Traditionally headphones have been thought of as an audio accessory. A necessary peripheral to be used when speaker listen is impossible or unfeasible. In the last 30 or so years headphone listening has taken off, as has headphone use for critical audio monitoring previously relegated exclusively to speakers. Still, the two Achilles heels of headphones remain – the in-your-head sonic image and the lack of a physical full body sound experience.
Combating the lack of visceral impact in headphones can be attempted with bass shakers like the one from Subpac. Some of our clients have actually learned how to mix bass on them. Taking care of the specific imaging a headphone offers is less straightforward. There’s the simple option of offering a frequency dependent channel bleed, called crossfeed. The next level is achieved by adjusting the timing of crossfed signals, after all – if a sound hits you from the left side, it takes some time for it to wrap around your head and get into your right ear. What these numbers should be can be calculated from real world measurements of a person’s head. Genelec has gone even further with their new Aural ID tech. The Finns promise to calculate one’s head related transfer function (HRTF) from a 3D model which in turn is created from a series of 2D image the engineer takes on their own with a smartphone. Products like Waves NX add the final touch – continuous head tracking, which is essential for sound source elevation perception. So, it sounds like we have this HRTF thing in the bag? Not quite.
An age old problem…
In all feature pitches for HRTF emulation one stage of the process is conveniently left out – the headphones themselves. All HRTF emulation relies on tonal precision to deliver the right kind of tonal alteration to the original signal, which is why it usually works on some headphones and misses the mark on others. Naturally the effectiveness depends on how close the headphones are to the implied target curve of the HRTF emulation solution. The problem is that frequency response of professional headphones (if there is such a thing) is all over the place…
The “trick” of creating an illusion of a listening environment on headphones is undoubtedly exciting to behold, however the pro audio community would do wise to remember that unlike speakers, headphones aren’t a mature technology. Choosing a pair can be a nightmare as consistency in tonal balance is rarely found even within the stable of the same manufacturer! Compare the Sennheiser HD600 (red) to HD800 (turquoise) which come from the same designer… for some reason the higher end headphone has a massive treble spike.
Get a pair of neutral headphones. Or at least know the limitations of your monitoring system. Or the shameless plug – apply calibration… 3D audio in no way protects one from inaccurate monitoring, regardless if it’s on headphones or a cyberpunk-looking sphere of three dozen studio monitors. Living in an illusion about headphone quality is lethal to the illusion of spatial audio and translation of the resulting material.