Opinions: Tweeter Crossover - High vs Low?

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Hey everyone - I'm looking for some opinions on this matter, in regards as to whether or not you guys prefer to cross a tweeter higher or lower (with regards to its operational range)

Do you prefer to cross higher, so as to have the tweeter play with potentially less distortion and such (due to it having to handle a lesser frequency range), yet you might have to deal with a 3-way, as well as taking into account the ctc of the drivers...

Or

Do you prefer to have the tweeter play more into the midrange, and worry less about ctc (and potentially be able to stick to a 2-way design), yet you'll have to worry about greater distortion and the possibility of burning out the tweeter...

But I'll digress, I don't think there is really a correct answer to this question...more so interested in a discussion upon this idea (obviously what is best will vary case to case)

Much love/many thanks in advance everyone for your thoughts
 
OK, this thread should have lots of opinions. Here's mine.

Short answer: the lower you can cross to a "tweeter" the better. 1.5kHz or less! But you need a driver capable of that.

Long answer:
1. A lower tweeter crossover point helps to prevent lobing and improves integration with the next lower band. The tweeter and mid have certain physical dimensions. This means they can only be put so close together, and usually this distance is a couple of inches. Keeping the crossover point low helps to prevent lobing that will occur because the acoustic centers are not coincident.

The problem is that your typical "dome" tweeter is on the order of 1" or so in diameter. It only has so much radiating area and Xmax capability, even if the unfiltered passband extends down into the 500Hz-1kHz range. The driver just cannot create enough SPL without reaching Xmax, or the distortion level starts rising to unacceptable levels. Usually a tweeter crossed to low sound "shouty" and this is apparent at higher playback levels. Even if you horn load the tweeter you still cannot really cross below 1kHz.

Using a larger format tweeter introduces a new set of problems. A larger "tweeter" e.g. a 35mm dome often has problems at its high end - narrowing off axis response the most common one. Some people like to try and use large ribbon or AMT type drivers. These also have the problem that, off axis and at high frequencies, the response droops badly. You are just trading off one problem for one or more different problems.

There is some evidence that crossing over in the range 700Hz-1kHz is "best". It's just not possible to do that with pretty much any tweeter available. But there is a type of driver that CAN be crossed over in that range (or even lower): the full-ranger.

Full rangers have improved quite a bit in the last 5-10 years. You can now find small full rangers that how sufficient sensitivity and HF extension to integrate with a woofer and serve as a tweeter. When they are small, e.g. 3" diameter or less, the off axis response in the highest frequencies is sometimes quite tweeter like, but this depends on several factors like motor design, cone, etc. I built several different systems using the Tang Band W2-800SL as a tweeter, which is a fine example of a full-ranger that can be used as a tweeter. For example, I used it in an MTM crossed at 750Hz. Very nice. More recently I found a BMR type driver with some promise, until I discovered some serious aging issues. The Peerless/Vifa TC9 and TG9 are good examples of full rangers that can function as tweeters, although they don't quite reach 20kHz. Not every full ranger driver is a good choice but if you look around there are a few candidates.

Finally, by using a full-ranger crossed low you can avoid a big hole or discontinuity in the power response around the M-T crossover point that is often found with M-T systems using a dome tweeter. Food for thought. But that's a whole new topic in and of itself, so I will stop there.
 
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frugal-phile™
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Hey everyone - I'm looking for some opinions on this matter, in regards as to whether or not you guys prefer to cross a tweeter higher or lower (with regards to its operational range)

I prefer to cross lower, such that the centre-to-centre distance from midbass to (mid)tweeter is at or less than the ¼ wavelegth of the frequency of the crossover. Usually 250-450 Hz.

dave
 
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frugal-phile™
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You can now find small full rangers that how sufficient sensitivity and HF extension to integrate with a woofer and serve as a tweeter.

My favorites are the Fostex FF85wk and the Mark Audio Alpair 5.2. Althou a 4” my absolute favorite is the Alpair 7.3.

Here the system i am listening to now with FF85wKeN & 2 sidefiring Silver Flute W14eN.

tysenV2-passive.jpg


dave
 
But I'll digress, I don't think there is really a correct answer to this question...more so interested in a discussion upon this idea (obviously what is best will vary case to case)

There is a correct answer: whichever approach that leads to flat and smooth axial response, and smooth off axis response. There's no reason to settle for less today.

As a general rule, that either means a 3-way with a small-flange tweeter and a small mid, some degree of waveguide loading on the tweeter, or both.
 
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The most satisfying speakers I have ever made have crossed over at 3Khz or above. I may be overly sensitive in the lower ranges but that is how I hear it. I find it interesting that despite all the directivity arguments, some of the most highly regarded British speakers ever from companies like Spendor (BC1, D7), Harbeth and B&W, are based on 7 and 8 inch drivers crossing over at or above 3 KHz. The BBC LS3/5A which was famous for its incredible midrange was based on a 5.25" driver crossing over at 3 KHz. SB acoustics in their new 2.5 way kits, cross both the Satori MW16P and 17MFC driver at 3 KHz.
 
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My Ariels are crossed over at 3.8KHz and that works well because the P13WH midbass drivers are smooth and have low distortion to that point. I've been playing with some SB12NRX drivers and while they're nice and smooth to 10KHz, they have a significant cone breakup spike that's seen as an increase in third harmonic distortion at 3KHz.

My plan with these speakers is to cross over below this point - probably at 2KHz if I can get away with it. See the attached plot showing the distortion peak at 3KHz.

I guess that means it depends. There's nothing arbitrary. Choose the point that gives the best performance for the specific set of drivers and their relative positions on the baffle.
 

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General rule (of mine) would be that if you want to cross it at 2KHz or less (depending on steepness of the filter), waveguide is perfect solution. It isn't a must have by any means, but it would be just silly not to use it. It controls the directivity (no blooming between 3-5KHz), it enables you to use electrical filter higher in frequency to achieve lower crossover point, it keeps the distortion low, it pushes the acoustic center of tweeter closer to the acoustic center of the mid-woofer etc.

The only thing that waveguide doesn't help with is center to center distance - but in my experience, in practice this has never been an issue. I have never heard any negative effects induced by ctc distance because of using a waveguide, and i listen from about 2,2m-2,5m distance. Further from speakers you go, the less of a "problem" it is.
 
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My Ariels are crossed over at 3.8KHz and that works well because the P13WH midbass drivers are smooth and have low distortion to that point. I've been playing with some SB12NRX drivers and while they're nice and smooth to 10KHz, they have a significant cone breakup spike that's seen as an increase in third harmonic distortion at 3KHz.

My plan with these speakers is to cross over below this point - probably at 2KHz if I can get away with it. See the attached plot showing the distortion peak at 3KHz.

I guess that means it depends. There's nothing arbitrary. Choose the point that gives the best performance for the specific set of drivers and their relative positions on the baffle.
Yet another person who only looks at the on-axis response! Ignorance is bliss I guess.
 
Bit harsh there, Mr. CharlieLaub. And what are you saying anyway? Something about dispersion? :)

Suzy's Ariels were designed by Mr. Lynn Olson, who is a great authority on tweeter distortion:
Lynn Olson said:
A common but unsuspected cause of sibilance is crossing the tweeter too low, or using a shallow-slope crossover. Many designers - unfortunately, a lot of them in the high-end biz - forget that direct-radiator drivers increase excursion at a rate of 12 dB/octave. Thus, it takes a 12 dB/octave highpass filter to merely keep excursion constant in the frequency range between nominal crossover and the Fs of the tweeter.

For example, if the tweeter has a typical Fs of 700 Hz, and the intended crossover is 2.8 kHz (again, typical), it takes a 12 dB/oct electroacoustical filter to merely keep excursion constant in the very critical 700 Hz ~ 2.8 kHz range. Part of the reason that this range is so critical is that audibility of distortion is at a maximum in the 1~5 kHz region. (Perception of distortion similar to, but not quite the same as, the Fletcher-Munson curve.)

Staying with the same example, if the electroacoustical filter is 1st-order (6 dB/octave), then excursion actually increases from 2.8 kHz on down, until 700 Hz is reached. Below 700 Hz, the excursion finally starts to decrease, but not very fast, only 6 dB/octave. This is troublesome because the maximum spectral energy of many recordings is around 300~500 Hz, so energy from this range can crossmodulate with the tweeter output.

This is why auditioning with little-girl-with-a-guitar program material and a full choral piece sound different. The LGWAG is spectrally sparse, and there isn't as much chance the tweeter will be struggling with IM distortion. Throw a dense, high-powered spectrum at the loudspeaker, though, and the tweeter will start to scream - and it is very audible on massed chorus as complete breakup.

At any rate, regardless of distortion of a particular tweeter (none of them are free of IM distortion), crossovers matter. Many designers want to take the tweeter as low as possible because the polar pattern is prettier and certainly measures nicer, but the inevitable price to be paid is more IM distortion resulting from increased excursion (the linear region is most tweeters is less than 1mm). Choosing a crossover is a difficult tradeoff between narrowing of the vertical polar pattern, IM distortion from out-of-band excursion, and how close the designer wants to approach the region of midbass driver breakup. The tradeoff is made more difficult when a rigid-cone (Kevlar, metal, ceramic, etc.) midbass driver is chosen, because the onset of breakup commonly falls in the 3~5 kHz region, right where the ear is most sensitive to distortion.

As you can see, the worst possible solution is a 1st-order crossover combined with a midbass driver that has a severe breakup region (Kevlar drivers, I'm looking at you). The 1st-order crossover fails to control out-of-band excursion, so program material in the 700 Hz-2.8 kHz region results in IM distortion in the tweeter's working range, while plenty of midbass breakup in the 3~5 kHz range gets through as well. And midbass breakup sounds the same as a bad tweeter, since the distortion and resonances fall in the same frequency range.

As a side note, most transistor amplifiers (including very expensive high-end products) go from Class A operation to Class AB around 1 watt. Feedback helps, but cannot fully overcome the two-to-one shift in transconductace as the AB region is traversed. In addition, thermal tracking is typically several seconds to a minute late (depending on the thermal mass of the heatsink and location of bias sensor), so the correct AB bias point is actually several seconds behind the program material. There are various sliding bias-tricks available (which avoid complete turnoff and associated switching transition), but they are all several seconds late. The more output transistors, the more AB transitions there are, since it is impossible to have transistors exactly match the switching transition - in production, they are matched for beta (current gain), but not usually for other parameters. Change the die temperature a bit, and the careful hand-matching goes away.

To recap, if you want lots of sibilance, use a midbass driver with severe breakup in the 3~5 kHz region (this is usually obvious from unsmoothed FR curves), pick a tweeter with limited excursion capability (not always spec'ed), select a 1st-order crossover at a low crossover frequency, and use an amplifier with a very large heatsink, many transistors, and somewhat unstable Class AB biasing (thermal overshoot). That should do the trick. Plenty of distortion from many different sources, even though the overall FR curves may look harmless.
http://www.diyaudio.com/forums/multi-way/190663-sparkling-treble-sibilance-2.html#post2604196
 
Bit harsh there, Mr. CharlieLaub. And what are you saying anyway? Something about dispersion? :)

Suzy's Ariels were designed by Mr. Lynn Olson, who is a great authority on tweeter distortion:]

Yes, it is the classic tweeter problem. You would like to cross low, but the dome tweeter cannot be operated low enough in frequency because either it doesn't have the passband extension (its Fs is too high) or when you try to use it with a low crossover point the distortion rises to unacceptable levels.

The dome tweeter IS the problem. That's why I advocate avoiding them alltogether and instead using a full range driver as a tweeter. Now that there are full rangers that are suitable for this task, why not use them?
 
Marco_Gea did a splendid cone tweeter design here, and I share your misgivings about dome tweeters:

500061d1440342286-classic-monitor-designs-system.jpg


http://www.diyaudio.com/forums/multi-way/147632-classic-monitor-designs-25.html#post4428857

He was honest enough to say it wasn't perfect, but for such simplicity a pleasing project.

I shared the opinion that the cone tweeter worked better on a third order at 3.5kHz, than at 2.5kHz on second order. IMO, the best of the domes are the bright old 19mm mylar ones. But those need a highish crossover to go loud.

In this MA7 one it is the usual 4uF, 0.2mH, 4uF 3.5kHz BW3 tweeter circuit in my estimation. The bass circuit is a three element BW3 too. Two coils and a 8uF cap. Nice little 5" 12L reflex speaker!
 

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Out of curiosity, could you guys elaborate on the detriments of a full range as compared to a traditional tweeter?

The reason I ask is that I was curious (with regards to some of the earlier posts - that crossing over lower is better), what are your guys' thoughts with a full range as compared to a compression driver at about 1kHz? (As a tweeter and such)

^I'm curious because some view compression drivers = distortion, so how would the two compare?

Much love/many thanks in advance everyone
 
This being the multi-way forum, careful about asking opinions on the differences between a compression driver and probably any other type of direct radiating driver as mid tweeter.

That said, any of the better quality modern small diameter "full-range" drivers can serve very well as mid-tweeters crossed over anywhere from 300Hz on up. My main L&R in surround / home theatre system incorporates exactly such a configuration.
 
Don't think I saw explicit mention of two important considerations (which are somewhat in conflict with one another).

1. logically in consideration of a range of physical issues, you'd want your drivers to handle comparable numbers of octaves; Parts Express has some nice 1-⅛ inch (3 cm) domes that go pretty low and hard to beat a dome driver for quality,

and

2. the music is in the middle frequencies and the woofer and tweeter are more like icing on the cake; so best to have a good mid-driver cover the whole middle and not to cross over in the middle of the middle.

That still leaves the question of where do the icing layers start? Others have mentioned good speakers with 3-3.5kHz crossovers. That leaves maybe 2+ 8/aves for the tweeter and, if you listen to the tweeter alone, you realize how little substance is north of 3kHz.

B.
 
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Out of curiosity, could you guys elaborate on the detriments of a full range as compared to a traditional tweeter?

The reason I ask is that I was curious (with regards to some of the earlier posts - that crossing over lower is better), what are your guys' thoughts with a full range as compared to a compression driver at about 1kHz? (As a tweeter and such)

^I'm curious because some view compression drivers = distortion, so how would the two compare?

Much love/many thanks in advance everyone

It used to be that a "full range driver" was something like a 6"-8" diameter paper cone with whizzer. Almost every example of this constructions has major flaws in terms of frequency response, distortion, and dispersion. I make the same claim for pretty much every coax that I have ever seen sufficiently characterized. Too many flaws.

These days you can buy small drivers that have extension on axis to almost 20k Hz and off axis the response is relatively smooth and tracks the on axis response. Not constant directivity, but close to that of a dome tweeter. Distortion will probably not be as low as a good dome tweeter, but is still well under 1% in some cases. Not every full range driver is good in these respects. No large full ranger can really work all that well unless it has a very special cone design, but I would need to see that (off axis response) well characterized by the manufacturer. Some efforts by Mark Audio seem to have promise, but I have never been convinced enough to pull the trigger on one to test it. Based on my own measurements, and those of others, the Tang-Band W2-800SL and the Vifa Peerless TC9/TG9 seem to work well in the tweeter role and can be crossed at 500Hz if you want to do that, and you can still use a pretty shallow slope. They lack a bit for sensitivity, coming in around 85-87dB/W, but that is about average for an "old school" dome tweeter. This also means that the output capability is not super high, because power handling is not very high either. For home use, that is typically not a problem. You will not find any full rangers displacing compression drivers in PA systems, however.

Compression drivers on horns are an interesting alternative, but they share some problems with the dome tweeter if you can believe it. They are designed for high SPL and not for high fidelity and often times the distortion performance of the CD is not great in absolute terms. But many people are not too concerned about that, and like that fact that a CD+horn has very high dynamic and absolute SPL capabilities. That said, if you want to cross over low enough to avoid the same lobing issues it's almost a losing battle. To cross over lower the horn mouth must be larger, which means more center to center distance. So you cross lower, and the problem grows even more. 1kHz is definitely doable with a 1" throat Cd, but not much lower. A 1.4" or 2" throat CD will go lower for sure, but you need a big horn to reach 500Hz for sure, and often the upper end starts to peter out and have lots of response ripples/ringing in the horn. Regardless of all this, I have heard some very nice Econowave type systems in the past and I am not trying to shove off all systems that use CD+horn at the top end.
 
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