midbass, chamfer or round sharp edges (including, in particular, the mounting hole’s
rear inner edge), flush-mount the speaker whenever possible, and use shallow, surface-
mounted hardware. Also, remove all unnecessary protrusions from the baffle surface.
Acoustic Treatment
A considerable benefit can be made to any mobile audio system with the select placement of
acoustic treatments. The purpose of using acoustic treatments is to reduce the amount of
reflected energy in the hostile automotive environment, and hear more of the direct sound
being emanated from the speaker. It is akin to the signal to noise (S/N) ratio in a piece of
electronics, where the signal could be considered the direct energy coming from the speaker,
and the noise could be considered the reflected waves off of nearby surfaces, such as
windows, hard center consoles and door panels, windscreens, and etc. A word of warning
though: there is a fine line between too little and too much acoustic treatment; just as some
vehicles can benefit from some selectively applied treatments, there is a point where the
vehicle can begin to approach “semi-anechoic” conditions, and lose its liveliness, which is not
ideal. Reflections are all around us, and are a part of our day-to-day lives. It is our opinion
that some lateral reflection is a good thing; it helps to establish stage boundaries, and gives
the recoded playback and more visceral and “believable” sound.
The first principle to understand is that below 200 Hz, acoustic treatments are rendered
virtually useless. It is Hybrid Audio’s assertion that only those frequencies above 200 Hz
benefit from the use of treatments, given that a 200 Hz waveform is about 1.7 m long; 1.7
meters is less than or equal to most vehicle widths. This is also the frequency where we
believe
pure tones
in the vehicle are going to be difficult, if not impossible to localize. Finally,
most vehicles exhibit a Schroeder Frequency (Fs) between 50 at 125 Hz; the Fs (or cabin-gain
frequency) is vehicle dependent, and is the frequency at which resonances become so tightly
packed in frequency and space that the acoustical properties of the vehicle behave quite
uniformly.
2
Acoustic treatment can be very effective above 200 Hz, depending mostly on the polar
radiation pattern of the speaker. In the case of the Imagine midbass, the polar radiation
pattern is quite large at lower frequencies, with a narrowing of the radiation pattern
(“beaming”) at frequencies into the treble bandwidth.
Should the Imagine midbass be placed in the kick panel locations, one may find that a notable
improvement can be made by adding acoustical treatments, such as open-cell foam, into the
underside of the dashboard. Likewise, should the midbass, or in fact the I1 tweeter be placed
up high on a-pillars, or in the dashboard, where comb filtering (reflective summation and
cancellation off of a hard surface, such as a windscreen) may become an issue, a dashboard
“mat” or other soft furnishing may be a noticeable improvement. It will require trial and error
to get it right, but the learning is in the experimentation!
2
As an aside, one significant benefit of car audio sound systems is that frequencies Hbelow the lowest room resonanceH increase at a
theoretical 12 dB/octave…it’s no wonder car audio systems have such great bass!
© Hybrid Audio Technologies, 2010
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