Ideal HT Speakers (Worried about Subwoofer Crossover)

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I'm wondering about how I should design a set of home theatre loudspeakers. If I make sealed enclosures, the dropoff begins at around 150Hz and has an F3 right at 80Hz. Now, according to some info I read, the THX standard sub crossover has an F3 of 80Hz. Doesn't this mean the satellite speakers would be 6dB down at 80Hz and the subwoofer 3dB down? If so, would I be more wise to vent the enclosures to produce slightly more deep bass so that they are linear down to 80Hz? Or maybe just use some equalization to smooth out the response? This system is hooked up to a computer that has hardware parametric EQ and digital coax output.

The speakers I would like to use for this are Axon 6S3's with Axon T-1S tweeters (and a shiva 12" subwoofer I have already built that sounds AMAZING and is linear to 30Hz with an F3 just above 20Hz).
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In a 12 db/octave crossover, both the upper and lower drivers are 6 db down at the crossover point.

At most, you might want to add a series inductor to make the satellites 6 db down at that point. Or you can make the satellites slightly smaller to be slightly more down at the crossover point.

There is all kinds of testing equipment on the web. Try Steve Ekblad's site at:

Test the subwoofer output at the amp. See where it is at 80 Hz. Remember: half the voltage means 6 db down. 3 db down is when the reference voltage drops to .7 of what it was.

A sealed system should roll off at 12 db/octave anyway. Your sealed system sounds close to that.

[Edited by kelticwizard on 10-18-2001 at 04:11 AM]
In an ideal world the mains should have good/strong performance to one octave below the XO point; and the sub to one octave above the XO point. This virtually eliminates any potential hole in the frequency response between the units.

IMO, don't get too hung up in THX specs, these are not really too important given the performance quality of most current systems.

I'd suggest using drivers in the mains that will give you good output to approx 40Hz. There are many inexpensive drivers with good performance that will do this.

Port tuning to 80Hz is pretty much a waste of time/trouble/money.
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