Overcoming the Problems of "Trunk Bass"
by: Grizz Archer
There are a few primary problems that are encountered when we install subwoofer enclosures into a sedan type vehicle with a trunk. The most obvious problem is the lack of bass in the main listening cabin. The next problem is that there is more bass with the trunk open than when closed. A third problem is the excessive rattling of the trunk body panels.
Lets take a good look at what is happening acoustically. We will find that everything makes sense and that the solution will be obvious...
Since the main goal is to get the loudest and cleanest bass in the cabin, lets address the loss of volume issue. Taking the logical approach, we would be curious to know how the bass is heard in the listening cabin when the box is in the trunk. If the car was a hatchback, we would have the box in the listening area. But since it is in the trunk, what pathway does the bass energy take to get inside? Well, in cars with foam-backed seats, the sound can propagate right through the foam. Suppose the sedan we are talking about is a Mercedes or BMW... Most of these cars have steel lined trunks. They pass even less bass. So, what pathway is left? Leaks! Thats it. So, the logical solution would be to increase the direct air coupling as much as possible. This would involve making some type of perforations in the rear deck, which would couple the trunk to the cabin. This seems like a simple solution, and works very well if done correctly, and if the consumer will allow you to cut metal. The last part of this article will give one format of making this modification solution.
If we put the enclosure in the front seat of the car and climbed into the trunk, we would have the same problem, right? So, the bass loss makes total sense, as will our next quick topic. If we can increase bass volume by making a pathway for the bass to enter the cabin, then what would happen if we open the trunk? It would be louder outside. People think that this is wrong, but it is, in fact, agreeing with the laws of physics. When we create more direct air coupling, this problem will be greatly reduced or eliminated, depending on which route is taken as a solution.
By now, the highly resonant trunk body panels should make perfect sense.
Why wouldnt it rattle and buzz if the box is trapped inside? We will find that all of the above mentioned problems will be cured with one simple solution.
Some people have realized that by simply cutting a hole in the rear deck, amplitude will increase. The bigger the hole, the bigger the gain. Other people do not want to decrease the structural integrity of the body. I have a technique that has worked for me for years which typically yields a gain of 7 to over 13 dB of gain.
The technique I use actually strengthens the body as well. I will start by removing the rear deck cover. I will then climb in the trunk and remove as much metal as possible, usually leaving a 1" frame around the perimeter and a 1" brace from front to rear of the deck. Try to visualize the left over metal. It is in the shape of the number "8", so to say. Assuming that the rear deck cover fits very well, I will trace the cover onto a piece of MDF or plywood that is 1/2" thick or thicker. When I set the 8 shaped piece of panel wood over the deck, it should fit tight. I will then proceed to climb back into the trunk to trace the holes that I cut out of the metal, onto the panel of wood. Then remove the panel and cut the holes out, leaving the same 8 shape as the metal rear deck. The next step is to cover the top of the panel with Chickenwire or metal grill material. Then I will finish the panel with color-matching acoustically transparent carpet, velour, grill cloth or similar material. To install the new panel, I like to use either 100% pure silicone or Liquid Nails as an adhesive. Lay down a generous bead of adhesive onto the surface of the 8 shaped metal rear deck. The press the new grill into place and allow to dry. The result is rear deck that is now over 1/2" thick as compared to 1/16" thick, so it is definitely stronger. We have a lot more acoustic coupling, so it will yield a lot more amplitude. The pressure will not build up in the trunk as much as before, reducing trunk panel vibration. As an added benefit, the rear deck speakers will not be forced to comply with the pressure of the larger subs in a "sealed" trunk, which will result in longer life for the rear fill drivers. This solution will work with acoustic suspension, bass reflex or bandpass enclosures.
Some like to run the port of a bass reflex enclosure up through the deck. I find this to be a bad idea as the only output that will increase will be the frequencies close to the tuning frequency. For those people who like the slammatotious bass of a psycho bandpass, you are in luck if you plan on building a custom enclosure. By simply extending the whole port up through the rear deck, you can focus 100% of the acoustic energy into the cabin with zero energy wasted in the trunk! The hole around the port must be sealed with silicone to assure 100% direct air coupling.
Suffice to say, hatchbacks are notorious SPL monsters and are naturally loud with little audio equipment. The above technique simply converts a sedan with a trunk into a "pseudo hatchback".