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. (As an aside, one significant benefit of car audio sound systems is that frequencies below the lowest room resonance increase at a theoretical 12 dB/octave…it’s no wonder car audio systems have such great bass!)

Acoustic treatment can be very effective above 200 Hz, depending mostly on the polar radiation pattern of the speaker. In the case of the Legatia 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 Legatia midbass and/or midrange 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, midrange, and/or 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 furnishings may be a noticeable improvement. It will require trial and error to get it right, but the learning is in the experimentation!

Continue to the conclusion.


Hybrid Audio Technologies Specifications and Parameters Spreadsheet (Google Sheets)Advanced System InstallationLesson One: Off-Axis ResponseLesson Two: Equalization of Pathlength DifferencesLesson Three: The Effect of HRTF, ITD, and IIDLesson Four: Point-SourcingLesson Five: ReferenceMounting Baffle ConsiderationsCrossoversTime CorrectionAcoustic Treatment — Acoustic Treatment — Conclusion