Calculating PA Size

[pulsemobiledisco]

Hi All

Hope you can help. Now I know that this will vary depending on room size and various other factors, but what would be a good calculation to use to give me a rough idea on the size PA needed for a gig.

The reason I ask is because I am doing a 6th form leavers prom at a posh private school in the middle of May for 300 people. Being a 6th form prom I'm sure they are going to want it loud so don't want to be underpowered.

My standard rig is 1.5K which has never let me down, but I've only ever used that for upto 200 people, anything bigger than that and I'm usualy working at my resident hotel and I use their PA for that.

I can push the output up to 3K by doing some mixing and matching with some backup/spare kit I have.

Thanks guys

Sam

[BBoy-Gnash]

Hi All

Hope you can help. Now I know that this will vary depending on room size and various other factors, but what would be a good calculation to use to give me a rough idea on the size PA needed for a gig.

The reason I ask is because I am doing a 6th form leavers prom at a posh private school in the middle of May for 300 people. Being a 6th form prom I'm sure they are going to want it loud so don't want to be underpowered.

My standard rig is 1.5K which has never let me down, but I've only ever used that for upto 200 people, anything bigger than that and I'm usualy working at my resident hotel and I use their PA for that.

I can push the output up to 3K by doing some mixing and matching with some backup/spare kit I have.

Thanks guys

Sam

We use 1k a hundred and it has always worked for us :)

[BeerFunk]

It's not something there is a finite answer for, there are so many variables involved. This is also where 'brand names' can make a difference (following on from Callum's thread (http://www.mobilediscodirectory.co.uk/forums/showthread.php?t=6672)). A Peavey or Mackie PA system is always going to be more efficient than a 'budget-end' branded system, watt for watt.

[flatliners]

there are alos lots of other factors to take into account size of room shape of room hight of room and position of speakers i think its about 100 watts for 10 people or something but not to sure

[bluishbob]

I used 1.5k for 250 in a large school hall...was quite ok but was on full most of the night.

[BeerFunk]

there are alos lots of other factors to take into account size of room shape of room hight of room and position of speakers i think its about 100 watts for 10 people or something but not to sure
As I said, you can't come up with a simple ratio to work with, because of all the variables involved, as you mentioned too.

[Danno13]

Just take way more than you need.. you can always turn it down.

I'd go for at least 4 subs and some decent high SPL tops though...

I'm doing a prom in June in a massive school hall, and will probably hire/beg/borrow 4 18" bins and use them alongside my MA F10s, which should keep up easily.

[BBoy-Gnash]

It's not something there is a finite answer for, there are so many variables involved. This is also where 'brand names' can make a difference (following on from Callum's thread (http://www.mobilediscodirectory.co.uk/forums/showthread.php?t=6672)). A Peavey or Mackie PA system is always going to be more efficient than a 'budget-end' branded system, watt for watt.

OK, we use 1k per 100 active Mackie 450/1501 :p

[gillo]

its not the amount of people its the size of the venue? obvoiusly a large hall with 10 people 100 watts isnt gonna be enough speaker positioning is also important if you place a sub and sat in each corner of the room then that will be more efficent then having 8 subs and 8 tops positioned in one place.

[colint]

It dosen't really work with the watts to people thing if you had a long through system (i.e. horn loaded) such as the void system you could proberly get away with half the amp power most Dj's would use there will never be a fixed format to work this out because each venue is so different even the hight of the stage can make such a big change in the sound!

[PropellerHeadCase]

As mentioned number of people and size of room are major and independent (or interdependent, if you want to look at it that way) factors. Height of room and number of bodies changes the amount of power required for the mid-to-high range, whereas number of bodies and length/breadth of room effects the over-all power required.

Another question is are you talking watts at the speakers or watts at the amps? Generally speaking your amps should be twice the ampage of your speakers. My speakers are 400W running off 750W per channel and my sub is 600W running off 1000W - I haven't had to turn the amps past three so far ;)

There was a post a short while ago discussing dimensions/numbers versus wattage and I think that was a reasonable rough guide for calculating amplifier power.

[Vibrant Sounds]

speaker positioning is also important if you place a sub and sat in each corner of the room then that will be more efficient then having 8 subs and 8 tops positioned in one place.

No actually you are wrong!

With Bass speakers particularly it is best practice to put them all together to utilise acoustic coupling and also prevent the cancellation of some of the sound waves when they meet in the middle if you deploy your bins as two satellites below your mid/tops. If you put two similar Bass Bins together of say 400watts each you will with the effect of acoustic coupling increase the audible power by about a factor of 3 over the power of one bin. If you seperate them, you will only increase the power by about 1.5 times due to some cancellation of the sound waves. No brainer really!

2x 400 watt bins apart= an audible power of about 600ish watts (1.5x)

2x 400 watt bins next to each other (acoustically coupled)= an audible power of about 1200ish watts (3x) Sound wave is an oval rather than a circle and of much more power.

Bass is multi=directional so it does not matter where you put them, but it is better to keep the bins together if possible. It might look better with them spread apart and the mid/tops above them, but it will not be better for either your sound or your power in that orientation.

Mid/highs frequencies are much more directional so it is better to have these speakers at the periphery of your rig with the mids about head height and the horns slightly above head height.

Dont believe me?

Throw two stones into a calm pond about 1 mtr apart and watch the ripples, see them cancel each other out where the two sets of ripples meet in the centre.

Now get one larger stone equal to the surface area of the two smaller stones (does not matter about weight) and throw that in, look at the size and power of the ripples.

[Solitaire Events Ltd]

I'm not sure you're getting what Gillo is saying Ed.

I think he's saying that the spread of sound would be better if you have cabs in each corner, and therefore the volume wouldn't need to be as loud, therefore making the overall sound more efficient.

I'm sure he'll comment though.

[spin mobile disco]

Nice analagy vibrant. On a similar subject how would you rate the wattage of a rig? by amp power plus speaker power? speaker rms? speaker peak?
I would like to work out how we all work this out.
I for example run 2 amps one as my main one as backup that can also power subs when used. The main is 1000 watts rms continuous the backp 625 watts rms continuous both at 8 ohms
the speaker i now use for subs are peavey 15st's at 175watts rms each (700 peak) and the tops are mackie s300z's 300watts rms each (750watts peak) both pairs are 8 ohms.

How would you rate that in terms of rig power? I knwo that it is suitable for over 300 people as I have used it for this amount before. but I wouldnt say it was a 3K rig would you?

Just interested thats all please give me your opinions or what you would class it as.

[Vibrant Sounds]

I'm not sure you're getting what Gillo is saying Ed.

I think he's saying that the spread of sound would be better if you have cabs in each corner, and therefore the volume wouldn't need to be as loud, therefore making the overall sound more efficient.


And I'm saying it won't be!

It may sound better at the periphery (which would be good if all your guests are just sitting around the walls), but it will not sound better further away from the speakers and especially in the centre of the dancefloor where the bass frequencies are being cancelled out, so you actually have to turn up the volume to get the desired effect on the dancefloor. This may well then lead to the bass at the periphery being overpowering and not give a nice sound.

Still each to their own. We all do things differently, and if he is happy with his symmetrical outfit then that is fine. Horses for courses.

But:

It is not more efficient to separate bass speakers, you will not get a better sound by separating bass speakers, and you will have to use more volume to get the same end result when you separate bass speakers. These are all physical facts, not perceived ones.

I have just told you what happens in physics, which is a fact. You can believe me or not, try it or not, it is each persons choice.

I think you will find though that your sound has a much better balance to it and is enjoyed more by those people who are not on the dancefloor if the bass has more power on the dancefloor (where you really want it) and tails off towards the periphery of the room than the other way around.

[Vibrant Sounds]

Spin,

The amount of power that the amplifier can give out is how you define the power of your rig. So, If you have a 1000 watt per channel amplifier and a 500 watt per channel amplifier then the maximum output of your rig will be (under perfect conditions) 3000 watts RMS.

This does not equate to how loud it will go, that is dependent on the efficiency of your speakers. And a 1000 watt speaker that is designed with an SPL of 100db will be 1/4 as loud as a 1000 watt speaker designed with an SPL of 106db. 3db=a doubling of power.

So a 200 watt speaker powered by a 200 watt amplifier with an SPL of 121 will be louder than a 400 watt speaker powered by a 400 watt amplifier with an SPL of 115. (SPL = Sound Pressure Level).

If I understand your post correctly, then you have a 1000 watt RMS amp running into a pair of S300Zs (please tell me that this isn't 1000 watt rms/channel) and a second amp powering your Peavey subs which you say is 625 watt rms (again please tell me that this isn't per channel).

If these two amps are 1000 watt per channel and 625 watt per channel then it would seem you have an amplifier system capable of a power of 3.25kw RMS. Whereas your speakers have a maximum peak of 1500 watt for the S300Zs (600 watt RMS) and your bins have a maximum peak of 1400 watt (350 watt RMS).

So yes your system would be capable of running at 3kw but for how long?

The bins are probably more likely to survive better with the smaller amp, I would not be too sure about the tops if you are regularly running a 1000 watt RMS through a speaker designed for 300 watt RMS.

If the amplifiers are mono and you are splitting the power into the pair of speakers, then the system is more balanced and only about 1625 watts rms. Considerably less then 3.0kw.

As for whether a system that is less than 3.0kw is suitable for 300 people, it really depends on the venue. I have done gigs with my compact system (800 watt RMS total but with high SPL speakers) of over 150 people and had no problem. The Bose L1 system will cope comfortably with upto 200 people yet it is only rated at 750 RMS total.

Your question and the possible mismatch of amplifier to speakers is why I endorse the use of quality powered speakers, as all the hard work of matching both amplifiers, x-overs, speakers and cabinets has all been done by the manufacturer.

[Solitaire Events Ltd]

Your question and the possible mismatch of amplifier to speakers is why I endorse the use of quality powered speakers, as all the hard work of matching both amplifiers, x-overs, speakers and cabinets has all been done by the manufacturer.

Another satisfied Mackie user. :D

[DJ Frankie]

Are you trying to blast out the whole venue or isnt the idea just to have enough sound around the dance floor? I would have thought 1.5K would be more than enough!!!!

[Ricesnaps]

WOW - what an interesting and informative thread!

Just been doing some investigation and my Mach M-Flex 15's have an SPL of 130db, which is comparable to the Mackies, so I'm feeling well pleased now!

[CRAZY K]

Hi All

Hope you can help. Now I know that this will vary depending on room size and various other factors, but what would be a good calculation to use to give me a rough idea on the size PA needed for a gig.

The reason I ask is because I am doing a 6th form leavers prom at a posh private school in the middle of May for 300 people. Being a 6th form prom I'm sure they are going to want it loud so don't want to be underpowered.

My standard rig is 1.5K which has never let me down, but I've only ever used that for upto 200 people, anything bigger than that and I'm usualy working at my resident hotel and I use their PA for that.

I can push the output up to 3K by doing some mixing and matching with some backup/spare kit I have.

Thanks guys

Sam

Im not getting too involved in this but I agree that you probably wont need blistering volume all round the room, presumably more at the front near the stage?

What I do think is it must be quite difficult to have speaker cables going to four subs all round the room---not only Elf and Safety problems but also some clever clogs disconnecting the speakers or EVEN cutting through the cables:eek:

The time to set up this would also be a pain.

Yes ive got the T Shirt and been there :D

CRAZY K

[spin mobile disco]

Just to clarify it is 1000 watts as in 500 watts rms per channel and the 625 is 310 rms per channel at 8 ohms both amps can do 4 ohms as well so i could for example run both pairs of of one amp to lower impedence and get more power out of one amp (about 600-800 rms per channel if i remember)
Tha makies are recomended for about 400 - 500 watts for music and I find there is plenty of headroom for peaks. Mind you with a larger audience you have to use subs to counter the powerful highs of the mackies.

[pulsemobiledisco]

Im not getting too involved in this but I agree that you probably wont need blistering volume all round the room, presumably more at the front near the stage?

What I do think is it must be quite difficult to have speaker cables going to four subs all round the room---not only Elf and Safety problems but also some clever clogs disconnecting the speakers or EVEN cutting through the cables:eek:

The time to set up this would also be a pain.

Yes ive got the T Shirt and been there :D

CRAZY K

Thanks for all the responses guys. I've been thinking about this and I think Subsonic and Crazy K are correct. Only needs to be loud on the dancefloor and 1.5K should handle that OK. I'm going to look at the room at the weekend though, just to be certain.

[Vibrant Sounds]

Just to clarify it is 1000 watts as in 500 watts rms per channel and the 625 is 310 rms per channel at 8 ohms

Thank god for that, you did have me a little worried!:D

[spin mobile disco]

The amps also list a typical bass music power output as much higher than the rms, i guess that is something that helps the signal peaks.

[Tom]

if you look at a touring pa system, lets say download. Notice that the subs are in a line across the bottom of the stage, more like on the floor from one side to the other. if you need to project bass its best is you keep the subs in a line, close to the floor and not 2 over here and 2 over there because thats where the other speakers are. we all know that to get you sound further you have to rise your speakers. well if the disco has to fill a big hall then rise your speaker higer then 6ft (if your stand can handle it???). i recently did a disco for about 300 people in a hall the size for about 600 people. i only have 800 watts total system with no subwoofer but because i raised the speaker i was able to project the sound further. yes i agree that the speakers were at there limits because i have to increase the bass as there was none but over all the sound was ok. so really it all depends on the size of the venue that you are playing the music in.

[jamesh]

Just take way more than you need.. you can always turn it down.

I'd go for at least 4 subs and some decent high SPL tops though...

I'm doing a prom in June in a massive school hall, and will probably hire/beg/borrow 4 18" bins and use them alongside my MA F10s, which should keep up easily.

Did the beg borrow of the 18" bass bins work dan :D ;) :P :whistle:

[Thames Valley Discos]

Well i did a similar bash in June. Used 1.5K 300+ people in very very large function room. I had no issues with the sound.
I think some of us get carried away with what k rig we need.
So long as it sounds clear and loud enough by the dance floor, thats all thats needed.
Don't forget we are mobile discos , not fixed installs in large clubs.

[Tom]

Don't forget we are mobile discos , not fixed installs in large clubs.


true but dont forget that different people require different size system's. thats why i think one dj should be able to cope with anything thrown at them. the dj can either hire or buy the equipment. in my case i want to buy all my equipment. i am aiming for the 6.4k

4x DB Sub 15 (800 watts rms each)
8x DB Arena 15 (400 watts rms each)

But also in time i will be a live sound engineer for band hire like download but a hell lot smaller