Improving my IEM measurement rig.

Discussion in 'Measurement Setups, Systems, and Standards' started by Serious, May 31, 2020.

  1. Serious

    Serious Inquisitive Frequency Response Plot

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    I've mentioned this again and again, but I was just not happy with how my measurements of IEMs came out. After a post in the Andromeda thread here, I decided for myself that I had to do something about it. Here's my reply:
    https://superbestaudiofriends.org/i...-dream-awesomesauce.2075/page-105#post-302546

    My current rig consists of an 8mm (inner diameter) PVC tube with a 6mm microphone inserted into a comply foam tip with the flat side facing the outer end. The distance between the opening of the tube and the microphone is 25mm. Here's a picture of how it looks like with my RE272 inserted into it:
    IEM_coupler_baseline.jpg
    This seemed like a good compromise in the past:
    • The 8mm tube allows me to insert small eartips, which also tend to fit my own ears. Usually there's no need to try different tips for the coupler than I would use for my own ears.
    • The comply foam provides some damping for the typical insertion-depth related resonances.
    • I can achieve similar insertion depths as with my own ears
    There are a number of downsides to this, though:
    • The tube is pretty stiff and getting a seal can sometimes be an issue.
    • I think the magnitude of those resonances is generally too high, hence the comply foam tip. It helps somewhat, but I think it's not enough.
    • I'm fairly certain the volume of this coupler is larger than that of a real ear canal. A 711 coupler is equivalent to 1.26 cubic centimeters at the reference plane (12.4mm distance from the eardrum IIRC). This should be even larger than 2cc volume, about 2.5cc. I think this likely explains why I thought the FR measurements didn't come out consistent between different types of in-ears.
      I'm not sure how I got to these numbers. Ear canal should be roughly equivalent to a 7mm tube at 1cc. 8mm tube is roughly 1.3cc. 711 coupler also simulates volume behind eardrum. http://www.aes.org/technical/documentDownloads.cfm?docID=177

    I am somewhat lucky (or unlucky) in that I can hear these insertion-depth related peaks myself. It seems other people on this forum cannot fathom hearing them - while I don't understand how you cannot hear them. So I think it's likely different ear anatomy plays a big part here. Also earwax build-up will change the "damping" and inner volume of your ear canal aswell. The new coupler I'm trying to make might be a good fit for me, but not so much for you.

    So for tonight I just tried damping the insertion-depth related peaks to a level that I feel is realistic. This is not a new idea at all and even G.R.A.S. have redesigned the 711-coupler in a way to minimize such λ/2-resonances: https://www.gras.dk/files/812-Whitepaper Hi-Res Audio Simulator.pdf
    In the case of the RE272 with shallow insertion I hear about a 3-4dB peak at 7.5kHz (relative to 6.2kHz). This is equivalent to a distance of 23 mm from the eardrum. Very shallow insertion. The configuration in the picture above also results in a peak at 7.5kHz, but instead of 3-4dB it's 10dB.

    You can very easily "fix" this by inserting damping into the tube. For now I simply chose to use sheep's wool. You can eliminate the half-wavelength resonance completely by doing so without affecting the rest of the measurement too much. However I don't want to get rid of it completely, I just want to damp it from roughly 10dB to 3dB. For that I ended up needing about 0.07g as per my tonearm scale. This is how it looks like:
    IEM_coupler_0,07g_wool.jpg

    Here you can see its effect on the FR, aswell as the effect adding slightly more and slightly less has.
    RE272 - IEM coupler tube damping comparison.jpg
    Note that this is entirely uncompensated. Normally I would raise the upper treble, but for this comparison it's fine this way. Also ignore the 50 Hz spike, which is hum typical to my setup.

    The advantage is obvious: Treble measurements are much closer to how they subjectively sound. But the disadvantage is that the sheep's wool will add to the already too large volume of our coupler. As you can see at lower frequencies the response drops as a result. The exact nature of this effect depends on the in-ear. I think this wrong volume is the main reason my in-ear measurements aren't as accurate as I'd like them to be. So this is not how it can stay. Obviously I will need to test other IEMs aswell.

    The entire rig will need a redesign so it hopefully gets more accurate. To be continued...
     
    Last edited: Jun 4, 2020
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  2. Biodegraded

    Biodegraded Friend

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    You could give yourself more tip options by sliding a bigger tube over the end of the first, like this:

    [​IMG]

    The inside diameter of the yellow one is 7/16" or 1.1mm, which fits most 'medium' size tips pretty well. Then you could more easily fiddle with insertion depth. You'd need to change the length (volume) of the smaller tube to compensate, obviously.
     
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  3. Serious

    Serious Inquisitive Frequency Response Plot

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    Yes, I was planning on doing something similar. Larger inner diameter near the exit and smaller diameter near the microphone/eardrum. Same total length of 25mm from the microphone. Our ear canals are quite similar in that regard after all. Actually your option seems too easy to not simply try it. Maybe this way I can reduce the volume to a realistic value. You could even slide a longer outer tube over the smaller tube to allow for eardrum-sized (8-10mm diameter) microphones. Add the right amount of damping near the microphone, done. Just need to find the right sized tubes.
     
  4. purr1n

    purr1n Finding his inner redneck

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    That is a great idea. Now if there were a way to make a tube from more pliable / absorptive material like human skin and add ear canal hairs. Maybe a roll of felt lined inside the tube?

    I've always felt the 711 couplers were an overly complex way to simulate the ear canal, and really only in one way, the input impedance, while ignoring many other more crucial aspects.
     
    Last edited: May 31, 2020
  5. Biodegraded

    Biodegraded Friend

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    I'd also suggest not being too rigid about obeying the 'anatomically correct' ear-canal volume. My twin-tube coupler is much smaller, 1.68cc including the space occupied by the tip, but the results match IEC couplers well up to 1k and fall off predictably afterward, and because of the small volume, put the half-wavelength resonance up out of the way at 12-13k where driver resonances often live and where nobody should believe the results anyway. Here are a couple of examples compared to results from an IEC711 clone from some random guy on the internet ;)

    [​IMG]

    [​IMG]

    With a free-field to pressure-field compensation like a more extreme version of this B&K one to correct for the microphone response and some damping to bring down the resonance it might get quite close.

    I did try a 'correct' coupler volume and could replicate the 8k peak, but then the bass was not a good match with other measurements and with what I heard:

    [​IMG]

    If you don't like IEC711 results, play with the coupler volume until you get close to what you hear. Main point is I don't think coupler volume should be set in stone.
     
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  6. Bill-P

    Bill-P Level 42 Mad Wizard

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    Please ignore gross finger. Here's how I do it:

    [​IMG]

    Milk tea straw + a layer of rubber on the outside. This goes over the UMIK-1 and leaves a distance of about 22-23mm (edit: got less lazy and measured. It's actually 24.5mm) to the opening of the straw. Oh, and at the base of the straw is a ring of felt to potentially simulate some human hairs. I don't know how effective that is to be honest.

    It's... a piece of straw so I don't have that much problem getting ear tips to seal (fit) well. Mostly, I only get errors with insertion depth, so I use RTA and a generated pink noise to align 8KHz, plus some real physical distance measurements for sanity check.

    This gives me pretty good approximation of what I hear, especially with regards to the 8KHz peak. I don't think it is as pronounced as it is in some measurements. In general, I find that the 8KHz peak may or may not be there with certain IEMs. I've done sine sweeps of some of them to compare and see. I think at least for Campfire IEMs, the peak may be real. For some others, a peak at 8KHz or even its absence doesn't really match up with my subjective impression when doing a sine sweep.

    Anyways, for reference:

    [​IMG]

    ^ this seems about right to me. Solaris does seem like it has that much bass to my ears. Note that when I do measurements, I use the GOV2+, which seemingly should have a close to 0 ohm output impedance.

    Nice idea on the sheep's wool by the way. I should try that next...
     
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  7. purr1n

    purr1n Finding his inner redneck

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    Would need further investigation, but I suspect super exacto correcto input impedance curve is rather unnecessary. Folks have been getting similar results to the 711 couplers from a variety of different IEMs from cheapo DIY eartubes. The resultant FR can be compensated with a static curve - it needs to be compensated anyway.

    Makes sense if you think about it. The output impedance of the air pressure from IEMs ain't gonna differ that much from one IEM to another (unless someone makes a horn loaded IEM); and if the input impedance is sufficiently high, it ain't gonna matter.

    Is the DIY eartube elegant and based on formal scientific and mathematical principles? Nope. But if it works, it works.
     
    Last edited: May 31, 2020
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  8. Serious

    Serious Inquisitive Frequency Response Plot

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    @purr1n I suspect a roll of felt around the inside would need to be quite thick to have a comparable effect. I'll try it out and report back. I also think sheep's wool is not a good choice. Other materials will not increase the inner volume the way sheep's wool does.
    The next step is getting exact subjective impressions for the FR of the IEMs I have here and measuring them on different couplers. I have: UERM, RE272, RE400, Carbo Tenore, Apple BA IEM (both versions) and Brainwavz B2. Not a great sample pack (not very representative), but I think it should be fine.

    The way I arrived at the 3-4 dB figure was by simply sliding the frequency with the szynalski tone generator until I heard a peak: https://www.szynalski.com/tone-generator/
    Then I opened another tab and set it to the frequency at which the "base" of the peak seemed to be (6.2kHz) and adjusted the level of the peak until they seemed to match subjectively. Convert percentage into dB and that's it.

    Now why it is only 3-4dB instead of 10dB+ like you'd normally get on the tube, I don't know. The ear canal isn't very straight and the eardrum is at an angle. 3-4dB seems like a lot of damping to me. Eardrum might be less reflective than the microphone. Maybe some earwax buildup, although I usually don't have very much of it.
    The 4.5kHz peak is subjectively around 9dB, very similar to the measurement. Basically if I didn't lose so much sub 1kHz this way I'd already be satisfied.

    @Biodegraded I've played around with smaller volumes before and I'm pretty sure the smaller volume is more accurate to my ears. The RE272 seems to be a decent example here since you actually lose some energy in the sub 1kHz region. Other IEMs seem to just not care. Rin Choi used to post measurements for different insertion depths. Some IEMs change their response in the lows with deeper insertion depth (less volume) while others don't really.
    I think you're right in that the exact volume isn't set in stone. Ear canal diameter and length seem to vary significantly from person to person anyway.
    I'll try a couple different things and report back. Maybe not today, though.
     
    Last edited: Jun 1, 2020
  9. Serious

    Serious Inquisitive Frequency Response Plot

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    Did some more reading. What I had mentioned before regarding the volume is wrong and I think the 8mm tube should broadly mimic the volume of the ear canal. However I think without middle-ear simulation I'm not going to get anywhere. And I don't think I can simulate the middle ear with my current DIY-ing possibilities.
    I found the Brüel & Kjær patent for their new 5128 HATS: https://patents.google.com/patent/US20150124978A1/en
    After looking at a bunch of older papers and articles comparing other simulators, I found out that Jude now has the 5128 aswell. He has posted measurements of the new Campfire IEMs with the new B&K and the GRAS HATS. Some guy posted low-resolution screenshots of the measurements from his video on HF:
    [​IMG]
    [​IMG]
    Damnit! The two are not alike at all! While the difference in the bass for one IEM is maybe 1dB, it's more like 5dB for the other! Even in the treble the response difference isn't the same between the two IEMs. No compensation curve will fix that. I haven't heard the IEMs, but based on what I read I think it's more likely the 5128 is the better ear simulator.

    I'll still try other damping materials and tube diameters. In fact I bought some new tubes and will take sets of measurements with different tubes. But I think it's futile. The kind of accuracy I'm going to achieve by that has been superseded by the Zwislocki coupler fifty years ago. Maybe even the ancient 2cc coupler would've been more accurate.

    Right now I think buying a fake IEC 711 off aliexpress and possibly damping it a bit is going to give me the best results. But even then it's inaccurate. And I've made bad experiences with fake aliexpress goods before (in that they don't approximate the real thing at all.). Real measurement equipment is way too expensive for me to buy. It might be a better idea to generate IEM FR graphs subjectively.
     
  10. purr1n

    purr1n Finding his inner redneck

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    Too quick to conclude that with a sample size of two and the possibility of placement differences. The Andromeda and Solaris have a very different fit, There are some similarities in the differentials, even in the highs were things can shift in frequency depending upon placement.
     
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  11. Serious

    Serious Inquisitive Frequency Response Plot

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    Placement on the two couplers seems to be consistent to me. The way the HF peaks carry on between the two IEMs on each coupler. Seal between both looks fine aswell. We will see once Jude measures more IEMs.
    I think it's more likely the difference is due to this:
    Eardrum Z 711 vs real eardrum.jpg
    That's true, the broad trends are the same. Maybe that's good enough. Now it would be interesting to see your measurements for the Andromeda 2020. There are some here (711): https://theheadphonelist.com/campfire-audio-2020-launch-impressions/4/
    Doesn't make me too hopeful for the older couplers, though.
    Anyway, I'll try some stuff - maybe tomorrow.

    BTW: Sample size of two and all, but here the DD bass IEM has more bass on the B&K coupler than on the 711-style coupler, like I mentioned in my other thread.
     
    Last edited: Jun 4, 2020
  12. purr1n

    purr1n Finding his inner redneck

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    I would say the dotted lines are more reflective of what I hear between the two. The Andro 2020 was obviously darker than Solaris 2020.

    The dotted lines are the new 5128 coupler which is super awesome right?
     
  13. Serious

    Serious Inquisitive Frequency Response Plot

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    Some observations after measuring multiple IEMs all in multiple different coupling combinations:
    • The damped 8mm coupler was a colossal piece of shit after taking other IEMs into consideration. The RE272's peak seemed to be damped to roughly the right level, but peaks at higher frequencies were way too smoothed over (IEMs that I prefer with deeper insertion). The wool is likely not dense enough to work equally well after 5kHz or so.
    • The usable range for diameters for the outer tube really isn't that large. At 12mm I can't get a seal. 8-10mm works, anything more or less doesn't.
    • Larger tubes obviously let more high frequencies through, but as a result they are less damped. Smaller tubes give more SPL due to the smaller volume. With most in-ears the difference up to the upper midrange is more or less just a level change, but some react differently to different volumes.
    • Sometimes there are dips in the treble when I hear a peak and no insertion depth change can fix it. To me this makes the measurements effectively useless.
    • I tried a 10mm microphone inside a 10mm tube and somehow that combination required no damping at all to get the 7.5kHz peak with the RE272 to the same level as the WM61A inside the 8mm tube. The microphone doesn't even have a piece of felt on it, but it seems to have a much higher acoustic input impedance*, more like what we want. This is the combination I like the most so far. Even then measurements are more or less random.
    • I was going to use heatshrink and shrink the center part, but after trying a smaller tube inside the 10mm tube I think the volume is too small rather than too large. The problem likely is that with the 10mm tube at low frequencies the volume is too small and at higher frequencies the volume is too large: https://www.bksv.com/media/doc/bn0221.pdf
    Some of the differences between 10mm tube and 10mm mic vs 6mm mic in a damped 8mm tube:
    • RE400 and RE272 are about 1dB higher in level up to 1kHz-ish
    • ADDIEM is 1dB lower in the 1-2kHz region.
    • Despite my best efforts to match insertion depth the UERM measures 2dB higher around 5kHz. Also has 1dB more bass to 1kHz.
    The first two I'd say are closer to what I hear, the UERM... not sure. With the UERM I hear a peak around 11kHz that doesn't show up in any of my measurements. Instead there's a dip (and it's not a peak in disguise - kind of thing). But the UERM isn't the only IEM I see such a problem.

    This also means that what I mentioned before about the wool raising the inner volume was wrong. It seems it actually makes the volume smaller instead.

    I still think that we won't be getting anywhere without additional volumes.

    @purr1n not sure if that's some sarcasm I don't detect, but it's the other way around. If that's what you hear that's... not very promising. Some people on HF seem to argue that the newer damped GRAS "high frequency" 711 is too damped and smooths over peaks. As for the bass difference, I'd generally trust B&K in their claims that their 5128 system is more human-like than a 711. We'll see once there are more measurements, which I think means we will have to rely on Jude for now.


    *I suppose the diaphragm is possibly less tight and less damped. Maybe. It is a higher sensitivity mic.
     
    Last edited: Jun 6, 2020
  14. purr1n

    purr1n Finding his inner redneck

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    Oh, that’s not good. The newer 5128 doesn’t seem right at all using those two IEMs as an example.
     
  15. Serious

    Serious Inquisitive Frequency Response Plot

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    Okay, scratch that. Not sure why I did it this way, but pictures are better. Here's what the coupler looks like when trying to measure my CIEMs: I decided to angle the microphone a bit which seemed to not change too much, but it does lower the Q of high frequency peaks a bit.
    10mm_tube_CIEM_measurement.jpg
    I use something like blu tack to get a seal. IME this actually seals a bit worse than a silicone tip, so there's a tiny bit of rolloff at 20Hz, but it's the fastest and most reliable way to do it.

    RE272 damped 8mm tube vs 10mm tube (insertion depth is not matched).jpg
    RE400 damped 8mm tube vs 10mm tube 2.jpg
    ADDIEM damped 8mm tube vs 10mm tube.jpg

    For the UERM I decided to raise the 10mm tube measurement a bit to better show the differences.
    UERM damped 8mm tube vs 10mm tube.jpg

    There's a λ/4 dip in all of these with the 10mm tube that's way too deep for my liking. Not sure I can find a way around it since the 8mm tube already had the same thing. Inserting a smaller tube inside the 10mm tube brought it to roughly the same level as I got with the damped 8mm tube, but not any better. I'll try the heatshrink to see if that can fix it.
     
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  16. Biodegraded

    Biodegraded Friend

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    The UERM has twin bores, right? I've always suspected that the complicated pressure relationships between the twin bores interacting with lack of coupler damping are what screws up my plastic-tube UE900 measurements. These look nothing like I hear, and even my electrical impedance measurement is way off published ones. Might be best to focus first on 'phones with simple bore geometries.

    Also, at least initially, why not focus on <10k and forget about the highs. I don't think any measurements from any coupler around so far come anywhere close to consistency let alone reality, and I suspect a lot of people would be most interested in seeing reliably comparable results from upper mids and lower treble (supplemented by good descriptions of how the higher frequencies are).
     
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  17. Serious

    Serious Inquisitive Frequency Response Plot

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    Might be. Another issue is that the tip of the bore doesn't seal with the tube (obviously) and instead it only seals at the end of the tube. That'll definitely change the resonant frequency and damping inside the tube. As such I'd really need a tube with a geometry like my ear canal to even attempt to get accurate results.
    I gotta say I was surprised by how well damped the 10mm mic was compared to the 6mm mic. Probably too much, so I'm going to try an 8mm mic. Might be closest to the size of a eardrum.

    I have a feeling both are interrelated. I doubt you can get good accuracy for low to mid frequencies while disregarding the treble. So I've thought about two options:
    • Measuring IEMs in two different tubes with known volumes, measuring the difference (to more or less get the acoustic output impedance) and then calculating the response for a coupler that's more similar to a realistic coupler. Kinda convoluted and likely inaccurate setup. It might work in theory, but I doubt it works in practice.
    • Making a tube into the shape of the ear canal. Then drilling tiny holes into the tube into which you can insert tiny tubes for attaching volumes. Basically a craptastic DIY construction of real measurement gear.
    Turns out someone has already done just that: https://www.head-fi.org/threads/relatively-cheap-headphone-measuring-kit.664900/page-3#post-13944137
    That might work. Won't be super easy to get it accurate, though. But I think it's really the only option other than buying a fake/cheap 711. That's definitely something I'm considering, but I'm not convinced the 711 is that good. That way you're not really limited to the 711 coupler. Would have to measure the response of the tube somehow.

    Not really related, but I also found this: https://www.reddit.com/r/headphones/comments/g18b86/comparing_headphone_measurement_systems/
    Pretty interesting.
     
  18. purr1n

    purr1n Finding his inner redneck

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    Not too sure about side volume compartments because that creates resonators on the side and may not be equivalent to different tube lengths. Perhaps another approach is a tube with adjustable length, take several measurements, and see what peaks move and what stays. I’m convinced our brains have inverse transfer functions that process these peaks according to our specific anatomies.

    What tube diameter are you using with IMM6? I should play with this.

    Already implied: perhaps DD type drivers have higher output Z in the lows compared to BA drivers, sufficiency higher to be affected by Zdrum. I feel that the DIY measurements I have underreport the bass output from DD drivers.

    Question: how do the 711 couplers fare compared to the DIY tube solutions when it comes to differences in the lows between BA and DD.
     
  19. Biodegraded

    Biodegraded Friend

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    5/16" ID, 7/16" OD. In the plumbing supplies aisle at the hardware store.

    Hmm, dunno. I haven't measured any BAs with Andromeda-style crazy low impedance in the bass. The 4BA UE900 is around 20 ohms there - not too different from the 2BA Westone UMPro 20 which didn't give me any measurement problems.

    Here's the TRN BA5 (what? A cheap 5BA unit) on a tube & mic identical to mine:

    https://i.imgur.com/8guY7NI.jpg

    on a 2.5cm tube designed to mimic the 8k resonance:

    https://imgur.com/8UuCJ2m

    and on an IEC-711 clone:

    https://i.imgur.com/OGIF6vK.png

    No impedance curve for this one, unfortunately, I didn't get to hear or measure it myself.

    Bonus pic for @Serious : here's a hybrid 1DD 4BA on 3 different IEC units, I think all measured by the same person:

    https://i.imgur.com/1fp6lEe.png
     
  20. purr1n

    purr1n Finding his inner redneck

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    Thanks for the links.

    For impedance, I was thinking acoustical impedance to go with Zdrum, not electrical impedance. People seem to parrot this German X coupler has impedance that matches human ear, and then another German or Danish company e-peens that they got some Y betterer ‘moar accurate" than X. Then a dozen /r/headphones boobs start parroting 711 5150 10-10 roger. However none of this means jack shit if we don’t even know simple voltage/pressure divider calculations and have zero data for acoustical Zout for IEMs.

    What I would like to see is sets of DD and BA IEMs on a DIY coupler and 711 type. This to infer acoustical Zout in the absence of any data. If we find that the acoustical Zout / Zin relationship matters, then there’s more reason to opt with a 711 or similar type. If not, screw that technobabble BS and simply provide static compensation.

    The peaks remain problematic however regardless. Perhaps GRAS has something new for Jude. But occasional one time uncompensated measurements from ever changing betterer and betterer systems provides us with incomparable data. It’s just measurement data for the sake of establishing (false) authority from a PR perspective.

    I am also curious how Zdrum is different from person to person.

    These Germans and Danes love to make stuff more complex than it needs to be. If it were up to them, we’d be using dummy heads, pinna, couplers, Olive’s shitty targets to measure speaker response instead of pointing a wand mic straight at the speaker, saying it’s good enough, and calling it a day.
     
    Last edited: Jun 7, 2020

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