Apr 28, 2010 at 11:08 PM

#### rimrocks

##### Member of the Trade: Smyth Research
Quote:

 Originally Posted by rsaavedra /img/forum/go_quote.gif Treble is known to go beserk with each little tiny movement of the headphone on the dummy head, you've said it yourself, Tyll.

It could also be true that the repeatability of positioning headphones on your own (real) head is greater (ie less deviation) than positioning headphones on a dummy head. When we wear headphones there's only really one good position - but that may not be so true for a dummy.

mike

Apr 29, 2010 at 2:37 AM

#### Tyll Hertsens

Quote:

 Originally Posted by xnor /img/forum/go_quote.gif So I don't think it's easy to say square wave A beats B. Some things you can say ... we'll learn more over time. Could you take the same headphones and do the same measurement again but with a 30 Hz square wave? (picture would do) When I get to the bit of measureing square waves we might could. Did you make those measurements at the same volume? Yes. Another thing: at headroom check out K701 and SRH840 frequency response and 50 Hz square waves and compare them.

Ahhh! Good eye.

The Shure 840 is a sealed can and has some porting that will allow the lows to phase shift and go negative after a bit.

Check out the Ulrasones too. Many sealed cans do.

Quote:

 Originally Posted by rimrocks /img/forum/go_quote.gif It could also be true that the repeatability of positioning headphones on your own (real) head is greater (ie less deviation) than positioning headphones on a dummy head.

I'll have to make a little video of placing the cans. What I'm finding is that it's hard not to do it pretty right.

Alrighty, I've gotten the routines and spreadsheets prepared. The first victim was the AKG K701

Here's the graphs:

First one is me placing the headphones: centered, front, back, up, and down. I don't think the movements were more than 5mm from the center position in any direction as you really couldn't move the headphones farther without running into the ear.

Also on the first graph is the numerical average of all five measurements. As you can see there's not as much variation with the 701 as there was with the HD800. The average of all 5 measurements is, I think, accurately representative of the five measurements.

On the second graph is the DF compenastion curve for the head in red; the average measurement from above in aqua green; and the subtraction of one from the other to make the estimate of what the cans sound like in the blue.

K701s are pretty neutral, looks okay to me I guess.

Next couple of days I'll be measureing that whole seiries of cans I just did again and we'll have a look at the data like above for a bunch of cans and see how they act.

Apr 29, 2010 at 3:07 AM

#### Roger Strummer

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? That doubt hasn't left my mind for the past several hours...

Apr 29, 2010 at 3:39 AM

#### rsaavedra

Quote:

 Originally Posted by Tyll Hertsens /img/forum/go_quote.gif I'll have to make a little video of placing the cans. What I'm finding is that it's hard not to do it pretty right. : First one is me placing the headphones: centered, front, back, up, and down. I don't think the movements were more than 5mm from the center position in any direction as you really couldn't move the headphones farther without running into the ear.

Also, the HD6x0's have an ovoidal cup, not exactly circular. And its longest axis is tilted from higher-back to lower-front. If it allows for the same 5mm displacement in exactly the same directions you used for the K701, then we are ok: measurements for the HD6x0 (or any other non-circular headphone) would be taken the same way as for the K701. Otherwise, it wouldn't really be the same way to take the 5 measurements. But it must be the same for all headphones --that aren't IEM's, that is.

One other thing, how exactly is the "center" position determined? Just visually trying to land the center of the driver right on top of the ear canal? Or are you using some sort of aid, like maybe concentric circles marked around the ear canal on the dummy head, a la shooting target? In fact, I think that could be very helpful to accurately place the cup either centered or at any of the 5mm front, back, up, and down displacements:

Apr 29, 2010 at 3:54 AM

#### rsaavedra

Quote:

 Originally Posted by rimrocks /img/forum/go_quote.gif When we wear headphones there's only really one good position

Hmm... says who? Some people wear the same types of headphones in different positions. Meier Audio says the following on his webpages:

Quote:

 Scientific research has shown that our perception of depth is increased when the driver is placed more forward and lower with respect to the entrance of the ear channel.

Source: Meier Audio - Tips & Tricks

PS. Addendum: there's also the AKG K1000, which offer a huge variability in how you place and orient the drivers with respect to your ears.

Apr 29, 2010 at 4:12 AM

#### rsaavedra

Tyll, just thinking out loud here. Another placement aid could possibly be two laser pointers somehow fixed symmetrically and appropriately on either side of the surroundings of the dummy head, and each laser aiming directly at each ear canal. Then you place the headphones on the head, and the laser spot on the back of each cup should help placing them right on center, or at any of your 5mm displacements as well.

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.)

Apr 29, 2010 at 4:37 PM

#### xnor

Quote:

 Originally Posted by Tyll Hertsens /img/forum/go_quote.gif When I get to the bit of measureing square waves we might could.

Might could? Cool!

I was mentioning this because, if the phase shift measurements from ryumatsuba are correct, the K701's square wave might look better then.

What I'm saying is that it might not be so easy to tell how the headphone really performs in the low end just by looking at a square wave.
For that I prefer the good old FR. Simpler to read, accurate, not so much room for misinterpretation or misreading (and I bet that many guys here on the forums, excluding sound science of course, don't even understand FR graphs).

Phase shift and maybe other effects, as can be seen above, skew the line making the headphone look "bad" (obviously by misinterpretation). However the headphone could be just fine and produce voluminous bass. I don't think that phase shift is something that you'd want / need to be as low as possible with headphones, cause it might not even be audible. Correct me if I'm wrong here.
It could take some time (and a couple of experiments) to find out and understand in detail what's really going on here, but it would make a lot of sense to correctly explain those square waves since you're going to provide them with your measurements.

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.
I don't know how exact you want the results to be or how exact you could possibly make them, but things are looking good.

Cheers

Btw, does anyone know how clamping force is defined in IEC 60268-7? Would be nice to get free copies of such standards..

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?

Apr 30, 2010 at 4:19 PM

#### pictureguy

Good stuff:
Now, from the perspective of someone who spent 30 years in metrology:
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)

Apr 30, 2010 at 5:02 PM

#### rsaavedra

Quote:

 Originally Posted by xnor /img/forum/go_quote.gif Btw, does anyone know how clamping force is defined in IEC 60268-7? Would be nice to get free copies of such standards..

Clamping force is something I've been thinking about as well. Don't know how it is defined in that standard though.

In spite of the headphone being at exactly the same location on the dummy head (thinking in laser-pointer or concentric-circle highest possible accuracy terms,) a stronger clamping force might bring the driver closer to the ear canal by a few mm, hence yielding a different distance between driver and ear canal (and ear lobe as well.) That might result in a different FR, at least in some treble frequencies, or in the relative emphasis between highs and lows (even when the positioning of the headphone might seem perfectly replicated.)

Also, in perceptual terms (maybe measurement-wise too,) I think a different level of clamping force might result in bass perceived somewhat differently.

This difference in distance between driver and ear canal might be due to not only differences in clamping force, but also differences in how soft/worn vs rigid/new the cup cushions or earpads might be.

PS. Keep in mind I'm mostly thinking about the repeatability/reproducibility of the method and observed measurements.

Apr 30, 2010 at 5:26 PM

#### rsaavedra

Quote:

 Originally Posted by xnor /img/forum/go_quote.gif start measuring the FR at 50 dB

And that is yet another source of possible variability in the measurements, the volume of the signals being played by the headphones.

In fact, not just the volume, but the signal itself.

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?)

Keep in mind that the FR can vary depending on volume. A headphone playing at low volume might have a flatter FR than when playing loud. Or viceversa.

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.

Apr 30, 2010 at 5:46 PM

#### xnor

rsaavedra, I don't know what Tyll is using as signal for FR measurements, maybe pink noise or a sine sweep.

Quote:

 Originally Posted by rsaavedra /img/forum/go_quote.gif Keep in mind that the FR can vary depending on volume.

Oh really, guess why I suggested to take a look at this?

As for volume matching I'd use the "standard method" with a 1 kHz sine.

Apr 30, 2010 at 7:04 PM

#### Tyll Hertsens

All good questions folks; I'm starting to write a post now, but I thought I should give you all a link to the PDF of all the measurements so that you can draw some of your own conclusions while I do.

It's all the headphones measured five times each in different positions.

Centered, forward, back, up, and down.

Here's a little video:

And a couple of pix to look at:

More tomorrow.

Apr 30, 2010 at 7:30 PM

#### rsaavedra

Quote:

 Originally Posted by Tyll Hertsens /img/forum/go_quote.gif All good questions folks; I'm starting to write a post now, but I thought I should give you all a link to the PDF of all the measurements so that you can draw some of your own conclusions while I do.

Very nice PDF Tyll!
Scrolling between the pages keeps the graphs locked, while the FR line and the headphone names change. Very cool!

I´m a little puzzled by the HD600 vs. HD650's measurements. The 650 has commonly been regarded as having this extra minor hump in the mid bass. In your measurements, however, the hump appears more pronounced in the HD600, and the 650 seems to actually have a flatter bass (in fact, flatter response overall, even though people tend to say it's the other way around.)

To my eyes, and from the graphs, the flattest response of them all seems to be the Denon D7000's.

Apr 30, 2010 at 7:37 PM

#### MrGreen

Quote:

 Originally Posted by Tyll Hertsens /img/forum/go_quote.gif Here's a little video:

I'd hate to complicate things, but what about the effects of rotating the headband forward or backwards?

Most headphones have a certain amount of give in the cups so they conform to your head at most reasonable angles. Wouldn't this have an effect on the results? How are you making sure that the angle is the same?

I suspect that this would be a major issue for angled driver headphones like the AD700 or T1.

Then again, I'm not at all knowledgeable and it's real late at night. But it was just an observation I made.

Keep up the good work, mate.

Apr 30, 2010 at 7:38 PM