8. subwoofer notes
4) JBL 2242H cone
* Cut four pieces of tube at 15" length.
* Rout baffle counter bore for tubes to O.D. of tube, 1/2" deep in back of baffle.
* Cut eight panels for baffles and backs, 29" square.
* Cut eight panels for tops and bottoms, 29" * 27"
* Cut eight panels for sides, 27" square.
* Attach two pieces, two-by-four, 24" long to each panel, screwed and glued on edge as for columns.1
* Butt-join all pieces together using cabinet clamps.
* Line all five surfaces except the baffle, completely with fiberglass. Take care to keep the fiberglass away from the vent duct end or it may be sucked in and sprayed into the room by the air turbulence at the inside end of the duct.
1 note: make sure brace pieces on the different panels clear each other when the box panels are assembled.
Construction diagram for subwoofer cube(s).
The Subwoofers pictured here, should generally occupy the space between your main speaker systems. The reason for this preferred location is the so-called propagation delay of low frequency sound from the sub units.
All loudspeakers are natural electromechanical filters and as such, they produce a delay of some size between their input signal and their acoustical output response-there is no exception to the natural laws that cause this type of delay-simply because sound waves vary in size.
In more technical terms, the amount of delay depends on the distance (in time) between the zero-crossing and the maximum pressure formed in the air by loudspeaker diaphragms along with the filter bandwidth and the amount of attendant phase shift inherent in the filter's band-limit or envelope shape and other characteristics. For example, a simple single-order or "single-pole" filter produces 90 degrees of phase shift at the frequency where the signal passes through the half-power (-3 dB) frequency point. Thus a subwoofer whose high-pass characteristic is a single pole filter will exhibit a 6 dB per octave rolloff below its operating band. If we use a sealed-box with a 30 Hz lower band limit defined by the -3 dB or half power point, then we will have 90 degrees of phase shift or a delay of 8.3 milliseconds at 30 Hz. This delay is equivalent to roughly 9 feet of sound travel through air.
At the crossover frequency of 80 Hz we are imposing on this system, sound waves are 14.1 feet long. We will want to know what order our crossover will be. We will assume the subwoofer upper band limit is way beyond 80 Hz and will not add significant phase shift to the crossover, (in fact the 2242H driver described could be used as the low end of a three-way system up to 300-400 Hz with no special consideration). For the sake of example, I will use the common 18 dB/octave crossover slope and calculate the position correction guess needed. First, each 6 dB/octave forms what we referred to as a "pole" for the calculation. Three poles gives us 270 degrees of phase shift or three-quarters of a sound wave at our 80 Hz crossover frequency. Three fourths of 14.1 works out to be 10.6 feet.
Fortunately for us, there is also delay in the column's low-frequency output due to the high-pass function and the built-in delays in the amplifier we have adjusted to get our acoustical alignment between cones and horn. This means we will not have to place the subwoofers 10.6 feet forward of the columns. In fact, because of the ear's forgiveness, you'll find there's a "window" of space for physical placement that allows a good deal of flexibility in setting the speakers into your listening space.
There is an alternative to the four expensive monster sub cubes, but it takes some getting used to; both in terms of the concept and the execution. Near-field subwoofers can be used near (directly behind) the listening position. Placing the subs close to your ears offers three advantages:
1. Smoother low-end frequency response, because of the direct-to-reflected ratio of the sound you actually hear. Standing waves create huge peaks and nulls in response depending on your room characteristics, but if you can get really close to the sound source, your proximity alone swamps out the peaks and nulls because of the lower the output level of the subs required to compensate the inverse square law distance losses. If you are listening to only (or mostly) direct sound and not room reflections, then you will hear sound as smooth as the loudspeaker can provide.
2. Since you're using less power (typically about a quarter as much) you can get away with smaller devices-maybe one cube or a custom box with a pair of cheap 12" subwoofer drivers inside. You will have to ensure that the box itself produces good, flat response, but even this is easier to get-and down to a lower frequency-when you use a near-field arrangement.
3. Your neighbors and your family will appreciate not having their teeth loosened by massive bass penetrating the entire house. Bass carries the most because its long waves are so much harder to absorb or bend. You've probably noticed (in L. A. we sure do) how the cars with monster audio systems only seem to make bass as they pass on the street outside. Bass carries like a fog horn, and near-field subwoofers can reduce the amount of sound energy you initially pump into the air.
Of course, careful balancing will have to be done to match levels with the front loudspeakers, and you will need a good audio delay line to delay the subwoofer signal between 5 and 50 milliseconds. I recommend using something like a pair of Plye 12" subwoofer drivers in a sealed box shaped something like a low boy table, placed directly behind your couch. The box should be about 10 cubic feet internally, and it should be stuffed loosely but completely full of R-30 fiberglass to ensure that it is over-damped. If you're serious about the project, you should also get a 1/3rd or a 1/6th-octave graphic equalizer to flatten out the box from the listening position.
© 1997 Drew Daniels