Some terms: layman explanation to advanced

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diyaudnut

Member
2015-02-24 5:41 am
1. Directivity
2. Dispersion
3. Beaming
4. Lobing


Please define, describe and elaborate on significance in loudspeaker development.

If the initial few posts are basic explanations then it will remain useful to beginners.

my apologies if this has been done before.

linking to earlier posts would be great too if thats allowed


thanks
jp
 

eriksquires

Member
2013-05-10 4:11 pm
Here is a basic article on the subject from Paradigm. It's focus is on explaining why their speakers are better, but the descriptions and drawings are great for beginners.

The thing to keep in mind is that directivity/dispersion are to be chosen. It's up to the designer to match the right characteristics to the environment and use. In my living room I like my dispersion narrow in both axis so I get more clear sound with less room treatment. I found out recently this is a "thing" and there's a real expert who has written on the subject. I thought I was being cheap. :D Others like wide patters at home so they can hear a stereo image regardless of where they are.

Regardless of your choice, narrow, wide or omni almost everyone wants even roll off. That is, if you are 30 degrees off center, you don't want to hear all treble, or all bass. You want to at least hear the full range, even if you are no longer hearing it at the peak volume.

Beaming refers to what happens when the frequency being reproduced is significantly smaller than the diameter of the driver reproducing it. You only need a single driver for this to occur. Look at any driver's frequency response and you'll see off-axis charts, showing reduced volume at the higher frequencies as you go off-axis. That's beaming. In the picture below you can see the FR at 0 (black), 30 and 60 degrees. You see the dropping off of the high frequency? That's because the top octave is "beaming" straight ahead, like a focused beam of light. Anyone located 30 degrees off axis from the tweeter will hear about 12 dB less at the topmost octave. While you can't prevent a particular driver from beaming at a particular frequency, you can control it by where you cross over from one driver to the next smallest one.

You can also see where these 3 lines meld together. Below approximatley 2kHz the angle makes no difference and (up to 60 degrees) the tweeter is radiating evenly regardless of the listener's location. Sadly, the tweeter's cut-off is probably between 1,500 and 2,000 Hz, so the range below 2kHz is just not very useful for us! :D
An externally hosted image should be here but it was not working when we last tested it.


Lobing is, as far as I remember, is an effect caused by the crossover and at least 2 drivers. It can cause the same kinds of effects as beaming, but instead of it being straight in front of the driver, you can have it go up, down even sideways if you have drivers arranged horizontally. In digitally controlled arrays it can even be actively controlled or changed on-demand. I'll let others explain this better than I can. Essentially it has to do with the difference in time that a signal arrives, and whether this causes destructive interference or not. In passive crossovers this occurs because of the phase shift and the driver locations. In DSP controlled speakers this can be deliberately induced by adding delay or phase shift.

Look for the recent articles on the BeoLab 90 for a thorough discussion of a "consumer" speaker that causes lobing deliberately using this technique, but many pro audio arrays do this as well.



Best


Erik
 
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