Q&A: How about woofers in the trunk?

A long-time close friend asked me a question that I felt merited sharing with everybody. He asked the following:

Question:

I wanted to get your opinion.  Looking at the Thiele & Small Parameters of this sub, I tried 1 and then 2 of them in Infinite Baffle. It worked okay, but didn't have a lot of pop. Which of these two options would you recommend considering the T/S Parameters of the sub?

1)  1 sub in a 1.7ft^3 sealed enclosure (either rear facing or down firing - opinions?) in the trunk right behind the back seat

OR

2)  2 subs in an aperiodic membrane, keeping in mind I have 3 openings in my rear deck, but no passthrough in the rear seat, so the rear seat won't ever fold down.

Answer:

When it comes to reproducing vibrations, which is what sound reproduction is, I believe that controlled and anticipated vibrations are much more desirable over uncontrolled ones. The reason is that controlled vibrations will at least have a degree of correlation and synchronicity with the music. On the other hand, uncontrolled vibrations may not. In other words, it is the degree of uncertainty that crates a problem. As a result, I prefer panels and enclosures that are as rigid as possible. I very much want for vibrations to be created only by the cone, ports or passive radiators based on the design.

JBL cheese

I have seen plenty of cheesy home speakers where the enclosure is said to enhance the sound through its vibrations. They somehow imply that the enclosure has become a musical instrument; like a violin that is supposed to vibrate as it creates music. The problem is that this is clearly no longer about reproduced sound. As such, those speakers will never reach a high degree of transparency; no matter how pleasant their new vibrations may be.

When placing woofers in the trunk of a car, I prefer to apply sound-damping material over the whole boundary section between the interior and the trunk. While not perfect, this material reduces leakage by a large margin, it's inexpensive and it's easy to deploy. I then cut openings only for the area where the woofer will play through.

If the trunk and the interior are also connected through the side panels, I would either apply expanding foam or stuff the cavities tight with fiberglass insulation.

The goal is to reduce the amplitude of any leaked vibration, whether airborne or not, by at least 20 dB's and to delay them by as much as possible.

A way to test this is by playing sweep tones and impulses through a wide bandwidth speaker inside the trunk before the final holes for the woofers are cut. Any vibrations, musical or not, that are audible will destroy image specificity and stage dimensions.

Here is where the problem lies. While the so called desirable vibrations of the cheesy enclosure described above may improve tonality, they will destroy spatial information. Even in the case where they project a larger stage, imaging will be bloated and all recordings will seem to be of the same stage size. This trick, again, should be left to poorly designed systems that attempt to impress unsophisticated listeners. While a neophyte will enjoy the cheesy system, they will intuitively know that something is wrong, even when unable to establish the source of the displeasure. 

Unfortunately when competing in sound-offs, we are exposed to many sophisticated listeners. I know that I would certainly be able to pick the flaws and that any scores will reflect them. Those who know me have seen me score poorly a vehicle that would otherwise get decent scores. With my experience, it is difficult to get excited about cheap tricks.

Following the points addressed above, I am not sure that one woofer enclosure over the other would be better. Moreover, the woofer that you are using already has incredible Thiele & Small parameters.

You see, the pop that you are seeking is actually a distortion due to damping at around 40 Hz.
I should note that I have yet to hear the sound of woofers when listening to live, un-amplified music; let alone woofers that pop. This doesn't mean that the pop you seek would not be unpleasant; it would just not be linear.

A tight small sealed enclosure dampens the cone more at this frequency than a ported, infinite or transmission line. So, if pop is what you look for, then the answer is clear.

Furthermore, when it comes to pop, two is better than one.

With regards to facing the speaker towards the rear of the trunk or firing it directly into the interior of the car, I always prefer great quality woofers being fired directly. Firing back increases efficiency (loudness) at the expense of delaying the signal and increasing the uncertainty-risk by a large margin. This also means that sealing the boundary between the trunk and the interior will be impossible as the system depends on the sound getting through where the boundary would be.

A woofer firing back in a trunk is therefore far from what I would consider for a system intended to have a high degree of linearity (accuracy). 

While uncertainty could be relatively reduced by using downward firing woofers, these still concern me enough that I would not use them as a top choice for a high linearity system. 

I like aperiodic enclosures better but these certainly lack the pop you are seeking. It was my experience that the lower the Qts, the better the speaker performed in an aperiodic. Your speaker seems to be right at the border of where it needs to be. You would just have to make sure to build the aperiodic rigid enough where there is no buzz coming from the fiberglass or packing material. 

I know that many judges like to hear some pop. This is the same challenge I faced when designing systems. I had to decide whether it was better to score high at a local level, where judging was poor, or to score higher at national competitions, where judging was better. My answer was to never compete locally where a bad judge could kill us in front of our crowd, to compete extensively at a regional level to compensate for any unlucky shows, and to focus on national quality shows instead. Thankfully, this approach also met my desire to build systems with superior linearity.