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vibro veritas, measuring our IEMs like a pro with amateur budget.

Discussion in 'Sound Science' started by castleofargh, Aug 12, 2015.
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  1. bartzky
    Well I've been missing something here, sorry! Thought you wanted to do a whole SPL-calibrated measurement :D
  2. CraftyClown
    No worries
  3. castleofargh Contributor
    REW is a very powerful tool for EQ, but it's clearly aimed at room equalization with microphones that are calibrated. I had a clear need to RTFM and look for some video tutorials. ^_^
    about setting a compensation curve, sadly I don't have a simple answer, unless you use some super expensive very well calibrated measurement rig(we don't), the raw measurement will most likely not look like most other raw measurements, so some ready made compensation curves will mean nothing.
    I wasted a lot of time trying to match my IEMs to online measurements, just to realize that from website to website they have sometimes significant variations, and from IEM to IEM it's really usual to have 2db or more variation between 2 samples in the audible range(often even between left and right driver).
    and how far away from the coupler did the online guy put the IEM? we often don't know. we basically have no clearly reliable reference.
    so the logical and easy choice, is to forget about compensation curves if you haven't already wasted your youth on it, and care about having a stable measurement so that all the IEMs you will measure over time will show relevant variations(even that is hit or miss IMO, like vented IEM vs sealed IEMs).
    take an IEM like the ER4, use that as a calibration(so that you get a flat graph with it), and use that as you basis.  from time to time, put the er4 in it again and check that you're still getting a more or less flat line just to make sure you haven't changed something in your computer or devices that you forgot to set back to reference.
    having a mean of control is the hardest thing to have and the most important. nobody should trust that IEM XXX has a bump of 2.6db at 3khz from your graph. what people should be able to trust is that IEM XXX at 3khz is 5db louder than IEM YYY matched at 1khz. 
    each measure alone is meaningless, it's the comparisons between measurements done on the same system that can have good informative value. and a proper compensation curve, or the lack of, doesn't impact such comparisons so it's not actually necessary.
    some may even argue that not trying to look like diffuse field may be better so that ignorant people won't just go and look at your graph vs the graph from another IEM on another website and assume it's ok to do so. seeing really massive differences might trigger enough doubt for the readers to think about it.
    about using a DAP instead of a loop for REW, well you can, but you're missing out on some stuff. like using the other channel as timing reference, or having easy fast sweep for frequency response instead of recording stuff and importing them, or using noise to make an average curve that of course will be a noisier than a clean sweep. but evidently if you have nothing with relatively low impedance for source, that can be a problem with many IEMs. I can't tell you to take the easy way if it's going to mess with the FR of most multidrivers.
  4. CraftyClown
    Okey dokey, so I think I'm almost there [​IMG]
    So following everyone's advice I did use the Geekout 100 with it's low impedance to calibrate REWs SPL meter.
    1. I connected a y splitter to the geekout and on one side I attached my Etymotic IEMs and on the other side I connected a cable with 3.5mm male jacks on each end
    2. I played a looping 1 khz tone through the geekout and then used my multimeter to measure the ac voltage by touching one prong to the ground of the 3.5mm jack and the other prong to either the left or right pole of the jack.
    3. I then attenuated the volume on the laptop until the multimeter read 0.201v
    4. I inserted the right ear of my ER4-XR into the Veritas, which is in turn inserted into my Startech USB soundcard
    5. I then opened REW and selected the SPL meter and then calibrated it by selecting calibrate by file and entering the sensitivity of my ER4-XR which is 104db
    *As I understand it, that is the Veritas now calibrated in REW*
    6. I then connected some of my IEMs (Tralucent 1+2s) to my DAP (Sony WM1A) and started playing music at a favoured listening level
    7. I inserted 1 side of the IEM into the veritas and pressed record on the SPL meter in REW and the watched the dbs get recorded
    So my final question (hopefully) is which figure is the relevant limit here? Is it the LCSmax or the LZpeak? I know one is C weighting and the other is Z weighting, but I don't know which one I should be concerned with. I'm hoping it's C weighting, otherwise I really need to turn my music down.
  5. castleofargh Contributor
    not sure I get what you problem is this time. whatever you use when you set the calibration with the er4, you keep when using other IEMs. for 1khz, A, C or Z shouldn't change a thing. and as far as microphone and calibration curve goes, the veritas doesn't have any, so it doesn't really matter because it doesn't really mean anything.
  6. bartzky
    The good news first: you successfully have SPL-calibrated your Veritas :D

    Now the bad news: the trouble starts at step 6. You have calibrated your Veritas @ 1kHz - music of course is not just a 1 kHz sine wave, so it's not suited for your measurement.

    So the way to go is:

    1. Play a song of your choice with your DAP at your desired listening level.

    2. IEM to coupler and play the 1kHz 0dBFS file used for your calibration at the same volume setting.

    3. Find out how much headroom the song of your choice has got. There should be software to do this, but I fear I can't be anymore helpful at the moment :D

    4. Subract headroom from your dBSPL reading.

    castleofargh I aimed for uncompensated 711/60318 measurements with my calibration.
    IMO it's about finding a good average between various web resources and IEMs. One measurement might show some more bass, the other one might have more treble. For me it's important to get readings somewhere between the different resources. It's all about not having a drift in the measurement procedure like ever measuring to much treble with various IEMs.

    If someone is curious, this is my measurement "database": http://headflux.de/messungen/
  7. CraftyClown

    OK so here's where I'm confused then:

    In my step 5 I calibrated the Veritas to 104db. Why would it matter if that was a 1khz tone I used to do so?

    I presumed that REW (when connected to my Veritas) now recognises what 104db sounds like and can therefore recognise the decibels of SPL being emmited from any IEM being inserted into the Veritas.

    I don't understand why I need to find headroom?

    Thanks again for your help :)
  8. bartzky
    Okay, I see what confuses you.

    To be precise: You SPL-calibrated your Veritas at 1 kHz. Etymotic, while saying your ER4 does 104 dB at 200mV, also refered at 1 kHz - that's the standard.

    And now it will get a bit more complicated. The Veritas is not an IEC coupler so it's not accurate across let's say 20-10000 Hz. I think you might have noticed this fact while reading through this thread :D Thanks to your calibration a measurement at 1 kHz should give you a reading that's at least close to IEC standards, if not the same. But that is not true for every other frequency. Due to the fact music does not only consists of 1 kHz sine waves it therefore is not suitable as a tone for your measurement rig.

    So you can:

    a) calibrate your Veritas across the whole frequency spectrum.

    b) do what I explained in post #96.
  9. CraftyClown
    Ah ok I see. So now we are dealing with a limitation of the Veritas?
    So could you explain a little more what you mean when you say "Find out how much headroom the song of your choice has got"  and  "Subract headroom from your dBSPL reading"
    also is it the LCSMax reading I should be most interested in?
    Sorry for all the questions. I have come this far and I want to make sure I get this right. My hearing thanks you  [​IMG]
  10. bartzky
    I don't want to make it even more complicated but there are a few questions coming to my mind as well:
    - will short but high peaks cause more damage than longer exposure times?
    - does the damaged somehow correlate with the frequency or not?

    Unfortunately I don't know.

    But there are a few things to consider for your measurements as well.
    First every Inear has a different FR, so one might be significantly louder at a particular frequency while generating the same SPL at 1 kHz.
    Second the music is an unknown variable. There are many different frequencies at different levels.

    So IMO it's impossible to say something like "the music plays at XX dB" because there simply isn't such a simple number. There are however many different SPL numbers that all are dependent of time and frequency.
    I think there might be some standard to mix all these factors together but I simply don't know.

    IMO a good and yet simple way might be to determine an average headroom for your song. FYI headroom is something like the space for digital volume. A few post ago I told you to generate tones at 0dBFS - that's the loudest existing digital volume. Music is usually not as loud as this, so the difference between the actual level of the music and 0dBFS is called headroom.
    Let's say your song plays at -12dBFS; this results in 12dB of headroom. If you determine the SPL with your 1 kHz 0dBFS tone you therefore will have to subtract 12dB from your SPL reading.

    If you want to state more precisely you additionaly can look for a FR measurement of your particular IEM and search for the highest peak. Then you should add the dB difference between 1 kHz and wherever the peak might be to your SPL reading.
  11. CraftyClown
    Ok so just for arguments sake, what if I was to play a random selection of music on my DAP for say 15 - 20 minutes at my desired listening level whilst my previously calibrated REW's SPL meter was taking readings from my connected IEMs.
    If I was to attenuate the volume so that the LCSmax reading never went higher than say 90db during that time frame (90db is understood to be safe for a couple of hours and I would rarely listen to IEMs for that long anyway) would I not have a level that was most likely fairly safe? 
    Or am I oversimplifying this?
    Would I have just been better off using a manual SPLmeter and attaching the IEMs to it?   [​IMG]
  12. bartzky
    It all depends on how accurate you want your measurement to be.
    IMO this not the baddest idea. I'd use the equivalent sound level (Leq) with A-weighting for this purpose. The only limiting factor will be the deviation of the Veritas compared to IEC standards.
    This might be useful: http://www.cirrusresearch.co.uk/blog/2015/06/noise-101-what-is-equivalent-continuous-sound-level-leq/
    I'd rather play the 1 kHz 0dbFS tone. If the SPL reading is below 90dB you are definitively safe [​IMG]
    Hell no [​IMG] 
  13. CraftyClown
    Just so I'm clear are you suggesting either of these options?
    Or are you saying I'm still better off playing the 1khz tone than playing music?
    If I go with tone, does it matter whether I choose a or c weighting?
  14. bartzky
    Well, those are two different approaches to approximate a problem - that's science haha :D. I can't really tell which one will be better. They both focus on different aspects. I'd suggest to try both.

    At 1 kHz the weighting doesn't matter:
  15. CraftyClown
    Ok so I went with playing my music for about 20 minutes and using LEQ and A-weighting.
    As you can see, the average dbs measured were 75.9 and the LAFmax was 84.3 which seems great and within a perfectly safe range for a couple of hours of listening.
    My only concern is the LZpeak which sits at 100db. I don't really understand what it relates to and whether I should worry about it or not. On the logging graph it's represented by the red line right at the top, however you can see the grey wave form that has been drawn out, never reaches any where near that high. 
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