[ampgalore]

Just use canalphones, offers more isolation than the current crop of active noise cancellation phones.

 

[commando]

I wonder what would happen if you had a canal phone that did noise cancelling?

 

[gerG]

In my experience canal phones can and do offer significant reduction in noise levels down to 20 hz. That is about where the felt vibrations start winning, for me at least. No data, but the ER4 are not far behind foam earplugs, which can cut >20 db down to 20 hz. I think that there are some bone conduction issues, but I think the "felt more than heard" thing is a greatly exagerated issue. My office has some pervasive and audible harmonics of a 7 hz tone (which I can't hear) generated by the ventilation ducts. They bug me until I plug in the Etys, then all is quiet. otoh if I wear them out near a turbofan test cell (output all the way to DC) I get pounded in the chest and my head rattles. Quite a bit of fun, actually.

I am still looking for active NC cans that will take me down to 15 hz or so. I fear the 250s have a cutoff filter, so I have avoided taking them apart. They also give me a headache after I wear them for a short while. Seems to be the result of that weird pressure sensation. Anybody else have issues with that? Do the Bose have it as well?

I have gotten very comfortable with the Etys (grey foamies) in the office. Install time is usually about 45 seconds. Only problem is that after 15 or more interruptions my ears start getting sore. I am thinking external mic and mixer. There is a switch, eh? Sealed headphones with a mic to let ambient sounds IN. I have tried it in a car, and it is very interesting.


gerG

 

[gerG]

btw, if it hasn't come up here, there is a design for an inverting amp for just this purpose over at headwize. I tried something similar, and it worked great. It starts falling apart when the phase shift is more than 20 degrees, so the mics need to go close to the earphones, but on the outside. The problem that I had was a cutoff filter that would stop the effect at frequencies above where the phase shift was unacceptable (otherwise outside racket gets amplified). The filter that I was using added it's own phase shift, and messed things right up. Next iteration I will just try cloaking the mics somehow.


gerG

 

[mustang]

Quote:

Originally Posted by gerG Do the Bose have it as well?

Yeah although not as much with the QC2s than with the QC1s. Really bugged me while I had em

 

[Zoide]

I guess the point of noise cancelling headphones is that, unlike canalphones, they mostly filter ambient noise as opposed to important things like your boss talking to you or the fire alarm going off, right?

 

[taylor]

Quote:

Originally Posted by bioxeed You're showing your ignorance here as (at least in the pxc-250) there IS a microphone in each earpeice. Also, just inverting the signal you receive at the microphone there is totally unacceptable - you're going to end up cancelling the music as well as the noise. While it isn't impossible to DIY it's not nearly as simple as you think. People aren't doing their phd's on active noise cancelling just for fun.

To validate your point, I found a large picture of the PXC 250.

http://www.gentrade.co.jp/sennheiser...der/PXC250.jpg

Can you point out the earpiece mounted microphones? I don't see any.

As for inverting the input from the mics, that would only cause a problem with open cans. Closed cans with a bit of dampening material between the driver and the mic work fine. WHo would try to make open noise canacelling headphones anyway?

 

[TonyTripleA]

I use my ety 4Ps for air travel but am a little frustrated with the low frequency intrusion which is background noise from the plane. Guess having those cheap seats up the back doesn't help either!

So, here's an idea... the etys do a partially good job that could be better. It appears the noise cancelling headphones are doing a partially good job as well. How about wearing the etys with the noise cancelling headphone over the top just to cancel noise (no music).

Hi Fi sound from the etys... attenuation of background before the etys...

Damn, I'm going to have to buy some noise cancellers just to try this.

Anyone willing to have a go and let us know?

TonyAAA

 

[Takashi]

I've actually tried to mod Sony Z600 (=V600) by swapping circuit from Philips HN100. HN100 was an OK-sounding phone but NC effect was minimum, so I thought if I put them in more isolating closed phone it would work better.
Here's the pic of HN100 inside the cup.

See the little mic on the cup? The little green board in the center carries no circuit, it's just a connection point.
I took the whole circuit (including mic and driver) from HN100 and literally shoved them into Z600. (made hole for mic, too)
Result? It actually ADDED ambient sound, so loud that it exceeded the level of "cancelling". Yes, it was noisier than passive isolation. I switched cables to see if I made wrong connection but that was not the case. Surely the added sound was amplified ambient noise, like sound of cars, TV, etc.
I think the circuit needs feedback loop to compensate the level of reverse phase sound, but obviously Philips' circuit didn't have it. In fact, HN100 doesn't have completely closed back, I think HN100 is mechanically balanced to match the level of incoming noise and reverse tone. Low cost, but won't perform well.

Here's the pic of PXC250, mic is located in the center of the driver.

If you turn on NC and gently cover the center of the driver with your palm or thumb, driver starts to beep. I think it's because of the feed back circuit.
Also, PXC250 has a small hole on the back of both cups. Don't know why.

Are these metal mesh grills mics?

No. If you cover these with your hand while NC is on, no matter how firm you press your hand, it doesn't change the sound. (Actually shuts down a bit of outside noise.) I believe mics are hidden inside the circuit.

I can't see it but I'm not ready to tear down seldom used $300 phone yet, so I'll leave it a big mystery.


Oh and use ER-4 beneath QC2? Maybe I'll try on my next flight(don't know when), but still you can't get rid of low freq noise transmitted from the seat to your skull. (bugs me when I try to sleep)

 

[taylor]

Makes sense that they would put it inside the enclosure. Now i'm beginning to see why it's so difficult - put it outside, and the ambient noises are louder because the phone is providing some isolation. Put it inside and ti cancels out the music.

 

[gerG]

Tony, I suspect that the problem on the plane is the vibration coming through the seats. Cars have the same issue. Try lifting your head off the headrest and see if it makes a difference. You could be getting some through the cables as well. The foamies eliminate much of that.

Takashi, good job on your experiment. I am surprised that it did not work better. Closely located mics on the outside of a closed headset should do an excellent job of cancelling noise. It should work up to about 1/10th of the wavelength defined by the distance between the mic and the driver. This assumes no delay or phase shift in the circuit. Beyond that frequency the inverted signal becomes less effective. At 90 degree shift it starts becoming additive. Filtering is critical for a simple inverted signal type of system.

I don't quite understand the systems with the mic inside the earcup. The principle makes sense: calculate the difference between the mic signal and the input electrical signal, invert, and add in. Requires some damping to eliminate feedback and instability, which will slow down the response. Seems like an awful lot of trouble to get to something that can be measured directly on the outside. The obvious advantage is that by monitoring inside the earcup, it compensates for changes in fit and seal, plus it corrects for the natural noise attenuation of the earcup. It would seem at first that it could correct music response as well, but I doubt that the circuit is that fast. I am just thinking aloud here. Anybody know of other advantages/tradeoffs?


gerG