August 17, 2009

Why two subs isn't enough - Part 1


Yes, you heard it right - two subwoofers isn't really up to the task. I know what you're thinking and no this isn't about insane output. The primary reason for using three subs is actually all about sound quality! The increase in headroom and output is also a key benefit, but not the main reason. Of course, not everyone has a dedicated room. The context of this post is a serious dedicated home theatre or music room in which we are aiming for accuracy and fairly serious output.

The problem with one subwoofer

There are two problems with attempting to do the job with just one subwoofer:
  1. Output - the excursion and thermal power requirements are expensive and detrimental to accuracy
  2. Room acoustics - room mode issues aren't properly addressed
First, let's consider output. High output at low frequencies requires moving a lot of air. The typical solution is providing drivers with increasing excursion capability. From a driver design point of view, this leads to expensive drivers and low efficiency hence a great deal of power is required. This in turn means larger voice coils with higher inductance. Again, keeping inductance down increases the expense further. Many high excursion drivers give up so much efficiency that their thermal rating is exceeded long before they run out of excursion.

If the driver is placed in a sealed box, the efficiency will be very low at 20 Hz. In many cases it will be as low as 70 db 1w 1m! Compared to a driver in a vented box, the excursion requirement is doubled. If we place this ultra high excursion driver in a vented box it now becomes very difficult to deal with vent chuffing with air moving at high velocity. At very high excursion it become impractical to design a vented box. The typical solution then becomes using two passive radiators. This removes the vent turbulence problems, but further increases the cost.

Of course, all of these challenges can be navigated and balanced - this is a part of good design. However, the purpose of this post is to present a sensible alternative.

Simulated example

Let's consider an example to illustrate. Simulations have been performed in Win ISD pro alpha.

1. 3 modest subs with Peerless XLS (12") in an 80L vented box tuned at 22 Hz with a modest 300 w input each.

2. One sealed sub with Exodus Audio Maelstrom-x (18") in a 200L sealed box with 1.5kw input power.

Max SPL chart - Red: 3 x 80L Peerless XLS 12" vented Green: Exodus Maelstrom-x 18" sealed

On paper it doesn't look like much difference, but in fact in an actual room the difference in output could in fact be easily increased. In a room with substantial gain, different filtering could be used to get more output in the top end, with room gain brining up the bottom end to match. The efficiency of each Peerless driver is about the same as the larger Maelstrom, and the total radiating area is greater.

The Maelstrom uses 33mm one way excursion to achieve this output, but the Peerless drops down to 5mm at tuning with a maximum of 12mm at 30 Hz. As a result, the Peerless can be used in a vented box and air velocity remains at an acceptable level with a practical vent.

Please note: the point is not to attempt to show that either driver is superior, but to illustrate a different approach to achieving a target. Both are excellent drivers which are also very different. Many would in fact use the Maelstrom in a very large vented box tuned below 20 Hz. Used in this way with a very powerful amplifier it could achieve things which the three Peerless subs couldn't.

So it can be seen that the three modest subs can actually achieve more output than the single large sub. They do it with less power and excursion, and will often fit more easily into the room.

The real advantage, however, is the way in which three subs can integrate into the room. This will be covered in Why two subs isn't enough - Part 1

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