Big post coming up.
First, I feel like I need to say that I love all the interesting comments and brainstorming, but I do have a mission I'm on, which is being able to measure headphones at CanJam, which is just a few weeks away. So I have to keep moving forward and staying focused on that. If at times I just sort of run by a few things I apologize, it's simply because I'm trying to stay on track. I can tell you that I am carefully reading the comments though and many of them have already been productive.
Before we get too far with this post, I'll point again to this
Positioning Tests PDF, I'll be referring to it.
On to some questions:
Quote:
Originally Posted by rsaavedra
- Do you know how treble-wise accurately you can reproduce the positioning of the headphones on the dummy head? Or in other words, do you know the standard deviation or variance you can get in your treble measurements when repeating measurements for a specific positioning of the headphones on the dummy head?
|
No, I don't. But from the test just done on position, it looks like:
Most headphones vary less than I was expecting. In these tests I generally just moved the headphones as far in each direction as I could without being in an obviously incorrect position. The only headphones that really moved very far were the HD800s, which have huge ear cups.
In most cases, as you look at the data, it seems like there is a characteristic cureve, and then amplitude fluctuations in the curve. This can generally be seen in the fact that the left and right channels seem to track each other fairly well.
Also, have a look at the first two K701 tests and the two D5000 tests. In both cases the two tests were taken two days apart. What I see in the data is two fairly strongly repeatable tests. The first D5000 test has quite a bit more deviation in the high frequency information, but when you look at the averaged data curves they look very similar.
Quote:
Originally Posted by rsaaveddra
- How exactly do you determine the five different headphone positions you choose to gather FR measurements from?
|
As shown in the little movie, it's become apparent that simply feeling for the right position is pretty much the only way to do it. At first blush I thought the laser idea might work, but what are you really targeting. It's the same with the square wave thing, I can tell if the seal is changing 'cuz I'll loos bass, and I can tell that things are changing in the highs, but as has been pointed out to me, who the heck know what's right.
Seriously, you should go try putting hradphones on someone elses head, and you'll see that it's fairly obvious when things are right.
I'll also add that how much you can move is a total crap shoot. Some headphones you can move a lot, some not so much. I've come to feel that headphone makers have control over these things (how close the want the cans to fit around the ear, how much space is available) so they are responsible for building a headphone that can be worn in widely differing positions or not.
Given that the method of taking five measurements seems to be repeatable and moderately insensitive to movement, I'm thinking that the "do it by feel five times with moderate movement in each direction away from center" is a good one ... in fact maybe the only practical one.
Quote:
Originally Posted by Roger Strummer /img/forum/go_quote.gif
Hey Tyll, I was wondering, since there is difference between channels, when one looks at the graphs in the headroom site (in the page where you can compare various headphones) are you seeing only one (arbitrary) channel, an average of both or what?
|
It's the left chanel --- the one on the head the seems to measure best.
Quote:
Originally Posted by rsaaverdra
I think the concentric shooting target circles around the ear canal, or these laser pointers, or even both strategies together, might really help making placements more accurately repeatable, regardless of headphone (non-IEMs, of course.)
|
Just wanted to add a point here that I can feel the seam around the ear which allows me to move the earpads fairly reliably to the five different positions.
Quote:
Originally Posted by xnor /img/forum/go_quote.gif
The graphs are as usual very nice. It shows that it makes sense to average a couple of re-placements. I wonder how much different the averaged result would look like if you re-did those 5 measurements.
|
The two D5000 and K701 graphs were done on different days.
Quote:
Originally Posted by xnor
Btw, does anyone know how clamping force is defined in IEC 60268-7? Would be nice to get free copies of such standards..
|
Just gave a quick flash through it, but I couldn't find any spec.
Quote:
Originally Posted by xnor
edit: Another thing that just came to my mind is linearity: start measuring the FR at 50 dB and double the voltage (approx. +6 dB) every measurement until you're at ~92 dB. (8 measurements)
If you put all the measured curves on top of each other the result should be a single perfectly overlapping curve, shouldn't it?
|
Quote:
Originally Posted by rsaavedra
It would be really interesting to have different FR's for different volume levels per headphone. That would mean, of course, more measurements and more work, I know. I'm just dreaming out loud.
|
I'll see if I can fit one of these tests in. Currently I'm doing most measurements at 90dBSPL at the ear.
While I don't think we'll see much of a difference in fr based on SPL, I do think well see some differenced in distortion as it gets higher. So I'm currently thinking I will have some Frequency vs THD and hamonic series spectra at different output levels.
My justification for using 90dB is that it's loud, but well within the range where the headphones should be acting normally.
Quote:
Originally Posted by pictureguy /img/forum/go_quote.gif
Are you going to build a dummy load for amps? By this I mean NOT just a power resistor and a DVM with signal generator / oscillator.
I mean a truly reactive load. I can find some schematics which the loudspeaker folks use. This will show an amps response into a more realistic load. I don't know if it is possible to make a sound transducer of pure resistance.
Also, as fun as it is to measure sensitivity and impedance and max spl of a transducer, wouldn't it be also nice to know the PHASE ANGLE of the load? Phase angle is important since increasing angle results in less power being delivered to such load. For example, at a given frequency, if the phase angle is 45degrees, the result is a loss of power of nearly 30%. (cosine of phase angle)
|
These are both good points. Would you like to help by building some dummy loads for amps? I won't be getting to that for a while yet as the headphone measurements need to be well along the way first, but I'd love the help.
We should be able to get the fphase information out of the impulse response data. I will be persuing that.
Quote:
Originally Posted by rsaavedra /img/forum/go_quote.gif
Clamping force is something I've been thinking about as well. Don't know how it is defined in that standard though.
Also, in perceptual terms (maybe measurement-wise too,) I think a different level of clamping force might result in bass perceived somewhat differently.
PS. Keep in mind I'm mostly thinking about the repeatability/reproducibility of the method and observed measurements.
|
These quotes are pretty right on. I do see bass warm up a bit as clamping force is increased, and the volume get louder overall.
And in terms of repeatability, I agree, too much external figiting with the cans (like putting a rubber band around the whole thing to improve the seal) is really beginning to over influence the test IMHO.
The manufacturers know (or should know) what the specified head for measurements is, a lot of work has gone into it, I'm of the opinion curerently that we should just place the cans as well as possible on it, and leave it at that. Though I would be willing to adjust the headband a bit (like with Grados, lets say) to make sure the fit as properly as possible on the head. Remember, the heas is slightly smaller than the average male head, so this can be a bit of a problem.
Quote:
Originally Posted by rsaavedra /img/forum/go_quote.gif
And that is yet another source of possible variability in the measurements, the volume of the signals being played by the headphones.
|
At the bigining of each test the volume is servoed to be 90dBSPL at 1000Hz.
There's a bit of a bug in the test somewhere, or the lines would be laying on top of each other better. I thinkk it's servoing at the beginning of the test, but not for each subsequent sweep. I'll be working on fixing this soon.
rsaavedra said:
Is the FR being gathered using pure sine waves or warble tones? And at exactly what level at which frequency? (e.g. 80 dB at 1 kHz?)]
It's a swept sin wave. The IEC spec says that swept sin, integrated pink noise, and calculation from impulse respose are all allowed, but they prefer impulse response as it also deliver phase info. We may go that way befor this is all over.
Quote:
Originally Posted by xnor /img/forum/go_quote.gif
As for volume matching I'd use the "standard method" with a 1 kHz sine.
|
Actooly, I'm thinking I should use something more like 500Hz as most headphone response is pretty well behaved and flat up to that point; above it things start wiggling around some and may result in fluctuation of over level of the sweep. I think it something like this happening as evidenced by the modest but clear level shifts between curves now.