Rational Acoustics



merlijnv
September 17th, 2010, 06:05 AM
Hello everybody,

Yesterday we did an open-air concert at a market place for aprox. 2000 visitors. We use the AXYS (Duran Audio) Target U-16 system. The crossover frequency between mains and subs is at 160 Hz. We flew arrays of 12 speakers left and right. I've used 4 dual 15" subs to create a horizontal sub array. The few subs left me with only 2 processing channels, but fortunatly the marketplace was relatively long and narrow. The sub array was phase alligned to the mains. This let to a nice even LF coverage throughout setup and soundcheck.

At 19h00 hours the seated audience gets in and show starts at 20h00. Right from the start I notice something was missing. I quick glance at smart showed me that the entire octave around de crossover frequency dropped aprox 6 dB's. Below 100 Hz everthing seemed to be allright. Bass and plosives where coming through. And form 250 Hz and up things where also ok. During the pause me and my colleague at monitor world, reasoned that the subs upper frequency's didn't get through the crowd. This left me with no other option than to use a low shelf on the mains in order to restore some of the warmth and intimacy, forcing something out of the mains outside it's opperating range (fortunatly we work at relativly low levels). Since pushing the subs would only have let to more stagewash and part of it was apparantly not getting through anyway. The solution worked though I must admit/guess at a cost fore the first 10 to 15 rows or so.

This let me to believe that frequencies of around 80 Hz or maybe a little higher. Didn't get through the audience argubely because of absorbtion/obstruction in relation to wavelenght, by the crowd. Resulting in losing half the contribution to my crossover. Interresting experience and next time I will definetly fly some subs with my mains.

What are your experiences in this matter? Is there a glass ceiling to the low frequencies you can force through a crowd? Or is it just a question of raw power. If so, how do you set it up in a empty venue? Since this might result in a non-flat response.


Your thoughts please,


kind regards,


Merlijn

Rasmus Rosenberg
September 18th, 2010, 01:20 PM
4 dual 15" for 2000 people??

merlijnv
September 19th, 2010, 06:09 AM
Hi Rasmus,

I should have added that we're talking about theater not rock. One Spanish guitar, a violin and two vocals. The area was aprox. 40 by 40 meters and my normal average level is about 83-86 dBc. It's the same setup we use indoors in 500 to 2.500 pax theaters and has thus far never been a problem. I'm aware that for this specific gig it was tight, since open-air abides to different rules. However it was all we could get under the circumstances. We did a similar concert last year for aprox 3000 people where we flew everything. Back then we used one hang of 16 speakers and 3 dual 15” subs on each side. I’d like to think it worked great for our production. This experience let me to believe that the setup mentioned in my first post, would work in terms of power for this gig. Though I wouldn’t have minded having more subs at my disposal. My intention was to improve LF coverage, by arranging the same amount of subs differently. Your response suggests that it is a matter of power. I'm curious to your experiences.

Kind regards,

Merlijn

Rasmus Rosenberg
September 20th, 2010, 04:07 PM
Hey Merlijn,
Hope you know, I meant no offence with my comment. Your set up is a bit backwards so I find it a bit hard to comment. I have had audience lower/absort the sub energy but not in the 6 db range. Also I haven't worked with speakers with such a high crossover frequence in years (that was Turbosound flash/flood 125-180hz) and back then we stacked the subs high and narrow or up on scarfolds. Covering 40x40 meters (not very long and narrow? or was that the other gig) with a center subarray it would take a hole lot more than 4 subs. I would think that 4 subs, spaced 1.3 meters apart (150 hz control) would cover maybe 7 meter wide (-6db) and maybe -12 db at 20 meters. With out audience. Try to play with it in Mapp (the meyer online LAC) or others.
What I like the best, is to have an array that can produce and control as much low energy as posible (you can "always" turn the mid/high down). Also when ever possible try to fly the subs. In my experince it takes more subs to do center arrays (or other) than L/R stacks. But again it, all depends on the audience area and logistik.
mvh
Rasmus

merlijnv
September 20th, 2010, 05:55 PM
Hi Rasmus,

Thank for the response and no insult taken at all. I agree with you and regret, regarding the high crossover frequency. However the system I’m using is the legacy of the former engineer, rental company and long lasting friendship that requires delicacy. However this could be resolved by flying both subs and mains using their default XO frequency and complement with extra subs at a lower XO frequency for horizontal array purposes in the future. However at the cost of extra equipment to compensate for the high XO frequency.

I stand corrected regarding the dimensions I mentioned. I reviewed the original simulation and the area was 20x40 meters. Sorry for the confusion. Like you mentioned, I started planning for this very gig with MAPP, using 600-HP’s because they best match our dual 15”s. I wanted control up to 100 Hz and used a maximum displacement of 1/3 wavelength. Resulting in aprox 1.14 meters from driver center to center or 55 cm from cheek to cheek in case of 600’s and 40 cm for our own system. You might say that I’ve been a little over cautionary and ˝ wavelength would have sufficed. Under the circumstances I was pretty content with the MAPP prediction. I ended up using 2.7 ms delay for the outer subs. Made extensive comparisons between hor sub array and conventional left/right at 100, 80, 63, 50 and 40 Hz. In both scenario’s I ended up having 9 to 12 dB’s loss from front to back. First row being at 3-4 meters. Nothing I could solve without extra equipment. However in the hor sub scenario the level drop left to right in consecutive rows was limited to 3 dB for the majority of the audience, without the typical nulls. Next I entered the same values in the proprietary software that comes with the system and the results were identical. I’ve attached screenshots from both programs.

Having learned from both experiences I’ll likely end up using both solutions next time, if production costs allow for it. However my original question seems to remain unanswered. Because I fail to see how more subs at the same level would somehow manage to get the 80 to 160 Hz octave through the audience. Maybe one could state that an average human, is an about 1.80 meter tall bag of water and hence absorbs some frequencies that I seemed to have “lossed” during the show. Or that compact groups of people either seated or standing don’t pass through certain bass frequencies. This of course being evident for higher frequencies. Resulting in a upper limit to the practical bass frequencies you can force through an audience. Having no resort but to send the remaining bass frequencies over the audience.

Kind regards,

Merlijn

P.S.: I’m still very much interested in anyone’s experiences. So please feel free to join in.

Ferrit37
September 20th, 2010, 08:16 PM
Hey merlijnv,
Most of the studies on Absorption seems to be based on octave spacings with nothing much below 125Hz,
see: http://www.sea-acustica.es/Sevilla02/arcgen016.pdf
and also : http://www.leoberanek.com/pages/sabineandeyringeq.pdf as typical examples.
In my experience it is the upper region of the subwoofer band that exhibts these problems. Flying the arrays gets better audience penetration.
It seems below 60Hz(ish) gets through clean.

merlijnv
September 21st, 2010, 05:57 AM
Hi Ferrit,

I suspected as much. The absorption seems to be relatively benign at those frequencies. Which leaves me thinking about causes like temperature, obstruction and diffraction. Although any changes in the speed of sound caused by temperature changes, seem to have little influence on the crossover behavior. At least not to offset it to the extent I have witnessed. During setup and sound check the chairs weren’t in place yet. However these being little open wooden folding chairs, I find it hard to believe that these would create such a big difference in relation to the proportionally larger wavelengths. What I found specifically interesting was the so called “Seat Dip Effect” described in one of the papers you mentioned. In our case the bottom of the mains was hovering a mere 1,2 meters above the audience. Over a distance of 40 meters that corresponds to an angle of 1,7 degrees. Pretty grazing I would say. When I think about it, I think I experienced it very clearly last week again. During the encores the venue was half empty and I instantly had to adjust all my lo-cut filters to compensate for the increase, or should I say return, of LF. However this seems to be a geometrical problem like comb filtering, and hence can’t be solved by equalizing. Since we play 100% of our time for a seated audience this is something I’ll keep an watchful eye on for the future. I’m inclined to think that the system arguably might have been suffering from both the “Seat Dip Effect” on the mains and obstruction/diffraction of the subs. Resulting in the 6 dB drop around the 160 Hz octave.

Kind regards,


Merlijn

Ferrit37
September 28th, 2010, 06:27 PM
Hey Guys,
just got Home from Zac Brown Band/MLA tour, where in a couple of arenas I tried a mono stack cardioid arrangement in front of the stage, it actually replaced the center section of the barricade. the "null" was timed to point up toward the stage/mic position and the outer elements of the wall were delayed to spread the beam out.
during the day (no audience) 20-100Hz was no problem. But when the "pattern-modifiers" (the audience) were installed (GA floor) the octave above 60Hz was absorped and when I went down to the stage you could here these frequencies being reflected back to the stage.
I changed the crossover freqs 20-60Hz on the subs and used 60Hz and up from the mains. this seemed the best compromise.

merlijnv
September 29th, 2010, 05:10 AM
He Ferrit,

Interesting experience. Last week I saw U2 360, standing aprox. 25 meters to the right of F.O.H.. I’ve never experienced sound that loud, that well. After the show I did some Googling . There sub setup seems to be entirely orientated at 63 Hz and below. Underneath the B stage they have 36 pairs of “2-element cardioid” proprietary Clair S4S sub (4x 18” per cabinet) over a aprox. 147 meter circumference on centerline. In other words at aprox. 4 meter intervals , which at half wavelength compares to 40 Hz. Each pair was spaced at 54” or 1.37 meter, which at quarter wavelength compares to 63 Hz. Makes you wonder if 63 Hz is the upper limit to LF that you can send/force through an audience.

Regards,

Merlijn

Ferrit37
September 30th, 2010, 03:16 PM
hey Merlijnv,

try : - http://www.essex.ac.uk/csee/research/audio_lab/malcolms_publications.html

scroll down to C139 SUBWOOFER POSITIONING, ORIENTATION AND CALIBRATION FOR LARGE-SCALE SOUND REINFORCEMENT, Hill, A.J, Hawksford, M.O.J, Rosenthal, A.P and Gand, G, AES Convention, May 2010

under conferences papers patents.

merlijnv
September 30th, 2010, 04:52 PM
Hi Ferrit,

Great document! Especially the part about the 4 piece clusters. Of course I immediately thought of Dave Rat’s “Vortex” setup. Whom they mentioned two paragraphs later. The way I see it now, is that you arguably have to make a compromise between: A) good horizontal coverage up to aprox 63 Hz with a evenly spaced ground solution and or L/R stacks/clusters, arguably risking absorption/diffraction/reflection by audience from aprox 63 Hz and up. B) good front to back coverage by means of flown sub systems without the previous mentioned problems at cost of a power alley (center flown subs not included) and power loss due to the lack of ground coupling. Great food for thought and lots to think about.

Thanks and regards,

Merlijn