PA Sound Installation Advice

Discussion in 'Location Recording' started by audioangel, Feb 16, 2011.

  1. audioangel

    audioangel Active Member

    Currently designing a simple installed PA system for our school hall. It's a big space and quite boomy. I'm looking at Mackie C300Z's with the FRS2800 Amp.

    What I'm wondering is, whether I should go with just a stereo pair or place a second pair further down the hall and possible a third pair at the back? (for flexibility at events)

    It is an very boomy space, however, I have used a pair of MAckie SRM450's with no problem in there, but I could place them more into the centre of the hall. Anything else I should be thinking about? I'm not sure our budget will stretch to subs, so I'm not looking at that right now.

    ALso I'm wondering if I can daisychain two C300Z's to each out on the FRS2800? That would save £600.

    Hall dimensions 17.75m x 12.5m
     
  2. vttom

    vttom Active Member

    I looked up the specs. The C300Z's are 8 Ohm speakers. The FRS2800 looks like it can handle down to 2 Ohms per channel. So, theoretically, you could wire as many as 4 C300Zs in parallel per channel of the FRS2800. The formula for parallel resistance/impedance is 1/R = 1/R1 + 1/R2 + 1/R2 + etc. So 1/R = 1/8 + 1/8 + 1/8 + 1/8 = 4/8 = 1/2 => R=2.
     
  3. dvdhawk

    dvdhawk Well-Known Member

    Can you give us the ceiling height as well as what kind of material the floor, ceiling, and wall surfaces might be? (glass, concrete, wood)


    A room that size can usually be covered using one good pair of speakers with good speaker positioning. The second pair introduces a whole new batch of potential problems. I'd rather start with one pair and use the money you save on finding creative ways to improve the acoustics of the room.

    Is your goal just sound propagation of background music and paging, or are you presenting some sort of program (musical, speech, multi-media) from an area designated as the 'stage'?

    In the first instance, go ahead and spread the speakers out wherever you feel you'll get the best coverage in your big boomy room and you'll have a big boomy mess of inarticulate sound in every corner that should be sufficient for background music and paging.

    However, if you're designating one area as a stage and you would like the sound to have a focal point (for lack of a better word) that coincides with the stage - we need to determine why you're considering the second pair of speakers.

    A) You're not getting sufficient volume at the back of the room
    B) You're trying to get some added clarity to overcome the room's poor acoustics

    In either case if you add a second pair 'further down the hall', that pair will need to be delayed a very specific (calculated) number of milliseconds to compensate for the distance and speed of sound relative to the first pair. Otherwise the second pair, is likely to do more harm than good in the battle for (B) clarity in a reverberant space. Even though the second pair may help project to the back of the hall (A), without a proper delay it will just be louder muddy sound at the back of the hall.
     
  4. audioangel

    audioangel Active Member

    Height wise - Not sure, as least as deep so at least 12m
    Concrete walls, down one side windows with curtains. wooden floor - not sure about the ceiling.

    All of the above, most likely, initially, speech, music playback, and multimedia. It's a very multi-purpose space.

    Basically, at one end of the hall is a stage. We don't use the area of the stage behind the proscenium arch anymore, so often we build staging out in front of the small stage area. It is for this reason I'm considering a second pair. If I position a set of speakers in front of the stage area when we don't have this staging, anyone sitting infront of the speakers will not get the best sound, of course if I position it behind, I begin to worry about feedback.

    What do you use to add delay? or is there a switch on the loudspeakers?
     
  5. audioangel

    audioangel Active Member

    Thanks!
     
  6. dvdhawk

    dvdhawk Well-Known Member

    I'm headed out to finish an installation tonight, I'll check back in on these questions when I get home.
     
  7. dvdhawk

    dvdhawk Well-Known Member

    I'll bet that is a boomy sounding room with the ceiling so high. I'm not sure you'd get much enjoyment from subs in there unless you were able to get really loud.

    POSITION:
    I'd try to keep the speakers well in from the corners. And if it were me, I'd consider mounting them in the horizontal position (horns toward the center of the room). The horns throw a 90˚x40˚ pattern, and I'd prefer to avoid reflections from the side walls by using the narrower pattern horizontally. If you can get them the right height and the right angle of tilt you'll get better clarity front to back too. Building the stage out into the room is going to be a problem you'll have to find a way to deal with. I agree with your assessment of the trade-off between speakers that shoot over the front rows of seats vs. having the mics in front of the speakers causing potential feedback.

    DELAY:
    To add a delay for the second pair you would need a special, very short, delay. Something like a dbx driverack type processor would have an alignment delay and some other handy functions too. (EQ, compression, limiting, feedback suppression). A lot of common digital delays don't go short enough for this job. Or for a little more money than the FRS amp, something like a Crown XTI series amp has some built in processing - including up to 50 milliseconds of delay. Then it's a matter of doing the math. If my math is up to par, 50 milliseconds would be appropriate if the second pair was about 56.5 ft (17.25m) from the primary pair. You would probably want the second pair slightly quieter than the primary too. You'd want them to reinforce, not overpower the primary sound source.

    OHMS:
    Your amp may be able to run at 2-ohms, but the added heat it will generate in the amp + the loss of tight control over the movement of the speaker make 2-ohm loads a last resort for me.
     
  8. audioangel

    audioangel Active Member

    Okay... so I have made a decision, i think!

    I'm going to combine the FRS2800 with the dbx driverack pa+.
    I really wanted to use the FRS2800 alone to power four C300z speakers, but I don't think I can if I am putting the driverack in the mix. So I need 1 of each for each stereo pair? Unless you can tell me how I might work it otherwise.

    In terms of placement, I'm going to place one set either side of the proscenium arch. And the second pair 4m down the hall on the walls. For when the staging is in use.

    The bill for the hardware is beginning to add up, so if anyone thinks I can get away without anything please voice your opinion.
     
  9. dvdhawk

    dvdhawk Well-Known Member

    What are your plans for mounting to the proscenium sides? [in terms of height, mounting brackets, suspension points]

    And how far does that put the speakers from the side walls?
    or, how wide is the mouth of the old stage and is it centered in the room?

    Is a center cluster out of the question?

    Is there any way you could post a photo or diagram?
     
  10. sheet

    sheet Well-Known Member

    Ok, this is what I do now, so forgive me if I get harsh. I am just being direct.

    What you have will not make you happy.

    Your acoustics need to be dealt with first. You need to be concerned with Alcons. A measurement will determine the intelligibility of the room. You need to know for the alarm system, paging system, etc anyway. This is a code issue. Most people will try to eq out the room's time, energy and energy issues and it never sounds good or works.

    Your speaker is not a permanent install speaker. How do you plan to mount it? How you mount that speaker could be a major liability and a violation of the fire code. A speaker should be chosen by application. For large spaces, you will likely need multiple speakers. The big thing will be pattern control down to the low mids. You will need something horn loaded down to 500Hz. The Mackie has poor pattern control at best. It is a plastic box and it radiates lows and mids like a big dog. Buy a contractor series box with a horn loaded mid/lf driver, that has good control. This will also lend itself to being more efficient.

    The Driverack PA is not a discrete left and right unit. If it were, you could use one unit, Channel A for the mains, and Channel B for the delays. It also makes a nasty pop upon power up, dumps it's memory presets, etc. If you get a DBX, get the 260, or better yet a big step up, the EV DCOne. It will provide six outs for the same money.

    I see schools with all kinds of junk installed in a McGiver fashion all the time. It makes me mad. I wish that people would consult someone and do things right instead of spending more money doing things wrong, not saying that the OP is. I am just ranting here.
     
  11. ACIDMAN

    ACIDMAN Active Member

    If you've got height to play with then I would thoroughly recomend flying your speakers. This will help lessen the db difference from the front of the room to the back and you may well get away with only one pair then. Just a thought !!!
     

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