Reducing Headphone Treble Response with Paper Filters in Front of the Driver

These are the four filters Takato14 sent in. Filters 1 and 2 appear to be very air permeable and thin. Filters 3 and 4 are the same and apear to be Tyvek (the material used for FedEx envelopes) and somewhat impermeable to air.

Takato14 is a freak for old headphones and regularly sends in boxes of vintage cans for measurement. (Thanks, mate!) Evidently, he's also a sucker for a lost cause and decided to try to get rid of the nasty 6kHz peak in the Sennheiser HD700 response.

His thesis is the pads were to blame and proceeded to perform a significant modification, which did net good results. He felt they could be further improved with modifications to the acoustic exit hole behind the driver, and small disks of filter paper in front of the driver. The filter paper was intended to reduce some of the treble response as he felt them a bit too hot.

I did some initial measurements on the mod as he sent them in, but there were some odd artifacts in the bass distortion. When I sent him the results, he asked if I could remove the filters and measure the cans without. I did so and found the distortion to be significantly reduced but, of course, the treble was back to being too elevated.

160109_Blog_TakatoHD700_Photo_FilterinHeadphone

Photo of a couple filters in place inside the headphones. Once the dust cover is installed it holds the filters gently, but not rigidly, in place.

Well, now I was curious. Where did the distortion come from? He included four filters for each earpiece, what would the response be with only one or two? And...there you go, a nice little rabbit hole to dive into. I proceeded to measure the headphones with a variety of damping to see what it would look like.

In addition to the filters Takato14 included, I also tried some toilet paper as a replacement for the Tyvek filters as they seemed to be rather impermeable to air. Here's a combined frequency response plot for all the headphones.

160109_Blog_TakatoHD700_Photo_Main

I think it's probably best not to read too much into any changes in bass response as that is somewhat sensitive to positional changes on the measurement head. For a number of these measurements I used my IEM testing routine, which only takes one measurement of frequency response and allowed me to complete the survey much more quickly. But it also means that the bass response is not averaged over a number of measurements, which means it may be inaccurate. For the same reason, treble response noise may be a bit greater on the unaveraged measurements.

It doesn't much matter though as I was more interested in the gross trends in treble response and how distortion would change as the amount of filtering increased.

In the legend of the above plot, the headphones are arranged in order of increasing amounts of filtering from top to bottom. I also made the color of the plots move from yellow through the color wheel to green so as to make the progression of change a little more easily apparent.

What is fairly readily apparent is that the treble does indeed reduce as more damping is added. (Duh.) But it's nice for the DIYer to know the magnitude of change as the filters are added, which is as much as 10dB at around 8kHz. That's a significant amount of tuning ability!

Unfortunately, there's a down side. Apparently, if you add too much filtering you end up creating a problem in that at some point distortion starts to markedly increase in the bass. My guess is this might come from two possible sources:

1) The filter papers (especially the Tyvek ones) might be mechanically moving along with the driver. When the driver pushes out, the filters will move away from the driver lowering the acoustic impedance they present. When the driver pulls inward, it may suck the filters inward increasing the acoustic impedance they present. This asymmetry of positive and negative going pressure would create even order distortion.

2) Because the filter papers are just resting between the driver and dust cover, they can move around. When they do, it's possible that that they make some small amount of noise on their own. Sort of softly clapping into each other. I thought this might have been the case with the Tyvek filters, but it turns out toilet paper in place of the Tyvek also causes a similar distortion.

Here's a .pdf booklet of all the measurements, again put in order of less filtering towards more.

One thing to observe there is the initial transient edge of the impulse response when using filters 1,2, and 3 (the Tyvek filter) is is reduced relative to the measurement of filters 1, 2, and two plies of toilet paper. However, bass distortion is worse with the toilet paper. This indicates to me that the Tyvek is slowing impulse response, possibly because the air isn't moving through it as much as driving the Tyvek back and forth acting as a second series diaphragm, which would delay fast transients.

At any rate, my take is that the very thin damping materials Takato14 used (scavanged from surgical breath masks) do provide provide roughly about 1-2dB of treble reduction per layer above 5kHz. A handy little rule of thumb for DIYers, I reckon. And secondarily, once you get to about 3-5 layers of that (or 1-2 plies of toilet paper) you bigin to get the possibility of distortion from the acoustic impedance of the paper. Of course, different headphones will respond differently, but I would think the technique is still broadly aplicable if you want to knock the treble down a few dB.

If you're interested in the full story on these cans, see this superbestaudiofriends.org thread. Thanks for the lost weekend, Tak!

COMMENTS
lmader's picture

Great work Tyll and Tak.

burhan11's picture

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Seth195208's picture

This might be a schiitty question, but what brand of toilet paper did you use. Really. There's a big difference between Scott and Charmin.

Tyll Hertsens's picture
I think it was Western Family brand extra large roll two ply...the "Not sand paper, but close, you cheap bastard!" version.
Argyris's picture

I remember when I tried TP to tame a Grado SR225i I had early on in my headphone experience, a sheet of two-ply did a nice job smoothing out the treble, but it also ended up blocking midbass, which was readily apparent when removing the TP and listening again. Ultimately I didn't really like either tuning and moved on from that set, but it was an interesting result nonetheless.

Beagle's picture

Coffee filters?

This is interesting. I wonder if similar results might be obtained for the HD800? Would save me $1500.

Sal1950's picture

A set of HD 650s would be a lot cheaper

Seth195208's picture

But the brown ones would probably be pretty natural sounding.

Sal1950's picture

Don't you think a good eq system would be a better solution than adding numerous veils? LOL

AllanMarcus's picture

Equalization is not always available.

AllanMarcus's picture

MrSpeakers has sold "doggie treats" and provides a "tune-up kit" with Ether C that essentially does the same thing. These products from MrSpeakers are filters to attenuate treble, and they do not cause bass distortion. You might want try them.

gibtg's picture

This is exactly what I wanted to read Tyll! Thank you for elaborating on this handy DIY trick!

ktmracer's picture

BTW, if you haven't checked out superbestaudiofriends, I suggest you check it out. It's like Head-Fi without the crap. (AKA the "High-Yield" stuff if you are in the education world.) Tyll is fairly active on it and he has a lot of interesting stuff to say.

drWho2's picture

Interesting that toilet paper was one of the SBAF filters. I'm literally on the throne, iPaddin' this comment, and squeezin' out some gooey No. 2 ... and I had an epiphany: if the TP was better than the stock foam on my HD700, could I re-use that foam for the real deal. I'm gonna need it in a minute!

Seth195208's picture

.

Bob Katz's picture

It seems to me paper has a stiffness and is less neutral with respect to frequency. I think you should experiment with breathable cloths of different thickness. I'll bet the cloth won't introduce the bass distortion either. A stiff piece of paper forms a membrane which is quite variable in response at different frequencies. Depending on the weave of the cloth, you can control the high frequency response.

drWho2's picture

Lotsa "fun" 'speriments fer kidz who live for this stuff! I'm sure the homeboyz at time-kill central (SBAF, H-F) can do the grunt work.
Then, Tyll can recap ... and we can all go the crafts store for more felt pads. And be happy and DIYed again.
What I'm interested in is Sennheiser's/AKG's/et. al.'s OWN experiments with materials -- any why they used the stock material that ships with their cans ... and why they don't offer aftermarket filters? E.g., too little ROI, etc.?
BTW: I'm outta TP and I feel 'nother Nr. 2 comin' on ;)

LytleSoudn's picture

You can add all you want between the driver and the ear, but 'filters' won't fix a resonance such as this. The resonance is inherent in the driver and changing the path won't alter it and may add distortion to the signal, either by adding in extraneous vibration or seriously changing the loading of the driver. The next step that would also meet with no real fruition would be to alter the ear and ear canal with inserts, so don't bother going there.

dogears's picture

I've been using coffee filters with my IEMs before :)

angelanaa's picture

I've been as of late thinking the way of earphone modding and how to legitimately assault it from a sound science point of view, instead of experimentation. I'm not a to a great degree experienced earphone modder, nor am I a sound architect, yet I'm searching for feedback of my earphone modding breakdown. Paying for papers

im533's picture

Glad to read your article. With HD800, I'm using the foam sponge of DT880, with the hole in the middle of it. It's resting between the driver and dust cover. I don't know about the FR of the modified HD800, but the sound is very good to me, it's my taste. Warmer and smoother but narrower soundstage.

And what about the outer paper filter? What effect does it have? Could you please let me know?

atifkhan's picture

in fact i occasionally play my favorite games Fireboy and Watergirl
and
temple run with these headphone so the quality is different in real time for me

Phoniac's picture

I seem to be not getting the point of all this. According to the above measurements the 6 kHz peak stays nearly unchanged, while all the nice sweet treble is reduced, turning the phones into squeeky and dull disappointments.

What did I miss?

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