[caption id="attachment_617" align="alignright" width="300" caption="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?"] [/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."] [/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."] [/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?"] [/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."] [/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."] [/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."] [/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."] [/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."] [/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."] [/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."] [/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."] [/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."] [/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 "] [/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"] [/caption]
And here it is off the home made tester.
Say! That looks pretty close! |
[caption id="" align="alignleft" width="298" caption="Shure SRH440"] [/caption]Here's the official measurement for the Shure SRH440. |
[caption id="attachment_642" align="alignright" width="300" caption="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"] [/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"] [/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"] [/caption]Last one, here's the Sennheiser HD800 headphones. |
[caption id="attachment_646" align="alignright" width="300" caption="Sennheiser HD 800"] [/caption]
Another surprisingly good result.
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