help pls, challenging design - portable PA


2008-01-28 5:18 am
Hi folks,

I have two daughters who are really big into synchronized swimming (to music at a pool) 11 months of the year, 5 days a week. Since they are so involved, I am too....and one part of that is to be the volunteer "AV guy" for their team.

We have a couple of sound systems, one of which I built (a huge and heavy fixed system), and we are trying to set up another one (a very portable system). I am always looking to improve on our design. There are lots of tradeoffs and challenges. Basically, I want to build a portable PA system that can be carried in ideally on a strap over the shoulder of one of the coaches (this sets the weight limit at max 50-60 lbs). We are constantly going to different pools, so portability and quick setup and breakdown is a big deal. Also, durability, reliability, and SAFETY are huge, particularly using 110v around water.

I do carpentry too so I can build a strong, lightweight, and compact cabinet with an over-shoulder strap for this new system. I am thinking of essentially a suitcase that would sit on a table with a top and maybe one side that could be removed. Lets say the dimensions would be 30x30x7 inches, with a carrying handle or strap on the 7 inch side (right, like a thin suitcase). When the "suitcase" would be laid flat on a table, handle/strap side towards the pool and towards the audience (in an indoor pool watching the swimmers), I would like to have a grille on each side of the handle (each about 11x7 inches) that would have some small, maybe 5 or 6 inch full range, durable speakers pointing towards the listeners.

The speaker chambers should be absolutely as small as possible, as the rest of the cabinet needs to hold 1) a CD player, 2) a small (probably 35w) PA amp, 3) a storage area for storage and protection of an underwater speaker (Lubell brand, see, 4) storage area for microphones, cables, etc, etc.

I think I can deal with all the questions except the speaker and enclosure design. It's something I could easily miscalculate from lack of background. Note at a competitive meet there can be 100-200 people listening to this kind of performance, in an indoor pool. Yes there is all kinds of echo with the music as the acoustics of a pool are not good, but we really shoot for strong CD quality music at a pretty high volume. Even some tight bass down to 80 or 100 Hz would be great if it was possible, but I may be asking too much.

How can I get max SPL AND decent sound quality from say four 5 inch full range transducers in a VERY compact enclosure? To keep it really simple, imagine I am building a speaker cabinet to house four 5 inch transducers and I want the dimensions of the cabinet to be 7Hx23Wx5D.....can I get the SPL? Should I seal the speaker area airtight, or port it, or ?

I appreciate any advice here. Its an interesting design problem.

I used to lifeguard (including syncro competitions) -- you have set yourself a challenge.

Bass extension, efficiency (ie sp you can go louder with power available), and size are governed by Hoffmann's Iron Law... to get one you need to give elsewhere.

7Hx23Wx5D ~13 litre -- that is 3.25 litre/driver.

You could really use more... can you expand the 7" to 8? And instead of a CD for source have you considered an iPod? That could free up space.



2008-01-28 5:18 am
your suggestions

Good suggestions Dave....thanks

I have considered using an IPod...the only downside is getting the coaches to use it. The habit has been to use and flip CDs. We finally rooted out the cassette tapes, which died hard. An IPod would be a good approach for our own team practices, and then we would need to add a CD player for meets.

I did some more scrounging around and found a very portable Fender model 1270 PA speaker. I am gonna buy one used from a guy locally here. Then the challenge of the system will be a lot less complex, and maybe its easier to carry two components, both on handles. The suitcase "sans speaker" should be an easier haul and my worst design challenge is obviated.

Thanks for your ideas!