August 24, 2009

Why two subs isn't enough - Part 2


In Part 1 I focused mainly on showing how using multiple moderate subwoofers can achieve higher output than a single monster sub. The real benefit, however, lies in the improvement in sound quality. The multi sub approach isn't new. The material here is largely based on online forum discussions with Dr Earl Geddes.

The problem isn't what you think it is

It's very easy to think that the way to achieve the ultimate bass is to buy the ultimate subwoofer. This isn't the case, although it's a good starting point. The reason is that the room itself introduces big problems. If you compare a cost-no-object sub placed in a room with unresolved acoustics issues, it's more than likely that three subs set up following this approach will be more accurate and cost less.

So what is the problem? In a nutshell - room modes. They are talked about a lot more than they are understood. At very low frequencies, a room is unformly pressurised and the wavelengths of these sound waves are large compared to the room dimenions. Here, we only need to move a lot of air and the output will be fairly even anywhere in the room. In the midrange and above, the wavelenths are small compared to room dimensions. Each sound wave will have peaks and dips, but they are so densely spaced that they are spatially averaged, and we don't need to pay any attention to these peaks and dips. In between these two regions, we have the "modal region" where our problems start. In this region, the modes are spaced far enough apart to become a problem. In one seat you have a peak in the response, but in the next you might have a dip. Typically there are many peaks and dips in all locations.

Typical solutions don't really work

There are a few different ways that this problem is often addressed:
  • live in blissful ignorance (this option is diminishing quickly for you)
  • room EQ
  • try to place the sub in the best position and hope for the best
  • opt for dipoles
  • treat the room with bass traps
None of these attempts at solving the problem are completely effective. The reason is simple - they fail to address the problem at the source, which is always the best way to solve a problem. A 3D problem needs a 3D solution.

Room EQ
It can often achieve a flat response in one location, but make matters worse at others. In well behaved rooms it is better than doing nothing at all, but it's certainly not the best solution.

Open baffle dipoles do have some merit. They share some advantages with infinite baffle, but their output is seriously limited. Their response is not necessarily smoother than monopoles, and multiple monopole sources achieve the desired goal more effectively.

Bass traps
Typical bass traps can help, but ideally the whole room should be designed to act as a bass trap. This makes it much easier to implement the multisub solution, but does not replace it.

This post is more of an introduction, but I'll soon write a more detailed account of the multi sub approach.

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