twgib
contributing
Posts: 59
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Post by twgib on Oct 29, 2016 17:22:00 GMT
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: 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 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 audble 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,
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solderdude
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measureutternutter
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Post by solderdude on Oct 29, 2016 18:46:57 GMT
Ports in headphones work very different from those in woofers. The acoustical mechanical properties are very different.
The impedance hump depends on a lot of factors. Driver compliance, amount of seal/leakage, acoustic motor (voicecoil/magnet) properties determine this. The ports on headphones usually affect larger parts of the frequency range and are often used to tune the sound of the driver to that of the earpads and enclosure.
In woofers the bass port is a certain 'mass' of air preferring to vibrate at a certain frequency. When such a port is tuned (diameter and lentgh) to a frequency below that of the woofer in that cabinet then the resonance of that port (when 'exited' with that frequency) will vibrate louder than the speaker itself. These loudness'es 'add' and the speaker thus has better bass extension at lower frequencies. But only in a narrow band. well below the port frequency the speaker acts like a closed speaker again. So it doesn't work as a bass extension but can improve the bass response of a speaker around the port frequency.
In headphones the acoustics differ. A driver has a (partially) 'sealed' enclosure at both sides of the driver. Often the pads colour the sound in an unwanted way which (partly) can be altered by feeding back parts of the rear acoustic energy to the front. It is also done to create a 'leakage' on purpose to make the effect of a different ear pad seal smaller for instance. Some 'ports' create a, for the driver seemingly, larger enclosure for certain frequencies but doesn't work as a bass port as in bass-reflex enclosures.
You can modify headphones by blocking ports, changing ports (more, less or different damping) making holes larger or smaller. Applying wool or other damping materials. Changing pads or ear-driver distances etc.
The problem is that there is no 'one solution' in that if you do this or that in headphone A that it will do a similar effect in headphone B. Both headphones may have paper ports in the baffle but sealing them (partly) or changing them may have a different sonic effect.
Another (rather big and not predicatable) issue is that most of the sometimes elaborate tuning is thought out carefully by the manufacturer. Most manufacturers actually know what they are doing, others less so.
Changing 'something' in a headphone to improve a certain aspect may well have the desired effect but most often also introduces other problems or also affects parts of the FR you didn't want/need to change.
So... there is no single solution that always works in the same way with headphones as it is an interaction between driver, enclosure, pads.
Then with modding there is the issue of having made a change which you have to determine from 'audible memory' to evaluate if it is an improvement. Also one can easily get used to a sound and have a wrong 'reference' for yourself.
Best way is to either measure the changes OR have another not modified version around OR use a known 'reference' headphone which you listen to for a while to 'set' your internal tonal balance and then listen to the modified headphone.
Young people prefer a hyped bass and warmth and subdued treble. This is because A: they hear a LOT more treble and B: they often listen to crappy streams or low bitrate MP3. Recording quality of pop music generally isn't as 'good' as in certain other genres as well. This explains the popular tuning for 'on the go' and 'fashion' headphones. Not because they are a standard but because the 'paying target' prefers that kind of sound.
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twgib
contributing
Posts: 59
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Post by twgib on Oct 30, 2016 5:13:49 GMT
I do appreciate your input. It is astonishing to me that the headphone acoustics are so cutting edge! In that, they don't appear to be well defined and are mainly empirical.
I do understand from your explanation that the holes in these enclosures don't act as bass ports such as in woofers, but if there are holes they must resonate at some frequency, correct? Then maybe we're always dealing with aperiodic enclosure types since the holes are almost always damped with acoustic resistors too... Are you aware if there has ever been any thought into using thiele-small parameters for headphone drivers?
I would really like to make a measurement system but I'm really unsure how to go about doing this. Even though I have some experience at university with taking scientific measurements, I don't have ANY experience with acoustic measurements. I could invest some in it but I don't believe I could swing a industry acoustic dummy head. Any advice?
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solderdude
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measureutternutter
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Post by solderdude on Oct 30, 2016 7:43:08 GMT
I do believe those HATS aren't really suited to measure headphones at all. The reason for that is that A: ear canals and Pinnae differ in size and dimensions so the theory that a HATS is capable of more 'accurate' measurements seems suspect. B: There are multiple resonances and changes in the FR because of various standing waves etc. in that fake ear canal. Yes, the big 'hump' you get can be easily corrected IF someone can definitely figure out HOW much of this should be 'compensated'.
There is a substantial difference in FR when a microphone is held directly in front of driver (at head distance) and when mounted in a short tube. There is also a difference between a free-air microphone mounted in free air or mounted in a baffle. There is also a difference between a mic on a baffle and a mic on a baffle measuring a sealed 'enclosure'.
Still, to me, the big issue is not that wide 'hump' in the microphone signal (raw measurements) caused by the Pinna and fake ear canal BUT rather the small narrow resonances that create high peaks and dips (over +/-10dB) that are not easy if even possible to 'correct' afterwards. Yes, averaging (smoothing) does help somewhat. Just compare all Tyll's plots and notice how they all have 9-10kHz peaks and dips before and after it. That just isn't what really exists and is a compensation error... to me that is.
In the end I want to know what soundwaves reach the entrance of the ear canal as these are the soundwaves I hear. My 'brain' has a perfect, always recalibrating, resonance filter after all so I don't need the soundwaves to be altered in a fake ear canal only to have to be undone again (which IMO is not possible to do perfectly). The soundwaves coming from the driver also interact with the material they are 'fired at'. So for that reason I mimic'ed my head in width (so pads compress in the same way as n my head) and made it out of wood which has somewhat similar properties as bone. Din't use cellfoam, cardboard, CD's etc for that reason. Above the bone there is a very thin layer of skin around the ears so used dense closed cell foam which looks a bit like skin in compliance and thickness. So no hard plastic or other materials bouncing sound waves back.
Then there is the issue of having to find 'compensation' for the effects caused by mounting a mic intended for free field measurements on a baffle. I still want/need to investigate this myself and have a freefield measuring mic for this (for measuring speakers) and still have to use tests.
I wanted to skip this testing so went the easy route for this. My reasoning was this:
Dummy heads may not be suited to measure everything accurate above 1kHz but do appear to be good at lower frequencies. What I did was compare a well known and quite 'consistant' reference headphone and measured it. Then looked at the various measurements of the same headphone made by others. What became obvious was that home made 'flat bed' measurement rigs all showed MUCH more lows than those made with (different) HATS.
on FB rigs that reference headphone seems to measure 'flat' to 20Hz where on all HATS measurements this same headphone measures about -5dB around 20Hz.
You might say who cares, we don't hear 20Hz and the lowest bass notes in music is around 40-60Hz for bass. True, yet the overall appreciation of a recording increases when that subbass is present... at least to me.
Anyway... I overlayed all these measurements made by HATS and other measurement rigs and noticed the HATS measurements correlated better with what I heard myself. Not surprisingly... HATS manufacturers are quite skilled and know how to measure very accurate from a few Hz to 1kHz for sure.
So.. I added the same compensation HATS use and applied it to my rig (hardwired filter, no afterwards compensation) My 'raw' signal from a few Hz to 1kHz thus matches HATS quite decently.
A downwards tilted FR on most home made rigs thus measures 'flat' on my rig. and a 'flat' home made rig thus will show some low bass roll-off on my rig.
Building the mechanics for such a heaphone test rig isn't that difficult. Finding a mic you can trust and compensate it for baffle mounting and 'trusting' the frequency response above 5kHz is another matter.
My reference headphone for this is the HD650 by the way. Often measured and Sennheiser knew what they were doing making this headphone. Just compare the populat DIY measurements (below 1kHz) to those made with HATS and see what is more accurate to what's heard.
Playing with equalizers teaches you about how 'peaks and dips' in certain parts sound which helps with evaluating by ear.
Compare HD650 plots from all websites around and you get a good idea how difficult it is to accurately measure headphones. a few dB is already audible.... you will find differences in the order of 10dB which is about a perceived doubling (or halving) of 'loudness'.
When modding though it is enough to measure before and after differences to 'see' what mods do. Tedious as sometimes you have to disassemble, mod and re-assemble it many many times to see what each 'port' does invidually AND how changing multiple ports in certain ways influence the other ports as well. I can be busy for several days playing with ports and pads as I not only measure but evaluate by ear as well comparing to my reference (HD650 on Kameleon).
When you have made measurements of a known good (not modified or old) reference headphone (HD600 or HD650) and know how and where they differ from yours and above all where these differences may come from then you can compare the modded headphone to a known reference.
Having been in audio (amplifier designing, building, testing) and having been in repair business (audio/video) for over 15 years, speaker building, sound measurements etc. for over 30 years and knowing how instruments and voices sound IRL and once recorded (band and studio) also helps I think.
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