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Old 30th January 2012, 03:10 AM   #1
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Default Flat FR, Flat Power Response, in-phase crossover?

Is this simply not possible?

Do you have to pick between Phase/Frequency Response and Power Response?
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Old 30th January 2012, 03:40 AM   #2
mdocod is offline mdocod  United States
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Any time you double the number of drivers performing the same task (like what happens in the middle of an ideal x-over), there is a doubling of sensitivity, so if you build the x-over with a power response goal, there will pretty well always be a ~3dB hump at the x-over as a result of the improved sensitivity there provided the listener is on-axis equa-distant from the drivers in question. I suppose by purposely selecting different slopes for each driver and combining that with driver placement it may be possible to get a mixed breed of power response and frequency response both staying reasonably flat as there are some losses to phase issues. I suspect that such an approach is probably not a good idea as it means that there is likely to be an off-axis peak at x-over somewhere in the room and that could make it's way to the listener as an unwanted reflection. Such a condition might screw up some buzzwords.

I have to ask, is there a stubborn valve amp involved?

Last edited by mdocod; 30th January 2012 at 03:49 AM.
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Old 30th January 2012, 03:49 AM   #3
AllenB is offline AllenB  Australia
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It is possible. In reality you make small sacrifices here and there but yes, you can have FR, power and phase looking good at the same time.
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Old 30th January 2012, 09:17 AM   #4
keyser is online now keyser  Netherlands
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"Flat FR, Flat Power Response, in-phase crossover?"

Is possible only if the inter-driver spacing is small in relation to the wavelengths involved.
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Old 30th January 2012, 04:22 PM   #5
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It is inherent in the nature of in-phase crossovers that two units will be contributing at the crossover point and there will be a 3dB gain of axial sensitivity over the individual responses. Assuming the units are fairly flat in their power response in the octave around the crossover point, then there will be typically a 3 dB hole at the crossover point. We primarily choose flat axial response and spread the crossover points such that the units are 6dB down at crossover (rather than, say 3 dB down for a quadrature summing approach).

Before everybody goes of debating the merits of flat power vs. flat axial for the next 1000 posts, let me point out that one thing the Toole studies always show quite clearly is that the minor holes in the power response found at the typical (in-phase)crossover point never preclude a speaker from being top ranked in any listening comparison.

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Old 30th January 2012, 09:21 PM   #6
AllenB is offline AllenB  Australia
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Thanks for stating that Dave.

I'd like to add that quadrature summing in non coincident drivers occurs on-axis, and to the best of my knowledge Linkwitz and Riley saw tilting of the lobes as a greater disadvantage.

RockLeeEV, were you specifically asking about the crossover region?
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Old 30th January 2012, 09:44 PM   #7
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Quote:
Originally Posted by keyser View Post
"Flat FR, Flat Power Response, in-phase crossover?"

Is possible only if the inter-driver spacing is small in relation to the wavelengths involved.
I'd just like to highlight keyser's point - most people assume that in phase crossovers (adjusted for flat on axis response) always have a 3dB hole in the power response.

This is only true when the driver spacing is half a wavelength or more at the crossover frequency. As driver centre to centre spacing is reduced below half a wavelength the power response hole is progressively reduced until by the time you get below about 0.1 wavelengths there more or less isn't a power response hole.

So if you had two drivers spaced say 200mm apart and crossed over at 4Khz they are several wavelengths apart and will have a 3dB power response hole. (In addition to any driver off axis characteristics) The same driver spacing with a 250Hz crossover frequency will have almost no power response hole despite the in phase crossover because they're only a small fraction of a wavelength apart.

As for tilted lobes in a quadrature summing crossover (true odd order crossover) I have to agree. I used to fall on the side of odd order crossovers from the perspective of power response but in the last year my opinion has changed 180 degrees (pardon the pun) to in phase tracking for a number of reasons, including avoiding off axis peaks in the response.

As above though, an odd order crossover will only have off axis peaks / offset lobes if the drivers are half a wavelength or more apart - if they are very close together (<0.1 wavelengths, eg woofer to midrange in most designs) it doesn't really matter whether you use in phase or quadrature summing filters - both will sum flat on axis, flat power response and will have a uniform lobe.

The even order L/R will result in 3dB less power dissipation near the crossover frequency though - as quadrature summing will "waste" some of the power due to incomplete addition.
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Old 6th February 2012, 02:45 AM   #8
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Quote:
Originally Posted by AllenB View Post
RockLeeEV, were you specifically asking about the crossover region?
Yes.

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it doesn't really matter whether you use in phase or quadrature summing filters - both will sum flat on axis, flat power response and will have a uniform lobe.
Hmm.. That's always been my thought about coaxials.. that if phase is identical off-axis, why would there be a power response dip? I guess there isn't one, then.
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Old 6th February 2012, 09:02 AM   #9
AllenB is offline AllenB  Australia
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For example, imagine affixing two pieces of string to a baffle, one where a tweeter goes and one at the centre of the woofer placement. Cut one string shorter by a half wavelength at some experimental frequency. Hold the two loose ends together between two fingers and pull them out and up, or out and down until they are both taut, hence delineating the lobe.

Using this technique on a coincident pair of drivers where each string is tied to the same baffle location, shows that in theory they especially need to be brought into phase, and that they will sum at all angles.
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Old 6th February 2012, 08:48 PM   #10
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Quote:
Originally Posted by RockLeeEV View Post
Hmm.. That's always been my thought about coaxials.. that if phase is identical off-axis, why would there be a power response dip? I guess there isn't one, then.
Coaxials are a great example of what I'm talking about. Because the two drivers are concentric the centre to centre spacing is zero, thus no power response hole due to using an in phase crossover.

Of course the driver itself - the low frequency portion, may start beaming if the crossover frequency is too high causing a power response dip, but that's due to the driver directivity, not the crossover phasing.

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Originally Posted by AllenB View Post
For example, imagine affixing two pieces of string to a baffle, one where a tweeter goes and one at the centre of the woofer placement. Cut one string shorter by a half wavelength at some experimental frequency. Hold the two loose ends together between two fingers and pull them out and up, or out and down until they are both taut, hence delineating the lobe.

Using this technique on a coincident pair of drivers where each string is tied to the same baffle location, shows that in theory they especially need to be brought into phase, and that they will sum at all angles.
I'm not sure that your string analogy says that the two coincident / coaxial drivers must be driven in phase. All it says is the relative phase won't shift as you go off axis. (Ignoring phase shift off axis due to beaming of the larger driver causing high frequency rolloff...)

If the two coincident drivers cross over at 0 degrees and -6dB or 90 degrees and -3dB the on axis result will still be flat and the off axis response between the two crossovers will be nearly the same as well since the only variation in phase off axis will be a result of driver beaming.

(Because the 0 and 90 degree crossovers will have different slopes, there will be a slight difference in the way that the beaming of the larger driver affects the summed response off axis)
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