lobing of drive units

[Professor smith]

Phase, Time and Distortion in Loudspeakers

As soon as the diameter of the radiating surface (the cone) of a driver becomes "significant" with respect to wavelength, you will have problems with lobing.

I want to ask about this quote which is taken from the link above.

Firstly, what is lobing in a way i can understand? Does this mean first order crossover speakers should be limited to very small 4 inch or less cones? Any larger and you get lobing which means you would have to decrease the crossover frequency which means increasing demands on the tweeter.

[gedlee]

When the wavelength of sound is larger than the source, then all sources act like monopoles (or dipoles) radiating sound in all directions regardless of how the source acts or its shape, etc. As the wavelength becomes comparable to or shorter than the size of the source, the sound can and will radiate differently ihn different directions - it is no longer a "simple source". "Lobing" refers to the characteristics of a typical source which sends a beam (or several) of ever narrower width out in what is called a "lobe".

"Lobing" or "beaming" is not necessarily a bad thing, I do it quite deliberately, but this requires that the source directivity be well controlled so that this "lobing" is consistant in space and in frequency. No simple source or piston source can do this, which is why people use waveguides, because they can.

[Professor smith]

When the wavelength of sound is larger than the source, then all sources act like monopoles (or dipoles) radiating sound in all directions regardless of how the source acts or its shape, etc. As the wavelength becomes comparable to or shorter than the size of the source, the sound can and will radiate differently ihn different directions - it is no longer a "simple source". "Lobing" refers to the characteristics of a typical source which sends a beam (or several) of ever narrower width out in what is called a "lobe".

ok but why does it do all of that?

[Ron E]

Lobing is essentially like a diffraction pattern. Perhaps the easiest way to understand is to look at "planar" waves hitting a perforated barrier in a ripple tank. If the wavelength is longer than the perforation, the perforation acts as a simple source and converts the planar wave into wave which radiates as a semicircle. If the perforation is wider than the wavelength, this same thing happens at the edges, but the stuff in the middle just goes straight through (beaming). Play with Falstad's ripple tank sim (hours of fun) Ripple Tank Simulation

[john k...]

Originally Posted by Professor smith

ok but why does it do all of that?

Any source of finite dimension can be though of as being composed of an infinite number of point sources distributed over the surface of the source. When the wave length is large compared to the source size, all these point sources sum in phase everywhere. Think of it like constructive interference. As the wave length becomes smaller the phase of each point source becomes dependent on the position in space where the the sum is observed with some point sources in phase and other out of phase. So there is both constructive and destructive interference between the sources. The results in a sound pressure that varies in intensity with spatial position. Lobing is a term used to discribe this variation in intensity with position. In two dimensions severe lobing looks like the petals of a flower. Each pedal being a lobe.

[DrDyna]

Originally Posted by Ron E

Play with Falstad's ripple tank sim (hours of fun) Ripple Tank Simulation

do NOT do this after you've had a few Tanqueray and cranberry, you'll be at it for an hour.

[AllenB]

Originally Posted by Professor smith

Does this mean first order crossover speakers should be limited to very small 4 inch or less cones?

Your crossover slope will affect the width of the region of overlap, but otherwise not the frequency at which it happens, like driver size will.

Crossover slope will be more of a concern when you deal with the other lobing in this article, which is a matter of joining the sound from two separate drivers, and matching them in phase.

[454Casull]

You won't get lobing, you'll get beaming.

You only get lobing from interference with another source.

[weltersys]

Originally Posted by 454Casull

You won't get lobing, you'll get beaming.

You only get lobing from interference with another source.

A large speaker, say 15", is not a single source at high frequencies. Run it full range, and it will both "beam" and "lobe".

[Professor smith]

so will any bookshelf speaker with a small drive unit beam if the crossover is set too high?
how does that tend to sound?