For several years, I have been using headphones exclusively for listening to almost all audio sources—such as music, videos, podcasts etc.—and have consequently always been very interested in obtaining the best quality sound from them possible, especially for music playback. This often meant subjectively trying out different headphone brands and models, trying to find ones with the "flavour" that I preferred.
As someone who prefers accuracy in reproduction, however, I always wished for a more objective approach for achieving it with headphones. Unlike with a loudspeaker/room system, the sound produced in the headphone/ear interface is not only dependent on one's individual auditory canals, it doesn't lend itself to easy measurement and correction.
So I was quite intrigued when, some time back, I ran across Siegfried Linkwitz's webpage on "Reference earphones," where he found that the perceived frequency response of headphones (relative loudness changes determined by sweeping over the frequency range with an audio signal generator) typically exhibited significant—but narrow—peaks, which could vary from person to person. Linkwitz's suggested solution was a simple passive LCR notch filter circuit—to be placed between a headphone amplifier output and the headphones—designed to achieve a uniform perceived amplitude response for a particular listener and pair of headphones/earphones.
This plot shows the voltage across the transducers (in this case Etymotic ER-4S earphones) with and without the notch filter equalisation circuit that Siegfried Linkwitz found was needed in order to achieve a uniform perceived amplitude response to sine waves when tuned for his ears. (Linkwitz Lab)
I still have plans to build and try this circuit, but since most of my headphone listening is done either on my PC (using a USB audio DAC) or an iDevice, my immediate thought was how I might implement this through digital/software means. The testing part is easy—there are any number of software programs and apps available capable of generating the sine wave tones needed for determining the perceived frequency peaks for a given headphone/ear combination. When I tested a couple of different high-quality headphones, I heard what appeared to be some significant response peaks, very similar to what Linkwitz described.
It was much trickier coming up with a playback equalisation solution that had the required parametric equalisation capabilities—i.e., the precise control over amplitude, centre frequency and bandwidth needed to implement notch filters. Fortunately, an extended discussion on Head-Fi ("How to equalise your headphones: A Tutorial") offered some suggestions.
For music playback on my PC, I ultimately ended up using the Electri-Q equaliser as a plug-in with the foobar2000 audio player to notch out the perceived peaks. The resulting difference between the equalised and unequalised sound was also as Linkwitz described—"switching the equalisation out of the signal path reveals a very exaggerated high-frequency emphasis of certain sounds and an overall colouration."
The real trick, however, is achieving this on an iDevice. Typically iOS apps all operate separately, each in their own independent operating space. So, unfortunately, this means it's not possible to process the output of your favourite music playback app through a separate equaliser app.
The only solution has been to find an appropriate equaliser app and then use it as your music player (a secondary function that any such processing apps have needed in order to be useful). Several parametric equaliser apps are available, including EQu, Equalizer, and Accudio. These may not represent ideal solutions, but will allow full control over the equalisation of music played back on the device—as long as the music is stored on the device.
Unfortunately, my favourite—and only—music "playback" app is iPeng. I don't have any music files stored on my iDevices. I use the "Playback" feature in iPeng to wirelessly stream audio from a Vortexbox media server (where my music library is stored) to my iDevice.
So I am still left without a software-based headphone equalisation solution on my iPad and iPod. However, there have been some interesting developments lately in another area of the iDevice audio space that could offer a solution. I'll be discussing those in an upcoming post.