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Need speakers that can rock with just one watt? You found da place.

name that tune

Hi jon77,

"Tuning" a ported enclosure just means selecting the right port dimensions so that the helmholtz resonance is at the frequency you want for a given box size. Box volume and port length & area are the parameters you'll be working with. Absorbing materials should have no effect - if they do, you're overdoing it.

Imagine blowing across a partially empty beer bottle. Waaaaaaaaah. That's a helmholtz resonance. The fat part of the bottle corresponds to the enclosure, and the narrow neck is the port. Now it's hard to change the port dimensions on a beer bottle, but you can change the enclosure volume by drinking the rest of the beer. This will lower the tuning frequency. Wuuuuuuuuuuh.

The way I determine the tuning frequency is to run an impedance sweep. There will be twin impedance peaks in the bass region, and the low spot between the peaks is the tuning frequency. But, this isn't necessary for your application because you can get close enough using a computer program.

Below is a link to a shareware program, and the upgraded version is only $20 as I recall.

The only parameters you need enter are driver Fs, Qts, Vas, and Qes. You can leave the rest of the driver page blank. It will calculate the efficiency for you, which will be lower than the published specs. Welcome to the real world.

Now you can play around with different box sizes and tuning frequencies. Use the port length calculator under "tools" to see how long a port you need.

For instance, suppose your design calls for a 1/2 cubic foot box tuned to 71 Hz. Let's try a 1" port diameter. Hmmm. The program says .11" port length. That's way too short to be practical, so we need to go up in port diameter. Let's say we try a 4" diameter port, and get a port length is 10.5 inches. Now, that's too long - it might not even fit in the box, and the longer the port the more likely you are to get midrange colorations from it. So, let's try a 2" diameter port. We get a length of 1.90 inches, which is pracical. A 3" port gives us 5.36" length. Either of these should work just fine, or pick some diameter in between and see what length you need.

Now before you obsess over cutting port lengths accurate to the nearest hundredth of an inch, you need to know that most programs over-estimate the required port length by about 5-10%. This is because they don't adequately model the turbulence at the ends of the port.

But wait - now that you're armed a computer program, you can try different enclosure sizes and different tuning frequencies. And you'll see that being off a little bit with either one doesn't ruin the speaker, it just changes it. You'll see that the recommended design is probably just one of many viable combinations. Once you cross the line and come up with what YOU think would be optimal, you're no longer just a DIYer - you're a speaker designer!

Be warned, it's a slippery slope. You might thank me now, but you'll curse me later.

Duke


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