Got it, and its wonderful. but yet I still have trouble figuring out the simple stuff. Is there any programs online for crossover determination and schematics?
Yes, but… please read the following advice once posted by Jeff Bagby on the Parts Express Tech Talk fourm
http://techtalk.parts-express.com/showthread.php?t=213239. I pasted that post here:
I was recently asked by one of the speaker supply companies to give them a breakdown of common crossover misconceptions. As I thought about it, it occurred to me that nearly all basic misconceptions regarding crossover design are the result of believing that textbook filters can produce an adequate crossover. Close behind that is the lack of understanding of baffle diffraction, phase and path length differences, how complex impedance impacts the design, etc. So I wrote the following for them to help some of their customers understand the differences and why a textbook crossover, then kind we learn about in beginning speaker design books, leaves a lot to be desired. I decided to post it here in case it will help some of the newer builders here as well. - Jeff Bagby
Common Misconceptions on Crossover Design
Most introductory books on speaker building deal almost exclusively with textbook crossovers. By "textbook" I am referring to crossovers designed by using standardized formulas (or look-up tables) to determine what value components to use. Learning about textbook crossovers is an excellent way to learn the theory behind how a crossover works, but learning it in this way also creates several misconceptions regarding crossover design. First and foremost is the misconception that a loudspeaker crossover can be designed using standard textbook formulas for 8 ohm drivers. There are multiple reasons why this is not true. Here several of them are briefly explained:
Textbook formulas will not produce a satisfactory crossover because…
- Textbook formulas assume a constant resistive load, like 8 ohms, when in reality a speaker’s impedance is reactive. It varies widely with frequency. As a result the only way to design a crossover that works as intended is to measure the real impedance of the speaker over the full acoustic bandwidth and use this complex impedance in the design stage.
- Textbook formulas assume a speaker has a perfectly flat frequency response, when in reality a speaker’s frequency response is never a flat line, always rolls off on both ends, and is seldom very flat in between. A crossover must be designed with the actual measured frequency response of the driver used in the design stage, and the circuit must be modified, as much as is possible, to account for the major response deviations of the driver.
- Textbook formulas do not take into consideration that when speakers are mounted on typical baffles there is a loss of support for lower frequencies. This is called baffle step and occurs when the wavelengths of sound become long enough to begin to wrap around the enclosure. It varies with the width (and to a lesser extent, the height) of the baffle. As a result, lower frequencies may be as much as 6 dB lower in response than higher frequencies. This must be taken into consideration when designing a crossover and usually results in lowering the overall sensitivity of the speaker as well. Without measuring this, or accurately modeling this affect, the speaker will tend to sound very thin in the lower mids and upper bass range.
- Textbook formulas do not take into consideration that woofers and tweeters are not coincident in space – meaning their acoustic centers do not come from the same point in space. Woofers are generally separated from the tweeter by being located several inches below the tweeter and, due to the depth of the cone and the basket, have an acoustic center that is further behind the baffle than the tweeter. The result of this means that sound from the woofer is delayed slightly in reaching the ear, and this delay changes the phase of the woofer’s response. This phase shift needs to be taken into consideration when designing a crossover. There are different techniques that can be implemented to do this, but it can't be ignored or the speakers will not sum to a flat response.
- Textbook formulas do not take into consideration the natural phase shifts that follow the frequency response of the driver. This is different than number 4 above but should be commented on. For the most part, speakers are "minimum phase devices". This simply means that the phase response of the speaker can be directly extracted from the frequency response. Unfortunately, since the frequency isn't flat, neither is the phase response. And like number 4 above, if this phase isn't taken in consideration in the design phase the speakers will not sum to a flat response.
- Textbook formulas do not take into consideration edge diffraction due to the size and shape of the baffle. This is closely related to number 3, but usually affects the response higher in frequency. Diffraction affects can be seen in the measured frequency response of a driver on the baffle and this is why these measurements should be used when designing a crossover.
- Textbook formulas do not take into consideration that drivers already have an acoustic roll-off, usually near the crossover frequency. In the end, in order for speakers to sum correctly it is the final acoustic roll-off that matters, not the necessarily the electrical roll-off in the crossover that is used. For example, many 4th order crossovers are achieved by simply using 2nd order electrical filters on the woofer and the tweeter, so that when the 2nd order crossover is combined with the driver's natural roll-off it results in a combined 4th order roll-off. This, then, is the actual acoustic crossover between the two drivers. These natural roll-offs need to be taken into consideration when designing a crossover.
- Some textbooks or guides to crossover design acknowledge that a driver may have an impedance that is not linear or "resistive". Often they refer to the fact that the voice coil's inductance generates rising impedance with frequency. They often refer to a small circuit called a Zobel that can be used to flatten this rising impedance. Other sources imply that you can simply subtract the voice coil inductance from the crossover inductor value and use that as your crossover because the voice coil inductance will create a roll-off of its own. There's more than one misconception in all of this. First, any roll-off due to voice coil inductance is already in the response of the driver, so you can't treat it like described and simply reduce the crossover inductor value. Contrary to what some people say, voice coil inductance is not part of the crossover and doesn't directly cause the response to roll-off. Many woofers extend flat much higher in frequency than would be the case if their voice coil inductance worked like a crossover.
- The second misconception here is that by using a Zobel to compensate for the rising impedance you can now use a textbook crossover because the impedance is now flat. However, this is not correct because it still does not address the fact that the driver's response is not a flat line and we still have to deal with the issues listed above.
- The third misconception is that the driver's voice coil inductance is defined by Le or one single value. Unfortunately, a driver's true inductance changes with frequency and using Le in a textbook Zobel formula may not result in a Zobel that flattens the impedance correctly. So again, we are back to taking our own measurements and optimizing the crossover if we want it to be right.
There are several other minor misconceptions that arise in the design of loudspeaker crossover but these are the main issues that results in less than optimal or even poor results. The best solution for contending for nearly all of these is to measure the actual driver impedance across the audio frequencies, measure the actual frequency response of the drivers mounted on the baffle, and use a good computer aided – crossover simulation tool to help design the crossover for the speaker.
