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Old 30th September 2005, 02:56 AM   #1
24kilo is offline 24kilo  United States
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Default Bessel Or power tapering?

What the Advatage and if it has a advantage. How do you wire 10 8ohm drivers?
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Old 30th September 2005, 03:09 AM   #2
simon5 is offline simon5  Canada
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Want to wire 10 8ohms drivers?

It depends on what impedance you want to end with my friend.

Power tapering is used to have less delay problems with the sound in a long line array if you plan to listen close to the line arrays.
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Old 30th September 2005, 04:31 AM   #3
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Quote:
Originally posted by simon5
Want to wire 10 8ohms drivers?



Power tapering is used to have less delay problems with the sound in a long line array if you plan to listen close to the line arrays.
I disagree.

Power tapering is used to control the beam shape while reducing or eliminating secondary lobes (sidelones or grating lobes) in the sound projection pattern at mid and higher frequencies where the distance between driver centers is a good portion of a wavelength.

Placing a line of drivers on a concave curved baffle where each driver becomes equidistant to the listener's ears is a mechanical technique which mitigates delay issues. Electrical delay lines to each driver in a flat array can do the same thing.

Rob
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Old 30th September 2005, 08:47 PM   #4
simon5 is offline simon5  Canada
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Did you read that document?

http://www.audiodiycentral.com/resource/pdf/nflawp.pdf

James R. Griffin had that to say about power tapering :

Quote:
Design Issues : Sound Bloom Caused by Unequal Vertical Sound Path Lengths Between Drivers and Listener in Near Field for a Flat Baffle. Mitigation Techniques : Use precedence effect via vertical power tapering
You just say plain and simple that power tapering is used to control the secondary lobes when in that document I read the only way to control that is to maximize the Active Radiating Factor via driver selection and placement and it's mostly caused by rectangular drivers not circular ones.

Anyway I'm not an expert, I'll leave you alone to argue with Griffin hehe!
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Old 1st October 2005, 12:11 AM   #5
Andy G is offline Andy G  Australia
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The normal way to wire a 10 driver (8 ohm driver) line array is top 3 in series, then the next 2, then next 2, then bottom 3. These series sections are then wired in parallel. Gives an overall impedance of 4.8ohms with a net gain of about 12 dB over a single driver.
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Old 1st October 2005, 04:39 AM   #6
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54lbs,

A bessel makes a line array disperse like a point source speaker. I'm not sure how you'd do a 10 driver bessel, but even if you could, I wouldn't because you give up the benefits of arrays. Other than for sound reinforcement, to me the only applicable uses of bessels for our home setups are as a horizontal array (like for a center channel), or a situation where you really needed the marginal sensitivity increase of a 5 or 6 driver bessel but still retain point source dispersion. One of the guys here on the forum reported success with 5 driver bessel arrays of tweeters. I personally messed around with some 5 and 6 driver bessels, but dropped them completely when I realized the total lack of imaging with bessels. It was really a strange effect between the stereo arrays, like a wall of sound eminating from between the arrays very evenly distributed.

Power tapering is a different issue and used to maximize stereo imaging of an array. You give up some sensitivity and power handling. It really depends on what kind of sound you want.

With a tall array with even power distribution you get a big sound, like a front row concert seat. Sometimes the audio image is distorted in size, but many like it because it is very different than a point source speaker. The cause is that you can literally hear the topmost and bottom most drivers because their sound is equal in magnitude but arrives to your ears last.

Power tapering generates more SPL from the center of your array, so you don't hear the latest arriving sounds from the end drivers because their SPL is significantly lower. This results in a more normal stereo image while retaining most of the dispersion benefits of an array. You do, however, lose some of the "big" sound characteristic of line arrays.
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Old 1st October 2005, 05:11 AM   #7
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Quote:
Originally posted by simon5
Did you read that document?

http://www.audiodiycentral.com/resource/pdf/nflawp.pdf

Not yet.

Quote:
[i]
You just say plain and simple that power tapering is used to control the secondary lobes when in that document I read the only way to control that is to maximize the Active Radiating Factor via driver selection and placement and it's mostly caused by rectangular drivers not circular ones.

Anyway I'm not an expert, I'll leave you alone to argue with Griffin hehe! [/B]
Well I am quite experienced with microwave antenna arrays and the physics is the same and correlates well so far with my more limited speaker building experiences. I am confident that I can stand by my previous statements.

I have a pair of very fine speaker prototypes here of my own design, that employ 23 drivers in each cabinet. 20 of them are 6" FR drivers in an array that utilizes power taper (aperture distribution) to shape the beam and also control azimuthal sidelobes for improved imaging.

As far as tapering the power to the outer drivers in a vertical line array to reduce temporal smearing of transients (image bloom was a term used), sure it will work but only the same way as dealing with a light beside the TV that is annoying by reducing its intensity. The best power taper in such an array to listen to nearfield without 'bloom' would be to taper the power delivered to all the drivers to zero except the center one. Addressing the time of arrival of sound from each driver, rather than turning down the ones that cause temporal smear is IMO the proper way to deal with smear. In my previous post I suggested two methods in which this could be accomplished in practise.

Rob
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Old 1st October 2005, 07:53 AM   #8
TerryO is offline TerryO  United States
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I have to agree that powertapering is just a stopgap measure to allow the speaker to sound better than it might otherwise. Only time delay or a correct physical alignment will give you the desired results. I have found that a focused array works well as long as you're within the window. I'm not sure how to post pictures, so here's a URL for a picture of mine:

http://speakers.sub-optimal.net/disp...album=16&pos=6

If you look closely it's clear as to the layout.
Best Regards,
TerryO
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Old 1st October 2005, 09:58 AM   #9
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Bessel arrays only theoretically work if you're sitting 10 times the line length away from the speakers, which means far-field listening. The point of using a hifi line array (to me, and going by Jim Griffin's work too) is to deliberately put the listener completely in the nearfield which slews the reflected to direct sound ration almost entirely in favour of the latter. Probably not worth the effort. That said, power-tapering the line is easy enough to do, as it only involves changing how the drivers are connected, so you wouldn't loose much other than 10 minutes of time by trying it. it can compensate for the greater distances from the listener the drivers toward the end of the line are compared to those at the centre of the line.
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Old 1st October 2005, 03:24 PM   #10
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Scottmoose,

Can you direct me to the source of the 10times line length for a bessel? It doesn't make sense to me because at that listening distance you will be in the farfield for most arrays. Even less than one line length of a bessel, it is pretty obvious that the line's dispersion is more like a point source than an array source.

Maybe the 10 times distance is required for a bessel to sound like a point source vs the strange imaging charateristics that I experienced.
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