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.