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Compression Chamber Sizing
In a back loaded horn, like the Medallion or Alerion, there is a compression chamber immediately behind the driver. Combined with the throat (beginning) of the horn, this acts like an acoustical filter, so that only the bass portion of the music goes through the horn. Sizing of this chamber is very important.
Because the horn is driven by the back wave of the driver, it is out of phase with the direct response given by the cone. Additionally, there is a time delay as the bass goes through the horn. When the system is properly designed, the sound coming out of the horn is in phase with the direct response from the driver at the crossover frequency.
In example, the Medallion cabinet has a crossover frequency of about 350 hz. Above 350 hz, the sound is coming directly from the speaker cone, and below 350 hz, the sound is coming out of the horn. If, at 350 hz, the sound coming out of the horn is out-of-phase with the sound coming from the driver, there will be a hole in the response at 350 hz. If those two sounds are in-phase, there will be flat response.
If your system has a hole in the response, the easiest way to cure it is to move the cut-off frequency. As the frequency shifts up or down, the relative phase between the horn and the driver will shift due. As your compression chamber has already been built, the only way to shift the cut-off frequency is to move it up by adding wood, or some other non-compressible material, into the compression chamber. By adding small pieces of material into the horn, you will get to a point where the horn and direct response as in-phase, and the hole in the response is gone. At that point, you are done. With some experimentation, this can be done by ear. Using a frequency response measuring tool allows you to work much faster though.
Using a Subwoofer
There has been a lot of discussion about using a subwoofer with a Lowther, and the need to have fast bass. This is somewhat of a misnomer, bass is by definition slow. However, mis-tuned bass drivers (typical of home theaters systems where low end impact is desired, and therefore accentuated) will "ring" and have slow decay time. Avoid these.
The single biggest problem with joining a subwoofer has been the result of mis-matched phase response when using with a back loaded horn, such as the Medallion or Alerion. WIth these speakers the bass horn is loaded by the back of the driver, and therefore is out-of-phase with the primary signal. If you do not reverse the phase of the subwoofer when trying to mate with one of these speakers, you will have an extremely difficult time. There will be a cancelation of bass in the crossover region. Boosting the subwoofer to fill this hole creates boomy bass, giving the impression of an ill-tuned system.
When connecting to an open baffle system, the subwoofer should be in-phase with the speaker. The bass is fed from the front of the cone, and therefore is in phase.
So please, buy a musical subwoofer (in example the Rythmic or the Nola Thunderbolt), and make sure the phase is proper for the speaker you are using.