Lobbing What is it in layman's terms? - diyAudio
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Old 10th November 2004, 04:45 PM   #1
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Default Lobbing What is it in layman's terms?

I am trying to explain the meaning of Lobbing in simple terms to some colleagues I understand it is when 2 drivers the same size are positioned close together and the sound wave from each intersects one another causing a degradation in the listening experience. Is there anywhere on the web I can go to for some info on this? What happens to the sound? What speaker design minimises the effect? Can one of you help ?

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Old 10th November 2004, 05:11 PM   #2
RJ is offline RJ  United States
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http://www.jblpro.com/pub/tour/AES%2...e%20Arrays.pdf

Here's a link that will show you the effects of lobing at higher frequency's and c-t-c distance between drivers.

If you haven't downloaded Jim Griffin's white papers on line arrays, here's the link;
http://www.audiodiycentral.com/awpapers.shtml

The simplest analogy to lobing that I can think of is to imagine a headlight ( sound radiation as light).
Below 1 wavelenth the lobing is not present so the headlight is a wide beam of light. As lobing occurs the headlight is narrowed (beaming) with side beams starting to develope.

I've built a line array with 5" woofers so the crossover was at 2500 hz.
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Old 10th November 2004, 05:12 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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Lobbing is derived from the word 'lob', meaning to throw something. The only time lobbing might be relevant to speakers is if you are so frustrated that you feel the need to throw them across the room.

Ahh, but you mean lobing!

Lobing is interference of the sound waves radiated from two sources, leading to an irregular off-axis response. That means that at some frequencies there will be peaks, and at others there will be troughs, varying depending on what angle you are at relative to the drivers.

To understand how it happens, imagine a sine wave being radiated from the two drivers. The wave has a particular wavelength. If you are at an equal distance from both drivers, then there will be the same number of wavelengths of the sine wave to each driver and they add together at your ears, resulting in you hearing something twice as loud as the output of a single driver.

Now imagine you are at an angle such that one driver is further away by half a wavelength. Now when the two waves reach your ears they are out of phase. Where one wave is at a peak, the other is at a trough, so they cancel completely and you hear nothing.

At a different angle the difference in distance may be a whole wavelength and the two waves sum together again.

This is the big drawback of multi-way systems. It can be minimized by placing the drivers as close together as possible, and by not having multiple drivers reproducing the same frequencies (e.g. have very steep crossover slopes). Positioning the drivers in a vertical line will also help prevent the effect in the horizontal axis.
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Old 10th November 2004, 05:16 PM   #4
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Me thinks you are talking of 'lobing'. Lobbing is what you do with grenades.

Two drivers set up within range of the listenner and fed identical samples of the same signal will generate an interference pattern where the distance and wavelength relationship causes phase additions and cancellations. This is the 'interferometer' principle.

Take a audio sinewave and apply it through two identical speakers placed side-by-side. Walk across the front of the speakers and listen to the sound. It will rise and fall in amplitude as a result of phase addition and cancellation because the distance change places you first where the distance from the ear to both speakers causes the peaks of both source sine waves to reach your ears at the same time (phase additive) and then again where the distance relationship places the sound from one speaker exactly 180 degrees opposite to the other (phase cancellation).

This is why there is the rule to NEVER place drivers in a horizontal arrangement on a baffle. The closer the drivers are to each other the larger and fewer the 'lobes' for a given sound frequency. In cases of three or more drivers in a tight row, reducing the power received by the outer drivers in relationship to the center one is one thechnique that can reduce the amplitude of the so called 'side lobes'. Side lobes are those sound beams (zones of phase reinforcement) that occur on each side of the desired main central sound beam.

Is this as clear as mud?
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Old 24th November 2004, 07:27 AM   #5
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What is the forumula for working out where lobing will occur, that relates distance and frequency?

eg. how do I find out if lobing will occur at 2.5k with x spacing ...
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Old 24th November 2004, 11:00 AM   #6
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So far lobing has been described by the interference between multiple drivers which results in comb filtering of the fq response at the listening position. But lobing is also caused by diffraction, or rather lack of it (a driver will become more directional at high frequencies). You can read more about it in this thread How does sound travel?

This is one of the reasons why you don't want to run say a 12'' woofer very high (others being cone breakup distortion and funky/falling fq response).
Choosing the crossover point between two drivers and thus mathing their directionality is critical to achieve a smooth off-axis response. This is (one) of the issues often neglected by too many designers (myself included in my early days). While you can always use an EQ to achieve either a perfectly flat on-axis response or off-axis response at some angle you can never get them both with a poorly designed speaker. This was discussed somewhat by SY and others in this thread Why the emphasis on flat speaker response?

/Magnus
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