Lobing vs. comb filtering

[Cal Weldon]

I'm not sure I know the difference. Can someone please explain in 'Cal' English for me?

[Andrew Eckhardt]

Someone is going to come in here and clean up with a much better response for sure, but lobing to me usually has to do with the individual or total off-axis response of a driver or speaker. It's usually a problem of narrowing in dispersion with increasing frequency, far as I know due to phase cancellation of radiation from a surface larger than the signal wavelength. Even if the entire cone is moving as a piston, because the distance from one point on the piston is farther away from another than a whole wavelength, "launched" waves have a possibility to arrive at one another with large phase differences, nulling each other. The only thing that winds up getting far away from the speaker at high frequencies is the radiation that has no unpaired inverted phase partner coming from a larger ring on the cone. Whether that's very accurate, comb filtering is distinct in that it is more of a broad-band problem occuring from multiple radiators interacting at some distance. If you ever hear comb filtering you'll remember it forever. As you move around signal shows up and drops out at different freequencies almost like you were running your open fingers slowly by your ears between you and a source. (only it can be far more pronounced of a sound)

[Andrew Eckhardt]

Selenium posts polar graphs of a lot of their drivers. You can look at those for their selected frequencies and note pretty clearly how the radiation pattern changes for them (generally narrowing at higher frequencies, and even going Behind them for very low ones). That's "lobing" in the frequency sense. There would also be polar response as controlled by horns or other means of directing the shape of the radiated pressure, maybe in a very broad range of frequencies, that you could unmistakenly call lobing. A lobe is just a lobe. It could just be on a cam shaft if it wants.

[TerryO]

Cal speak:
The lobe is just a balloon shaped pressure wave radiated out from a point source (your driver). As the frequency increases the outside dimensions of the balloon begins to narrow, based on the frequency and the diameter of the cone. This is what is referred to as beaming. Small drivers begin to beam at much higher frequencies than large drivers. This is taken into account by speaker designers when designing a crossover in order to match the off-axis power response of the woofer with the tweeter's so that, as you walk around the speaker, there won't be any sudden discontinuities or frequency imbalance in the sound of the woofer/tweeter combination.

Comb filtering is when the center to center distance between drivers exceeds a certain distance based on a fraction of the wavelength frequency that is being generated. This allows a null between the drivers, due to beaming at their high end. In the case of a line array this beaming or comb filtering would be detected by a sonic Venetian Blind effect to the sound in the vertical axis.

It is for this very reason that the most advanced theorists/ speaker builders recommend the use of a focused array, which can automatically eliminate comb filtering (and it's evil cousin, time (phase) misalignment).


I hope that some, or all, of this makes sense.

Best Regards,
TerryO

[cocolino]

Originally Posted by Andrew Eckhardt

.....lobing to me usually has to do with the individual or total off-axis response of a driver or speaker. It's usually a problem of narrowing in dispersion with increasing frequency,.....

What You have decribed is beaming.


Lobing involves 2 or more drivers radiating the same range of frequencies. It denotes the frequency dependent change of the angle of the main beam axis.

Comb filtering is caused by the same mechanism as lobing, namely different time delays of frequencies radiated by different sound sources.
Comb filterung is what does the effect of lobing to the frequency response at the point in space where the receiver (listener, microphone) is located. So it is sort of the result of lobing.
Also, lobing is a speaker only related thing, whereas comb filtering can include listening room related effects as well.

[Andrew Eckhardt]

Originally Posted by cocolino

What You have decribed is beaming.


Lobing involves 2 or more drivers radiating the same range of frequencies. It denotes the frequency dependent change of the angle of the main beam axis.

Got it, and I figure that's exactly what Cal was asking about too. Thanks for the cleanup.

[Cal Weldon]

I kind of understand cocolino and if I may ask:

If I understand correctly, when I am listening to an MTM on it's side, the cancellation effect I hear as I move side to side is comb filtering?

If that's true under what conditions will I hear lobing and what will be different in what I hear?

Thank you for your answers so far.

EDIT: I have re-read Andrews and that does seem to be what I have understood as beaming.

Reading Terry's I seem to get that comb filtering is what I always thought it was and now I think I have no idea how I would detect lobing or what I would be listening for.

[chris661]

From what I can tell, lobing is when you move around and different frequencies cancel in different positions.
(note: the dips in response here are at a single frequency. There would be more problems higher up, similar to comb filtering.

Comb filtering is, when you're stationary, there are dips in the frequency response...


Chris

[cocolino]

Originally Posted by Cal Weldon

I kind of understand cocolino and if I may ask:

If I understand correctly, when I am listening to an MTM on it's side, the cancellation effect I hear as I move side to side is comb filtering?

If that's true under what conditions will I hear lobing and what will be different in what I hear?

Cal, I think wether this effect (cancellation) is called lobing or comb filtering might just be a semantic issue. Essentially it`s the same thing.
A distinction may be that lobing generally refers to what the speaker(s) does to the angle of the main lobe axis at discrete frequencies and comb filtering is what this has an effect upon the frequeny response at a fixed position in the room.
Or to put it more simply, one thing is what the speaker does at certain radiating angles and the other is how You perceive this at a certain location.

This would be also in good accordance with what "chris661" said here:

Originally Posted by chris661

From what I can tell, lobing is when you move around and different frequencies cancel in different positions.
Click the image to open in full size. (note: the dips in response here are at a single frequency. There would be more problems higher up, similar to comb filtering.

Comb filtering is, when you're stationary, there are dips in the frequency response...

[soongsc]

I believe lobbing descibes the equal pressure line at specific frequencies, while comb filtering describes the frequency response over a spectrum. Both are the results of the same phenomena involving interference of waves from different sources.