How to Avoid “Trunk Bass”

There are several problems that we come across when we install a subwoofer enclosure into a sedan type vehicle with a trunk. The most apparent problem is the reduction of bass in the main listening cabin. Another problem is the situation that results in more bass being heard with the trunk open than closed. A third problem is the annoying rattling of the trunk body panels and lid.  Proper damping will significantly reduce the  vibration of body panels and yield slightly higher SPL in the vehicle, but will not fix the other issues apparent with sedans.

Let’s take a look at what is happening in your trunk acoustically. Everything will make sense and that the solution will be quite obvious…

Since the goal is to get the cleanest and loudest bass into the main listening area, let’s address the issue of loss of volume. Taking the logical approach, we would be curious to know how the bass is even heard in the listening cabin when the box itself is in the trunk. If the car in question is a hatchback, the box would be in the listening area. But since it is in the trunk, what pathway does the bass energy have to travel to get inside? Well, in cars with foam-backed seats, the sound can propagate through the foam itself. Let’s suppose the sedan we are talking about is a Mercedes or BMW… Most luxury cars have a steel lined trunk or masonite-backed seats. They pass even less bass into the listening area. So, what pathway is actually left? Leaks! That’s it. Is it any wonder we don’t get a lot of bass in the cabin?  So, the apparent solution seems to be to increase the direct air coupling between the trunk and cabin as much as possible. This means making some kind of perforation in the rear deck, which would help to couple the trunk to the cabin. This seems like a simple solution, and works very well if done right, and if the consumer will even allow you to cut metal. The last part of this article will give one format of making this modification solution.

So far, the high resonance of the trunk body panels should make sense.  Why wouldn’t it rattle and buzz if the box is trapped in the trunk? All of that bass energy can only escape through poorly damped body panels.  We will see how all of the above mentioned problems can be cured with one simple solution.

Some people have noticed that by simply cutting a hole in the rear deck, amplitude will increase. The bigger the hole, the bigger the gain will be. Other people do not want to decrease the structural integrity of the vehicle’s body. I have used a technique that has worked quite well for me for years and typically yields a gain of 7 to over 13 dB of gain.

The technique I use doesn’t weaken but actually strengthens the body. I start by removing the rear deck cover. I then climb in the trunk and remove as much metal as possible, just leaving a 1″ frame around the edge and a 1″ brace from the front to the rear of the deck. Try to visualize the left over metal. It is in the shape of the number “8″, so to say. If 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 drop 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 chicken-wire or metal mesh 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 8x thicker than the stock 1/16″ thickness, 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 (if located here) 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 people think they’re a genius because they ran the port of a bass reflex enclosure up through the deck. I find this to be a dumb idea as the only output that will increase will be the frequencies close to the tuning frequency of the port. For those people who like the slamming bass of a custom bandpass, you are in luck. By simply extending the whole port(s) up through the rear deck, you can focus 100% of the acoustic energy into the cabin with none 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”.