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Multi-Way Conventional loudspeakers with crossovers

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Old 12th January 2019, 08:47 AM   #1
fatmarley is offline fatmarley  England
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Default Why simple crossovers, tuned by ear, donít work

It seems like almost every day we have a new person asking about online crossover calculators or ready made crossovers.

What we desperately need is a sticky that explains clearly why ready made crossovers don't work, but explained in a way that a person who has no understanding of loudspeaker design understands. Then all we have to do is link to the sticky. I'd do it myself but I think others would do far better job than me.

Anyone up for it?
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Old 12th January 2019, 09:47 AM   #2
wintermute is offline wintermute  Australia
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I started doing a "designing a loudspeaker from scratch" series of posts, but I stopped before I got to the crossover design part. I really should finish it some time! Since it was something that I needed to do for that thread I'll see if I can come up with something Don't hold your breath though. I started that back in Jun 2015

Tony.
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Old 12th January 2019, 10:24 AM   #3
wintermute is offline wintermute  Australia
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Designing the crossover.

This is the easy bit right? Just go to an online calculator put in the frequency you want to crossover at and it will spit out the values right? Or just go to partsexpress and purchase a ready made crossover that crosses over at a suitable frequency.

If only it were this easy. If speakers had a completely flat frequency response, and a completely flat impedance, and the distance from each driver to your ears was the same then yes that would work*, however that is not the case in reality and we need to take these things into account.

A crossover filter does not just cut off the frequency at a particular frequency, it gradually rolls off the speakers responses over a fairly large range. In this range BOTH drivers are adding together to form the sound. For this to work ideally, both drivers need to be adding together equally and need to be in phase with each other through this range.

Lets see what happens with our theoretically flat speaker with a completely flat impedance using our text book crossover. For this we have two speakers which are both 90db efficient, completely flat and have a completely flat 8 ohms resistance. Now if you had such drivers you wouldn't be making a crossover in the first place, but it serves to demonstrate what is happening.

The schematic is for a textbook 2nd order linkwitz riley crossover at 3Khz. Calculated using this online calculator Speaker Crossover Calculators by V-Cap

Note if the images look a bit small try clicking on them and they should expand to full size.
Click the image to open in full size.
The graph is showing the summed response of the two drivers and the rolloff of each driver. See how wide the range is where it is both drivers that are summing together to make the flat response (It's almost from 200Hz through to 20Khz)!
Click the image to open in full size.
Note how there is a step in the frequency response. This is due to the resistance inherent in the inductor which is in series with the woofer. For this sim I used 0.29 ohms for the DCR of the coil. It is not too significant but it does show that even with our perfect drivers we have not got a perfectly flat result with out text book crossover.

Next we will show what happens if we use actual impedance curves for real drivers (but still with our perfectly flat frequency response). I've chosen a Dayton RS150P-8 for the woofer and a Morel DMS37 for the tweeter, as I happen to have frd and zma's for both. Everything is the same except for the real-impedance curves being used in the sim.
Click the image to open in full size.
Whoah!! What happened to our nice flat response!! Textbook crossovers only give a text book response with a flat impedance, which the above demonstrates very well . Now it is possible to greatly improve this by using impedance compensation on the drivers, but that is not necessarily something you need to do as you will see later when we do a more optimal crossover.

So now we add in the real responses of the Dayton and Morel drivers and see what we get.
Click the image to open in full size.
Probably not really what we want! So you can see that if we just used our text book crossover with our two new drivers, that we would likely have a bit of a tweaking job on our hands!

Note that even the above is a very crude estimate of what would really happen, as we have not taken into account how the baffle effects the frequency response of our drivers, or how differences in driver offset on the baffle affects their phase relationship to each other, in reality the above probably looks even better than it would in real life!

One of the things that people starting out with crossover design fail to grasp is that it is the acoustic rolloff of the speaker that is the real goal not the electrical order of the filter. When we say we want a 2nd order linkwitz riley crossover, that doesn't necessarily mean that it will be an electrical linkwitz riley filter (for newbies I would actually suggest that a fourth order LR or bessel filter is much more likely to give them results that they will be happy with, 2nd order acoustic filters are not easy!) After having said that, what I come up with later, does appear to be a 2nd order electrical filter on both the woofer and the tweeter, but it doesn't always have to be. When doing 4th order targets I have achieved it with 2nd order on the woofer and 3rd order on the tweeter, it all depends on the drivers and the frequency you cross them at.

We can demonstrate this concept by adding some target slopes to the third graph. The red lines in the fourth graph are our target 2nd order L/R at 3Khz. These show how our speaker should roll off to get the best blending (assuming we also get the phase right, but more on that a bit later).
Click the image to open in full size.
As can be seen the drivers are not following the target curves very well at all and the resulting frequency response is pretty awful. I'm sure if you got a result like this you would not be happy and wondering where you had gone wrong! We would be looking at quite a lot of tweaking by ear to sort out this mess, and without actually knowing whether it was the woofer, the tweeter or both that is causing the issue we would be stabbing in the dark.

So instead we can do some virtual tweaking and see what the results are with our sim. As previously stated I have not properly prepared these FRD's or put in proper offsets for drivers so this result is invalid, but is useful for demonstration purposes.

I spent maybe an hour playing around with some tweaks to get a result that is roughly +- 1.5db from approx 70 hz to 16Khz. It also has a good deep reverse null (meaning the drivers are in phase at the crossover frequency). Now of course I already have experience so that makes it faster, but I'm sure that trying to tweak this by ear would take an awful lot longer!

The new circuit is below:
Click the image to open in full size.
You can see that there has been a few additions, and also changes to values from our original textbook crossover. I used the optimizer on C2 and R4 which is why they have strange looking values, I would normally change them to the nearest standard value but was being lazy.

Below is the resulting frequency response using this new crossover.
Click the image to open in full size.
Looks quite a bit better doesn't it?

Next is the reverse null response which shows how well (or not) the drivers are in phase at the crossover frequency.
Click the image to open in full size.

Now we show the individual driver responses and how well they track to the target LR2 curves. Also this graph shows each drivers phase. They do overlap at the crossover frequency but in general the overlap is fairly small, ideally you would have a much wider range where the drivers are in phase...
Click the image to open in full size.
The coil and resistor on the tweeter circuit are there for phase adjustment only. Without them the tweeter follows the target curve better at lower frequencies but the phase alignment is bad at the crossover point.

One last image shows the difference between our custom crossovers response (black) compared to our textbook crossover (blue).
Click the image to open in full size.

Hopefully this has helped show why a text-book crossover almost certainly will not give you the result that you want.

Tony.

*if such drivers existed, there would not be any need for a crossover in the first place
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Last edited by wintermute; 15th January 2019 at 07:20 AM.
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Old 12th January 2019, 10:38 AM   #4
marco_gea is offline marco_gea  Italy
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This page is a good start...

Quote: "The question is whether we can make a crossover at all without measurements - and the answer is NO.
It cannot be done, and crossovers cannot be calculated."


Marco
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Old 12th January 2019, 10:52 AM   #5
korpberget is offline korpberget  Norway
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Wow!
Go ahead. Take the fun out of diy. I dont care how it measures. At the end of the day it is how it sounds to me and my ears in my chair in my listening position in my room that will tell me if it sounds good or not.
I have a load of crossover parts and like to experiment.

It can be done, no problem.
Maybe a flat response speaker does not appeal to me? Then what? Tweak by ear is the answer.
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Old 12th January 2019, 11:32 AM   #6
fatmarley is offline fatmarley  England
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Quote:
Originally Posted by marco_gea View Post
This page is a good start...

Quote: "The question is whether we can make a crossover at all without measurements - and the answer is NO.
It cannot be done, and crossovers cannot be calculated."


Marco
That's just the sort of thing I'm looking for. I'm sure some will still disregard the information in the link, but you can't help some people - it should help a few though.
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Old 12th January 2019, 11:41 AM   #7
fatmarley is offline fatmarley  England
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Quote:
Originally Posted by korpberget View Post
Wow!
Go ahead. Take the fun out of diy. I dont care how it measures. At the end of the day it is how it sounds to me and my ears in my chair in my listening position in my room that will tell me if it sounds good or not.
I have a load of crossover parts and like to experiment.

It can be done, no problem.
Maybe a flat response speaker does not appeal to me? Then what? Tweak by ear is the answer.
You could tweak by ear forever and still never achieve a flat response at your listening position.

It's only when you have accurate measurements imported into crossover design software that you can see with your own eyes how bad simple crossovers will sound and how they react to tweaking by ear (rarely as you'd expect).
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Old 12th January 2019, 11:47 AM   #8
picowallspeaker is offline picowallspeaker  Italy
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Articoli Archives - AudioReview
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Old 12th January 2019, 11:49 AM   #9
Vintage Audio Projec is offline Vintage Audio Projec
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I have to agrea here with korpberget a little bit

So ok you measure and design a loudspeaker with the "perfect" crossover from scratch getting a flat frequency Then you put it in a normal room that has reflections and al sorts of imperfections.... I mean nothing is perfect.....

If you just want simple 2 way PA tops for DJ-ing on party's with a lot of drunken people..... they won't bother as long as there is enough Bass and Sub.

It is however good to design crossovers from scratch or to adjust ready made crossovers. It is not that difficult to use the speakers DCR etc... and make the crossovers adjusted for your speakers. To measure the exact response is more difficult and not always needed. Many commercial speakers aren't measured also.

And if you make something that sounds good to you and you can enjoy then just enjoy it. I think some people are just constant nagging and never listen to music.
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Old 12th January 2019, 12:13 PM   #10
fatmarley is offline fatmarley  England
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I'd be interested to know what commercial speakers aren't measured - Do you mean designed by ear?
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