The ''No-Crossover-in-the-Mid-range'' Dogma...

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#102
Consequently and presuming drivers with a linear response past their passband Butterworths produce a 3dB hump in the FR which should be particularly audible in the higher mid range.
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Butterworth crossovers do indeed have a disatvantage but it is not the one you mentioned - simply because it is not true !
The Butterworth disadavantage is the asymmetrical lobing.
They don't have a hump in their response - neither on-axis pressure response nor power response.
The 3 dB dip in power response is actually the disadvantage of LR crosssovers.

There is no best crossover unfortunately ! As always in engineering you have to balance all properties against each other in order to get as close to your target as possible.

Regards

Charles
 
#104
I'll have to check wheter this is true for the even order Buttwerworth filters but I seriously doubt it.
At least for the third order (and all higher-odd-order) BW filters flat on-axis pressure response AND power response are a given while you can only achieve on-axis pressure response with LR crossovers.
This is actually not that difficult to follow given that twice the same signal 3 dB down but both "versions" out of phase by 90 degrees sums to one in terms of SPL and voltage but at the same time twice half the power sums to one as well - while in the LR case only the the pressure and voltage sum to one (two times one half, in phase) while the power will sum to half the power (two times one quarter).

Power

Regards

Charles
 
#105
Charles,

This is a common misunderstanding. Two coincident sound sources combine plus 6dB. There is 3dB from doubling the number of drivers, and a 'free' 3dB because of better acoustic coupling of the drivers. If you don't take this 'free' 3dB into account, and have both drivers down 3dB at xover, you get a hump. LR does it right, by having both drivers down 6dB at this point. There is no 3dB dip in the power response.
 
#107
Why is power response down? With what assumptions? How is power response calculated in this case?

I must say that I don't understand these "rules" at all. And do they actually work in real life with non-coaxial and nonlinear transudcers etc. ?
 
#108
Juhazi said:
Why is power response down? With what assumptions? How is power response calculated in this case?

I must say that I don't understand these "rules" at all. And do they actually work in real life with non-coaxial and nonlinear transudcers etc. ?
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I'm not sure I get it either. I assume that "Power response" refers to acoustic power measured with a calibrated mic, and the other measurement might be voltage sent to the speaker drivers. It seems that the acoustic output is what matters the most (?), so is that really what the Linkwitz/Riley (LR) crossover does?
 
#109
There is nothing more to this than a choice of phase quadrature on axis. You can either get a hump or not with the butterworth depending on whether it is in phase on axis or out by 90 degrees.

Why would you take phase in to account with regard to power response. -3dB is half power and two of them is back to flat power, that's it.. unless you want to investigate interactions that occur beyond the speaker itself.

By combining 2 x -3dB and a 90 degree phase difference, you get flat power and flat axial response with a tilted lobe pattern. I don't see how filter order is going to stop us, considering there are more degrees of freedom available ie: acoustic centres, artificially added delays or assymetrical filters if required.
 
#110
AllenB said:
Why would you take phase in to account with regard to power response. -3dB is half power and two of them is back to flat power, that's it.. unless you want to investigate interactions that occur beyond the speaker itself.
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-3dB is half power, but if you combine two coincident sound sources at -3dB each, the end result is +3dB. Efficiency increases. This is how large bass stacks can reach 25% efficiency or more.
 
#111
I completely forgot about the coupling of sources, sorry. But keep in mind that this accounts only to certain situations.
When it comes to closely spaced subwoofers at very low frequencies we have a clear situation where this rule is definitley true.
When it comes to crossovers the situation is not that clear anymore.

Regards

Charles
 
#113
vacuphile said:
-3dB is half power, but if you combine two coincident sound sources at -3dB each, the end result is +3dB. Efficiency increases. This is how large bass stacks can reach 25% efficiency or more.
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A slightly different example. Say a tone is played, and there is a certain amount of power in the room. Also say this power is coupled to a room mode so that the pressure you measure depends on where the mic is?
 
#115
AllenB said:
A slightly different example. Say a tone is played, and there is a certain amount of power in the room. Also say this power is coupled to a room mode so that the pressure you measure depends on where the mic is?
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Yes, exactly, there will be different SPL's measured by your mike at different locations in your room. Only by allowing a time window so that only direct sound is measured, this can be avoided.
 
#116
Only by allowing a time window so that only direct sound is measured, this can be avoided.
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YES and/or NO - depending on what you want to measure exactly !

If you want to measure th eon-axis FR of a speaker for instance: Use a short window.

If you want to measure the SPL level relevant for subjective perception of loudness: Use longer measuring windows.

While we are at it: Some recommend to use LR crossovers for speakers in non-reverberant surroundings and the use of Buttwerworth in reverberant surroundings in order to achieve the best FR linearity at the listening position.

Regards

Charles
 
#117
Bob Richards said:
I assume that "Power response" refers to acoustic power measured with a calibrated mic,
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You can calculate power by combining Spl measurements, eg from around the room.

phase_accurate said:
Some recommend to use LR crossovers for speakers in non-reverberant surroundings and the use of Buttwerworth in reverberant surroundings in order to achieve the best FR linearity at the listening position.
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This sounds like a good rule of thumb, but that isn't the goal. You want to match the FR to the power, and get a flat or similar response.
 
#118
So basically, "power response" takes into account how the room affects what gets to the listeners ear, as opposed to "frequency response" which usually refers to the speaker in an anechoic chamber. Power response will be a function of the speakers FR, it's off axis emissions, and the acoustics of the room. It might be nice if they were to match, but not likely in most typical rooms and/or with most typical speakers.

I try to avoid abrupt changes in the off axis FR, and thereby the power response, by using a smaller diameter driver from 500HZ on up (such as the 3 inch Peerless/Tympani TG9FD1008), so the change in off axis response isn't too abrupt when a tweeter takes over at the high end. I also try to keep that change, which isn't likely to be precisely balanced, above 6kHZ, so stereo imaging effect is minimally damaged.
 
#120
Toole was reffering to power response being more informative
curve because it respresents the weighted average of all the
measurements taken vertically and horizontally around the
speaker. Since there is far more measurements points close
to on axis, the on-axis curve has low weighting on the sound power.

Any bumps that show in power response are bad resonances.
 
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