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Re: Porting midbass
Here is what noted car audio equipment designer and enthusiast Andy Wehmeyer says about ported bass drivers--
"The port is basically a speaker that's designed to play loudly at one note. Its response is a peak. We design the port to have a response that compliments the response of the woofer. We hear the sum of the output of the woofer and the output of the port. If we increase the tuning frequency, we have an area where the port's response and the woofer's response have a small gap--that creates a peak in the response in those frequencies. If we move the port to a lower frequency, we have a dip in the frequencies in between. Usually, the resonance frequency is chosen to extend the low frequency response as low as possible while maintaining flat response. For small bookshelf-style home speakers, a little bump is often helpful in providing some additional bass.
At the frequency where the box is tuned, the port plays, but the woofer hardly moves. This is because the acoustic impedance (pressure) inside the box is much higher. Yes, the pressure inside a ported box is HIGHER (at the tuned frequency) than in a sealed box. Below the frequency where the box is tuned, there is much LESS pressure than in a sealed box--hence the need for a subsonic filter. So, at the low frequencies where the box is tuned, the woofer doesn't move much, so we don't need a heavy long coil. That makes it easier to make a more efficient woofer, since we don't need so much moving mass.
OK, so what does all of this mean?
1. If you're going to use lots of power and a sealed box, then you need a woofer with a long coil.
2. If you're going to use a vented box, a woofer with a shorter coil will be fine.
3. If you use a woofer designed for a small sealed box (low Q) in a vented box, the box volume requirement will be small and the port frequency requirement will be low. That means the port will be very long and the box will be difficult to build because the port will be difficult to fit in the box.
4. If you use a woofer with a short coil, designed for a vented box, in a sealed box with high power, the woofer will run out of coil and you'll hear distortion. Because the condition that produces the distortion is symmetrical (coil leaves the gap in both directions), the distortion will be mostly odd-order, which sounds nasty. "Brap Brap Brap".
So, what's the conclusion?
Your power requirement and choice of box type determines whether a long coil (woofer with high Xmax) is necessary. Almost all woofers will work in a sealed or vented box, so long as the woofer's Qts is lower than your target Qtc. A woofer with a Qts higher than .707 will have a peak in its response NO MATTER WHAT KIND OF BOX YOU USE. The rest is a matter of compromising between box size, power required to hit a target SPL and required low frequency extension.
When you put the box in the car, the transfer function (car's frequency response) will be added to the response of the woofer. this will produce roughly a 12dB per octave increase in level as frequency is reduced starting at about 50 or 60 Hz, depending on the size of the car. A vented box will give you a big peak and a sealed box will not. If you have an EQ, then you can reduce the peak by reducing the power the amp has to provide at those frequencies. I think a vented box and EQ is always the best way to go, so long as you can afford the space and the EQ. "
Last edited by Andy Wehmeyer; 09-16-2011 at 11:17 AM..
"THE BASS OF THE PEOPLE IS THE GOD OF GOD"
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