Relatively cheap headphone measuring kit?

Discussion in 'Sound Science' started by takato14, May 21, 2013.
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  1. yuriv
    Yeah, the Startech is pretty good most purposes. But if you can build that battery adapter, it would be just as convenient as using the Startech. If you have to assemble the damn thing using alligator clips all the time, then it's not worth the effort. Speaking of an audio to-do list, I've been playing with a new audio toy and the my first project with it may or may not be on topic for this thread, so I'll also post it elsewhere.
    The new toy: The Web Audio API. A web browser apparently has some pretty sophisticated audio tools already built in, and a programmer can quickly build synthesizers, recorders, FFT-based analyzers, and custom audio file playback systems. The API gives you access to oscillators, biquad filters, splitters, delay and gain modules, a convolver, etc. And you can work at the sample level if the processor you're looking for isn't already built in.
    Unfortunately, I don't know anything about web programming, so these nifty tools are out of reach. I wanted to learn the minimum to use these tools. Some of the videos on YouTube show how you can build an adjustable sound source with a toy example that takes only a few minutes and a few lines of html and Javascript. Eventually I'm thinking of using it to build my own custom EQ/DSP/convolver/crossfeed playback system, and even using it to measure one's own hearing for a better idea on what the various parameters should be. Here's an example, my first project, starting small using the built-in oscillators. Perhaps it's relevant to this thread as you're using your own perception to "measure" your own hearing cheaply: YuCH
    That link is flaky, so it might take a few tries to get it. An app like this has been made before, but using Flash. Maybe, as the project improves I'll rename it to MEH (misguided EQ helper) or SICH (self-inflicted calibrator for headphones)  Lol. Some more simple measurement ideas: maybe a finely and easily adjustable sine oscillator with a readout to make it super easy to find resonances and nulls quickly, a utility for experimentally determining what the ITD and ILD can be used as parameters in a crossfeed system. (I wonder if it can be done by using an earplug on the left ear and then using a the Web Audio API's oscillators to determine how much delay and magnitude on the left speaker can null the output on the right speaker the most.)
    Let me see if I can find a more appropriate thread to re-post this info. 
  2. prescient
    Samsung phones have something similar built in (adapt sound). Instead of asking for the people to evaluate equal volume the app asks you to indicate whether you can hear the tone or not. I suspect this is probably a better approach as it is a binary evaluation of the noise.
    Edit: NM i forgot about the fletcher munson curves so maybe binary isn't a great idea unless you apply some sort of correction on top of the EQ according to the curves.
  3. castleofargh Contributor
    yeah I gave up last time I couldn't find my breadboard and bought a soldering iron. last time I used one was about 20 years ago and it shows [​IMG].
    there are a few topics with tutorials on how to EQ by ear, to make copies of a signature by ear, or just to get an idea of flat. the eternal problem being to keep consistent loudness for the equal loudness contour to stay the same.
    for ILD ITD and more funky stuff, I'm not sure. usually you end up with stuff to use in matlab and that's where I stop[​IMG]. something like the razer surround software could probably be a starting point, to "calibrate" you point at the direction you're hearing things, it is incomplete but a pretty fine idea IMO. I'm guessing this could then be used with impulses to get the loudness/EQ/timing applied. and then use that on our own as basis for crossfeed or convolution.  nothing we can't do on our own with something like xnor's crossfeed, music isn't always the easiest sound to set those things.
    I ended up with  picking the few ones that felt ok in the 0 to 45° area, and used the 30° ones as starting point for my kind of custom convolution. but getting a more balanced EQ seems like a good idea before messing around even more with positioning cues. so EQ stays a very relevant first step IMO.
  4. prescient
    If you want to put in the effort I suspect some audio processing in Matlab or equivalent could probably make it pretty easy to shift EQ by loudness contour. A very layman's perspective, but I'm thinking you setup your EQ at one Fletcher Munson curve volume. As you change volume in the digital domain you update your EQ based on a relative change in fletcher munson curve values. Maybe I should learn some matlab/octave as this seems like it could be fun experiment. 
  5. janosch simon
    i watched some of daves videos and this one was very interesting does any of you try this method?
    i wrote him an email and wait for the program could be also nice to opimise my diy electrostatic headphones :)

    cheers janosch
    tmuka likes this.
  6. castleofargh Contributor
    as anything requiring actual work and probably more than a few tries to get used to the method/software(like where to start to avoid clipping), don't expect a lot of headfiers to try. I don't mean it as a critic but as an observation, most people just didn't come here for that.
    there you have a dedicated topic with a few feedbacks and ideas
    it is indeed a cheap but less reliable method compared to buying in ear couplers and measuring a speaker, then the headphone. the very obvious bonus with Griesinger's method is that you can also try with IEMs.
  7. yuriv
    Here’s a follow up to my post last year about the coupler based on the Dayton Audio iMM6 measurement mic. I abandoned the project several months ago because I found a way to get two IEC711 clones from China. And I’ve already put them to good use while modding cheap IEMs. But I think the results of my experiments with the iMM6-based coupler are interesting in their own right. Maybe somebody else here might find the following an interesting read.

    Basically, I wanted to see what could be done to improve the coupler’s response. Sure, you could EQ the output or come up with a calibration curve. But Isuspect that while this approach can get you a little closer to a more accurate response, it may be the case that you’d need a different calibration curve for different IEMs—or at least for different kinds of IEMs. The acoustic circuit to model the system looks like this:

    Source --> IEM’s acoustic output Z --> coupler’s acoustic Z --> iMM6 acoustic input Z​

    What if the IEM has a very different acoustic output impedance from the one you used to make your initial calibration curve?

    An IEC60711 (or IEC60318)-compliant coupler tries to mimic the acoustic transfer impedance of the average human ear. One of the ways it does that is to tune its response using side volumes (really Helmholtz resonators).

    IEC711 coupler 2D section view.png IEC711 coupler 3D section view.png
    (Google image search found nice pictures in a thread by Jude)​

    It’s not just a simple tube with a mic at the end. Maybe something like this can be done with a DIY iMM6 rig. Unfortunately, the hobbyist may have no reliable and inexpensive way of measuring the acoustic impedance of the coupler. Perhaps the best that can be done with limited means is to emulate the output of the IEC711 coupler for as many kinds of inputs as possible:

    Input --> black box --> output ​

    If the black box behaves just like a copy of the real thing with most inputs, then for our purposes, who cares? We’re good to go. Lol. I know, lots of assumptions. But please bear with me.

    Here’s another look at the IEC711’s side volumes:

    As it says in page 11 of the slides (shown above), the IEC711 coupler has two Helmholtz resonators tuned to match drum impedance. A frequently-given example of a Helmholtz resonator is a bottle. If you blow across the top, you’ll hear a sound at its resonant frequency, which depends on the bottle’s volume and the neck’s geometry. That’s pretty much what’s going on inside the ‘711 coupler. The narrow slits are the bottle necks that lead to the side volumes.

    Page 10 above shows the system’s lumped parameter model (LPM) as an LC transmission line (Ma,4, Ma,6, Ma,8, Ca,4, Ca,6, and Ca,8). Each Helmholtz resonator is modeled as an RLC branch (R_, M_, and C_ a,5 for the first one with the opening near the coupler’s reference plane, and R_, M_, and C_a,7 for the second one near the mic).

    For each resonator, M and C determine the resonant frequency and R determines the damping, just like in an electrical RLC circuit. In fact, the slides show that this is how they have traditionally modeled the behavior of the coupler. The values of R, M and C are on page 11, and it shows how we can compute the values of R and M from the dimensions of the slit. Later, on page 21, they give the measurements of the first one: the opening is 0.17 mm x 1.11 mm (for a cross sectional area of 0.1887 mm^2) and the length is 2.57 mm.

    From the values of M and R given on pages 11 and 14, we can compute the angular velocity at resonance, ω = 1/√(MC), from which you can get the frequency. Plugging in the numbers gives 1.19 kHz and 3.5 kHz for each resonator, and these are where the nulls occur in the graph of the undamped response on page 13:

    With damping (energy losses from R), the response looks like this:

    I circled in green the effect of the first resonator, the one with resonant frequency 1.19 kHz. It has a relatively large contribution to the response, compared to the second resonator, whose effect is subtler. This gives a 4 dB to 5 dB rise to the ‘711 coupler’s response. Our base iMM6-based coupler doesn’t have this behavior. Yet.

    Have a look at my first attempt at placing a side volume on the iMM6 coupler:

    iMM6, ER4PT, black syringe and needle as Helmholtz resonator 600x450.png

    I drilled a small hole in the side of the tube and stuck in a 19-gauge blunt-tip needle and a 1 mL syringe. Earlier, I mentioned that the ‘711 coupler’s first slit had a cross-sectional area of 0.1887 mm^2. By my calculation, this falls between 24 and 25 gauge. At the time, I didn’t have a 25-gauge needle, so I used 19-gauge, which is much thicker (0.6528 mm^2 cross section). Here’s its effect with an Etymotic ER4PT in the tube:

    ER4PT FR into iMM6, 1mL syringe, 19 gauge needle.png

    As the syringe volume gets smaller, the resonant frequency increases and the response swings less. You could compare this to a spring-mass-damper system. The air volume in the syringe is like a spring, as air is compressible and elastic. The mass is the mass of the air inside the needle’s tube. The damping comes from energy loss in viscous flow. Smaller springs and smaller moving masses start off with less energy, so less swing.

    Another observation is that the wiggles in the graph look like behavior that's somewhere between the undamped and damped responses in pages 13 and 15 of the AES slides shown earlier. So it looks like we need more damping if we want to copy the damped response on the ‘711 coupler. Also, the red curve above is the response with the syringe already at its minimum volume. To increase the resonant frequency to something close to 1.19 kHz, we can’t decrease the air volume in the syringe any further, but we can still shorten the needle’s tube or increase its width.

    I decided to saw off the end of the needle. I didn’t need to make it as short as the ‘711 coupler’s first slit (2.57 mm) because the 19 gauge needle’s larger cross section gives a much higher M (related to mass really) for the same length. Here’s what it looks like:

    iMM6, ER4PT, black syringe and needle as Helmholtz resonator 2 600x450.png

    The needle is a bit shorter now. The picture also shows a bit of earbud foam; part of the piece that’s missing was placed in the air volume to increase the RLC branch’s series R for damping. After playing with it for a while, I discovered that if I instead placed the damper in the needle’s metal tube, then I needed very little foam to get the same result. Also, since the air volume needed to get the resonance close to 1.19 kHz is less than 0.1mL, I found that I could do away with the syringe and just use some putty (Blu-Tack) to seal the air in the needle’s plastic sleeve. Here’s the result:

    ER4PT FR into iMM6 base response.png
    Before: the iMM6’s base response without the resonator. (Ignore the bass roll-off for now. After taking apart and putting the iMM6 back together, it now has an air leak that I should take care of some day.)​

    ER4PT FR into iMM6, shortened 19 gauge needle, foam damper.png
    After: the response with the resonator. Frequencies above the bump effectively get around a 3 dB boost. If we subtract the two responses, we see that the biggest difference from my ‘711 clone comes at around 2.3 kHz, where the iMM6 measures approximately 4 dB lower compared to the rest of the spectrum. Also, the peak is a different spot.​

    I thought about the placing the second resonator near 3.5 kHz, just like on a ‘711 coupler, but maybe being slick about it and getting the resonator’s dip to partially cancel the right side of the 3 kHz peak, thereby shifting the peak lower in frequency.

    I also thought about placing a third resonator to fix the response at 2.3 kHz. Maybe if I had used an 8-mm tube instead of 7 mm, I wouldn’t need this fix. I don’t know. I abandoned the project before I could explore it. Here’s where I left off:

    iMM6, silicone tube and three Helmholtz resonators 600x450.jpg
    Dayton Audio iMM6-based coupler with three Helmholtz resonators, with resonances near 1kHz (19 gauge shortened needle, the one with the black plastic sleeve), 2 kHz (16 gauge, green, damper in tube), and 3.5 kHz (18 gauge shortened needle, orange).​

    Here were the kinds of measurements I was getting before I put the project aside:

    ER4PT FR into iMM6, three resonators, two attempts.png iMM6, ER4PT, three Helmholtz resonators and FFT Plot in IOS 667x375.png

    I’m not completely satisfied with the results, but maybe someone out there will have better luck. The seal with the putty isn’t permanent, but it allows for quick changes. After a few months, the parameters probably have drifted. If I had been satisfied with the results, maybe I would have made the seal more permanent.

    On the other hand, I’m not completely dissatisfied with the results either. I had some fun playing with it. Here are a few more comparisons:

    Philips SHE3905 frequency response into iMM6 with three resonators (red), IEC711 clone (green).png
    Philips SHE3905​

    Sony XBA-C10ip frequency response into iMM6 with three resonators (red), IEC711 clone (green).png
    Sony XBA-C10iP​

    I stopped working on the project, but I still find it interesting. If you already have a measurement mic, the material cost of constructing and tweaking a coupler is very low, even with these resonators. An added advantage is that you can see the IEM inside the tube and you can mark off where the reference plane should be.

    So, if you have a DIY IEM measuring rig, please share what you did to move beyond just having the mic in the tube. Or share any other ideas anyway. For example, I suppose you could buy a ‘711 clone without mic and use whatever measurement mic you have at hand. But I’m glad I ordered the ones with the microphones already included.

    Even if this project leads to a dead end, I still found its experiments fascinating. I’m thinking of using some of the ideas here to mod cheap IEMs, maybe even full-size cans, to further tune their response. (Yes, I'm aware that these are old ideas that have been done to death by manufacturers and hobbyists alike.)
    Last edited: Dec 30, 2017
    bartzky likes this.
  8. castleofargh Contributor
    it's fun how we end up using similar IEMs for those test. the other day I posted this and picked randomly 3 of my IEMs. guess what? an er4 (although not the same model), and a xba-c10 ^_^. I had a Philips on the table and ran some measurements in it too, but I'm not sure about the exact version number so I decided not to use them.

    and I have been thinking about the same sort of things, but for my EMC8000+coupler, because the extrusions on the side of the mic are adding free space that I didn't want to have. and I I struggled a little trying to find a solution the wouldn't move with time yet let me still use the EMC8000 alone for room measurements. I gave up and just placed a thin aluminum sheet over it with a hole over the capsule. it doesn't totally remove the effect of the extrusions around the EMC8000's head compared to filling them with blu tack, but came close enough IMO for an easy and fully reversible option. having very low expectations really helps to set the "close enough" level.
  9. yuriv
    The ER4 series IEMs are an obvious choice for a sample response because they’ve been around for a while and everyone knows what they sound like and how they measure. I happened to have my XBA-C10 on the workbench because it developed a channel imbalance, and I’m trying to fix it. Ha. But maybe should have a used a different one for the other sample graph as it’s also single-BA like the ER4.

    The ECM8000 gave me some unexpected results too when I first used it with some vinyl tubing. It’s right here in this thread, post #14 on page 1. Looking back and reading it again, I can see that the rig gets the correct frequency of the peak on my ER4PT. It agrees with the Chinese IEC711 clones and the FR graph that comes in the box. It’s my iMM6 coupler that has the unusual response at 3k. It’s the odd one out, so I’ll have to investigate the problem some time.

    Your iMM6 doesn’t seem to have the same weird response. This is why it’s good to see what others are doing with the same gear. Maybe we can get really good results with properly functioning iMM6s with just a single resonator. It’s really the one near 1 kHz that makes the big difference. The one near 3.5k is just fine-tuning the response. I’d love to see what that would look like on an ER4SR.

    It’s not hard to make a resonator that gets the resonance's f and Q that’s close to the ‘711 coupler’s first slit and side volume, even if the resulting R, M, and C are very different. I used a 19-gauge blunt needle with the end sawed off, some earbud foam, and some Blu-Tack for making quick changes. I bet that you can get the response closer to the graph in the box than what you’re getting with your other rigs: The EARS response looks way off. The ECM8000’s looks rolled off on both ends. The Veritas response looks like it’s falling off a cliff. Those might be harder to fix. Your iMM6 rig’s response, on the other hand, looks like it’s already close above 2k. The effect of the first resonator will be to bump 1k and lower by a good 3 dB down, preferably closer to 5 dB.

    BTW, I’ve had my eye on that EARS rig, but I didn’t pull the trigger. Are you planning to write a review before Tyll and Bob Katz get a chance to write about it?
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