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Author Topic: 6dB attenuation at multiplying distance by 8.  (Read 5228 times)

Sebastian Rivas Godoy

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6dB attenuation at multiplying distance by 8.
« on: July 21, 2013, 06:31:02 pm »

Hi. This image caused some fun at facebook, so here`s again. Is interesting how using a very old approach we can get just 6 dB variation from 10m to 80m (like 30 ft to 240 ft). That, for the frequency band between like 500hz to 10khz. (beyond that the air absorption  plays a role, and the bass, you already know that his polar pattern is almost omni-directional, so there you have just 6dB per doubling. Anyway, i think this is cool. The math involved in the calculus is simple wave equations and attenuation.
« Last Edit: July 22, 2013, 10:17:36 am by Sebastian Rivas Godoy »
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Corey Scogin

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #1 on: July 21, 2013, 10:37:29 pm »

Is interesting how using a very old approach we can get just 6 dB variation from 10m to 80m (like 30 ft to 240 ft)...

... The math involved in the calculus is simple wave equations and attenuation. And, the polar pattern of the J3, that is the key for doing this, was measured by Pat Brown`s ETC. Regards.

Care to explain the "very old approach" or the math involved?
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Mac Kerr

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #2 on: July 21, 2013, 10:42:14 pm »

Care to explain the "very old approach" or the math involved?

And do it without using Danley products as an example. Since you work for Danley you are not permitted to promote their products here. You may answer questions about them when a question about a product has been asked.

Mac
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Doug Fowler

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #3 on: July 22, 2013, 01:18:08 am »

And do it without using Danley products as an example. Since you work for Danley you are not permitted to promote their products here. You may answer questions about them when a question about a product has been asked.

Mac

I'll chime in as well: this is NOT permitted.  Please read the rules regarding manufacturers' participation, and abide by them.

Thank you for your cooperation. 
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Sebastian Rivas Godoy

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #4 on: July 22, 2013, 10:25:47 am »

Ok please excuse me about the logos and names of boxes. I edited strongly the post to take off logos and brand names. The thing is that you don`t need exclusively one brand to do this, and the explanation is simple. You use a constant directivity box and you choose a heigth and rotation angle so that the last rows of the audience get the benefit of being "on axis". On the other hand, the first rows obviously get more spl because of the they are near to the source. But, they also receive attenuation due to their off-axis location. That way we can get this even coverage of 6dB variation from 1x to 8x. Even if you can`t get 6dB, certainly using this you will get less than the 18dB that would result from duplicating 3 times distance.
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Mac Kerr

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #5 on: July 22, 2013, 11:50:24 am »

Ok please excuse me about the logos and names of boxes. I edited strongly the post to take off logos and brand names. The thing is that you don`t need exclusively one brand to do this, and the explanation is simple. You use a constant directivity box and you choose a heigth and rotation angle so that the last rows of the audience get the benefit of being "on axis". On the other hand, the first rows obviously get more spl because of the they are near to the source. But, they also receive attenuation due to their off-axis location. That way we can get this even coverage of 6dB variation from 1x to 8x. Even if you can`t get 6dB, certainly using this you will get less than the 18dB that would result from duplicating 3 times distance.

This is a good illustration of the advantage of flown PA over ground stacked PA. As long as the off axis response is good enough to cover the audience with the on axis aimed at the back of the audience you get the benefit of less differential in the distance, and the natural roll off off axis. The higher you can fly the PA the better for minimizing the distance differential.

While I'll probably never see one in real life, this begs a question of the Danley folks about the JH-90. Does the 40 vertical coverage act like 1/2 of an 80 vertical coverage, ie 0 is the on axis hot spot, or is it a tilted down 40 with the real on axis point at -20?

Mac
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Mike Hedden

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #6 on: July 22, 2013, 12:32:50 pm »

This is a good illustration of the advantage of flown PA over ground stacked PA. As long as the off axis response is good enough to cover the audience with the on axis aimed at the back of the audience you get the benefit of less differential in the distance, and the natural roll off off axis. The higher you can fly the PA the better for minimizing the distance differential.

While I'll probably never see one in real life, this begs a question of the Danley folks about the JH-90. Does the 40 vertical coverage act like 1/2 of an 80 vertical coverage, ie 0 is the on axis hot spot, or is it a tilted down 40 with the real on axis point at -20?

Mac
Hi Mac,
The J1's horn design is indeed 1/2 of 80 so one can to use all 40 degrees of vertical so that the zero axis is the aimed at the farthest seats and you aren't throwing the top half of the  horn away. It also has a -20 degree down tilt built in as the original idea was the cabinet height would be 20-30' above the listening plane and the back wall was 150-200' back.  Its rarely used in those applications as they immediately found homes in stadiums many times larger but its the same idea.

Thanks

Mike Hedden
Danley Sound Labs, Inc.
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Brad Weber

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #7 on: July 22, 2013, 12:47:54 pm »

This is a good illustration of the advantage of flown PA over ground stacked PA. As long as the off axis response is good enough to cover the audience with the on axis aimed at the back of the audience you get the benefit of less differential in the distance, and the natural roll off off axis. The higher you can fly the PA the better for minimizing the distance differential.
It's also a good example of why not just the nominal coverage but the actual pattern and how it varies with frequency can matter not only off axis but even beyond the nominal pattern.
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Mac Kerr

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Hmmm...
« Reply #8 on: July 22, 2013, 01:04:36 pm »

Hi Mac,
The J1's horn design is indeed 1/2 of 80 so one can to use all 40 degrees of vertical so that the zero axis is the aimed at the farthest seats and you aren't throwing the top half of the  horn away. It also has a -20 degree down tilt built in as the original idea was the cabinet height would be 20-30' above the listening plane and the back wall was 150-200' back.

OK, I'm confused. Is the highest high frequency output at 0, or at -20? You seem to be saying both in your answer.

Mac
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Art Welter

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Re: Hmmm...
« Reply #9 on: July 22, 2013, 02:03:12 pm »

OK, I'm confused. Is the highest high frequency output at 0, or at -20? You seem to be saying both in your answer.

Mac
Mac,

The JH-90 nominal coverage pattern is  90 horizontal x 40 vertical (0 on axis, 40 down -6dB).
Wish DSL would just hire me ;^).

The problem with a typical single "constant directivity" source is the  higher you fly it the greater the  disparity between HF and LF  level in the front seating coverage areas.
Since  a single constant directivity source needs to be directed to the most distant coverage area, the upper half of the coverage energy is put in to the reverberant field, or wasted in the outdoor sky.
Products like the DSL DH-60 and JH-90, the EVI-15, JBL's Screenarray series, various Peavey downfiring horns all address that problem to some degree.

Mid 1980's JBL made an "unusual looking" horn similar in concept to EV's Variable Intensity horn which has a more narrow "long throw" upper portion blending to wider "short throw" downfiring section.
The look of that horn gave it an unpopular nickname, can't remember the model number but it seems to be missing from internet history.

Art
« Last Edit: July 22, 2013, 02:09:40 pm by Art Welter »
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ProSoundWeb Community

Re: Hmmm...
« Reply #9 on: July 22, 2013, 02:03:12 pm »


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