Originally Posted by dsully444
Total NOOB question, what is a HP (high Pass) filter? Is there a link you point me to so I can read up on this? ...
Wiki is your friend; redirects rumble filter as well. Scroll down to audio section.http://en.wikipedia.org/wiki/High-pass_filter
The idea is that the driver is supported by the air in the horn, but that air is only supported by the horn walls and it's own mass in the throat. As driving frequency drops below tune (20Hz here), 2 things happen.
- the mass of air in the throat now has time to escape, so air pressure at the driver drops, reducing cone support and allowing greater excursion.
- the driver impedence falls from a peak at horn resonance toward DCR at 0Hz. Look at the impedence charts in post #1 to see what I mean.
So, below Fc, current to the driver increases but acoustic loading is reduced, so there's nothing to prevent overexcursion at power levels much lower than you might want to drive them.
The fix is a simple electrical filter that reduces amp output below the tuning frequency. It allows frequencies above, say, 15Hz to pass. It's a "high-pass" filter because it lets through higher frequencies. Many sub amps have one built in. Here's an example (see Chart 1 on page 12).http://www.oaudio.com/docs/manual_v1_6.pdf
You also use a high-pass filter in a tweeter crossover circuit, but at a very different frequency.