How To's

Measuring Headphones with Studio Six Digital's Acoustics Analyzer for iPod Touch and iPhone - Pt 3.

I've spent quite a bit of time measuring headphones on our lab's Audio Precision tester and Head Acoustics Aachen head, and I knew it would be very difficult to replicate it's measurements. In fact, I almost abandon the idea right from the start. I really thought it wouldn't be remotely useful, but somebody had to give it a try. And now that I'm a professional headphone geek, I thought, "Why not?" Boy am I glad I gave it a shot!
[caption id="attachment_617" align="alignright" width="300" caption="Chop ... chop ... let's build a gadget!"]Chop ... chop ... let's build a gadget![/caption] Over the weekend, I drug my chop saw out of the garage and found a nice plank in the wood pile and started cutting, gluing, and screwing. I made a base and a sliding part to adjust for headphone width and clamping pressure. I put a little shelf in there with a hole for the Thumbtack mike to stick through.
[caption id="attachment_618" align="alignleft" width="299" caption="Hey look! A headphone tester. I wonder if it'll work?"]Hey look! A headphone tester. I wonder if it'll work?[/caption] It took a little while to get it all together. I had to route a little slot for the iTouch to go into so that the mike was even with the wood's surface; and I had to use some Sculpy clay to seal the hole around the mike. Yeah, that might work.
[caption id="" align="alignright" width="306" caption="Official measured response of the Denon AH-D1001."]Official measured response of the Denon AH-D1001.[/caption]I had a pair of Denon AH-D1001 around, so I decided to use it as a Guinea pig. For reference, here's the measurement from our website for these headphones.
[caption id="attachment_620" align="alignleft" width="281" caption="Just the mike level with the surface and sealed."]Just the mike level with the surface and sealed.[/caption] The first measurement was just the mike sealed in it's hole with some Sculpy clay.
[caption id="attachment_621" align="alignright" width="300" caption="What's with the gap between 4-8kHz?"]What's with the gap between 4-8kHz?[/caption] Well ... that doesn't look too much like the regular measurement.  :(: A big gap between 4-8 kHz, and a bunch of ringing up in the highs. Not really surprised. I suspect the chamber is too "live" with all the bare wood and sound bouncing around within the headphone's ear cup.
[caption id="attachment_624" align="alignleft" width="243" caption="A little felt to damp the chamber."]A little felt to damp the chamber.[/caption] So the next step is to put some felt in the chamber to see if it will settle down.
[caption id="attachment_625" align="alignright" width="300" caption="A little better ... but still not there."]A little better ... but still not there.[/caption] The measurement is a little better, and we see the big notch at the 8kHz beginning to damp, and the ringing in the highs is lower.
[caption id="attachment_626" align="alignleft" width="300" caption="Middle bump displaces the volume of the outer ear."]Middle bump displaces the volume of the outer ear.[/caption] I know that acoustic couplers for measuring headphones have a large bump in the middle to displace the volume of air that would be taken up by the ear within the chamber. Here is a G.R.A.S. Type 43 AS Ear Simulator.
[caption id="attachment_628" align="alignleft" width="292" caption="A simple ear to displace some air volume."]A simple ear to displace some air volume.[/caption] I figured I would need to displace some interior volume so I removed the felt and slapped a simple clay ear on the tester and remeasured.
[caption id="attachment_630" align="alignright" width="300" caption="Wow! Now we're gettin' somewhere."]Wow! Now we're gettin' somewhere.[/caption] As you can see, we've gotten rid of the big gap between 4-8 kHz, but still have some notches.
[caption id="attachment_631" align="alignleft" width="259" caption="Ugly, but effective at stirring things up."]Ugly, but effective at stirring things up.[/caption] At this point I suspect the ear is just too simple and I need some more features on the ear to defract sound around within the ear cup, which will prevent some of the resonances by stirring things up a bit. Ten minutes later I stick a rather more complicated ear on the fixture.
[caption id="attachment_632" align="alignright" width="300" caption="Much better ... but a little to responsive up top."]Much better ... but a little to responsive up top.[/caption] As you can see, this got rid of the notches at 4 and 8 kHz, but the rising frequency up top is not what we see on the real measurements.
[caption id="attachment_634" align="alignleft" width="279" caption="My felt ear took about an hour to make."]My felt ear took about an hour to make.[/caption] I decide to go back to the idea of damping the chamber, and fashion a fake ear out of felt that should absorb some of the energy in the ear cup. I build it up by cutting a bunch of layers of felt and glue them together, and then put a cover of felt over it. Then I trim it up with the scissors.
[caption id="attachment_635" align="alignright" width="300" caption="D1001 measured with felt ear."]D1001 measured with felt ear.[/caption] Suffice it to say I played around quite a bit more, but this is about as close as I got. Not bad though. Let's see how some other cans measure up.
[caption id="" align="alignleft" width="298" caption="Sennheiser HD 448 "]Sennheiser HD 448 [/caption]Here's the Sennheiser HD 448 as measured by our lab system.
[caption id="attachment_638" align="alignright" width="300" caption="Sennheiser HD 448"]Sennheiser HD 448[/caption] And here it is off the home made tester. Say! That looks pretty close!
[caption id="" align="alignleft" width="298" caption="Shure SRH440"]Shure SRH440[/caption]Here's the official measurement for the Shure SRH440.
[caption id="attachment_642" align="alignright" width="300" caption="Shure SRH440"]Shure SRH440[/caption] Oh, well, not quite as close. Still, it's not too bad; let's look at a few more.
[caption id="" align="alignleft" width="298" caption="Sony MDR-V6"]Sony MDR-V6[/caption]Here's the Sony MDR-V6 --- a headphone that measures surprisingly flat.
[caption id="attachment_644" align="alignright" width="300" caption="Sony MDR-V6"]Sony MDR-V6[/caption] It's a little tilted. I think I had a little air leak on this one. The felt ear is a little stiff, and the MDR-V6 has a shallow earcup, so I think the ear was pushing the earpiece off the wood and preventing a good seal, and reducing the bass. But otherwise a decent match.
[caption id="" align="alignleft" width="298" caption="The Sennheiser HD 800"]The Sennheiser HD 800[/caption]Last one, here's the Sennheiser HD800 headphones.
[caption id="attachment_646" align="alignright" width="300" caption="Sennheiser HD 800"]Sennheiser HD 800[/caption] Another surprisingly good result.  :clap
I've got to say, I really didn't think I'd get any useful data out of this little experiment, but it turned out much better than I expected. If any of you decide to build one of these little gadgets, please feel free to send me some pictures and some of your results and I'll be glad to post them. Phew, that was too much fun. :knary

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