Let’s first cover the mounting baffles for your speakers – most importantly for the midbass, but also for the midrange as well. To begin, construct mounting baffles for your speakers. I use Baltic birch typically, as it is stiff, easy to work with, and won’t deform like MDF should it get wet (a really important consideration for doors and other wet environments). You could go a step further and make friends with a local machinist and get baffles made of aluminum or even steel, which would be even better. The mounting baffle and mounting location should either be secured extremely well to the vehicle’s body, or completely isolated from the vehicle’s chassis. The reasoning is that the speaker baffle panel will vibrate and will radiate sound. Baffles need mass and need to be tightly clamped. Even small vibrations can result in the baffle itself radiating more sound than the actual speaker at certain frequencies. In many cases, using a thicker baffle panel in concert with self-adhesive sound damping is advantageous, provided the rearward wave of the speaker has no obstructions created by the baffle itself. Multiple layers of dissimilar material works well, such as a sandwich of materials, e.g.:
“sound damping -> steel/birch/aluminum -> sound damping -> vehicle chassis“
If possible, the speaker should also be mechanically decoupled from the baffle. This can be something as simple as a layer of self-adhesive foam tape or sound damping, to more exotic examples of decoupling, including rubberized rings or multiple-layer septum shielding.
Now that the midbass is sorted, let’s dive into the satellite speakers; the midrange and tweeter. I like using 3-inch cone midranges for pillar installs – no bigger and no smaller. This diameter usually gives the optimum balance between robust lower-octave authority, and transparent and believable upper-octave representation of vocals, even extending up well past the vocal range into the treble range. In my experience, smaller diameter midranges in the sub 3-inches size struggle terribly tracking the low vocal range sub-400 Hz, and especially at 250 Hz +/-. Larger diameter midrange drivers perform well in the lower octaves but tend to lack the efficacy in the upper octaves (sans wide-bandwidth midrange drivers of course). Plus, the 3-inch cone size is ideal for most pillar installations (as an aside, I have built and tuned many championship cars that use only a 3-inch wide-bandwidth midrange to cover all frequencies above 250 Hz using no tweeters; perhaps this would make a good future article?)
As discussed above, 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. That means when deciding where to put your midrange driver with respect to the tweeter driver, the midrange should always be mounted further away from you, while still keeping a “line of sight,” i.e., the tweeter nor the panel itself blocking the midrange. This is because your midrange will likely be playing, at least partially, in that critical ITD band (up to around 500 Hz), and the entire band where a combination of ITD and IID are dominant (approximately 500 Hz – 2,000 Hz).
About now you should be sitting at the edge of your seat, waiting for me to tell you how to aim your midranges. I can’t. Only you and your ears can tell you how to aim the midranges. I can tell you, however, a decent place to start. In the last article, I mentioned:
“When a speaker system is placed in an automotive environment, we hear the direct (shortest path) and reflected (longer path) sounds, such as resonances and reverberations. The two sounds are processed by the brain as one sound, and this influences our perception of height, width, and depth of soundstage, as well as rearward ambience. For this reason, the off-axis radiation pattern of any speaker in a vehicular environment has a significant influence on how natural the music sounds. Most mobile audio sound systems benefit greatly from having the front stage speakers at least partially “off-axis.” Off-axis means that the speakers are not pointing at you, but rather at some angle less than 90 degrees away from you.”
I believe the same applies to a-pillar midranges. I usually start by mocking up my midrange pillar pods using a birch ring and popsicle sticks, glued temporarily in place with cyanoacrylate glue (Super Glue©). I begin to aim the pods at a common point in the headliner, usually about half-way between the rear view mirror and the dome light. It’s usually a good starting point. I then listen to just the midranges with typical crossover, with a towel covering the back-wave of the speakers (very important), to see if it sounds reasonable. I’ll then play around by popping free one popsicle stick and shortening it to change the angle. The pods don’t necessarily need to be aimed at the same point. This process might take many hours or a few minutes, depending on how lucky you are! Once the angle is figured out, it’s time to fabricate. Break out the fiberglass resin, cabosil, and chop mat. (Don’t know what these items are? Well then you might want to get a GOOD custom installer involved right about now).
Does your midrange require a sealed enclosure, or does it like to be “infinite baffle?” That is a question for the tech support staff of your favorite speaker brand. The speakers I design for Hybrid Audio Technologies all require a simple seal between the front and back-waves, i.e. “infinite baffle,” with unrestricted access to airspace. When a speaker is mounted in a small closed box, it radiates as much energy forward of the cone as it does rearward of the cone. All speaker cones (diaphragms) are a weak sound barrier at best, and the result of the high amount of energy being “pushed” into a small enclosure is the energy transmitting through to the outside of the cone (an additive phenomenon to the incidental wave). The speaker should not be significantly prone to enclosure back-pressure and sound coloration when placed infinitely baffled. The “infinitely large” enclosure, such as one might find in the unrestricted airspace gap between the plastic a-pillar and the vehicle’s structural metal is usually perfect for this installation technique. Plus, I hate building enclosures! That being said, should your speaker choice require a sealed enclosure, do a good job of sealing it to the desired internal volume and be sure to damp resonances with thick baffle walls, or even go as far as adding mass in the form of BB’s or pellets, etc.
Aiming the tweeters is a little bit easier, as their dispersion pattern is quite narrow in comparison to the midrange, they’re smaller, and there are no acoustic time considerations to keep in mind, just acoustic intensity. I usually just start with the tweeters aimed at the same spot as the midrange, for consistency’s sake. The tweeters do usually end up a bit more on-axis than the midranges though, after careful listening tests. But it is very much vehicle specific!
Generally speaking, the tweeter should be relatively close to the midrange, as we can effectively mitigate anticipated environmental conditions and reduce the number of variables in this type of installation. When the tweeter and midrange are placed close to each other, the relative amplitudes (volumes) are equalized to within each speaker’s sensitivity, which can then be level-matched using digital signal processing or amplifier gain structure.
One of the most important things to do before completing your installation is to get a reference for your future listening tests. There is no substitute for the visceral impact and emotion of live music. Nothing else in life can touch your soul the way music does. Whether it’s a 200-member orchestra, or a four-piece fusion band, nothing compares to the phenomenon of live music. You’ll need your reference for when you join me next time: how to tune your newly-fabricated pillar install!
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