Headphone Port Tuning Frequency - Possible Sennheiser Mods

Discussion in 'Modifications and Tweaks' started by gibtg, Oct 29, 2016.

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  1. gibtg

    gibtg New

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    The biggest question here is: Do 'vented' headphone driver enclosures work upon the same acoustic properties as a ported subwoofer?

    After inspecting several Sennheiser 'cans and wishing to mod them I notice some similarities and I have assumptions of how they may act but I'd like to some input from those possibly more knowledgeable than myself. Case in point the Momentum cans have two round holes in the enclosure behind the driver with acoustic dampening cover them. Do those act as bass ports? I notice in impedance measurements a 'spike' at around 80 Hz, is this the tuning frequncy of these ports? The Urbanite XL shares a very similar ported arrangement. The Momentum 2.0 has a similar arrangement but with a third, smaller hole that is undamped. However all have a enclosure within a enclosure design ,so to speak. There is a sealed small enclosure behind the driver and then that chamber is mounted in the exterior ear cup. I'll post a picture courtesy of Tyll at Inner Fidelity as an example:


    [​IMG]
    Does the air leak caused at the top of the cup, around the rubber cable guide serve any acoustic purpose? Does this act as a simple ported arrangement or possible a rear band pass enclosure? These headphones isolate extremely well, I imagine due to the chamber within a chamber design but the acoustics of the Urbanite XL are not top-notch and neither are that of the Momentum 2.0, both which appear to share the same midbass hump. Heard, to my ears, as slightly excessive warmth. Both also show some sub-bass roll-off unlike the original Momentum.

    I'd like to mod the Urbanite XL as I believe if the port tuning frequency could be lowered and possibly the port gain diminished (to reduce distortion) that these would be a terrific sounding headphone. I had been comparing the Audioquest Nighthawk to the Urbanite XL and surprisingly both do show some similarities. The nighthawk obviously sounds more open with the large vents, has better bass extension, and less distortion, but both have similar treble levels and both have an overdamped treble that is excessively smooth. They also have a very similar midrange response that many say is too warm. I don't find either extremely offensive sounding and I can get the Urbanite XL for peanuts compared to the nighthawk [​IMG]

    Back to the possible modifications, would blocking off one of the ports in the Urbanite XL's enclosure change its tuning frequency or just lower the port gain? I experimented with removing the acoustic damping material and blocking off one hole and the result was a bass distortion disaster, I imagine caused from audible port noise. Blocking off one of the holes while leaving the acoustic resistor in place seemed to net good results to me but then the treble level may be slightly high. I thought possibly some felt may be in order but after doing so the bass distortion also became problematic once again and the treble already lacks sparkle - however I'm not ready to give up on these cans just yet. If possible a tiny bit of drilling could be done to the enclosure I'd gladly risk it to properly tune the acoustics for what I believe could be a terrific response. Please give your thoughts. Thanks,

    Read more: http://diyah.boards.net/thread/1418/headphone-acoustic-frequency-sennheiser-examples#ixzz4OUpwl19R
     
  2. Armaegis

    Armaegis Friend

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    The best I can give you is some very fuzzy "yes and no" answers.

    A ported speaker/subwoofer utilizes (Helmholtz) resonance to give you a boost around the port frequency. There is a one cycle delay before the reinforcing sound comes out, so ultimate resolution is a teensy bit off. A headphone enclosure isn't quite the same, because the ports lead to outside and not to the ear. The system of chambers is also far more complex. A speaker has inside the box, and outside. A headphone has the cup enclosure, the earside (plus to a lesser degree the ear canal), and the outside, with varying degrees of venting/leakage across each.

    As near as I can tell, no one has really fully figured it all out yet (or they're keeping it a closely guarded secret... but I doubt it). It's an incredibly complex system of chambers and acoustic/atmospheric impedances. We can't even fully agree on how to measure the earside frequency response, nevermind multi-chambered reveberations and backscatter and voodoo levels of distortion and delay and the fact that acoustic theory on wave propagation for speakers is monkey bollocks when you're working at the small headphone scale when people should be looking at acoustic impedance and finite element instead... but I digress.

    Get yourself a Beyerdynamic COP with the sliders to play with the bass vent. That is an extreme example of how much the port opening affects frequency response. I did a mod and stuffed a T50rp driver into one of those and the effect was significantly less pronounced.

    Check out Bluemonkeyflyer's massive thread on T50rp modding over at headfi for a wealth of information on the effects of port venting (both cup and baffle). There are no answers per se, but a good stack of empirical data from which to draw some hypotheses.
     

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