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Subwoofer integration

[Paul Spencer]

Some enthusiasts "don't like subwoofers." The reasons for this may or may not be related to the natured of subwoofer. If a subwoofer is lacking in accuracy, then it is going to be related to at least one of the following:


The latter two aspects are covered here.


The first is what most commercial subwoofers focus on. As they are sold seperately to your mains, they have to offer standard methods of integration. Correctly implemented, a custom integration will be far superior. When you include this in the design, you then can choose the filter slope which is critical to getting a correct transition from your mains to the subs. If you don't get it right, you may have a dip in the crossover region, or a peak. If you get it right the sub should not draw attention to itself.

The second aspect is where diy really comes into its own. Few commercial subs contain any tools capable of enabling you to correctly deal with room issues. Velodyne Digital Drive series subwoofers are an exception. This second area can actually be quite cheap to implement.

Active Filters


The first and most cricial aspect is crossing over to the mains. What you want to achieve is match the corner frequency (fc) and roll-off slope. Suppose your mains have an F3 at 40 Hz with 4th order rolloff. The simplest way to cross over is to add a 4th order electrical low pass filter on the subwoofer with an fc at 40 Hz. The mains should not have a high pass in this situation.

Suppose you wanted to cross over the subwoofer higher. You might convert the mains to a sealed box with an F3 at 80 Hz and a 2nd order roll off. You would then have two options. The simplest is to add a 2nd order low pass to the subwoofer and run the mains without a filter. An alternative is to add a 2nd order high pass to the mains with an fc at 80 Hz. They will then have a -6db point at 80 Hz and an effective 4th order rolloff. You would then add a 4th order low pass to the subwoofer with a matching fc.

Note: You should at least simulate the combined effects of acoustic and electrical roll-off as they will often not be what you expect. WinISD pro is suitable for this.

Another often neglected filter is the rumble filter!

[Bill (Collo) on building your own rumble filter]

Room Acoustics


Most subwoofers are placed in a room without an awareness of how much damage room acoustics imposes on the sound. As a result, the potential of a subwoofer for accuracy is often not realised, and usually underestimated. Many decide that they dislike subwoofers, and poor integration with the room may be one of the primary reasons.

There are two primary considerations:


Room gain can be resolved without too much difficulty. These 2 strategies can work equally well:

1. Design the response to work with room gain
2. Ignore room gain in the design but use eq to achieve the desired in-room response

A sealed subwoofer may suit applications where there is a lot of room gain. You may simulate an F3 at 40 Hz (anechoic) but in-room you may get it down to 20 Hz. A sealed sub will work well using the first approach. Using the second approach, you may choose a vented subwoofer. Room gain may shift the in-room F3 down very low, or perhaps result in a peak where the response slopes upwards, yielding an exaggerated bottom end. Eq can then remove the boom and give some extra dynamic range.

Room modes are more challenging to resolve. There are two approaches which can reduce the problem dramatically:


The first approach can yield very accurate results but with a dramatic loss of output capability. At least four times the displacement (SD x xmax) is required.

The second option places the sub as close as possible to the listening position. This option is discussed in the wiki page on placement. It is not a cure-all and is not favoured by most but may be worth considering.

For conventional subwoofers, there are two primary strategies to deal with room modes:


If a choice is to be made between the two, then room treatment is the preferred option as this deals with the problem at its source. However, better still is to use both.

To eq peaks out of the response requires parametric eq. Behringer have a number of units suitable for this task, and they are attractively priced. [1] Consider Feedback Destroyer or Ultracurve DEQ2496 or Ultradrive DCX2496. The latter also provides a digital active crossover.

Room treatment that is effective at taming modes is limited:


The first two options are expensive if commercial products are used, but there is scope for diy versions to be made. This type of treatment is common in recording studios. Note: Room treatment that is effective in the midrange is not likely to have any impact on room modes. Special treatment here is needed.

The latter option is covered by Dr Earl Geddes in his home theatre book. He recommends constructing an enclosure with two layers of drywall (platerboard) onto furring channels with liquid nails or other flexible adhesive holding them together. The advantage of this approach is that there is no visual impact on the room yet the effective area is much greater than bass traps.

Conclusion


Getting the best out of a subwoofer involves a little more than many consider. The issues involved are not difficult, however, to get an accurate subwoofer, it is necessary to come up with a way to resolve all the issues. If you place a subwoofer in a room without giving thought to integration issues, it's like trying to pin the tail on the donkey blindfolded when the donkey is somewhere on a football field! You may get it to sound decent, but its very unlikely that you will achieve the best possible integration. This is a problem that can't be solved by spending more money. It requires some custom design work. It's satisfying to know that with some skill in resolving these issues, your diy subwoofer can surpass the performance of more expensive commercial subwoofers.