Optimum crossover frequency?
A lot of the time I find myself taking a step backwards and looking into the supposedly trivial to find a solution for the complex.
Often it is much more difficult to something simple than something difficult however strange this may sound.
Albert Einstein said "Everything should be made as simple as possible, but not simpler".
And I believe he was right. In simplicity lies the genius.
So onwards to the question at hand.
What are the optimum crossover frequencies one should aim fore and why?
Let's make life difficult and start with a 2-way speaker.
Even though I have read a bit in the past it would be great if you consider me a n0ob for the discussion's sake.
Obviously the drivers will post limits but lets assume we start with the crossover points and choose drivers to match.
Fletcher-Munson curves and the theory behind it talk about hearing sensitivity dependant on frequency. Where will the crossover be most detectable to the human hearing and where is it easier to get away with?
Most of us I believe like to have bass packing a good punch but we dont want to have a bad midrange.
Talking about bass we probably want to go low as well and that will have a serious impact on the choice of driver.
Then we have the matching of phase, impulse response and directivity.
Everything is supposed to blend nicely togeather.
And last but not least we have sonic signature and type of enclosure. Not everything will blend well.
How do you do it?
Where do you actually start?
What is the best target frequency for the crossover?
Is there a way to have it all?
What are the compromises?
In this particular thread I would like to focus on finding the optimum crossover frequency taking into account human hearing and perception.
If you consider directivity down to the hundred hertz and up to the Khz area to be important, a two way becomes very difficult. If you go really low, and loud, you get doppler distorsion too. Depending on your room, you may also not want the lows to be placed at the same physical point of the midrange.
What amazes me is that you still read in French Hifi magazines about these two ways that cross the tweeter around 3k that they avoid the most sensitive zone, which is pure BS, it's a lie, that's exactly the MOST sensitive zone and you know why they do it, because a tweeter that crosses lower is expensive. And people don't check, whilst they woofer is beaming like hell.
I actually read Greisinger's last paper again yesterday, if you want to have a look..the one about Pitch. Really interesting.
I start with a driver that tickles me, see where it needs to be crossed with regards to dispersion, FR and, if known, distortion and then find another that is suitable.
But may be I'm doing it wrong.
Other than that I generally agree with lolo. 3k is usually way to high for 2way unless you use the tiniest of midwoofers and the 2.8-3.5k region is where the ear is the most sensitive.
I also like to cross drivers steeply (and actively) within their linear response region so I don't have to bother too much about electrical and acoustical slopes.
There is one, and only one, correct answer to your question: "It depends"
Some simple questions just plain have no simple answers. Even two designers building a speaker in the same box will have very different ideas. Go look at the ZAPH design competition. Same two drivers, same box, many different "optimal" choices.
Ah, the engineer answer... ;)
I'm not so much looking for a single answer as much as a discussion where one can make up his/her own mind based on your own preferences.
3k sounds awfully high and smack in the middle of the sensitive area.
I was tinking maybe 1.5kHz +/- 0.5kHz?
I don't have an answer myself so I'm curious to hear what you people thinka about the subject.
If you want some opinion backed up by expertise, experiment read Dr Geddes justification for directivity control with CD waveguides for small room acoustics - resulting phsyics constraints leading to his 2-way speaker designs
http://www.gedlee.com/ and here at diyAudio
very aproximately - need tweeter waveguide and mid/woofer dia to ~match for smooth directivity over XO frequency range
lowest possible XO frequency is enabled by compression driver with large dia waveguide's efficiency - below 1 kHz with Suma 15" waveguide and 15" LF driver
another engr answer: cross outside 1k-4k area. Again, impossible without major compromises in a 2way. Maybe a 'small 3way' instead?
It's pretty much like Lolo says, it all hinges on the directivity of the drivers, which for best result should match around the crossover point.
Do read the above discussion by the good Dr. Earl.
If you insist on some elaboration,
In practice.... I find very few conventional one inch tweeters that behave in acceptable distortion or energy decay below 2K or so. SB29, lower, XT25, higher. It depends. I find precious few mid-bass that can get much higher than 2.6K without impinging on breakup problems. Metal being more difficult than coated paper. It depends. Slope matters, to keep away from resonance peaks and breakup modes, a steeper crossover may allow a different crossover than a shallow crossover. It depends. Does it give that much advantage in more uniform off-axis response, or is the complexity of the crossover going to overshadow the gains? It depends. Does you environment depend more or less on diffuse or primary reflections? It depends. How loud? In my office, I may get away with a lessor slope, or lower crossover because the levels are not what I use in my living room. It depends.
For reference, my most successful to date are LR4 acoustic at 2.6K. Seas drivers. My next most acceptable pair are Daytons with Seas tweeters at 2.4K, asymmetrical 3rd/4th acoustic crossover. These were finalized by the physics of the drivers, my bank account, and when I just plain reached as far as I wanted to go on a given project. The pair I am working on right now is first order at 4.5K. How did I go so high? Fountek FE85 which is a 2 1/2 inch full range, A.K.A. a midrange. Computer desktops so 125 F3 is just fine. It depends.
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