Acoustic Issues

[munster]

Hi,

 

Just wondered if someone could offer some advice. I have been asked to design a PA for a Les Mis production in an old fort. The whole event will take place in a horse shoe shaped space. The flat edge or top of the 'U' shape will have the stage situated there. This will then face the 'U' shape. Behind the stage is a grass slope to the top of the fortress. The 'U' shape is a twenty foot concrete wall.

 

My concerns are the reflexions the speakers will produce bearing in mind I am using up to 14 lav mics. The area is roughly 80yards by 55yards.

 

My thoughts are to go with three Arcs a side and one SB218 all L'Acoustic. Why L'Acoustics? Because I am very familiar with them and easy to get hold of in my area. Accurate directivity whilst keeping the volume low (ish). As it is open air I feel that it should be able to handle the conditions if the wind picks up. We are right next to the sea.

 

Ideally the PA would point to the grass slope but his is not possible due to other factors such as power and postion of sun for lighting (there is no roof on the stage).

 

I really need to know if anyone has experience of working in similiar environments and what I can expect from working at medium to high SPL's in a fort.

 

 

Many thanks

 

Munster :D

[Rob_Beech]

Coverage and dispersion is the key here.

 

You need to ACCURATELY work out where the sound needs to go, but equally importantly you need to look at where is DOESN'T need to go.

 

Remember the ARCS boxes are only 20degrees, is 3 per side going to be enough to cover the 55yard width evenly? SPL wont be an issue here, although SPL before feedback is something that you need to look at.

 

I'm not one for suggesting such grossly over rated designs but there is a possibility that the dvdosc would be a better bet? (sticking with the same manufacturer that is, there are other systems I'd spec infront of that).

 

 

As for subs, whilst I appreciate that the LF content wont be high, 80yards is a lot to cover outdoors with just a pair of subs. 6dB for doubling them!

[Bobbsy]

I'd second what Rob says here. Carefully controlling your coverage is going to be the key to getting a good sound here and some protractor work (or modelling software work if you have it) is needed before deciding on the cabinets to use.

 

However, one thing you said jumped out at me, namely when you described Les Mis as a "medium to high SPL show". I've mixed Les Mis several times and love it...but at most I'd describe it as a medium SPL show, at least in SR terms. Certainly, on the solos and duets I've always gone for a "natural" rather than "amplified" sound...and on the big choral numbers everyone remembers, if you cast is half way decent, they'll take care of most of the SPL (which can get pretty loud) and your job will be to add a bit of fullness and emphasise the odd solo line rather than blast the audience out of their chairs. A reasonable expectation of SPL will certainly help your problem of potential reflections aid in the prevention of feedback.

 

Bob

[munster]

Hi thanks for replies. I am sure that the horizontal coverage is more than adequate. I guess that two 218's a side would be a better option. The show is actually a very highly regarded youth am dram. There will be the usual problems of gain into mic.

 

I don't intend to blast out the audience I just wnat to make sure I have enough headroom in the system.

 

I do know a v-dosc engineer do I guess I'll test the waters with regards the software.

 

Thanks any more advice would be welcomed.

[Rob_Beech]

3 ARCS is only 60degrees horizontal. My concern would be the middle of the front rows, and the edges of the front rows. The points where the sound is still a narrow beam.

 

Whilst the coverage would be better further back, a more off axis response would greatly reduce SPL. 55yards wide is a fair bit, especially against 80yards long.

[soundiesam]

this might sound really silly but how is the audience going to be sitting in this performance space and if its chairs how r they going to be spread out?

[johnhuson]

3 ARCS is only 60degrees horizontal.

 

Just for accuracy a single ARCS enclosure is 22.5 degrees so three would actually be 67.5 degrees, although I'm sure the OP already knows this if he's experienced in using the system.

[Rob_Beech]

You are quite right, though of course, you own some and I don't :o

[munster]

this might sound really silly but how is the audience going to be sitting in this performance space and if its chairs how r they going to be spread out?

 

There are PA wings so the audience will go out no further than the outside Arc speaker. The outside arc will be square with the stage. The audience will be lined in rows from the stage Mix position is three quarters of way between the grass slope and the wall . Hope this is clear :)