I’ve read somewhere long time ago about crossover point selection trick that one could make a steeper acoustical slope by applying an electrical slope combining with a natural roll-off of the driver. For example, a conventional dome tweeter could be considered its low-frequency loading as a closed box system since it has been sealed on the back. As a result, the tweeter would have a natural roll-off at 12dB/octave rate. Once we apply a 12dB/octave electrical filter to it, the final acoustical response should have going to be a 24dB/octave slope by implication.
First of all, is it true?
Then, by performing that, is there a risk for damaging the tweeters, because the selected crossover point would be located at quite low and close to the Fs of the tweeters?
Moreover, since the x-over point is near Fs, will it introduce a large distortion to the speakers?
Finally, would the principle still be valid for the dome midranges, as their constructions are identical, except for the size?
First of all, is it true?
Then, by performing that, is there a risk for damaging the tweeters, because the selected crossover point would be located at quite low and close to the Fs of the tweeters?
Moreover, since the x-over point is near Fs, will it introduce a large distortion to the speakers?
Finally, would the principle still be valid for the dome midranges, as their constructions are identical, except for the size?
First question yes tweeter slopes plus Xover slopes sum in terms of the roll on slope as you say.
You just need to get them to the manufacturers recommended Xover frequency.
Manufacturers often specify their tweeter Xover recommendations as say, 12db second order, at 2.5Khz which allows a basic comparison. between tweeters. Its a good place to start, and to keep in mind.
You can cheat on this if you don't play loud. But, If your wife or children don't know this fact they may unknowingly blow a tweeter.
If you want to adopt a lower than recommended Xover it is also worth looking at the distortion measurement of the tweeter with the designed Xover.
Impedance chance at FS will affect the crossover slope in that area, which means it can be beneficial to add an impedance equalisation network to allow the correct Xover performance. This should be viewable in Xover sims.
You just need to get them to the manufacturers recommended Xover frequency.
Manufacturers often specify their tweeter Xover recommendations as say, 12db second order, at 2.5Khz which allows a basic comparison. between tweeters. Its a good place to start, and to keep in mind.
You can cheat on this if you don't play loud. But, If your wife or children don't know this fact they may unknowingly blow a tweeter.
If you want to adopt a lower than recommended Xover it is also worth looking at the distortion measurement of the tweeter with the designed Xover.
Impedance chance at FS will affect the crossover slope in that area, which means it can be beneficial to add an impedance equalisation network to allow the correct Xover performance. This should be viewable in Xover sims.
Yes. It rolls off below resonance frequency (similar to a sealed woofer).
But tweeters are not made to be used down to the resonance frequency:
- They will easily get damaged by too much power and high excursion
- They will produce lots of tumbling movement, leading to high harmonic distorsion (they don't have a double suspension as most cone speakers do)
- The electric crossover will be badly influenced by the driver's resonance frequency impedance peak
The same applies to dome midranges - but it can be even worse.
You could try a small fullrange driver as tweeter. That might work!
Good data examples here that reinforce what everyone said:
http://www.zaphaudio.com/tweetermishmash/
Many tweeters Zaph tested have rising distortion with decreasing frequency significantly above Fs.
One example: "Vifa XT25 ($48) - Smoothest and most extended response curve in the group, and resulting CSD is excellent. Good tall order HD above 2kHx, but average 2nd order HD. Poor HD levels of all types below 2kHz, even considering the extended low end. It may have a 500 Hz Fs, but don't think about crossing it below 2kHz LR4 or 2.5kHz LR2"
http://www.zaphaudio.com/tweetermishmash/
Many tweeters Zaph tested have rising distortion with decreasing frequency significantly above Fs.
One example: "Vifa XT25 ($48) - Smoothest and most extended response curve in the group, and resulting CSD is excellent. Good tall order HD above 2kHx, but average 2nd order HD. Poor HD levels of all types below 2kHz, even considering the extended low end. It may have a 500 Hz Fs, but don't think about crossing it below 2kHz LR4 or 2.5kHz LR2"
In fairness you have to be a mite careful when reading HD plots as it depends on conditions. A tweeter with a lower Fs and / or flatter response to a low frequency will tend to have a rising LF distortion trend relative to another of equivalent size & with an equivalent quality motor. Assuming you're not actually trying to use that extended LF to a significant extent though, once HP filtered, the significance falls away.
The XT25 is an interesting unit & has to be treated with a degree of caution since in terms of Sd, it's actually a 3/4in tweeter, although it often gets thrown in with 1in types, which doesn't help it in that regard. That said -you can cross it successfully lower than John suggested, but realistically, only for systems intended for modest levels, and you're accepting there is some compromise in doing so. I wouldn't myself, as there are better choices, but it's been done. A comparison to the still highly-rated Seas Millennium is interesting: with the exception of HD2 (since the XT25 is essentially just a couple of concentric roll surrounds) HD is actually lower across the board, although the Millennium does have more excursion available. More to it than that (a lot more
) on both sides of course, but notable.
The XT25 is an interesting unit & has to be treated with a degree of caution since in terms of Sd, it's actually a 3/4in tweeter, although it often gets thrown in with 1in types, which doesn't help it in that regard. That said -you can cross it successfully lower than John suggested, but realistically, only for systems intended for modest levels, and you're accepting there is some compromise in doing so. I wouldn't myself, as there are better choices, but it's been done. A comparison to the still highly-rated Seas Millennium is interesting: with the exception of HD2 (since the XT25 is essentially just a couple of concentric roll surrounds) HD is actually lower across the board, although the Millennium does have more excursion available. More to it than that (a lot more
) on both sides of course, but notable.
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Yep. It's not a 'trick' though: accounting for the driver's actual response is a natural part of filter design.
Yes.
It isn't necessarily quite low / close to Fs -depends on the design. For e.g., ferrofluid has fallen out of favour in recent years for various reasons, but automatically tended to put a damped LF response trend to most tweeters using it in the octave or > above Fs.
Damage is possible, depending on how much electrical power gets to it & what the excursion is. You need at least 2nd order slopes just to keep excursion constant, while LR4 in theory (or at least as far as excursion goes) provides the highest power-handling of any regular flat-summing topology. Depending on how hard you're pushing the tweeter, you may need to consider the electrical power handling on top of just the acoustic slope though, so a 3rd order or > electrical may be needed.
Per the above really: it's not necessarily all that close to Fs, and distortion depends on the tweeter itself. You're pushing harder than if you were crossing higher, but many can easily take it if used well -at least within the context of what they are. 2in vintage horn-loaded Altec compression drivers capable of 120+dB they ain't, so they'll never rival those in that regard -but they weren't designed for the same purpose.
Yes.
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When a manufacturer states that a driver has a certain power handling it coms along with a x-over frequency and steepness. This is the limit - it doesn't matter where the driver falls off without a filter. It's already in the calculations so to say. So yes, exceeding these numbers puts the driver at risk.
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I'd rephrase this a little. The final resposne curve of any driver is the combination of the driver's natural rolloff plus that of any crossovers or filters applied before it. I wrote about this here. See my pretty pics.
Well, it's the maximum power it can take under IEC or equivalent testing (usually 2.5KHz with a B2 high pass). That doesn't mean that's the lowest safe crossover frequency though, it's just the maximum short / long term electrical power it can take under those conditions.