How do we measure distortion? - AVS Forum
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post #1 of 41 Old 09-23-2008, 07:19 AM - Thread Starter
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I never really figured out how distortion is shown in any measurements.

Ie...can I see a plot of the distortion for my subs or speakers?

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post #2 of 41 Old 09-23-2008, 09:30 AM
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REW can do it. You use the spectrum display and click the calculate distortion plot. Then you use the generator to run a sine wave through the speaker and it will calculate the distortion. You stop the spectrum analyzer to freeze the graph and then save it. It's pretty easy when you get it down. Be careful with the sine waves though. You'll get something like this. This is a distortion reading at 12.5hz the THD is 5.7%. Ideally you would have just one big spike at the fundamental tone being used and nothing else. The second harmonic would be 25hz, third 37.5hz, etc...and the rest is noise.



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post #3 of 41 Old 09-23-2008, 09:35 AM - Thread Starter
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Thanks!

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post #4 of 41 Old 09-23-2008, 09:50 AM
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Also look at the actual levels of the harmonics, looking at the simple THD% number is meaningless.

~Brandon


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post #5 of 41 Old 09-23-2008, 10:01 AM
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When measuring distortion using REW, can you measure distortion with an SPL meter, or do you need to measure with a microphone?
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post #6 of 41 Old 09-23-2008, 11:26 AM
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It should be mentioned that there are a lot considerations for the testing environment and equipment setup you have to solve. Getting good results is not as easy sticking a mic a meter away from a speaker sitting in the middle of a room.

~Brandon


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post #7 of 41 Old 09-23-2008, 11:47 AM
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If you have the right software and use a logarithmic sine sweep, you can make a continuous plot of THD% against frequency. WinMLS can do this among other software packages. It can also show the contributions of individual harmonics at each frequency.

THD% as a pure number unfortunately tells you nothing about how distorted something sounds. You have to take into account the ear's equal loudness contours, transfer function of the ear and psycho-acoustic masking effects to get anywhere near a metric that will tell you how bad a given level of distortion sounds.

Measuring in-room will also completely mess up any objective measurement of distortion level as the room will boost/cut fundamental and individual harmonic levels in a way that renders the results pretty meaningless for anyone but the originator.
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post #8 of 41 Old 09-23-2008, 12:21 PM
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Quote:
Originally Posted by cjwhitehouse View Post

If you have the right software and use a logarithmic sine sweep, you can make a continuous plot of THD% against frequency. WinMLS can do this among other software packages. It can also show the contributions of individual harmonics at each frequency.

THD% as a pure number unfortunately tells you nothing about how distorted something sounds. You have to take into account the ear's equal loudness contours, transfer function of the ear and psycho-acoustic masking effects to get anywhere near a metric that will tell you how bad a given level of distortion sounds.

Measuring in-room will also completely mess up any objective measurement of distortion level as the room will boost/cut fundamental and individual harmonic levels in a way that renders the results pretty meaningless for anyone but the originator.

Thanks, CJW, and nice to see your input in this forum, as always.

You can use TrueRTA to calculate in-room harmonic distortion manually, but as CJ points out, there are sympathetic vibrations and room influences involved.

You can run a sweep to get the in-room FR at the mic position, save it on the graph and refer to it in your calculations of the individual harmonics, adding/subtracting for the rooms boost/cut influences. This, IMO, is a decent enough reference for those who are never going to consider dragging everything out to a parking lot, although much more tedious.

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post #9 of 41 Old 09-23-2008, 06:11 PM
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Quote:


Measuring in-room will also completely mess up

A good thread for beginner likes me. CJW i often see they measure in the cars. is it very bad measurement?(noise and reflection in car)...somo
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post #10 of 41 Old 09-23-2008, 06:47 PM
 
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How do we define the audible distortion? How do we define standards used to measure distortion? How do we differentiate between different sources of audible distortion and a possibility of compounding certain types of distortion? How do we establish consistent limits to consider distortion as a limiting factor? Everything seems to distort in one way or another, why is it ok to call certain types of distortion a tonal signature while manifestation of others may be considered a failure? Why be concerned with measuring what you cannot hear? Why pay more for what you cannot qualify or quantify without using instruments? Why rely on parameters supplied by the manufactures when they do not reflect measurements reflecting actual purpose of the equipment being measured? Someone enlighten me...
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post #11 of 41 Old 09-23-2008, 07:13 PM
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Well gee, you do what any scientist would do: you set up a study that can isolate a variable and then survey how people perceive changes to the variable. With enough testing you can begin to quantify the results and make predictions with good correlation to actual measurements.

Some of the most recent surveys I know of have been done by Toole, Geddes, and Klippel which correlate what we hear with what is measured. Now you are free to disagree with their methods or completeness or results, but the basic idea is simply the Scientific Method which has been used for several centuries...surely you are aware of it...

You seem to think you are so clever, yet your comments are not only tiresome but indicate to me that you were probably stuffed in your locker too many times as a kid and now want to get back at the world by this "clever" banter.

~Brandon


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post #12 of 41 Old 09-23-2008, 07:24 PM
 
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Quote:
Originally Posted by augerpro View Post

Well gee, you do what any scientist would do: you set up a study that can isolate a variable and then survey how people perceive changes to the variable. With enough testing you can begin to quantify the results and make predictions with good correlation to actual measurements.

Some of the most recent surveys I know of have been done by Toole, Geddes, and Klippel which correlate what we hear with what is measured. Now you are free to disagree with their methods or completeness or results, but the basic idea is simply the Scientific Method which has been used for several centuries...surely you are aware of it...

You seem to think you are so clever, yet your comments are not only tiresome but indicate to me that you were probably stuffed in your locker too many times as a kid and now want take it out on the world by this "clever" banter.

Any reason for your hostility? I asked questions and while reading through your tactful references to my character and speculations about my childhood, I did not find anything meaningul to suffice as answers. Again, I asked questions, I would appreciate some meaningful answers. I am not smart as you, I would like to learn. I would like to know, if I turn up the gains on my amplifier and hear hiss (I think it may be called noise floor?), is it distortion? If I turn the gains down, and do not hear the hiss, there is no distortion or is it there but I cannot hear (could I still measure it though?)... So please, no need to be so insulting toward someone less knowledgeable than yourself
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post #13 of 41 Old 09-23-2008, 07:29 PM
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ARTA (actually the companion program Steps) does it automatically. Do a sweep and plot the % of the various harmonics. It also does IM distortion.


LL

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post #14 of 41 Old 09-23-2008, 07:35 PM
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Don't bother Anarchy with the facts, Auger. Trolls aren't interested in the facts.

Quote:


You seem to think you are so clever, yet your comments are not only tiresome but indicate to me that you were probably stuffed in your locker too many times as a kid and now want to get back at the world by this "clever" banter.

Crap, you made me spit coffee all over my keyboard!

Dennis H
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post #15 of 41 Old 09-23-2008, 07:38 PM
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Quote:
Originally Posted by anarchyhifi View Post

I am not smart as you, I would like to learn.

Ah, yes, playing the simpleton *nudge nudge* *wink wink*

See that stuff is just not as clever as you think. Heck I can go to any computer game forum and there are dozens of sad souls like you that display the same childish behavior that only half bright children with serious social defects display. After some digging I think I've discovered your identity, didn't you star as Syndrome in The Incredibles?




~Brandon


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post #16 of 41 Old 09-23-2008, 07:40 PM
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Quote:
Originally Posted by catapult View Post

Don't bother Anarchy with the facts, Auger. Trolls aren't interested in the facts.

Actually he does bring up a good topic, how we perceive harmonic distortion. Here are a couple links for the OP: http://www.klippel.de/pubs/default.asp http://www.gedlee.com/distortion_perception.htm

Klippel has a test on his site anyone can take for themselves and I've done a tutorial that others can try at home: http://htguide.com/forum/showpost.ph...89&postcount=7

~Brandon


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post #17 of 41 Old 09-23-2008, 07:49 PM
 
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Quote:
Originally Posted by augerpro View Post

Ah, yes, playing the simpleton *nudge nudge* *wink wink*

See that stuff is just not as clever as you think. Heck I can go to any computer game forum and their dozens of sad souls like you that display the same childish behavior that only half bright children with serious social defects display. After some digging I think I've discovered your identity, didn't you star as Syndrome in The Incredibles?




well, go ahead and ignore my questions, I thought these were legitimate questions... Perhaps you can point me to someone who may know? I would still like to know if partial phase shifts would count and I am getting chastised for asking questions... I just hope you are not a teacher somewhere, I would hate to see the reaction of the kids being sent to prinicipal's office after asking some questions important to them that you decieded were a trick or a joke?
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post #18 of 41 Old 09-23-2008, 07:52 PM
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Although he comes off a little brash. It is also good to have someone question what we all do. If everyone just simply agrees with eachother, no progress will ever be made. He could probably be a bit more tactful, but some of his posts actually bring up good points.

And this is coming from someone whose build he ragged on in one of his first posts.
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post #19 of 41 Old 09-23-2008, 07:55 PM
 
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Quote:
Originally Posted by augerpro View Post

Actually he does bring up a good topic, how we perceive harmonic distortion. Here are a couple links for the OP: http://www.klippel.de/pubs/default.asp http://www.gedlee.com/distortion_perception.htm

Klippel has a test on his site anyone can take for themselves and I've done a tutorial that others can try at home: http://htguide.com/forum/showpost.ph...89&postcount=7

I noticed your output file had the level attenuated to prevent clipping? Clipping is not distortion? Can you audibly tell when signal becomes slightly clipped? Anxious to find out, unless you decide to belittle me again...
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post #20 of 41 Old 09-23-2008, 08:00 PM
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If Anarchy really cared about the subject, he'd withhold comment until he had read the links Auger posted and tried the test for himself about what level of distortion is audible.

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post #21 of 41 Old 09-23-2008, 08:04 PM
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Quote:
Originally Posted by anarchyhifi View Post

I noticed your output file had the level attenuated to prevent clipping? Clipping is not distortion? Can you audibly tell when signal becomes slightly clipped? Anxious to find out, unless you decide to belittle me again...

I'll play. Yes clipping is distortion, that's why the level is reduced...so that the file doesn't clip.

About the phase of the harmonic you mentioned: here is your chance to be constructive and tell me what you would do to determine what polarity to designate the harmonics in the tutorial I did?

~Brandon


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post #22 of 41 Old 09-23-2008, 08:25 PM
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Clipping is distortion due to the over driven of the amplifier from it's limit. The amplifier no longer is able to reproduce the amplified signal as an input of the (sinusoidal / sine wave). The clipping occurs only when the amp is over driven and reaches more than the rated RMS power output. There is no power reserve left in the amplifier circuits and the DC is induced at the output level when the amp clips and that is very serious for your speaker (which fuses the VC wires).

Distortion is measured in the deisgn phase and during the final phase of the amplifier design. All IC / Tube / Transistors have distortion (THD in the spec) before you design the amplifier. SNR (signal to noise ratio) is a way to describe and meansure distortion.

You need in the very basic "Digital Scope, Signal Generator / Input signal" that are calibrated to measure distortion accurately. Distortion does and can occur at any level even if you can't hear it. You need to use electronics measuring equipments to accurtately measure distortion. Signal distortion is due to the design of the amplifier (design and components) in 99% of the time.

Clipping is 99% due to the human nature by over driving the amplifier due to too much input or too high of the gain (reduce the damn volume).

Seriously, have you ever hard of distortion ratings given for a speaker?

Some says the glass is Half Full, and some say the glass is Half Empty. I say the glass is holding 50% of it's capacity. :)
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post #23 of 41 Old 09-23-2008, 08:48 PM
 
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Quote:
Originally Posted by augerpro View Post

I'll play. Yes clipping is distortion, that's why the level is reduced...so that the file doesn't clip.

About the phase of the harmonic you mentioned: here is your chance to be constructive and tell me what you would do to determine what polarity to designate the harmonics in the tutorial I did?

In the real life, where would you encounter this type of distortion and under what circumstances?
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post #24 of 41 Old 09-23-2008, 09:00 PM
 
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Quote:
Originally Posted by megasat16 View Post

Seriously, have you ever hard of distortion ratings given for a speaker?

all the time as a matter of fact... the question lingers though, how meaningful are they... and even more funnier, why be concerned with amp generated overall distortion when there is such a disproportional difference.... Also, I get confused between interference and distortion... may not be the same thing?
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post #25 of 41 Old 09-23-2008, 09:04 PM
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So what you you guys think about sonotubes?

Dennis H
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post #26 of 41 Old 09-23-2008, 09:55 PM
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Quote:
Originally Posted by anarchyhifi View Post

all the time as a matter of fact... the question lingers though, how meaningful are they... and even more funnier, why be concerned with amp generated overall distortion when there is such a disproportional difference.... Also, I get confused between interference and distortion... may not be the same thing?

They are very meaningful....The question you asked is very much like "What does 1oz Gold Means?" For Gold, it's the weight measurement. For Amplifiers, the THD and SNR is very meaningful as the weight of GOLD. It's for the ones who cares though.

Distortion is already part of the amplifier due to the components and design (good vs. bad). and you can never get rid off regardless of whether you can hear or not. It's imperfection / flaw is very much like us and designed by us - imperfect beings. Intereference can be from the amplifier itself (due to the AC source such as transformer) and from other sources outside of amplifier but does not necessary exist within amplifier (could come from nearby audio / video / power sources). A good and well design amplifier has internal shielding for transformer induced interferrence and from AC sources.

Now, seriously serious, do you really don't know this **** or you just enjoy making funs of others in the thread???

Some says the glass is Half Full, and some say the glass is Half Empty. I say the glass is holding 50% of it's capacity. :)
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post #27 of 41 Old 09-24-2008, 01:40 AM
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I think he knows more than you are giving him credit for, yet continues these little rants and banters for what reason I can't hope to guess. Anarchy has contributed a few times around here, enough to not make him a complete troll IMO. I do wonder what he is really looking for in these threads though, he knows more than the newbies think, yet seems to fallow a code to his posting habits and thinking process that I just don't get. One second he asks for info, when its presented he acts the expert (sorry, I know how you feel about that word, anarchy) and tries to shoot everyone else down without actually posting a single factual comment in rebuttal, just replies with his typical remarks that he posts if for no other reason than to amuse himself and satisfy his very unique perspective on audio.

I can't say his post on this thread is useless, I have personally found THD+N measurements to be somewhat worthless in actual listening sessions. I am sure most anyone on here would heartily attest to their own ability to hear distortion, yet if a true test were conducted to see how this correlates with real distortion data and our listening sessions, most would find themselves very surprised with just how hard it can be isolate what problems (if any) are being heard and how to distinguish them. The klippel test is a great one to try if you want to learn more about isolating distortions. I have found that even though I tested fairly well with them, I still can't isolate which distortion I am actually hearing if its level isn't quite loud. Just because I can hear something is wrong with a recording when I hear it played next to the "right" two seconds later, doesn't mean I can readily identify what that is in a real world situation amongst the plethora of other issues that present and when my audio memory isn't constantly refreshed.

It may surprise you to know that I have discussed these issues before with certain audio "celebrities" on this forum via PM in the past. Just because anarchy posts it in his own way doesn't mean the thought should be discounted.
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post #28 of 41 Old 09-24-2008, 05:11 AM - Thread Starter
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This is my thread! My question!!

if people think its dumb and useless do not reply, Anarchy is on ignore for me so I do not care what he posts.

Also this is NOT a debate about IF certain measurements are useful either, its simply a question and more about the TOOLS and METHODS involved then discussing the theory about it all.

Lately, things go down hill fast. I know people seem to be more tense these days and I know guys like Anarchy get off on pissing others off because he does know about audio and he thinks others are stupid and ignorant....I do not care for his opinion or input, I do not care if he is the smartest audio guy in the world.

I do not care for anyone that does not learn how to interact online properly. He obviously can not interact with people!!

It is not "open-minded" to reject knowledge - Bob Lee
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post #29 of 41 Old 09-24-2008, 05:53 AM - Thread Starter
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More to the points...take your battle somewhere else!!

It is not "open-minded" to reject knowledge - Bob Lee
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post #30 of 41 Old 09-24-2008, 06:08 AM
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To be fair, your original question was "how do we measure distortion?", not, "how do we measure THD%?". All I did was attempt to point out that while THD% is not hard to measure, it is not particularly useful either. There is rather more to the subject. In any case, I shall leave it at that.
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