I think the problem may be that you are under the impression that an 80 Hz low pass filter passes everything below 80 Hz and removes everything above that. While it is possible to design a filter that approximates that behavior, that is not the kind of filter we are talking about here.
I am aware that a crossover is not a brick wall. It has a slope with a rate of attenuation. With low-pass filters it's usually 4th-order, but if it's 4th-order at 80 Hz, by the time it reaches the kinds of frequencies I'm talking about in a passive speaker crossover, it would be too far down to interact in any meaningful way. So while I agree there may be measurable interaction, however slight, there is no possible way there could be actual cancellation or any audible interaction given how far apart the crossover frequencies are.
In that, I believe my friend is COMPLETELY wrong. I hope I have explained myself more clearly.
But I don't understand how filters can combine if they are operating in two different ranges! How can the slopes combine if one is at 80 Hz, for eg, and another is at 500 Hz?
I refuse to believe my friend is right.
Speaker crossovers CANNOT cascade with the amplifier crossover.
Goodness, I had a discussion with a few EE's in another thread and they agreed that the filters cannot cascade. Now you are saying my friend is correct. WTF?
Because on starts a drop a bit above 500 Hz and goes down, and the other joins in a bit above 80 Hz. They both combine at, let's say, 25 Hz.
Read this reply from the member towards the bottom. http://www.avforums.com/forums/speakers/17...r-question.htmlHe is an EE. What qualifications do you have?
Please stop with the inaccurate to the point of being nonsensical filter responses!
You have a 12 dB/oct filter at 600 Hz. Down at 80 Hz that filter will provide approximately 96 dB attenuation.
For one, you keep talking about filters that are seemingly highpass filters, whereas the OP has been asking about lowpass filters.
You should now see that the figures you gave correspond to the response of a filter with a slope of 24 dB/octave.
This is quite basic stuff, so it’s best not to get things like this mixed up whilst attempting to explain things to someone who is greatly confused about similarly elementary concepts… not to mention ever more obstinate as time goes on.
The 25 Hz point was just any random point below the cut-off point of both filters (assuming, erroneously, they were high pass filters).
Quote from: julf on 20 April, 2013, 07:40:42 AMThe 25 Hz point was just any random point below the cut-off point of both filters (assuming, erroneously, they were high pass filters).Assuming they are high-pass filters and assuming that by "combine" you mean the point at which the slope changes, that would occur at 80Hz, not at some random point below 80Hz.
The point I was trying to make was that at that random point, below the cut-off point of both filters, the gain would definitely be affected by both filters - so the effects would "combine".
And the point that various members have being trying to make is that the gain will – equally “definitely” – be affected at any point that is reached by the slopes of both filters. Including yourself!
Your latest invocation of a “random point” seems to refer to some point at which this interaction becomes noteworthy. I don’t wish to speak for greynol or anyone else, but I think that’s what is causing confusion/bemusement. The conversation is about defining the point at which the effects of any two filters combine, not some subjective definition of when that effect is significant. The filters will already be interacting at any frequencies where they are both applying attenuation, which will presumably be some point before the nominal cut-off of the higher LPF, to a degree depending upon the specific way in which it is implemented.So, the “random point” is either well-defined for any two filters of known implementation, or it’s a subjective marker for when the (always present) interaction becomes worthy of discussion. If the latter, what are your criteria?
Hopefully the scilab plot explains it better.
A graph is worth a thousand words. Thanks for taking the time to make it!