Lesson Three: The Effect of HRTF, ITD, and IID
Head-related transfer function (HRTF), interaural intensity differences (IID), and interaural time delay (ITD) all play a key role in the optimum placement location for the Legatia SE component speakers.
A sound wave approaching the eardrum from your chosen speaker location is shaped by interactions with the size and shape of your head, torso, and outer ear, resulting in the HRTF. More specifically, the HRTF is the ratio between the sound pressures of the wave at the eardrum, as compared to the sound pressure that would exist at the center of the head if the head were removed. In general, the sound arriving at the ear further from the source is attenuated and delayed relative to the sound arriving at the ear closer to the source. This generates an interaural intensity difference (IID) and an interaural time delay (ITD). As a sound approaches the head, the ratio of distances from the speaker location to the near and far ears increases, and the effects of head-shadowing are amplified, causing the IID to increase. The spectral shaping caused by the head and the shape of the outer ear may also change. The ITD, which results from the absolute difference in path length from the source to the ears, remains approximately constant as distance decreases. From this we learn:
- ITD is the dominant factor for frequencies below about 500 Hz;
- A combination of ITD and IID are dominant for frequencies between approximately 500 Hz and 2,000 Hz; and
- IID, in concert with HRTF, are dominant above about 2,000 Hz.
These are generalizations and are subject to the size and shape of one’s head and torso, and size and shape of the outer ear (the folds and ridges of the ear), but in general, the above is a good guideline for establishing ITD, IID and HRTF thresholds for the human auditory system.
Because of the majority of Hybrid Audio midbass drivers’ abilities to play into the sub-200 Hz range, an effect clearly dominated by ITD, up to and including frequencies exceeding 6,000 Hz, an effect clearly dominated by IID and HRTF, placement of this driver is extremely important. The driver should be placed as far forward as possible in the vehicle to optimize ITD. Lateral (forward to back) placement is much more important than horizontal placement (up and down). This is because of the brain’s ability to process sounds such as spectral envelope cues, and use a phenomenon known as the “precedence effect”; the brain can be easily “fooled” into thinking a sound stage is high with kick panel or floor-mounted speakers (a word to the wise: the best place to put a set of speakers is not always “up high”, as most vehicles do not offer an amicable location in the dashboard or a-pillars for good image placement for both seated passengers, especially in the critical frequencies sub-500 Hz). In addition to the time equalized placement of the drivers, the end-user must also consider that above approximately 2,000 Hz, intensity plays a key role in good sound staging and imaging. Therefore, the Hybrid Audio midbass’ should be placed in an area where intensity differences can be equalized, either mechanically or electronically, to ensure good imaging and sound staging.
The lesson to be learned is that, like Lesson Two, the Hybrid Audio midbass should be placed as far forward from your listening position as possible, and every effort should be made to optimize time and intensity domain characteristics of the installation.
Hybrid Audio Technologies Specifications and Parameters Spreadsheet (Google Sheets) — Advanced System Installation — Lesson One: Off-Axis Response — Lesson Two: Equalization of Pathlength Differences — Lesson Three: The Effect of HRTF, ITD, and IID — Lesson Four: Point-Sourcing — Lesson Five: Reference — Mounting Baffle Considerations — Crossovers — Time Correction — Acoustic Treatment — Acoustic Treatment — Conclusion