[Jon Sonne]

Dear Sounds Science subforum,

Recently we have seen that headphone manufacturers have developed closed-back headphones which tries to sound more like open-back headphones. One example is the Sennheiser HD 820, which uses concave gorilla glass and sound absorbers to remove resonances associated with closed-back headphones, in order to sound more like a closed-back headphone.

I was wondering if active noise cancellation could be useful in developing new closed-back headphones that sound more open, by removing unwanted resonances associated with closed-back headphones. The idea is simple (see figure below): The main driver of the headphone creates sound waves that propagate towards the ear. The back side of the main driver also creates sound waves. These sound waves propagate into the back-chamber of the headphone. Inside the back-chamber, a microphone measures the sound/resonance inside the back-chamber. A second driver situated further back in the back-chamber plays the inverse phase equivalent of the sound/resonances the microphone picked up. In this way, resonances created by the sound waves propagating backwards into the back-chamber of the headphone could be cancelled.

I would like to hear your opinion - If you believe this idea could be a plausible way to improve the sound of closed-back headphones, I might try to put a pair of small open-back headphones inside an active noise cancellation headphone to test the concept. If the open-back headphones still sounds partially open when inside the noise-cancellation headphones, it might be a viable strategy to create a more open-sounding closed-back headphone.

Cheers,
Jon Sonne

 

[castleofargh]

the principle seems cool, but practical application might be a nightmare. in your drawing, the outside driver has open back? or is the headphone is still closed back behind the second driver?
if open back, well then why bother at all with the second driver?
if closed, we will either have the second driver creating it's own reflections in the back. and depending on where it's placed, I suspect we could run into issues like maybe a lack of air to move around.
also I wonder about where to place the mic. dead center with the aim of it measuring as little signal as possible would require pretty serious processing, I don't really see how that would be done in a rather passive way like some noise cancelling designs do. but I'm too much of an acoustic noob to even consider that I can predict what's going to happen inside such a space. I'm just a little concerned by the fact that the mic will also capture the "returning signal" from the back driver.

 

[Zapp_Fan]

In theory it might work, but you might do better with a fixed filter on the secondary driver instead of using a mic plus driver. Having the secondary driver in the proper phase to cancel reflections from the primary driver will be tricky at best.

Mics are vulnerable to interference, whereas if you have a very well-measured acoustic profile of the inside of the headphone, you already know what the resonances/reflections will look like and therefore know how to cancel them. This also reduces the difficulty of dealing with very short or very long wavelengths.

On the other hand, if you had such good measurements and realtime DSP I guess you wouldn't really even need the second driver...

This might be why passive absorption is more popular. A bunch of cotton balls or weird reflectors are much cheaper than a quality DSP setup, or a pair of drivers and mics...

 

[MF Kitten]

The effort involved VS the payoff isn't really worth it, when physical modding (absorbers inside the cup) will do SO MUCH to get you closer to that exact result. You want to "eat" the reflections while maintaining a lot of air movement. So larger bass vents and absorbers together could probably do it. The factors that go into the sound of a closed can is mostly just high air pressure and reflected backwaves causing a little bit of comb filtering (you can see this as peaks and dips through the FR graoh).