Thứ Tư, 15 tháng 6, 2011

Speaker Workshop Project Crossover Design

Speaker Workshop Project
Crossover Design
Crossover design is the most artistic part of speaker design. Any set of drivers with an enclosure will work with many different crossovers. For this system I tested 4 completely different crossovers, all of which had good looking frequency response charts and all of which had specific pluses and minuses. The general methodology is...
  1. Ensure that you have actual impedance and on-axis frequency response datasets for all of the drivers in the system. The on-axis response should be stored in the "frequency response" part of the driver properties. If you merged a port and nearfield and on-axis response that works also.
  2. Create a network (crossover piece) for each driver. I name mine Woof and Tweet, usually. Start with the woofer and tweeter in each network.
  3. If desired, add impedance compensation to the woofer (the tweeter rarely needs it).
  4. Calculate frequency response and impedance for the resultant circuits.
  5. Create goals for the woofer and tweeter circuits based on your desired crossover point and slope. I usually use an absolute value for the goal value based on eyeballing the woofer and tweeter response.
  6. Insert stock crossovers for the woofer and tweeter using the same frequency points and slopes.
  7. Use network optimize to get optimum networks for the drivers.
  8. Create a new network using all of the speaker drivers and copy/paste the networks from each indivudal into that. Calculate that frequency response, using the correct offset for the drivers.
  9. Tweak the final crossover.
Here's one crossover I did using 4th order Linkwitz-Reilly.
It sounded good and measured well. I also tried creating a third order butterworth. That turned into this
Finally, I built a third order butterworth without impedance compensation, based on seeing that the impedance rise is mainly after the crossover point. That turned into this
This crossover actually tested very close to the 4th order Linkwitz-Reilly, is much simpler, sounds good, and uses way fewer components so this is my final crossover. The measured frequency response of the system is ±1.5dB from 40Hz to 20KHz.
The woofer circuit induces some droop in the low frequency response. This compensates for the baffle step. Baffle step is a 6dB rise in response caused by the front baffle size. At low frequencies the sound wraps around the speaker - producing sound in a 360 degree arc. At higher frequencies the sound radiates towards the front - a 180 degree arc. Thus, at higher frequencies you get a 6dB increase (doubling) of the forward facing sound. This will be measured acoustically when you do an on-axis measurement, although since some of the low frequency sound gets bounced and reradiated I think it may be an overstatement of the audible change. Nevertheless the woofer crossover here does compensate for the baffle step - producing a very smooth measured on-axis acoustic response.