equalization of pathlength differences for most vehicles. And the reason why this is a good
choice for most vehicles is defined in the second full paragraph of Lesson Three, below.
The lesson to be learned here is that by taking a few moments to evaluate the potential
mounting locations in your vehicle, in a very short period of time, you will be able to find the
best location for your I6 midbass by determining the location with the smallest PLD.
Lesson Three: The Effect o HRTF, ITD, and IID
f
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 Imagine
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 the I6 midbass’ ability 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 Imagine midbass’ should be placed in an
© Hybrid Audio Technologies, 2010
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