In the race to bring the best-of-the-best to discerning buyers, Mercedes-Benz has taken a new approach to bring audiophile quality sound to an open top roadster. In the upcoming SL series, M-B has incorporated footwell mounted woofers directly in the frame of the car. The ~8.5″ woofers are mounted into the front bulkhead and then hidden from view with the normal carpet that you would expect to find under your feet. Why would they go to such trouble? Read on to find out.
Without going completely Bill Nye (Mr. Wizard for our older readers) on you, producing high fidelity sound in an open top roadster is not easy. The high notes, such as cymbal crashes and vocals are easier to produce. The reason for this, is your ears can hear high notes directly as they come out of a speaker. So, in a convertible, manufacturers have done quite a few things in order to make sure you have a speaker or two pointed directly at you. This includes speakers mounted directly into your seats or headrest (remember the Fiero?) The issue with having speakers located so close to your ears is it gets difficult to create “stage.” Stage can be thought of as the depth and location of your music. The goal isn’t to make it feel like your favorite singer is screaming in your face its to make it feel like you’re at your favorite theater. If the band that you’re watching has a trumpet player on the left side of the singer, you should hear the sound emanating from the right-front of the stereo. Today, cars like the Infiniti M have personal speakers located in the seats, but they are more for surround purposes than delivering vocals. Most other manufacturers have gone to a system of putting the high note-creating speakers (also known as tweeters) either on the dash or on the doors to provide as much space between you and them as possible.
What is much more difficult in an open top roadster is reproducing the mid and lower level tones consistently. With top up or the top down the interior of the car is significantly different. The issue here is that the vehicle becomes a chamber when the roof is up, but when it is down there is nothing to trap the sound and create sound pressure. It is significantly easier to recreate low tones in a chamber. Want an example? Take your home theater sub woofer and put it in your front yard. (Lets face it, if you’re in the market for an SL, you can afford a real stereo for your Blu-rays) Most likely you’re only going to hear a bunch of wind noise from the moving speaker and very little actual bass. This concept has just less than 400,000 levels of complexity and the large majority of them are beyond my grasp. In summary, a convertible is not a friendly place to create powerful and consistent bass. During the days of soft-tops that did not take much space in the truck, locating woofers in the truck of a car was a viable option. It ensured that the chamber of the trunk was consistent and then the sound could travel through the rear seat to the vehicles passengers. Here again is one of those interesting things about very low notes (bass): it is mostly omnidirectional. In English, unlike high tones, whose location you can determine the direction of the source, bass is much more difficult to locate. Finally, bass doesn’t have much trouble traveling through things like seat foam and carpet.
Wow, don’t I sound like a fun dinner date? Back to the Frontbass Audio system in the upcoming Mercedes-Benz SL. M-B has put a woofer in the footwells of all of the SL models. Putting low-note creating speakers here has quite a few advantages. First, the chamber of the footwell is consistent top up or top down. The sound created in this space is then directly funneled to the passenger. In the past people would put woofers in the doors of cars to attempt to create this same effect, but that always had its issues. Beyond being a large and heavy speaker, these speakers generate a lot of vibrational energy that tends to create rattles in something like a door. By essentially frame mounting the speakers M-B has eliminated parts that are easily vibrated and also carefully controlled the air volume behind the speaker.