Rational Acoustics



merlijnv
January 5th, 2010, 03:24 PM
Last year, I did a show in a 3000 pax 25 by 50 meter venue. The place used to be an old German tank factory, all concrete and steel. It’s called the “Phoenix Halle” in Mainz, Germany.

We were forced to stack the P.A. left and right of the stage, because there were no means to fly. Since our system has a 90 degree coverage angle, my colleague and I had a brief discussion about the distance of the P.A. stack to the wall.

I felt, since interaction with the sidewall would be unavoidable, that being closer to the wall. Might result in stronger reflections but, also meant that combfiltering would start at an higher frequency. It also would minimize overlap along the center. Specially in the back of the audience. You could say that, I choose to create a coupled point source system with the wall.

My colleague on the other hand, would rather increase the distance to the wall. Speakers facing slightly more outwards. In order to decrease wall interaction at the cost of more overlap along the center and, in my opinion, slightly less stronger reflections that start combfiltering at a considerably lower frequency.

I was curious how you might feel about this.


Kind regards,


Merlijn

Ferrit37
January 6th, 2010, 02:37 PM
Hi Merlijn,
interesting question......
Bob Mccarthy dealt with this in his book (you can buy it here- shamless plug :) ) pgs 143 to 149.
Placing the speakers next to the wall and angling them will of course give you a version of a splayed coupled point source array, which as you move off axis degrades but you do get extra LF.
Moving the speakers away from the wall emulates an uncoupled point source, the distance to the wall has a range shortening effect, and a null in the LF appears at about 30 deg off axis on the wall side (dependant on spacing).
Looking at the two simulations in Bob's book (figs 2.92 & 2.93) it looks like against the wall and angled is the lesser of the two evils.

hope this helps,

merlijnv
January 6th, 2010, 03:19 PM
Hi Ferrit,


Thanks for the response! Mccarthy's book was the very reason I tried it this way. Like you said, under the circumstances, it was IMHO the lesser of two evils. Yet it raised some eyebrows overhere (read The Netherlands). That made me wonder, how readers of the forum might look at this. Since it felt like an unorthodox solution. I have to admit that it worked reasonably well.


Regards,


Merlijn

Ferrit37
January 6th, 2010, 03:57 PM
Hey merlijn,
It's amazing how many people "listen-with-their-eyes"
Now that modelling/prediction programs are available, people are beginning to get a clearer idea of these interactions, but in the end "did it sound better?"
that's what its all about :)

Ferrit37
January 7th, 2010, 01:02 PM
"One man's comb filter is another man's stereo image"

Arthur Skudra
January 7th, 2010, 09:56 PM
The fine art of compromise at play here. Isn't that what professional audio is all about? We manage many variables, and find the perfect medium as our answer. Just curious how much of a delta path difference would you estimate these adjacent walls created in terms of beneficial direct:reflected energy in the affected seating areas? I would guess that if it was beneficial, it would be roughly 10 to 30 or 40 ms difference between the two.

James Woods
January 14th, 2010, 02:46 PM
Hi all.

This reminds me of another kind of dilema, where manufacturers take very different stances:

Lateral Line Array fills at 7 meters (for the first null to be very low in frequency and then "filled in" with room sound)

or

Lateral Line Array fills right next to the main array since the loudspeakers ought to be phase coherant between them.

Although I have my own thoughts, but have found that depending on the setup, venue, and system tuning, that you can get reasonable results with each. However, what works at one venue with a particular system does not necesarily work well at a different gig....

I would have set up the PA closer to the wall and angle in towards the audience. However in smaller theaters with classical music I try to keep the loudspeakers in closer to keep the imaging towards the performers on stage (not really using any sub frequencies at all).

In the end, all of these things are techniques that you can carry around in your bag of tricks. You will have to try each implementation yourself, and decide if it worked or not. In any case, you may be the only one not happy with the result, as the band, the manager, and the audience may not give a ****, which is a different subject altogether.