Introduction to designing crossovers without measurement

Top notch tutorial! I do wish you would have made mention of driver power ratings compared to total system power, with reference to power demands at a given frequency and resistance? For example: I, as well as others I've read, often express concern in finding a tweeter capable of handling much, if not all of a systems total power. I'd appreciate some information clarifying the relationships?
 
If you look at the voice coils of each driver, it is clear that there are fundamental power limitations in each. For example, a 25mm dome tweeter will have a very small coil made with very fine wire. Realistically it can only handle 5 watts or so. A midrange may have a 38mm voice coil with a more solid construction, while a woofer could have a voice coil between 50 and 100mm that is designed to handle a lot of power.

Typically this is ok, because a lot of power is in the bass, while the midrange and treble can have a lot of power for short periods, but the average power is quite low - most music does not contain extended high frequency wine waves. If you overload the amp, a lot more power is generated at higher frequencies because of harmonics, and this can damage tweeters.

With a good crossover design, you can ensure only the correct frequency ranges make it through to each driver, so the fact that a HiFi tweeter can only handle a few watts is generally not a problem. If you are designed for PA situations, or situations where amps may be overdriven, it is a good idea to choose drivers than can handle higher powers.

You should check manufacturer's specs for each driver. Typically they specify a crossover frequency, and sometimes specify that second order crossovers should be used.
 
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Sorry for the late reply, and thanks danoz.

There are two limitations of this nature: power handling and excursion. Either of these could be, and one normally is, the limiting factor. I have found that with ordinary drivers used in the normal way, and where you can hear when they are straining and you never push them further, you'll have no problems with damage. This gives you the opportunity to decide whether to change their crossover so you can turn them up further. Similar drivers such as 1" domes will tend to behave similarly.

Much of the load on tweeters can be eased by making sure the bass is kept away from them. It takes more energy to produce bass tones. Using a single capacitor on a tweeter in a first order filter will reduce the bass, but a bass tone will still be causing more stress on a tweeter than something higher up. Even though you won't be able to hear it happening it will be running the dome out for no good reason. If you move up to a third order filter the bass will still play through the tweeter but won't challenge it as much as the treble naturally does.

On the other hand even if you use a very high order filter the tweeter will seem to have limits due to the passband (from the crossover point and up), so setting a higher crossover point will also increase power handling.

Another reason to limit the energy reaching the tweeter is due to the heating of the voice coil causing distortion. This is not always an obvious distortion and for some people it is not an issue, but some will notice a tweeter sounding different as the volume goes up. When free of this problem a tweeter will sound the same at high levels as at low levels, which should mean it sounds effortless, and as if it were capable of more.
 
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Thanks for taking the time to respond! I understand the principals behind coil size and power handling capacity... I guess I was looking more for the mathematics behind it. No one wants to spend money on driver components (or filters) only to find out "the hard way" that they were a poor choice. What is incorporated into a "tweeter protection" circuit and how is it calculated? If you combine a tweeter rated for 40w with a woofer rated for 100w, with filters appropriate to the drivers specs, is system power handling said to be 40w, 100w, or somewhere in between?

Thanks again.
 
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It's been a while since I've gone over this but I'll see what I can recall.

It has been suggested that the power half way mark in music is around 600Hz, but it depends on the music and isn't fixed. Obviously, Metal music will demand more from a tweeter than Bluegrass played at the same power levels, and there are guides to be found. Before you can say that the tweeter is handling music above a certain frequency and therefore handling a certain percentage of the power, it needs to be crossed appropriately.

There is more power in a lower frequency tone of equivalent level than a higher frequency one. The relationship is at 6dB per octave. So firstly if you don't use a filter of greater than 6dB per octave (such as 12dB/octave or greater) then you can't say that the tweeter is only receiving significant power from above the crossover point.

The next point is that for a single driver with a fixed diaphragm, lower frequencies will take an increased excursion in order to be reproduced at the same response level. The relationship here is also at 6dB per octave. This brings it up to 12dB per octave so far, but note that the filter slope and the response slope are usually different. It is not unusual for a 12dB/octave filter to contribute to a 24dB/octave response slope.

What kind of tweeter protection did you have in mind?
 
Thanks AllenB. I've owned many commercially made speakers and some have had either a replaceable fuse labeled "tweeter" or a breaker that can be reset, and some speakers have a fuse "protecting" the entire system... I once blew both protection fuses in a pair of Cerwin Vega DX9 (30hz - 20kHz Power(RMS): 5/400 Watts Efficiency: 101db 4ohms 2.06 Tweeter 6.5 Midrange 15 Woofer Crossover 250 hz, 5kHz @ 12db per octave Self resetting tweeter protection and overall system fuse)

1) I wonder what impact using such a device can have on the sound and at what point do you decide to use one.

2) I imagine there are other ways to produce protection, for example simply using a tweeter that is spec'd with similar power handling as the rest of the drivers, but then that at least as "a rule of thumb" would mean that many of the first time DIY'ers are correct... that you *should* ideally use a tweeter that is of similar power handling capability... and yet you don't have to search very deeply to find others, presumably with more DIY experience, suggest that this is incorrect, that the tweeter (for whatever reason) does NOT have to have as a high a power handling as the other drivers. All in all this seems pretty vague and I have never read any sort of mathematical formula describing the power handling of a tweeter that could be used as a guide to selecting a "safe" tweeter option.

3) Is it better to use a tweeter with a much higher sensitivity and then attenuate to match the rest of the system than one that is closer "out of the box"?

4) What consideration must be made incorporating a 4 Ohm tweeter into an 8 Ohm target system and how does this coincide with tweeter and overall system power "rating"?

5) How do commercial speaker builders determine what is a good maximum power handling rating for their speaker system? Is it entirely arbitrary, just marketing, or is there some sort of reliable way of determining such a thing?

6) Back onto the original topic, what considerations do you need to make concerning a tweeter (and it's power handling, xover frequency and impedance) in the design of the crossover? When you start talking about exotics like AMTs that can cost several hundred dollars each, how is a DIY'er supposed to approach them, from a crossover point of view, with protecting their investment in mind?

On a tangent I am slowly cooking up a build myself and simply want to make sure I do it all "right". It is strongly influenced by one of my favorite commercially built speakers of all time, the Teledyne Acoustic Research AR9 . Like the Cerwin Vegas I mentioned above these are rated as having a rather high power handling capacity... 150w nom/275w max (clipping 10% at 400wpc). Certainly the tweeter can't be a special 275w (or even 150w) version and likewise for the 400w CVs... a 400w tweeter in a home stereo speaker?

I know my questions likely stray out of the context of this excellent tutorial thread you have started and for that I apologize! I'd simply like such an excellent reference to crossover building to include something on the tweeter topic that many (including myself) still seem to be a little bit hazy on.
 
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To question 1, anything with contacts, resistance or metals whose structure may change, such as fuses, may have an impact on the sound. It is rarely major, but I don't use protection myself. Fuses are also not known for their accuracy (or timing). Another option is to use a light bulb of a specific rating in series with a driver. This offers a gradual overload as the filament resistance increases with heat.

Question 2. A DIYer will want to take learning their system's capabilities slowly. This may mean listening to a selection of songs at a moderately loud level, and over a period of days or weeks trying greater levels. Do they sound strained or do you notice them clip on a loud passage? After playing your speakers at a new high level, go over to them and feel and smell the components for signs of overheating.

I've not had a problem where either the tweeter or woofer is the limiting driver (the first to overload). It may be frustrating for others when there are 'instructions' on how to listen to your system, but some of the choices that some of us make can at first glance seem unusual.

I found a sample power distribution that you may be interested in looking at
The following data helped me a lot, they were published in a french audio magazine by Pierre Etienne Sirder about twenty five years ago. They are based on Harwood's works at the BBC. For a maximum power of 55 W, the power distribution in bands is:

Band (Hz) -> Power (W)

32 - 63 -> 4.4 W
63 - 125 -> 8.8 W
crossover at 125 Hz, total bass = 13.2 W

125 - 250 -> 10 W
250 - 500 -> 10 W
500 - 750 -> 7 W
750 - 1000 -> 5 W
1000 - 1500 -> 3.5 W
1500 - 2000 -> 2.5 W
crossover at 1500 Hz, total medium = 35.5 W

2000 - 3000 -> 1.75 W
3000 - 4000 -> 0.875 W
4000 - 6000 -> 0.44 W
6000 - 8000 -> 0.22 W
8000 - 12000 -> 0.11 W
cross over at 1500 Hz, total tweeter = 5.9 W

If your crossover is at 125 Hz and 1500 Hz, then the electric power reaching each unit is the sum of the powers in the band 32 - 125 Hz (bass), 125 - 1500 Hz (medium), 1500 - 12000 Hz (tweeter).

By scaling to your maximum power and your crossvoer frequencies, you can estimate the power requirements of your amplifier and speakers.

~~~~~~~ Forr

Question 3, one reason would be so that when you decide to change your design you'll have room to move. By the way, efficiency can work in your favour. This would mean finding a woofer with a sensitivity as high as the tweeter. Some would disagree as to whether this is a good goal, but either way it can open up amplifier options and give you loud music with lower power.

Question 4. Sensitivity ratings are often listed with Watts rather than Volts which can sidestep the impedance issue. If you know the level it is playing at you can estimate the power.

Another concern is ensuring the impedance doesn't drop too low for the amp you are using. A DIYer may not see the need to conform to 8 ohms in the normal way. You can sometimes get away with pushing it a little at the higher frequencies (also amp dependent) but please, only if you know what you are doing.

Question 5. There are rules about commercial speaker ratings, and they are there to ensure that ratings actually make sense. Realistically though, probably all of the above.

As for exotic drivers it would be wise to find others with experience with them. Some kinds of driver for example are delicate. What I've read about the AMT leads me to believe that they give few clues as to their best cutoff frequency.
 
Thanks AllenB for the informative reply! The power distribution scale that you posted/quoted is the first of it's kind I've seen and may be quite useful. The light bulb idea is something I hadn't considered either! Interesting...

I wholeheartedly agree that especially first time DIY'ers be quite careful about impedance matching their final product to the appropriate amplifier... More than once I've read threads where fairly exotic home brew loud speakers are being used with relatively inadequate amplifiers, most often home theater receivers... In the United States at least commercial companies must conform to certain power handling standards to receive a "UL Listing". In a nutshell it's to prevent over heating and causing a potentially catastrophic fire, something that can occur if drastic mismatches encounter a worst case scenario (some tube amps can be used for space heaters in a pinch!) It's also why commercial companies tend to offer an impedance rating... a home theater receiver that is certified by the UL for being 8 Ohm capable means that it is unlikely to catch your house on fire when combined with 8 Ohm rated speakers. Of course we all know that impedance is rarely a perfect 8 Ohms and most amplifiers and speakers have a range that they will feel most comfortable with rather than one single number "carved in stone".

Coming from a background with high end commercial products into DIY I can testify that in my experience one of the biggest differences between an expensive amplifier and one at the less expensive end of the scale is how stable they are dealing with different impedance for extended periods of time... A less sensitive speaker that will spend much of it's time presenting itself as a 2 Ohm load will take a very good amplifier to keep it happy for more than a few seconds... Example: The infamous Infinity Kappa 9, aka "the amp killer", was sold by the company knowing it could easily dip to 1 Ohm and maybe below...

Anyway, I apologize again for taking your wonderful thread so far off topic! I have no intent to run AMTs in a home made system and will likely use something in my next build that's a little less painful to replace if/when something goes wrong.

Cheers!
 
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Hi Jim. Cone breakup is dealt with in different ways depending on several things.

When you first approach the problem it is hard not to focus on the large response peak and the temptation to notch it. However the peak usually looks worse when you look at only the on-axis plot, which is not a complete report of the overall character of the sound you'll achieve.

The work 'breakup' also sounds synonymous with things like 'harshness'. However one of the concerns I have with the breakup region is that the sound to all directions becomes inconsistent.

There is not a lot you can do other than to avoid it. It will probably begin earlier than the largest peak you can see and I don't usually focus on the peak when dealing with breakup for this reason.

If you would like to pose this on your other thread I will help you through using the online calculators and see if I can show you an example of dealing with breakup without resorting to a notch filter.
 
If you would like to pose this on your other thread I will help you through using the online calculators and see if I can show you an example of dealing with breakup without resorting to a notch filter.

I thought id opened the discussion there to not only notch filters, but more fundamentally to dealing with cone breakup. If there is more to say on that issue, I invite further comment.

Now, I didnt chime in on this thread just to further my cause on the other thread. I felt since notch filtering was part of the introductory framework. Knowing how to derive what components you need through knowing the parameters of the peak ( I suppose the center freq, the -3db points, and its depth) would be very helpful. I have looked for such a tool and havent found one yet (for the series notch filter anyway).
 
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I see your point. I thought long and hard about whether to include notch filters here and finally I decided against it. Since they can often be avoided with wise crossover frequency choices, driver choices and tweaks to normal filters, their potential complexity seemed unnecessary. To know whether to choose series or parallel, where to put them and why to use them, to understand its (varying) effect created by the existing crossover and to understand (and compensate for) its (varying) side effects against the existing crossover, would have warranted its own tutorial.
 
AllenB, you're right. Your tutorials add a lot to general understanding of what can be changed to achieve particular corrections in speaker response.

Still I believe that to achieve results close to excellent we must align phase first and then see if we need any further tweaking at all. So I created alternative tutorial for those who'd like to take a somewhat more complex but in my opinion more rewarding path. It involves measurement :) so I started it as a new thread here
 
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Yes, with measurements it can be taken further indeed, although I disagree with the value to this extent of phase alignment at this early stage of the game.

During the '80s I wrote some software which I used to precision align phase between drivers, but it didn't bring me satisfaction after several attempts. I learned that while phase aligns at one place, there is a room full of sound that needs to be accounted for at the same time. This will limit the ability of phase alignment on-axis as being able to either make or break a speaker's response. Phase distortion, of itself, has also been shown not to be a major factor.

What matters in the end is the frequency response. This of course is phase dependent in the direct field but this then becomes virtually irrelevant by the reverberant field. At this early stage of the game, it is within the realm of being judged by ear or using a broad frequency response measurement.

All the best with the next step, PRTG, I'll drop by next chance I get.