Specialist subs - moving beyond sealed and ported subs

When it comes to mass produced subwoofers, it makes sense to manufacture generalist subs. In 2 channel systems, this will mean moderately sized sealed subs. For home theatre, it usually means ported subs. These conventional designs are good all-rounders. They can easily cover 20 Hz - 80 Hz and even dig deeper than that.

So what happens when you want to go beyond what you can get out of one or two fairly conventional subs? If you're buying from the conventional hifi retailers, the price goes up quickly as you start to move into 15" or 18" subs. You pay a lot more but do you actually get that much more?

In over a decade now of going into rooms and testing how they perform, one thing has become clear. You can do a lot better, when you know your room. Once you do understand your room acoustics and how the sub will perform in different positions in your room, this gives rise to some different goals. Many people assume that all subs in a system should be the same so that one doesn't bottleneck the system. In reality, the opposite is often true. When you have more than one sub, often they are not all doing the same job.

The heavy lifter

In some rooms, you may have one position that is ideally suited to very low bass. It's usually due to having the best room gain in the bottom octave. It could also be because this particular position allows a larger sub than other locations, which might be limited by things like windows, doors, speakers, furniture. In a multi sub set up this sub doesn't necessarily need to cover the upper bass. This type of sub suits a ported, bandpass or horn alignment. Where this type of sub is used, normally it will handle the lower bass without any help from the other subs. Hence, it will allow any other subs to be smaller.

The infill sub

If this is the second sub to be paired with a heavy lifter, it won't need to cover the low end. That means generally it can be more compact. Typically an infill sub will cover 40 - 80 Hz. It might be a conventional sub with a standard hifi driver in a sealed box. Its purpose is to fill in dips caused by room modes, where we can't find a single position that avoids all dips. 

The thumper

This is the name that I give to a sub that functions as an infill but that is designed for higher output. We can achieve this with a bandpass design, giving up some extension to get higher sensitivity. This is the case with a recently designed sub that I cal "lil thumper." It has 92 dB sensitivity. We can also achieve this using pro drivers, where the sensitivity might range from 93 - 97 dB.

The stealth infill sub

Sometimes we need an infill sub to fill in some dips, but we might need that sub in a position that might localise. This can happen where the sub is located around the back of the room and quite close to the listening position. If you aren't careful, you may be able to tell where the sub is located and that can be distracting. For this type of sub we have to be careful to avoid any features that would give away the location of the sub. This can come from air leaks, turbulent ports that chuff, ports with port resonances above the passband or horns with ringing above the passband. Even a sealed sub, which by design avoids all these things, can still localise.

This type of sub is tricky, especially where it must fill in dips in the mid and upper bass. Extra care must be taken.

Does this all sound confusing? Watch this space, as I will be adding some example that will make the use of these specialist subs more clear.

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!