HRTF stands for Head Related Transfer Function.

In other words: the phase and frequency response of our head. In fact, a transfer function is a particular mathematical formula that groups both data sets.

Basically, it is the way in which our head changes the sounds that reach our eardrums.

These changes are dictated by the structure of our head: nose, forehead, mouth, hair, bone density, auricles… every feature of us that the sound hits before reaching our eardrums. And, in the event that the sound is coming from below, our shoulders too.

Every struck “obstacle” is going to faintly change the sound, altering frequencies and phases.

Depending on where the sound comes from (in front, behind, above, below), is going to encounter different obstacles. And different acoustic alterations.

Our brain has finely memorized these peculiarities, and it takes advantage of them to understand which direction the sound is coming from.

That is the reason for which, even with our eyes closed, we can still understand the position of a sound source.

The organ that influences these alterations the most is auricles: all their twists are needed to extensively characterize the auditory changes, by having the latter “clash” onto them.

(You’ve finally understood why ears have such a “weird” shape, instead of simply being flat.)

Here’s an example of front (continuous line) and back (dotted line) HRTF.

If we apply the continuous line frequency response to a signal, our brain will understand that the source of the sound is in front of us.

If we apply the dotted line frequency response to a signal, our brain will understand that the source of the sound is behind us.

Needless to say that everyone of us has it’s own physical structure, and for this reason the HRTFs are never going to be identical. However, there’s a slight resemblance between all the HRTFs that allows our brain to effectively interpret signals that have been elaborated with other people’s HRTFs.

Moreover, are you aware that there are systems to record in HRTF, so that our recording isn’t just going to identify left and right (monodimensional), but also above, below and to the sides (tridimensional or binaural)?

Here’s one of the most common ones: a dummy head.

 

Dummy head (Neumann KU100)

 

That is, a tool that simulates the shape of a human head, with microphones instead of eardrums. The attempt is to effectively record the HRTF information.

And it works pretty well: listening to a recording made with this tool feels like being on stage.

Software-wise, there are HRTF decoders that allow HRFT data to be linked to a signal, therefore giving it 3D spatiality.

Another interesting implementation of HRTF is to be found in almost every modern headphones: to try and avoid the “sound inside the head” effect, a frontal HRFT impression is imprinted into the headphones.

Which is also the reason that headphones frequency responses can’t be interpreted “with the naked eye”.

By the way, here’s another example of a professional pair of headphones.

http://www.headphone.com/

Even if the curve is quite irregular, the auditory result is going to be reliable anyway, given that the anomalies are due to a particular frontal HRTF impression.

Actually. not exactly frontal, since the typical position of two speakers is a the vertexes of an equilateral triangle, with our head corresponding to the third vertex.

If you want to know why this tutorial was made, you’ll find out more in this post:

Our first post.

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