Posted by Wayne Parham on May 14, 2003 at 13:53:38

In Reply to: Basically, yes, but... posted by mollecon on May 14, 2003 at 13:15:58:

Yes, tuning the Helmholtz frequency to match that of the motor will reduce its excursion. This in turn will reduce distortion. But it is also important to note that system tuning affects a region and not a specific frequency. The mechanical resonance of the woofer is shifted upwards by putting it in a box, and this new resonant frequency is denoted as F_o.

In a bass-reflex system, you have five frequencies of interest - F_s, F_o, F_b, F_h and F_l. F_s is the woofer's free air resonance, F_o is the mechanical resonance that's been shifted by putting the woofer in the box and F_b is the resonant frequency of the box, it's Helmholtz frequency. F_h is the upper frequency of highest impedance, sometimes called the upper resonant frequency, and F_l is the lower frequency of highest impedance, the lower resonant frequency.

The upper resonant frequency (F_h) is usually nearly coincident with the enclosed woofer's resonant frequency (F_o). They actually aren't the same, but they are near enough that the port damps the woofer's resonant frequency by a great deal. So for the octave above woofer resonance (approximately F_h) down to resonance (F_o), the port is tightly coupled with the woofer, damping its motion and making the system more rigid. Distortion is reduced because excursion is limited. Then, as frequency drops near F_l, the port begins to augment system output. In this mode, the port isn't limiting motion but it is increasing output so that the woofer doesn't have to move as much. You can find the formulas that define the relationships in the post called "Measure impedance."

• Excursion damping bandwidth? - mollecon 14:43:06 05/14/03 (1)
  • Re: Excursion damping bandwidth? - Wayne Parham 16:36:42 05/14/03 (0)