Positioning Subs and Venting Trunks

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With the article on the BMW 550i which shows you how to create a vent into the vehicle cabin we thought it would be appropriate and helpful to some to go over some fundamentals on which way to point woofers and how to address issues related to the trunk. —Ben Oh

Questions:
I have an '06 Buick LaCrosse with 12s in a sealed enclosure pointed forward in the trunk as far as I could get them to the back seat until the seatbelt bolt sticking down from the rear deck stopped me in my tracks. Then I screwed it from the rear deck intot the top of the box to secure it. Which way should the subs have been directed to be the most efficient? Some years ago I saw an article that showed subs in different positions in a trunk. There were pics of the front waves and the rear reflections for each position. Should I point the subs back or up? I've heard that in order to point them forward you have to isolate the entire area behind the baffle, which I haven't done. I thought that pointing them backwards would just cancel the bass out by the time it got through into the cabin. In my experience, pointing them up sounded too flat. I've pointed subs forward before, but the baffle isolated the whole rear seat area from the trunk. Will it be OK to point a box forward pushed up in the trunk without sealing the whole trunk off from the baffle or should I just point them back? I'm not trying to make a one-hit wonder with this system. It would be nice to have a broad spectrum of sub harmonics.

I have a '93 Custlass Supreme coupe with two Alpine SWR-1241D's and an Alpine MRV-T757. The subs are in a custom sealed box in the trunk, which is completely sealed off from the cabin of the vehicle. How do I get more sound into my cabin? I love these subs for SQ, SPL, aesthetics, everything. But I want to be able to push more boom every once in a while. Is there any way to port my trunk so the sound will flow into the passenger area?

My bass isn't as loud as I'd like it to be (isn't that always the case?). I have an '85 Corolla and one 12" KIcker Comp VR in a sealed 1.2 cubic ft. box, running off a Profile Cali 600msx (600-watt mono) mounted in the trunk. It hits pretty hard, but I know it wants to hit harder. It's my trunk that's holding it back. What can I do to get more dB from my Kicker sub? I've considered adding ports into the trunk. How many should I add and where? Is this the best solution? And what is the best direction to have my woofer face? I've read a bunch of articles (don't quite understand them completely) and noticed they recommend a ported box facing toward the cab with the port pointed toward or even going straight into the rear deck. They state this based on the fact (they think) that the bass increases inside with the trunk open. I don't know about everyone else's car, but with my trunk open the bass sounds like crap.

Derek Lee: Regardless of the vehicle platform this is a common issue, one that affects all different brands of equipment (although it varies with cabinet style). Add on a variety of Internet articles and random opinions and you have a huge field of confusion. Let's see if we can put some of the voodoo into context.




There are several options for how you position a subwoofer. First, if the speakers aren't in cabinets, they must be prevented from suffering bass cancellaton by mounting them to a baffle that keeps the wave emitted from the back of the speaker from finding the front of the speaker cone. This is a crucial part of the installation of what is known as a "free air" application. If your speakers, however, are in a sealed enclosure, there's an entirely different set of rules to consider.




Firing Upward
First, in a sound quality audio system, our industry is obsessed with the "bass up front" philosophy. That means your bass should sound like it's coming from the front of the vehicle even though the subs are behind you. This is considered essential for proper staging and imaging. To achieve this, the question of where the speakers are firing is important. When firing upward, the higher frequency harmonics that the cone of the subwoofer naturally produces will reflect off of the window, causing your attention to be pulled to the back of the car. The bass will seem to be coming from behind because of the sonic cues that you hear, defeating our goal of "bass up front."




[Of course you could install a woofer in the front as in this sound quality competition vehicle]

An 8" woofer on the dash of the Dodge Neon.


Firing Through the Seat
Firing speakers through the seat is thought of as being the wrong thing to do, yet it's a very common technique and for good reason. The seat will act as an acoustic filter that prevents the high-frequency sonic elements from wiping out the "bass up front." While purists with too much free time like to argue this concept, it just works and is a long-standing trick of the trade. Essentially, the foam or other semi-porous materials in the seat will allow the deep bass to conduct its energy into the vehicle, but high frequencies are turned to heat.




The Importance of Front Speakers
Since deep bass is omni-directional, we can't tell where it is coming from if two things are in place. First, there can be no higher-frequency harmonics to reveal the real source of the sound. Second, there needs to be some dominant sound coming from where you want the bass to also seem to be originating. If you had only a sub firing through the back seat, but no front speakers at all, you'd still be able to figure out the sound is originating from the back of the vehicle. Add front speakers and now the sub bass can "hide" within the music.

Seat Port Superior to Rear Deck Opening
As a result, I'm not a big fan of doing a lot of extra work just to fire subs through a rear deck opening if I can filter through a back seat. However, if the subs are in the trunk of a sedan or coupe with a pair of rear deck speakers, then the bass can find another route into the vehicle through the rear deck speaker cones—not good! The pressure produed in the trunk by the subs will beat up the small rear deck speakers unless they're isolated. This means that the rear deck spekaers are in their own tiny sealed cabinets that prevent any pressure in the trunk from pushing them around.

Another option would be to create some kind of duct or port to carry bass into the passenger compartment without firing through a seat. This would make more sense to me in an SPL competition vehicle where it's important to squeeze every last bit of energy into the passenger compartment. For sound-quality applications, it isn't the best way to go.

Size of Vent
The key problem is that if the vent isn't large enough, it will begin to behave like a resonant tuned port as found in any bass reflex (ported) speaker cabinet. This will cause the bass to become very unnatural as the vent begins to add its own sounds to the mix. If you plan on creating a pathway for the bass to enter the vehicle, it needs to be short and wide. For those of you worried about efficiency—I'd be happy to trade off some efficiency for more natural and realistic-sounding bass in a system of my own.

Which Direction to Point a Cabinet and Transfer Function
Another concern is the direction to point a sub cabinet for best results. You hear about people who've placed cabinets against the back of the seat, in one vehicle with the subs facing forward with that not working and in another car with the subs backwards and that not working as well either. In yet another vehicle it didn't matter—facing frontward or backward resulted in poor bass. What's up with that?


One thing we miss about older cars--the huge trunks that can accommodate this much gear.


Ever notice that when you move your home stereo speakers, the sound changes? If you play a great-sounding set of speakers in a school gym, the sound really falls apart. One concept we need to factor into the mix is that you're essentially sitting inside a box with a speaker cabinet. How everything reacts within the bigger box is often referred to as the "transfer function". Just about everything that can conduct sound has a transfer function, including the speaker itself. It takes some pretty complex math to determine the transfer function of a group of objects, so we just never bother, opting instead to work with the "trial and error" method of determining how to deal with the acoustic environment.

There are many vehicles in which you can predict whether the cabinet position is correct or not, based on experience in other vehicles. That doesn't help, however, when you encounter a new vehicle shape. For instance, installers learn very quickly that placing a cabinet at the middle point of a van will cause problems. As they experiment with positioning, they find that everywhere you locate the cabinet will result in different bass characteristics. This is where changing one or more variables in the transfer function changes the whole result.

An installer's typical rule is to "go with what you know". By sticking with approaches that work well, the results can be predicted to ensure that the customer gets a predictable result. But what about when the vehicle is a fresh unknown, or the installer is too inexperienced to have encountered the various differences?

Test Enclosure Locations
Experimentation is the key to keeping yourself in the ballpark. I like to take a "test cabinet", which is any old subwoofer I happen to have mounted in an enclosure gathering dust. I connect the cabinet to any amplifier and play bass into it as I move it into different positions in the vehicle. I note the differences in sound character and specifically where the real problem areas are located so I don't build the vehicle around a bad location. You'd be amazed at how many installers will design a system based on dropping subs in where they want them rather than on where they perform the best.




Try this technique for yourself: move a cabinet into a variety of different locations while it's playing. A surprise you might discover is that one of the better locations for delivering deep bass is to position the sub enclosure at the back corner of the trunk. Whether it's the act of loading the cabinet into the corner of the vehicle or that it's the greatest distance from the listening position might be a good guess—only the transfer function can tell you that. The key is that I changed my methods for designing systems as soon as I discovered this. I no longer stick cabinets into positions that I haven't tested or experienced first, unless I have no choice.




Buying tires for your ride is a similar example. If you had a set of tires that were perfect in every way, they would change the performance of each vehicle you put them on in a different way. Is there a problem with the tires? No, it's a matter of the combination of variables.

Transfer Function Database?
I was excited many years ago when I heard that a car audio company was beginning to run tests to map the transfer fucntions of all the different cars on the road. This was going to be huge. We would have a way to plug the unique trasnsfer function of the vehicle into the box design software on our computers to determine the exact construction of the cabinet that would work best in a particular vehilce. For years we had been able to plug the speaker specs into a program that would determine the perfect box—if you were playing it in a test chamber. Put that perfect box-cabinet combination into a vehicle and a new random variable ensured that your perfect cabinet sounded like trash!

This new mapping would ensure that the sonic characteristics of the room (car's interior), the cabinet and the subwoofer were all balanced to work together for the best overall performance. Unfortunately, the company called off the project at some point and we never did get the benefit. I suspect it was because there's a differenct transfer function for each location within any vehicle, so the database would be unimaginably large and expensive to collect. And so it's with aftermarket car audio. Even without a database, we customize based on matching our wants, or the customer's, as the case may be, with the appropriate product that will do the best job. And the extra effort is worth it because an aftermarket system still beats factory sound hands down—notwithstanding the difficulties discussed here.

Derek Lee is the former owner of Mobile Dynamics, Toronto, Canada and was a contributing tech editor to Car Audio and Electronics magazine.

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