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Author Topic: MEASUREMENT FAQ  (Read 4807 times)

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spoony

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Re: MEASUREMENT FAQ
« Reply #40 on: May 28, 2015, 06:12:47 PM »

Thanks all for the insight. A question for measurement takers: If my understanding is right, when you record the input signal at higher sample rates it allows you to use longer windows for analysis, enabling the display of CSD data below certain threshold, say 200 Hz, why not do this?
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Marvey

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Re: MEASUREMENT FAQ
« Reply #41 on: May 28, 2015, 10:56:33 PM »

Not higher sample rates, but longer window sizes. Bass waves are long and take a long time to propagate. Bass tends to be sensitive. A truck 1/2 mile down the street results in a lot of subsonics. I've wanted to look at bass decay for headphones on a longer scale, to the 10s of mSec, but haven't gotten to it. I still have a lot of raw data. Don't know if there would be anything enlightening or not.
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ultrabike

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Re: MEASUREMENT FAQ
« Reply #42 on: May 28, 2015, 11:19:59 PM »

Yup. Longer windows would be required, but sampling rate could actually be reduced as it mostly affect resolution much needed for high frequency characterization.

In fact, say you are constrained by the number of data samples you can take (limited buffer size). Then when doing measurements, going for 96 kHz would yield information just below 48 kHz in your measurements. But since you keep the number of data samples fixed, reducing the sampling rate in effect allow you capture a larger time frame which in turn permits much better resolution of lower frequencies. This is readily apparent in REW when one selects 44.1 kHz sampling vs 96 kHz.

As Marv said, large frequencies have larger periods. Therefore a few cycles of decay (or even fractions) in low frequency may correspond to a crap load of cycles on high frequency decay. IMO we would have to have two or more sets of CSDs for certain frequency ranges. Resolution may be frequency dependent. One could get an idea of decay though by looking at how smooth the low frequency responce is. Likely the smoother and flatter, the fastest the decay.
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spoony

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Re: MEASUREMENT FAQ
« Reply #43 on: May 28, 2015, 11:50:16 PM »

Understood, thank you both.
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money4me247

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Re: MEASUREMENT FAQ
« Reply #44 on: May 29, 2015, 06:58:35 PM »

@purrin & @solderdude,

Thank you both so much for responding in detail to my query and I also appreciate the more layman  usage of language!!!

@purrin, I will look into attempting a spl calibration on my mic. No idea how to do that, but will research the pertinent background information prior to immediately shooting out tons of other questions. I will also look into acquiring a O2 on the cheap just to ease my concerns about the amplifiers I am currently using. thanks again!!!

@solderdude, if you don't mind me further pestering you, I have a few more questions related to your direct responses (which were extremely helpful, though quite revealing of my still very incomplete grasp on everything). if you prefer to discuss this via PM, please let me know.

1) what measurement software do you personally use? I will like to give it a try :)
2) mmm... I guess I am not sure if I fully understand. This is how I am currently interpreting your statements. Please let me know if I am misunderstanding. Further explanation would be much appreciated!

So I am assuming that I can find from the manufacturer the reference they used and this spec will be listed as a certain SPL at 1kHz (or using noise). And then I will run a measurement trial, I will attempt to match the threshold of the measured distortion to what they say the spec should be using the gain settings?

if further explanation is too complex, I don't mind just resource referrals where I can slowly learn about it on my own free time.

3) that makes a lot of sense. sounds like you are saying I probably shouldn't worry about my amplifier as my mic and usb interface is the limiting factor? thank you
4) what program would I use to determine the THD? any recommendations?

I am very appreciative of the support shown to my probably very newbie-ish questions. Thanks again to both of you for taking the time to respond!! :)

edit: lol I just realized that the "@" tag does not operate the same here as on head-fi. just kind of a habit now to respond that way =S. hope that is okay!
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Solderdude

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Re: MEASUREMENT FAQ
« Reply #45 on: May 29, 2015, 08:16:40 PM »

1) what measurement software do you personally use? I will like to give it a try :)

REW 5.11 (http://www.roomeqwizard.com/)on Win XP, soundcard = EMU0204 on 192/24 samplerate, rig = WM61A on homemade rig + dedicated and compensated pre-amp.
The WM61A has a peak at 16kHz which I the pre-amp removes.
Furthermore a mic that is designed to be used in free air (a measurement mic as well) has a different LF response when mounted on a(n infinite) baffle so must compensate for this as well.
These compensations can also be applied in REW but haven't experimented with this.
The output of my mic amp is thus already compensated 'flat' (at least as far as I know) and outputs line level.
I use a homemade amp for driving the headphones.
It's irrelevant what you use as long as it has enough output power to reach 90dB SPL in each headphone (+ headroom) and doesn't alter phase and FR and has low distortion.
All amps that meet those minimum requirements ALL will give the exact same readings on the same headphone.

As Purrin also explained, REW needs also needs a separate dB meter for calibration (only for the SPL part).
Bought a cheap one from fleabay and already had a (modified) Realistic meter which I calibrated a while back when I had an opportunity to use it alongside expensive mics that were being calibrated.
The cheap fleabay meter is about 0.5 to 1 dB 'off' so cheap meters MAY be accurate enough (you never can know for sure though).
It really doesn't really matter if the actual SPL is 88, 90 or 92dB when it displays 90dB.

You set the SPL meter in the 90dB range and crank up the amp/vol control sliders in the calibration mode till it reads 90dB (using noise generated by REW)
Make sure you seal the headphone (using a CD with some felt on it ?) and poke the mic though the hole in the CD.
ENSURE you have a good seal around the mic and headphone to get a proper SPL reading.

Put that headphone on the rig.
Then you adjust the input of the soundcard so it is around -10dBFS (leaving headroom while maximizing S/N ratio)
Then you tell REW that its input level is 90dB SPL and never touch any of the controls again or note the settings so you can set them in the same position later on.

So I am assuming that I can find from the manufacturer the reference they used and this spec will be listed as a certain SPL at 1kHz (or using noise). And then I will run a measurement trial, I will attempt to match the threshold of the measured distortion to what they say the spec should be using the gain settings?

the dB spec the manufacturer gives is irrelevant, it only says something about its sensitivity at a certain frequency.


4) what program would I use to determine the THD? any recommendations?

I use REW (it can also give distortion in % and or dB) but ARTA (properly set up also gi ves good results.

Measuring headphones is not as easy as it looks and every one that does is produces different plots, some are somewhat comparable, others are not.
Some use smoothing others do not or use less.
A measurement is just an 'aid' and lies to you.
It's up to you to know if the rig is telling lies or not.
That takes knowledge and experience ... and thus takes time and lots of practice as well as understanding what the plots mean and learnig to correlate aspects to sound.

There will NEVER be a 100% correlation to the sound for many obvious reasons but such a rig can be helpful when modifying headphones or when looking for major flaws.



« Last Edit: May 30, 2015, 02:18:09 PM by Solderdude »
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transparent201

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Re: MEASUREMENT FAQ
« Reply #46 on: May 30, 2015, 01:58:12 PM »

Marvin, I saw a video of you on youtube talking about the problems with professional rigs, like the not accurate artificial ear canals which might cause resonances.
Instead of trying to build the perfect-according to your standards-measurement rig wouldn't it be better to buy a pair of relatively cheap binaural microphones and perform the measurements on yourself?
I think that you would discover relative differences between cans that no mannequin can show and you could understand better why headphones sound different when they measure about the same(on various rigs).
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spoony

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Re: MEASUREMENT FAQ
« Reply #47 on: May 30, 2015, 03:01:38 PM »

There's already measurements like that on this site, Hans' specifically.
BMF on Head-Fi also used to measure using in-ears. The downside is the same, if my pinna is bigger than your pinna, then my curves won't match yours to a certain extent.
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transparent201

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Re: MEASUREMENT FAQ
« Reply #48 on: May 30, 2015, 09:01:12 PM »

There's already measurements like that on this site, Hans' specifically.
BMF on Head-Fi also used to measure using in-ears. The downside is the same, if my pinna is bigger than your pinna, then my curves won't match yours to a certain extent.

Correct. But the same holds true for dummy heads. Only if you measure the response of top speakers in an accoustically  treated room with the use of a dummy/human head will you have a good reference. Otherwise you can only make comparisons and even then you are not 100% sure.

I downloaded some time ago an extract from a german audio magazine(Profesional audio Musik and Equipment-Juni 2009), pertaining measurements of many models and the DT 990 pro measures ruler flat on  Neumann KU 100).

Here is a picture from the pdf.

http://s13.postimg.org/yvx9xwsaf/Capture.jpg
http://s8.postimg.org/lotgax9wl/Capture1.jpg
Sorry for the quality. It's not my fault.

The caption reads "The third Beyerdynamic, DT 990 pro shines with an impressively linear frequency response". Even better than the HD800. And not radically different.

Would you trust this?
Would you dare saying that FR tells almost nothing about subjective experience when you don't know if the FR you have in front of you is correct in the first place?

EDIT: Because the scale on the left is not visible here is the result of a more colored can for comparison.

http://s14.postimg.org/ojqjymwch/Capture2.jpg


« Last Edit: May 30, 2015, 09:18:16 PM by transparent201 »
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Marvey

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Re: MEASUREMENT FAQ
« Reply #49 on: May 31, 2015, 02:22:27 AM »

Would you trust this?
Would you dare saying that FR tells almost nothing about subjective experience when you don't know if the FR you have in front of you is correct in the first place?

As long as you have other measurements to compare to using the same measurement method, should be OK. Also should correlate to what experienced listeners hear.
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