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Author Topic: 6dB attenuation at multiplying distance by 8.  (Read 5229 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 40º vertical coverage act like 1/2 of an 80º vertical coverage, ie 0º is the on axis hot spot, or is it a tilted down 40º with the real on axis point at -20º?

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 40º vertical coverage act like 1/2 of an 80º vertical coverage, ie 0º is the on axis hot spot, or is it a tilted down 40º with the real on axis point at -20º?

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 0º, or at -20º? 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 0º, or at -20º? 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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Mike Hedden

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Re: Hmmm...
« Reply #10 on: July 22, 2013, 03:50:30 pm »

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
A significant difference is that most all the previous attempts only worked on the HF area. With Synergy Horn technology and Shaded Amplitude Lens Texhnology we are taking this to a much lower frequency.  Physical size of the horn of course playing a huge part as well

Mike Hedden
Danley Sound Labs
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Tom Young

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Re: Hmmm...
« Reply #11 on: July 22, 2013, 04:16:25 pm »

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.

FWIW- Altec had one, as well.

I cannot remember any of these model names.
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Mac Kerr

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Re: Hmmm...
« Reply #12 on: July 22, 2013, 04:44:13 pm »

FWIW- Altec had one, as well.

I cannot remember any of these model names.

Nexo also has the PS series, PS-8, PS-10, PS-15. All of these (Nexo, and the others previously mentioned) theoretically increase the delivered SPL at distances by decreasing the horizontal coverage at the top of the coverage pattern, thereby approximately maintaining the "sound density" if you will. There are several manufacturers of cinema sound loudspeakers who have products like this.

The J1-94 (nee JH90) seems to maintain the horizontal coverage, but increases the acoustical power to the long throw parts of its coverage pattern through the use of vanes. What I am curious about, and what hasn't been made clear to me, is whether or not the maximum mid and high frequency output is truly at 0º with a steady rolloff to -40º where it is then 6dB down.

If this is true it would be nice to see this type of technology used in a smaller lower power speaker that could be used in situations where speakers like the J1 are too big, to heavy, and too expensive. It is understood that making the speaker smaller raises the frequency where the pattern control starts to fall apart and head toward omni, but having a useful 40º of vertical coverage as opposed to 20 useful degrees of a 40º pattern would be a good thing.

Am I wrong here?

Mac

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Mike Hedden

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Re: Hmmm...
« Reply #13 on: July 22, 2013, 05:20:17 pm »

Nexo also has the PS series, PS-8, PS-10, PS-15. All of these (Nexo, and the others previously mentioned) theoretically increase the delivered SPL at distances by decreasing the horizontal coverage at the top of the coverage pattern, thereby approximately maintaining the "sound density" if you will. There are several manufacturers of cinema sound loudspeakers who have products like this.

The J1-94 (nee JH90) seems to maintain the horizontal coverage, but increases the acoustical power to the long throw parts of its coverage pattern through the use of vanes. What I am curious about, and what hasn't been made clear to me, is whether or not the maximum mid and high frequency output is truly at 0º with a steady rolloff to -40º where it is then 6dB down.

If this is true it would be nice to see this type of technology used in a smaller lower power speaker that could be used in situations where speakers like the J1 are too big, to heavy, and too expensive. It is understood that making the speaker smaller raises the frequency where the pattern control starts to fall apart and head toward omni, but having a useful 40º of vertical coverage as opposed to 20 useful degrees of a 40º pattern would be a good thing.

Am I wrong here?

Mac
As the salt patent recently issued more are coming including some ceiling speakers that will no doubt create a stir in Hopkins Stryker land. I'm picking Tom up to go assist ivan, chad and myself with the LSU commissioning and will point this thread out for him to chime in

Thanks

Mike Hedden
Danley Sound Labs
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Ivan Beaver

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Re: Hmmm...
« Reply #14 on: July 22, 2013, 08:56:53 pm »

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

Mac
Here are a couple of coverage maps with a single speaker 25' in the air-facing straight forward.  You can see the down angle.  A straight firing one is in the works.


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A complex question is easily answered by a simple-easy to understand WRONG answer!

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PHYSICS- NOT FADS!

Art Welter

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Re: Hmmm...
« Reply #15 on: July 23, 2013, 11:40:34 am »

Here are a couple of coverage maps with a single speaker 25' in the air-facing straight forward.  You can see the down angle.  A straight firing one is in the works.
The down angle of the J1-90 coverage in the air facing straight forward appears to be at -20º with the -6 dB points at -40º and 0º, as would be expected of a conical horn with a built in down angle of -20º.

This is at odds with the J1 specifications, which state a coverage pattern of 0º on axis, 40º down -6 dB.
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Tom Danley

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Re: 6dB attenuation at multiplying distance by 8.
« Reply #16 on: July 25, 2013, 01:00:14 pm »

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

Hi Corey, Guys
I am sorry I couldn’t answer this earlier but I have been out of town,  but think I can explain the “very old approach”  without using any names. 
If you measure a vertical axis polar plot of a well behaved single horn loudspeaker,  what you see is a single “lobe” or sort of an ellipse which is the equal loudness contour or a line which represents the same loudness. 
 
At the first Synaudcon meeting I went to in the late 80’s, Don talked about the fact one could use the bottom of that contour to make a point source to produce a near constant SPL over a large distance. 
The idea was that one aims the lobe at the farthest seat and then mounts the speaker up in the air so that the bottom of that lobe presents the seating plane with a much more constant SPL than the “on axis” spl with always falls off at the inverse square (-6dB per doubling of distance).

This worked great IF one only needed the frequency range a single horn could produce but there are potential flies in the ointment.   The polar plots one sees are shown at ONE frequency and if one is producing a range of frequencies, then one finds the shape of that balloon can change with frequency and then the assumption this is based on goes out the window.   For example, curved wall horns normally produce a narrower pattern up high vs low and all horns have a “pattern loss:” frequency related to size and wall angle.

 For this to work over a range of frequencies requires the lobe shape to be “constant” over that band and if it’s not, then the frequency response changes as a function of listening position and frequency.      This is one of the things which drove the development of the constant directivity horn, the desire to have the same spl and have it sound as close to the same over the listening area, over a range of frequencies.
For musical reproduction, the frequency span is far larger than what a single driver can cover and also the acoustic power that is needed is often larger than a single driver can produce which then requires multiple sources.   
   
As John Murrays paper on line arrays discusses to a degree, an array of sources produces a Huygens summation into a new wave front but less understood is they also they continue to radiate as individual sources with their individual directivity and so in addition to the desired result , they also radiate a great deal of energy (lobes) in “other “ directions like up down to the sides and rear, directions you do not want the sound to go if one wants to use this old time approach.   

Also with the arrays of sources, one finds a strongly frequency dependant behavior and so what you measure / hear changes a great deal with the position and distance.   Also, that configuration radiating as individual sources produces a complex interference pattern which is a problem if the wind starts to blow, the interference from individual sources is so dense it isn’t noticeable as a discrete effect until there is wind or movement,  or one measured the time aspect when the individual arrivals spread out any impulsive / transient information..

On the other hand “IF” there was a way to produce a wide bandwidth horn that had constant directivity, then the old time approach becomes a very powerful tool in large scale sound for music.
 
“IF” there was a way to make one of these powerful enough for a large space, then one can use it here as well. 
By radiating as a single acoustic source and not radiating a complex interference pattern, then the effect of wind is also greatly reduced and the “time variable” part of musical information is also preserved.

The degree these things effect large scale sound is not generally appreciated and the directivity behavior is what normally limits the effective working distance of loudspeakers and is why systems often need delay rings etc.   Conversely, if the sources are horn loaded and close enough to combine coherently such that they act like one source, it takes far fewer drivers and power to do a given job. 
Here are a couple video examples of such an approach in action in a large space I was just at.  These were from a handheld video recorder and taken at several distances.  Note the one with my daughter showing that if one is not radiating an interference pattern, even a  strong wind has a minimal effect.  Try headphones.

 https://www.dropbox.com/sh/lvtdlbna0fj47vz/JLcYLguKTe

The shaded amplitude sources are a way to alter or shape the underside of that radiation lobe to produce an even more constant spl  over distance but like all horns, there is a pattern loss frequency where the radiation pattern is not constant with frequency.   The upshot of Don Keele’s pattern loss formula is that the large the horn mouth is and/or the wider the horn angle is, the lower the pattern loss frequency is.    The best, most constant results are from the largest horn possible for whatever the needed angle is.
Best,
Tom Danley
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Re: 6dB attenuation at multiplying distance by 8.
« Reply #16 on: July 25, 2013, 01:00:14 pm »


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