HRTF (Head-Related Transfer Function)
What is HRTF?
Humans perceive sound in 3 dimensions: above and below (Elevation Plane), left and right (Azimuth Plane). No one hears sound in the same way. The sounds which we perceive are affected by factors such as: the size and the shape of the head, pinna and torso. Sounds are also affected by the environment in which sounds are being heard, elements such as diffraction and reflection cause alterations in sounds perceived by the ear. Consider a listener in free space and a far field point source, the acoustic wave is approximately planar by the time it reaches the listener.
As one moves through the environment sounds change in relation to the position of the receiver, this principle is otherwise known as Head Related Impulse Response (HRIR). The ratio of the amplitude of the received and incident waves are referred to as the HRTF for that angle of incidence relative to the listener. Assuming that air is uniform, and the size of the sound pressure fluctuations is not large, then the HRTF can be modeled as a linear time-invariant LTI. All source signals for a given direction are related to the signals received at the listener’s ears by a pair of LTI filters
HRTF forms the cornerstone of contemporary binaural displays. Efficient modeling and implementation of HRTF has been the subject of much research throughout the last two decades. The most extensive research carried out on HRTF was compiled by Kemar in the MIT anechoic chamber. Kemar have been heavily involved in researching HRTF to create new recording techniques. In may 1994, Kemar released it’s Dummy Head Microphone. Based on the average human Male and Female torso dimensions, the dummy head allows for the recording of sound as they are heard by a real person in the environment. The company continued to invest their time in researching methods of sound recording and advancements in previous technology.
The principal of HRTF is used almost everywhere today due to the advancement of technology in the computer, film and music industry using surround sound. There are many surround sound setups. Examples include 4.1, 5.1, 7.1 and 7.2; the number preceding a decimal refers to the number of loudspeakers placed around the listener while the number following the decimal refers to the number of sub-woofers playing low frequencies.
In a study carried out by LucasFilm whilst testing the THX standards, it became apparent that decent sound can actually fool the brain into thinking the picture is better. In the study, one group of people that where shown a movie with average sound, then the same movie with better sound actually commented that the picture seemed sharper too.
The development of surround sound means that sound cards manufactured by companies such as Creative Labs, Diamond and Aureal allow for the use of multiple speaker set-ups which are ideal for computer gaming.
- Modeling of Head Related Transfer Function through Parralel Adaptive Filters by Kenneth John Faller II, Armand Barreto and Naphtali Rishe [sourced on 4th November 2010] http://resources.metapress.com/pdf-preview.axd?code=jg08062303k5u857&size=largest
- Modern Audio Technologies in Games http://ixbtlabs.com/articles2/sound-technology/
- A Model of Head Related Transfer Functions based on a State-Space Analysis by Norman Herkamp Adams http://deepblue.lib.umich.edu/bitstream/2027.42/58485/1/nhadams_1.pdf
- Enhancement of 3D Sound Using Psychoacoustics by Kyosik Koo and Hyungatai Cha http://www.waset.org/journals/ijbls/v4/v4-3-27.pdf
Examples of the Effect of HRTF in Recording
- Antiphony Video on Vimeo http://vimeo.com/2503188
- Virtual Haircut Video on Vimeo http://vimeo.com/16114037