# "Rotating" frequency response with passive crossover

#### Bryguy

Hello. I have been reading up on using horns and waveguides with tweeters, and a few times I've heard mention of developing a passive crossover to "rotate" the frequency response to account for the rolloff in the high frequencies you get with horns. The higher frequencies will gradually and consistently taper off at the top end, so a passive network is devised that somehow rotates the entire frequency response such that the response is flat, instead of tapering downward as frequency increases. I don't see many people talking about this, or how to do it. Is there anyone who can point me in the direction of where to start? I am still working on developing an understanding of the effects of placing different components in different parts of the crossover, but I can't find any resources that explore this in depth. Thank you 🙂

#### AllenB

Paid Member
This falling happens when directivity is being held at or towards constant. The simplest circuit involves a series resistor with a capacitor in parallel.

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#### Bryguy

I understand this is not a simple question to answer, but if anyone else has advice I would appreciate it

#### Bryguy

The simplest circuit involves a series resistor with a capacitor in parallel.
Thank you. I'll input that into my simulation software and see what I can do

#### AllenB

Paid Member
If you get stuck, feel free to post the files or ask questions. There's always a way.

#### Bryguy

If you get stuck, feel free to post the files or ask questions. There's always a way.
Thank you

#### Azrael

The higher frequencies will gradually and consistently taper off at the top end
Strictly speaking, it's the other way around: the waveguide/horn provides a boost at the bottom end, whereas at the top end the tweeter radiates at the level it would achieve without the waveguide.

There are also waveguides/horns that do not cause a frequency response drop, but these are not constant directivity waveguides/horns: they radiate narrower and narrower with increasing frequency.

The tip to use a capacitor in parallel to the series resistor of the voltage divider for the tweeter has already been mentioned.

Many greetings,
Michael

Translated with DeepL

#### Bryguy

Strictly speaking, it's the other way around: the waveguide/horn provides a boost at the bottom end, whereas at the top end the tweeter radiates at the level it would achieve without the waveguide.
Sure. I figured as much. Sorry I did not reflect this in my post
The tip to use a capacitor in parallel to the series resistor of the voltage divider for the tweeter has already been mentioned.
I'll play with VCAD when I get home. Though, there seem to be many crossover designs out there that do not use this method, but do have unfiltered measurements that have that rolled off top end. They solve the issue in other ways, apparently. To my eyes, components seem almost placed at random. I can recognize a ____ order filter or an LCR circuit, a few other things, but in many crossover designs there are components placed in the crossover in places that I don't recognize as established filter types. Or, components will be placed (usually resistors?) in filters that I otherwise recognize. I didn't really make mention of this in my original post, but I'd like to learn to make sense of this sort of thing. People seem to move beyond using preset filter types and sinply finding the right values for their use case, and seem to have a deeper understanding that makes deviations from established filter types the norm? Or otherwise, people seem to use filters that I have never heard of. This understanding seems to me to have been cultivated by many people, and I guess I just want to get an idea of how it's done. I want to learn a working knowledge of the consequences of placing any component anywhere in a crossover. For example, what's the difference between placing a resistor in series vs parallel? Or, what's the difference between placing a resistor before or after a capacitor? Stuff like that, but with regard to everything in a crossover.

I have attached an example of a crossover that is beyond my reckoning. It's for the VBS 10.2. In the case of this crossover design, what is the benefit of placing the LCR filter in the middle of the 4th order filter, rather than before or after? For the tweeter circuit, why are there so many resistors? What is the filter in the HPF that uses an inductor+cap+ resistor? I know there are reasons for these things, but I don't know what they are. I probably speak non sensibly, but it is hard for me to speak sensibly when it doesn't make sense to me. Anyways, if anyone knows of any resources that makes sense of crossover design in a deep enough way for me to make design decisions like I'm describing, I would really appreciate it

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• VBS-10.2 v2.1 Schematic.png
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#### Robh3606

The easiest way to see what's going on if you have the schematic in a sym is to just add switches. That way you can just look at the response changes
as you shunt it out of the circuit. If you look at the attachment you can see the switches added for all 3 notches. I used the switches to see exactly what the notches did to tailor the Frequency Response.

Rob

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• 9900 Network with Switches.jpg
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#### DonVK

Hello. I have been reading up on using horns and waveguides with tweeters, and a few times I've heard mention of developing a passive crossover to "rotate" the frequency response to account for the rolloff in the high frequencies you get with horns. The higher frequencies will gradually and consistently taper off at the top end, so a passive network is devised that somehow rotates the entire frequency response such that the response is flat, instead of tapering downward as frequency increases. I don't see many people talking about this, or how to do it. Is there anyone who can point me in the direction of where to start? I am still working on developing an understanding of the effects of placing different components in different parts of the crossover, but I can't find any resources that explore this in depth. Thank you 🙂

The horn/ waveguide output decreases with frequency (slopes down), acting like a low pass (LP) filter, decreasing by 6dB/octave. It's preferred to have a flat response so the freq response sloped "line" must be "rotated" back to flat (level). The way to correct (cancel) a first order LP (pole, parallel cap) is to add a first order HP (zero, series cap). Typically, the LP is rolling off at 2Khz (starts decreasing) and the HP is around 10Khz (stops increasing).

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#### Azrael

The horn/ waveguide output decreases with frequency (slopes down), acting like a low pass (LP) filter, decreasing by 6dB/octave.
As I said: this is only the case for constant directivity: for waveguides/horns, which radiate narrower and narrower out the top, it can look significantly different. Energy conservation, you know....

Whereas: I would prefer Constant Directivity, even if it increases the circuit complexity. But tastes are different: even a P.Audio PH230 as in my "Moppel 101" doesn't sound bad, even if it's not CD.

Many greetings,
Michael

Translated with DeepL

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#### DonVK

^^ The "constant directivity" means the beamwidth is constant, and does not narrow (over a limited range), hence acoustic output power is constant. The compression driver's motor force is relatively constant, so its throat velocity decreases with frequency, resulting in the waveguide (constant directivity) SPL output decreasing because of its constant beamwidth.

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#### Bryguy

The easiest way to see what's going on if you have the schematic in a sym is to just add switches.
That is good thinking Rob. Thanks for the info

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