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Old 22nd February 2013, 11:07 PM   #101
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roughly how many dB is lost/dropped at the point of baffle step?
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Old 23rd February 2013, 01:30 AM   #102
JackNZ is offline JackNZ  New Zealand
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Theory sais 6dB drop down those frequencies where wavelenght exceeds baffle dimension.
In real world it is less.
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Old 23rd February 2013, 09:29 PM   #103
JimT is offline JimT  Canada
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AllenB,

Thanks very much for this. I have to admit that speakers were my first effort at DIY audio and almost the end as I nearly gave up. I could never get them to sound quite right. I moved to amps, but alway wished I could someday make some good speakers. Your explanation on tweaking will have me pulling out those dusty old half built speakers and modifiying those crossovers with more insight.

Thanks again
Jim
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Old 28th February 2013, 06:07 PM   #104
AllenB is offline AllenB  Australia
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Regarding the baffle step, What we are actually talking about here is directivity rather than loss. There wouldn't be much sense in giving you a number between 1 and 6dB as the 'lost' sound is still interacting via the room.

So when tweaking by ear it is helpful to know that a baffle step can be dealt with in a crossover, but that the room can change things again to the point where it is hard to tell what's going on or it may even be the more significant issue. Sometimes when the room changes the sound it cannot be fixed again through a crossover, and must be dealt with in other ways.

I'll describe the baffle step, some crossover suggestions and the room issue.

A baffle is often made a little wider than the woofer, and woofers are often crossed just below their breakup region which is related to the size of the cone... so, the baffle step usually occurs within the woofer's passband and reaches up to somewhere near the crossover. I'll assume this is the case here.


Crossover Tweaks.

The inductor, resistor and capacitor as used in this thread can often deal with this issue at the same time as crossing over the woofer. All three components get used as free points of adjustment. From the basic crossover you'll want to reduce the upper woofer response gradually starting an octave or two below the crossover point. Below this the response shouldn't need to change. This region just below the crossover is referred to as the knee of the filtered response, as that's just what it looks like.

Reducing the knee is fairly simple. You can increase the value of the resistor. In doing this I might start by trying a value up to twice the original, and see if I can hear what changes. If you feel you want a value much higher than double you should try changing the other components because as the resistor gets higher, the woofer's highest frequencies above the crossover point will come back into play. It may not be significant, and if you can't hear it then there's probably something else worth chasing instead. By the way, a very large resistor effectively disables the capacitor.

You can increase the value of the inductor for a similar effect. Taken too far, this may reduce lower frequencies too, but won't have the issue of the higher ones coming back in. The filter rolloff will probably be more correct for the lowered top end of the woofer response as well. The inductor is also best not overdone unless your individual speaker responds well to it.

To reduce the knee and maintain the balance with larger adjustments, you should change all three components at once. For example, you might double the inductor and resistor values and halve the capacitance. The response should be similar to before above and below the knee, but the knee will be reduced. You can now change the resistor up or down a bit from that point.

Don't forget to lower the tweeter level as required. You'll notice this if it gets too far out of line because the tweeter will begin to stand out as if unsupported. If you listen to the tweeter with the woofer disconnected you'll hear this sound, it will seem out of place at it's lower end where it cuts off. When a woofer is properly integrated below it this bad sound disappears.


The baffle itself.

The speed of sound will be the same for all frequencies. The low frequencies cycle so slowly that by the time the cone has moved in and back out again, the pressure waves have moved significantly beyond the baffle. Effectively they can't 'see' it as it is too small. This pressure can then radiate freely in all directions.

The higher frequencies will already be radiating in a dome pattern from the front of the flat baffle (nothing to do with a tweeter dome), before they've even reached the edge.

The distance sound travels during one cycle is called its wavelength. The sound pressure measured from the front (say, outdoors in an open place) would be 6dB higher for the high frequencies, so if not for the room I'd probably say just use 6dB.


The room

Rooms give reflections. As the frequencies get lower and reach around the baffle, they can bounce off the rear wall, but all walls have their effect. Since there is a delay involved in the extra travel time, they may meet back up with the direct sound and add constructively, cancel, or anywhere in between. Usually you get a bit of everything.

Under some circumstance some of this energy may fill a part of the baffle step back to where it should be, and some will create dips in the response. Another concern is the midrange frequencies, that when exposed to the room, may give the impression of a lack of bass. The location of your speaker, the way it's facing, and room treatments can be worthwhile.

Last edited by AllenB; 28th February 2013 at 06:14 PM.
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Old 28th February 2013, 08:42 PM   #105
JackNZ is offline JackNZ  New Zealand
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[QUOTE=AllenB;3389914]Regarding the baffle step, What we are actually talking about here is directivity rather than loss. There wouldn't be much sense in giving you a number between 1 and 6dB as the 'lost' sound is still interacting via the room.
QUOTE]

That's why I said in theory the higher the frequencies the woofer has to reproduce, when wavelength get small enough to see the baffle it changes from 4P radiation to 2P radiation and would pick up another 6 dB in level on axis.

In our usual room then the speakers will be close enough to the floor and other boundaries, so that lower frequencies reflected from there will still be in phase with the initial wavefront to fill up the "loss".
So the 6 dB in reality never really happen.

If the woofer is crossed over too high, even if it has a very good cone and it does not break up quick, its surface will get too large to radiate higher frequencies because the frequencies from middle od the cone to the side of the cone will seen from an angle be severly out of phase. They will cancel out gradually and will cause a narrow coverage. A 6.5" woofer will radiate 2 kHz with 90 and above that narrow rapidly. A 8" woofer will be there at 1700 Hz.
That is lower than intuition would lead us. Constructions with a 8" woofer crossed over to a tweeter at 3K will have to deal with severe problems in coverage over frequencie and will sound different in every room.
The higher the woofer runs, the more periods away will the tweeter be to the center of the woofer and phase coherent crossing will be impossible to find.
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Old 28th February 2013, 10:51 PM   #106
AllenB is offline AllenB  Australia
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Yes, it's not very exacting when doing things this way...you just need a good starting point. For many the baffle step will happen a bit lower than the crossover, requiring extra components. Not sure where I might have suggested this but in some cases adding a second inductor in series with the first, with a resistor across it the same value as the nominal driver impedance can give a starting point for speakers with a lower frequency baffle step.

@ Jack.
The thing with directivity through cone size is that it can come with side lobes. By the way, my first floor cancellation happens at just under 200Hz.
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Old 28th February 2013, 11:06 PM   #107
JackNZ is offline JackNZ  New Zealand
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Of course there are sidelobes. Another reason why the woofer should be crossed over not to high. There will be also a lobing between woofer and tweeter. Not useful for a good reproduction.
Floor cancelation @ 200 Hz maybe meassured on axis but will change under angle and will be averaged out by the room. Still not to relevant for stereo. Do you know about your cancelation or do you really hear it?
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Old 1st March 2013, 01:56 AM   #108
AllenB is offline AllenB  Australia
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A crossover is by definition, a compromise. If we didn't need them we wouldn't have them, but it's not all that bad. Some things we just can't hear and these are being proven even in recent times.

I can't agree with your first two points. I run some woofers right on to the edge of their first breakup mode so I can exploit their narrowing pattern (and the response behaviour to increase the rolloff). The side lobes ask only to be plotted and accounted for, plus a nice round edge on the baffle if it's possible. My tweeters are also more than a wavelength above the woofer centre (44cm).

This may be getting a smidge off topic, but 200Hz is below the Schroeder frequency. Pick your room mode weapon of choice. Mine is the multi-sub approach.
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Old 1st March 2013, 08:41 AM   #109
JackNZ is offline JackNZ  New Zealand
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You can run a midbass higher than a wavelength of its diameter. You can also have the tweeter further away than a wavelength at crossover. For listening music in a average room this can work. But if you look (listen) at your onaxis response compared to the overall generated magnitude vs frequencie, the diffuse field will not be flat when onaxis is.
If my speakersystem must work in a given room which is live, I depend on the reflected and direct sound to be same.
Maybe I should mention that I am constructing sound systems for professional use where people use microphones and speach intelligibility is essencial.
We can not have side lobes as they can catch in the microphones and will colour the allover tone. My crossovers often are in the 2k range and it is important that the drivers are time alligned and close as possible.
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Old 16th April 2013, 03:33 AM   #110
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Great post - extremely valuable. Bookmarked.

I wish there were a way to measure the signal-out of my dbx Driverack 260 and translate that into accurate x-over components.

I understand I should use the actual measured impedance of the driver opposed to the nominal impedance given:

Do you know this to be an accurate method to acquire impedance values from each driver?: placing a 10 ohm resistor in series with the negative of the loudspeaker, running a tone at the chosen x-over freq (i.e. 1.6k Hz for a 2-way x-over) at 1V (max) output and with a digital multimeter
Click the image to open in full size.,
completing the following formula?:

Vr = Vin - Vs
Where Vr is voltage across resistor, Vin is unloaded voltage, Vs is voltage across speaker

I = Vr / R
Where I is current, and R is the value of the resistor (10 ohms is suggested)

Z = Vs / I
Assume an input of 1V, R = 10 ohms and Vs = 400 mV

Vr = Vin - Vs = 1 - 0.4 = 0.6 V
I = Vr / R = 0.6 / 10 = 0.06 A
Z = Vs / I = 0.4 / 0.06 = 6.67 Ohms

I found this information HERE

I don't have WT2, WT3 or DATS and I don't feel I have enough time to learn and troubleshoot the software I have installed on my computer. As long as I have time to get this crossover in the ballpark for a show on the 20th, I can tune it for higher accuracy afterward, while re-measuring the components after they're broken-in.

I'm considering using Xover Pro (same company as BassBox Pro) to get me in the ballpark for now. The program offers impedance correction as well as EQ correction. But, I would like to acquire accurate impedance values to start with.
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