What happens when a bass box is too large

Recently we had a client bring in a custom loudspeaker cabinet which had been built for some vintage drivers, featuring a 15" woofer. Much to my suprise, initial modelling indicated an enclosure that is much too large. More commonly, cabinet size is compromised and smaller than ideal. We usually want a box to be smaller than it could be. Hence this is a surprise when it goes the other way. This prompts a fair question: how do we decide how large a ported enclosure should be? 

 

How low can you go? 

 

First we consider the maximum bass extension that we can achieve with a given driver. Typically we start with the driver free air resonance (fs). This provides an initial guess at the potential bass extension. Of course, the actual potential depends on all parameters. So the quick way to find out is to model a standard alignment such as Chebyshev. Shown white in the plot below.

Green shows an EBS alignment, in which we give up 3 dB of sensitivity to achieve lower bass extension. This type of alignment involves a trade-off in which we give up maximum output in order to achieve lower bass. We should think carefully about this in this instance, as we're dealing with a 12" driver with limited output at very low frequencies. It may be worthwhile in small rooms with considerable gain. 

 

The next example (red) shows a compromise we might make to downsize the cabinet. The white alignment is 90L but we can get it down to a more modest size (65L) in this example without giving up much.

So far these are all workable choices, with typical trade offs with sensitivity, extension and size. But here is an example of a badly designed bass box - one which is larger than beneficial. 

 

Blue - an oversized bass box. In this case, the cabinet is 300L. 

What's wrong with this box?

Of course, there is always the practical issues like WAF, aesthetics and the extra challenges that go with very large boxes that are hard to handle and fit in your room. Aside from these issues, we also have a box that is hard to properly brace without it becoming extremely heavy.

Like with the EBS cabinet, we've given up some midbass punch, but we've also got an undesirable peak around tuning.

What can we do with an over sized bass box?

 

After the box is built, we can sometimes rescue it by changing the tuning. 

Here, all 3 cabinets have the same volume (300L) but different port tuning. We can remove the peak at tuning by lowering tuning by 2 Hz. If we tune higher, the peak gets worse (dark blue). 

So we can see that if your cabinet is larger than required for a balanced design, the best passive solution is to lower the tuning and effectively convert to an extended bass shelf design. If the extra extension isn't wanted, then there are two choices: EQ or rebuild. If EQ is used, we can overcome any sound quality issues and we expect to achieve higher SPL and hence more dynamic range. On the other hand, if we rebuild the cabinet, it's likely that we can end up with a more desirable result. The new box can be more aesthetic and we can also include more cabinet bracing, given that this tends to reduce net volume.

SPL vs extension

 

Quite commonly, audiophiles who don't understand the SPL cost of bass extension often will choose bass extension over maximum output. This is why we now have the modern trend of subs with impressive extension that actually lack the dynamic range required to fully appreciate it. There is actually no point in having bass extension to 12 Hz if you don't have enough SPL capability to actually appreciate that bass extension.

So let's compare the above alignments in terms of their actual SPL. In each case, power level is chosen to reach xmax above tuning. 

Red: 65L cabinet tuned at 20 Hz with 210 W
White: Chebychev alignment 90L tuned to 19 Hz with 160W

Green: EBS alignment 225L tuned to 15 Hz with 83W  

Blue: Oversized cabinet 300L tuned to 14 Hz with 67W

Excursion - we can see that all these subs would require a rumble filter below tuning

In the case of this specific sub, I'd argue the larger cabinets are not good choices for all but very small rooms with a large amount of gain. 103 dB modelled at 16 Hz is not a very good result. In many larger rooms in Australia, the low bass output could be very unimpressive.

On the other hand, the much smaller 65L cabinet achieves 110 dB in the midbass. If we carefully optimise position, this sub will likely offer impressive performance in a much more attractive package. The 2 dB greater output compared to the larger version will actually sound about one third louder. Bigger is not always better!