1st order XO design logic?

[fernandov]

Here's a theoretical question for the crossover gurus on this site relating to two drivers of the same impedance and SPL level using a single cap on the high and a coil on the low. For the purpose of this question, please disregard other parameters that are often considered while designing a proper crossover.

Scenario: A LF driver with a nominal impedance of 8 ohms is mated with a single 1.0mH coil. It is therefore at full power up to 1273Hz, down 6dB at 2546Hz and down 12dB at 5092Hz. The HF driver is then mated to a single 1.0uf cap which is at full power at 19900 Hz, down 6dB at 9950Hz and down 12dB at 4975Hz.

Question: The -12dB point of the two drivers (underlined above) is roughly equal. Is this an appropriate goal in designing a 1st order electrical network? What other factors should be considered?

Thanks in advance!

[Sydney]

The crossover addresses the electrical signal.
The drivers themselves have mechanical performance properties as well as electrical.
2nd paragraph:
http://www.linkwitzlab.com/crossovers.htm

"It is the acoustical summation of the signals radiated by the low and high frequency loudspeakers that is of interest."
Sound system engineering By Don Davis, Eugene Patronis
( page 294 )

[Sydney]

Question: The -12dB point of the two drivers (underlined above) is roughly equal. Is this an appropriate goal in designing a 1st order electrical network? ...
Not the -12db point...
http://www.rane.com/note160.html

[fernandov]

Since all drivers exhibit their own electrical and mechanical properties, I guess what I'm asking relates more to the mathematical formula itself. If you suggest it is not proper to target a design where the smallest driver overlap occurs at the 12dB down point (1st order electrical), then what other criteria should be used? For the moment I'm only considering amplitude, and not phase or group delay response.

[Chris Roemer]

Here's a theoretical question for the crossover gurus on this site relating to two drivers of the same impedance and SPL level using a single cap on the high and a coil on the low. For the purpose of this question, please disregard other parameters that are often considered while designing a proper crossover.

Scenario: A LF driver with a nominal impedance of 8 ohms is mated with a single 1.0mH coil. It is therefore at full power up to 1273Hz, down 6dB at 2546Hz and down 12dB at 5092Hz. The HF driver is then mated to a single 1.0uf cap which is at full power at 19900 Hz, down 6dB at 9950Hz and down 12dB at 4975Hz.

Question: The -12dB point of the two drivers (underlined above) is roughly equal. Is this an appropriate goal in designing a 1st order electrical network? What other factors should be considered?

Thanks in advance!

No, -12dB is never a goal.

Depending on a driver's natural phase, their relative distances to you based on how they're mounted and the baffle orientation, and the phase shift caused by the cap and coil, if the sound from both drivers reach you (or your measurement device) exactly in phase, they will sum to flat if they're both down -6dB at the crossover point. If they're 90* out of phase, they sum nominally flat if they're both down -3dB at the Fc.

But, your example's rolloff rates would only apply to drivers that have flat responses (no natural rolloff, which is never the case), AND flat impedance curves, which is also not true for moving coil drivers. A Zobel can be fitted (sometimes for as little as a dollar) across the woofer's terminals to effectively flatten its impedance throughout the XO region, which minimizes ONE of your 4 problems.

Chris