you refer to is usually based on listening tests vs measurements, and that's a slippery slope.  Listening tests can be quite valid and revealing, but not unless expectation bias is eliminated from the test using some form of double-blind test methodology.  When that is done, the ability to detect differences diminishes dramatically. The "listening is all that matters" group usually blanches at the double-blind test methodology as being too stressful, or forcing a quick decision, and thus clouding otherwise easily discernible differences.  But it's also undeniable that expectation bias has a powerful and measurable effect on judgement, and for sensory based judgement to be accurate, bias must be removed.

The problem here, really, is one of belief systems.  Even though my own lean strongly to scientific method, I also don't enjoy bursting peoples bubbles.  If someone hears something wonderful, even if I could prove it's imagined, why would I?  There's really no point.  My reality is based on the tangible, provable, and repeatable.  Someone else's reality may be based on legend, myth, pseudo-science, visual suggestion, or a whole palette of other influences.  If we both have the enjoyment of good sound as our goal, I'm no longer certain that it matters much which is right.  I've pursued truth in audio for over 4 decades, and yet I still see people becoming ecstatic listening to high-end systems with no measurable (or provable) advantage over something far more common.  I follow the one principle I absorbed from my high-school economics teacher: Economics is maximizing happiness!  What that means for me is as valid as what that means for someone else.  For some, buying is highly enjoyable.  Buying expensive and exotic things is a very powerful tweak of the pleasure center.  It's as valid as maximizing sound quality for the minimum expense, or any viewpoint in-between.

Hows that for dodging the bait?

Quote:

Every nook and cranny?  Hard to say. Most of the tests we now use are 30 years old or older, and outside of using computers and software, we aren't really doing much testing that's different today.  There've been no significant new tests added since TIM surfaced, became popular to test for, and then later was summarily dismissed.    Remember, FFT analysis was around in the late 1970s, and I was driving an old Crown (Techron) TEF analyzer in the 1980s.

But ask yourself this: if we test with music as the test signal, what is the analysis mechanism?  All we can really do with software (or hardware for that matter) is analyze the total spectrum over time, and look for a difference from the original, which may or may not tell the whole story.  But that's not what you're getting at, is it?

The claim you refer to is usually based on listening tests vs measurements, and that's a slippery slope.  Listening tests can be quite valid and revealing, but not unless expectation bias is eliminated from the test using some form of double-blind test methodology.  When that is done, the ability to detect differences diminishes dramatically. The "listening is all that matters" group usually blanches at the double-blind test methodology as being too stressful, or forcing a quick decision, and thus clouding otherwise easily discernible differences.  But it's also undeniable that expectation bias has a powerful and measurable effect on judgement, and for sensory based judgement to be accurate, bias must be removed.

The problem here, really, is one of belief systems.  Even though my own lean strongly to scientific method, I also don't enjoy bursting peoples bubbles.  If someone hears something wonderful, even if I could prove it's imagined, why would I?  There's really no point.  My reality is based on the tangible, provable, and repeatable.  Someone else's reality may be based on legend, myth, pseudo-science, visual suggestion, or a whole palette of other influences.  If we both have the enjoyment of good sound as our goal, I'm no longer certain that it matters much which is right.  I've pursued truth in audio for over 4 decades, and yet I still see people becoming ecstatic listening to high-end systems with no measurable (or provable) advantage over something far more common.  I follow the one principle I absorbed from my high-school economics teacher: Economics is maximizing happiness!  What that means for me is as valid as what that means for someone else.  For some, buying is highly enjoyable.  Buying expensive and exotic things is a very powerful tweak of the pleasure center.  It's as valid as maximizing sound quality for the minimum expense, or any viewpoint in-between.

Hows that for dodging the bait?

Haha nice. Utility maximisation ironically is what I am doing right now. Bloody microeconomic principles II EC202. FML lol.

But I must say utility maximising via wasting money on a distorted perspective seems very saddening.

Quote:
Originally Posted by uchihaitachi

Haha nice. Utility maximisation ironically is what I am doing right now. Bloody microeconomic principles II EC202. FML lol.

But I must say utility maximising via wasting money on a distorted perspective seems very saddening.

Sad from a particular viewpoint, extremely happy from another.  I once calibrated a system that cost \$350,000.  The client so happy when I was done he was almost in tears.  But, if he'd asked me how I would have spent the money, I'd have had to tell him, "\$50,000 on the gear and room, \$300K on a nice summer home", and he'd have had better sound.  But it was his money, and he was very, very happy.

I got paid, so I was too.

Two of my favorite quotes:

"Happiness is a choice"

"That which you think becomes your world"

Quote:

Sad from a particular viewpoint, extremely happy from another.  I once calibrated a system that cost \$350,000.  The client so happy when I was done he was almost in tears.  But, if he'd asked me how I would have spent the money, I'd have had to tell him, "\$50,000 on the gear and room, \$300K on a nice summer home", and he'd have had better sound.  But it was his money, and he was very, very happy.

I got paid, so I was too.

Two of my favorite quotes:

"Happiness is a choice"

"That which you think becomes your world"

What was the system out of curiosity? And do you not feel epic face palm?

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