August 2012

Distributed Bass
A simple execution for the distributed bass concept.
Article By Jeff Poth

Difficulty Level

  As I mentioned in my Kaboominator article, I destroyed one of the woofers that would have made my project more complete (two kaboominators rather than one).

Distributed bass is one way of saying that the bass loading in the room is intentionally varied in order to reduce the influence of the room. The room is the dominant factor in low frequency performance for most loudspeaker systems, in most rooms, below some 200 Hz or so. By distributing where the bass sources are the distances to various boundaries are also varied. Note the sine waves below.

If you were to sum this first set, the waves which are exactly 180 degrees out of phase will cancel and there will be no net SPL, or sound. If the first wave were the sound coming from your woofer, and the second were a reflection within the room, you can see how this frequency would be dramatically reduced compared to the "correct" sound coming directly from the subwoofer. This sort of cancellation causes a notch in the frequency response and is very common in typical rooms. In other words, your woofer is working against the room, and the room is winning. Likewise, where the waves are closer to equal phase, they will sum and you can have peaks, instead of notches, in the frequency response.

These cancellations and reinforcements from standing waves and other room/boundary effects are a major hurdle to good frequency response in the bass. There are a number of ways that modal behavior in rooms can be limited, all of which are intended to reduce the influence of the room. One is to use bass horns. When true basshorns are used, the sound is directed into a specific beam which is focused towards the listener. A significant reduction in energy to the sides and rear takes place, and thus the room plays less of a role. Basshorns reduce the influence of the room by increasing the ratio of direct to reflected energy. The drawbacks with bass horns are that they are extremely large -- a proper bass horn needs to be large relative to the wavelengths produced. With approximately a 10 foot wavelength @ 100 Hz and 20 feet @ 50 Hz, proper deep basshorns are bigger than cars.

Dipoles are another method, which have a narrower beamwidth than the omnidirectional behavior of typical sealed or vented box subwoofers. Dipoles radiate equally (ideally) to the front and rear, and dramatically less to the sides (or above). Instead of one directional beam, we have two. The room is excited, but still to a much lesser extent than with a normal monopole subwoofer, which sends bass frequencies equally in all directions. With dipoles, however, the system requires a lot of woofer to make up for the lost SPL from dipole cancellation. This can mean large systems with multiple 15" or 18" woofers and lots of amp power, though more modest systems can be made at the sacrifice of either max SPL or bass extension. Larger baffles (and again, large relative to bass wavelengths) can reduce the influence of the cancellation, but they also give up the desirable directional characteristics and limited room involvement of a smaller dipole frame/baffle.

Line arrays can control bass, and used carefully can help control sidewall reflections via directionality and minimize the involvement of boundaries by loading the room from many locations. This distributes the phase relationships of the reflections very similar to our last, and possibly best, choice. Line arrays, like dipoles and bass horns, can be quite large and expensive, and their ability to spread modal behavior in the room is lessened as bass wavelengths get longer.

The final choice is distributed bass. This is a method with many proponents, including Earl Geddes, and Duke LeJeune, both of whom sell subwoofers for "Swarm" subwoofer systems, another way of describing distributed bass. Below is Dr. Geddes whitepaper on these. He favors bandpass subwoofers for their high power handling and low distortion

gedlee.com/downloads/rooms%20and%20sources_norm.pdf

gedlee.com/Subs.htm

Where Mr. LeJeune utilizes more conventional sealed or vented systems.

AudioKinesis.com/product_ak_sealedswarm.html

The idea is simple, using multiple subwoofers to spread the distances to room boundaries around. In doing this, there will be an averaging effect, wherein the peaks and notches associated with the room are reduced. Imagine if the output from the first graph above (which results in no sound at the listening position) were combined with a third wave, representing the output from a second subwoofer, with a different phase relationship, and a fourth representing the reflected energy from the second location.

This four wave model is grossly simplified, but serves the purpose of explanation- the more different sources one has, the larger the variety of frequency and phase responses that will manifest themselves at a given listening position. The complex sum of these will tend to average to minimize the influence of the room, and thus give smooth bass response. People may wonder "Doesn’t this averaging make for less clean bass since the timing is all different?" In the bass, time behavior is not as readily discernible, and is dominated by standing waves in the room- that is unless you manage those standing waves. A multi-source bass system reduces the influence of standing waves and thus can achieve better, "Faster" sounding bass.

Typically distributed bass is achieved with multiple active subwoofers, each with variable phase, allowing for the use of the phase control along with the placement variation to minimize the influence of the room. Distribution of the subwoofers in this manner is ideal for minimizing the influence of the room, but doesn’t work with every setup. The need for wiring to each sub, both power and line-level, can be a major challenge in some rooms, as can the multiple locations.

Such is the case in my room so I had to create the best solution I could, and that was by stacking the subwoofers so as to spread the driver locations over a wider area. The kaboominators have two large dimensions and one short, making them perfect for stacking on their sides. I rotated one sub 180 degrees from the other, placing one manifold on the rear, top, left of the enclosure, and one on the bottom right front. While this is not the ideal case for distribution, it still creates some significant amount of variation in the interaction with the room. The acoustic sources are arranged over a 30" range vertically, out of a 96" typical ceiling, this is enough variation to reduce the consistency of standing waves between the floor and ceiling, and thus reduce the depth of the notches and amplitude of the peaks. There is a similar amount of distribution in the other dimensions, and while it’s not enough to fully distribute bass down to the lowest octaves, the high-q resonances in room response may still be compensated for even in frequencies where 30" would seem to be minor.

This is a fairly simple execution for the distributed bass concept. Wish I could squeeze more locations into my room, but even audio reviewers sometimes have to sacrifice performance for livability. With a dedicated room, one would likely use a variety of locations with infinite baffle manifolds.