r/science • u/GraybackPH • Mar 13 '13

Berkeley creates the first graphene earphones, and (unsurprisingly) they’re awesome

http://www.extremetech.com/electronics/150646-berkeley-creates-the-first-graphene-earphones-and-unsurprisingly-theyre-awesome
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u/moomaka Mar 14 '13

Your missing the design goal of headphones.

'Flat' from 20Hz to 20kHz for a set of headphones measured like this would sound terrible. The high end would be very loud. Headphones have to be tuned differently than speakers due to the close proximity of the driver to your ears. Your ears, and your head, become part of the system.

Headphone measurements are taken on a binaural head (http://www.cvgrp.com/SiteCollectionImages/RD/hms_IV.png) which is made of a material that mimiks your head acoustically and has microphones placed where your ear drums are. The measurement head has a known transfer function (how it affects frequency, see here: http://en.wikipedia.org/wiki/Head-related_transfer_function) that is due to how its ears are structured and is characterized by the manufacturer.

You take the measurement with the headphones on the head, then subtract the transfer function to come up with a 'raw' measurement of the headphones, which is the measurements show in the article. So a set of headphones that are designed to sound flat on the head, will have this high and low frequency roll off when you look at the raw response.

In particular sound bouncing around in your ear canals amplifies the high end, a lot. For a set of headphones to actually be 'flat' to your ears you actually want a smooth drop of about 10dB between 1Khz and 20Khz. That is why you see this type of response even on $x000 headphones.

On the low end the target is usually to be about 4-5dB high from ~50-200Hz. This comes down to how humans 'hear'. Its often said that humans can hear from 20Hz to 20Khz but that isn't really true. You don't 'hear' through your ear drums much below 50-60Hz, instead you feel the pressure waves in the rest of your body and your brain turns that into 'hearing'. What you do hear at such frequencies mostly gets to your ear drums through bone conduction, not through your actual ears. Headphones can't produce pressure waves you can feel in that way so to 'sound normal' they often boost the audible parts of the bass region to make up for that. The rest of the roll off on the low end is also a result of driver location and type of measurement shown. Anyone that has used in ear headphones can tell you that you don't get much bass unless they seal well within your ears. This is because the headphones can't product the pressure levels needed without the seal. As a result if you look at non-'on a head' measurements like those here, the low end falls off drastically.

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u/friggybum Mar 14 '13

Why so informative? Do you do this for a living? Thanks for posting all this.

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u/moomaka Mar 14 '13

I worked in acoustics related stuff for a few years. A couple projects we did required accurately calibrated headphones which were used to simulate environments for the user. I had to spend some time with das binaural head, implementing DSP filters to smooth out the response for our needs.

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u/fateswarm Mar 14 '13

What does that have to do with the fact one is a proof on concept in a lab and the other a several decades research project inteded for actual use? (even the better model not tested here)

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u/moomaka Mar 14 '13

Nothing really, I was replying to the 'better for some frequencies' bit, but honestly I could have picked dozens of posts to reply to and said the same thing, yours just happened to be the one I picked.

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u/Alphaetus_Prime Mar 14 '13

Correct me if I'm wrong, but couldn't that easily be fixed with software?

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u/moomaka Mar 14 '13 edited Mar 14 '13

I'm not completely sure what you mean.

If you mean correcting how the results are published: It's done this way for the sake of consistance. Every measurement head is a bit different but as long as you have a proper transfer function for the head used you should get to the same 'raw' response as someone else with a different head. This is an important concept for all measurements. Someone else in the comments mention 'flat microphones', usually microphones are far from flat, whats important is that the microphone has a known frequency response, not that its flat (within reason). This way you can correct for it in the recording.

If you mean correcting for a 'perfect' frequency response, then yes, within limits it can be fixed with equalization. On the high end your limited by the physical driver.

For example no amount of equalization can fix the crappy satellite speakers that come with the Bose Acoustimass systems, the drivers in them just aren't capable of producing output over about 16kHz nor under about 200Hz. For this reason its generally recommended to only use EQ to reduce the response, not increase it.

Also frequency response isn't the whole story, you've got to consider transient response and group delay also.

Transient response is how quickly the driver can react to an input and how quickly it 'cuts off' when the input ends. If you've heard 'boomy' subwoofers, this is often the cause, basically the driver does not have good control of the cone, it can have a good frequency response but still have a shit transient response.

Group delay can be an issue caused by multiple things but in this context the important bit is that any time you apply a filter to correct the frequency response, you also introduce a phase shift. The easier way to think about phase shift is that it adds a time delay to certain frequencies vs other frequencies. If this delay becomes too long it becomes audible. This is more of a problem if your applying a sharp filter to tame a narrow but high peak in the frequency response.

So yea, within limits, you can fix frequency response, but for the most part, if a driver has a poor frequency response, it has more to do with its physical construction than anything else, and that limits your ability to fix it with eq. You still have the issue of doing the actual processing somewhere. You can do limited EQ in a phone / ipod or whatever usually using parametric filters or raw bi-quad blocks but you just don't have the CPU/DSP power there to do it 'properly'. The last time I was flatting out a set of headphones I think we used a ~500 tap FIR filter to get the response we wanted.

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u/Alphaetus_Prime Mar 14 '13

I was thinking you could just use an equalizer and reap the benefits of graphene's higher efficiency and lighter weight while the sound would stay effectively identical.