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DariusMyszka

Joined: 20/07/12
Posts: 4
How to calculate amount of speakers needed
#998930 - 20/07/12 11:37 AM
Hi everyone, im new here and i had a few questions that i think the community could help me out with

First off, how can i calculate the amount of speakers i need for an event for x amount of people?

Say i have:
http://www.yorkville.com/products.asp?id=1&cat=1&type=29 - these as tops
http://www.yorkville.com/products.asp?type=29&cat=2&id=380 - these as subs

How can i figure out how many i would need for a set amount of people just by reading the specs? Obviously the acoustics and if its indoor or outdoor make a huge difference. But lets just say it is an outdoor event and the temp. is at nominal level (velocity of sound is 344m/s).

mpostor
member

Joined: 04/09/03
Posts: 409
Loc: S.W. London
Re: How to calculate amount of speakers needed [Re: DariusMyszka]
#998933 - 20/07/12 12:12 PM
Hmmm....
Homework question, or question from someone looking to put on an event...?

It's not so much how many people, but how large an area you need to cover.
Also, what volume level do you need and how far from the stage do you need it at that level.

It also depends on where you put the speakers as to how many you'll need.

Generally speaking, you'll need more speakers than you would for an equivalent sized gig indoors.

Google for the inverse square rule and speakers. That's very applicable to this sort of application.

Stu.

P.S. Variances in temperature only tend to be used when calculating delay timings for delay stacks. If you're thinking of using these, get help from a pro. It's not for the faint of heart!

Sheriton

Joined: 27/01/03
Posts: 1554
Loc: Leicester, UK
Re: How to calculate amount of speakers needed [Re: DariusMyszka]
#998936 - 20/07/12 12:25 PM
Also, it's not just a simple case of adding more speakers. More speakers will give you wider coverage (assuming that they're actually designed to array together nicely) but won't give you significantly more volume. Louder speakers will do that job.

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There's nothing we can't face... Except for bunnies

DariusMyszka

Joined: 20/07/12
Posts: 4
Re: How to calculate amount of speakers needed [Re: mpostor]
#998963 - 20/07/12 01:57 PM
Well its more of personal education.. i can do an event without a problem, its just i never had a calculated way to find the amount of speakers needed, i just kind of used my intuition for that.
I know how to calculate speaker delay and all that stuff but with the subwoofer power each person absorbs a certain amount of energy if im not mistaken.

Ive calculated before that i can get roughly 90db at 100m, but i can get that sort of number with one speaker, or 10 speakers.. so this kind of thing has always confused me. And also i know the inverse square law, but i just dont know how exactly to apply it to answer my questions

Joined: 21/11/04
Posts: 3768
Re: How to calculate amount of speakers needed [Re: DariusMyszka]
#998984 - 20/07/12 03:05 PM
Quote:

I know how to calculate speaker delay and all that stuff but with the subwoofer power each person absorbs a certain amount of energy if im not mistaken.

Calculating speaker delay has little to do with power and much more to do with timing.

And bass frequencies are very good about bending around large objects, so you shouldn't worry to much about what effect people have on it unless they are significantly large people, in which case you have far larger problems most likely(No bad pun intended). People do tend to become good absorbers of high frequencies, but for a person to be a good absorber for bass frequencies you would have to have someone multiple times the average human size.

Quote:

Ive calculated before that i can get roughly 90db at 100m, but i can get that sort of number with one speaker, or 10 speakers.. so this kind of thing has always confused me. And also i know the inverse square law, but i just dont know how exactly to apply it to answer my questions

Exactly how are you able to calculate that without knowing how to apply the inverse square law?

I would suggest you likely aren't that accurate in your calculations given what you have said. First in a strict theoretical point of view, ignoring cancellation right now, every time you double the number of speakers, you would be adding 3dB as you are doubling the output power. So for 10 speakers, it would be [Email]90dB@1[/Email], then [Email]93@2[/Email], then [Email]96@4[/Email] and finally [Email]99@8[/Email], so you would end up probably about 100dB all together. The fact that you came up with the same number leads me to believe you aren't calculating this properly as well obviously.

Finally you would need a fairly powerful point source speaker (Such as a KF-850z) in order to maintain 90dB at that distance, within its coverage pattern, at PEAK output(Which is intended to be very brief) in an ideal environment ignoring things like uneven frequency response, or frequency rolloff over distance and just going by the numbers posted.

The basic formula for determining dBSPL at distance with a point source is xSPL = aSPL - 20log(d)
Where
xSPL == dBSPL at the desired distance
aSPL == SPL at 1 UNIT from the speaker (ie. 1ft or 1m)
d == distance in UNITs from the speaker (In other words if aSPL is measured at 1ft, d needs to be in feet. Or if aSPL is measured at 1m, d needs to be the distance in meters)

This formula is directly derived from what is commonly known as the inverse square law(Which is really inverse distance), which states that for every doubling of distance you will lose about 6dB of power.

For the record, A single driver being used for that distance is just unreasonable. In fact any number of point sources without delays is, and to be honest so would a single line array in most cases. In my systems I aim for +/-10dB across the entire listening area and try to get as close as I can to +/-3dB average across the entire frequency spectrum in that area. This requires a far beefier system then the straight math above would lead you to believe as you have to account for uneven response in the cabinet, or across the listening area, etc.

Edited by seablade (20/07/12 03:06 PM)

DariusMyszka

Joined: 20/07/12
Posts: 4
Re: How to calculate amount of speakers needed [Re: seablade]
#998988 - 20/07/12 03:29 PM
I know the delay has nothing to do with that i just was stating i know how to calculate it because of your previous comment. I would just like to point out im finishing my first year at SAE and im only 17 so i dont have a massive amount of experience. So if my calculations are way off its cause im most likely forgetting something.

And i calculated that using the rating of 1w@1m
using the tx8 as an example i took the HF driver 110dB and used the 1w@1m to find out the max db it can dish out at 1m
since as you said sound is logarithmic so multiplying the wattage by 10 will give an increase of 3dB.
So,
113dB (2w@1m)
116dB (4w@1m)
118dB (8w@1m)
121db (16w@1m)
124db (32w@1m)
127db (64w@1m)
130db (128w@1m)

and since the driver is only 120, i estimate around 130db @1m by pushing max power into the speaker.
Of course i forgot to leave out that last part in my last reply, but yes having 90db continuous at 100m is a challenge with one speaker.

And for how i got that 90db @ 100m i used the double distance - 6db rule

130db @1m
124db @2m
118db @4m
112db @8m
106db @16m
100db @32m
94db @64m
88db @128m

Also for the square inverse law, they never connected it with what you are saying (double the distance is a 6db decrease, i knew that from before i just didnt connect it to the square inverse law because of the way it was taught to me)

But thank you for all the replies, it really did help me out a lot.

dmills

Joined: 25/08/06
Posts: 2129
Re: How to calculate amount of speakers needed [Re: DariusMyszka]
#999004 - 20/07/12 05:10 PM
I don't know where you got a power increase of 10 times giving a 3dB increase, doubling the power gives a 3dB increase!

When designing a rig you should generally start with the assumption that as far as possible there should only be a single source at any given frequency covering any location within the desired coverage pattern.
Thus adding more (arrayable) boxes will make the pattern wider but will not increase the volume at any given distance, the only way to get more level is to use more efficient (generally narrower pattern) boxes, with the increased number needed to put the pattern back where it needs to be.

Thus you can back calculate from the required level and distance, say 32m, and 100dB for example. log (base2) of 32 is 5.
5 * 6 = 30.
100 + 30 = 130 dB @ 1m.
If I have boxes that can manage say 107db @1W 1M, then I need 130 - 107 = 23dBW power input to make that level with that box, so 200W, but I like headroom, say 10dB over my average level, so I use a 2KW amp.
If instead I needed 110db @ 64M (A stupid requirement!), I would be in trouble with that box, because I would need 16dB more power, raising the power level from 200W to 8000W which would exceed the speakers rating, so I would need to pick a narrower box which will be more sensitive within its coverage (maybe 10dB more sensitive!), meaning I could make the required level with only 800W per box, but I would need more boxes to cover the required width.

Now there are caveats to the double distance = -6db thing, it ONLY applies within the critical distance and outside the near field of the source.
Practically this means that for a line source array within the critical distance you get only -3dB with each doubling of distance (The near field extends a long, long way), and that indoor gigs usually have the law break down fairly close to the array (short critical distance).

The thing about people as absorbers sort of has validity indoors, but not for the reason you would expect, adding bodies reduces the reflections and thus increases the critical distance (The distance at which the reverberant energy equals the direct energy) so extending the distance to which the -6dB with doubling the distance works, of course this also increases the distance to which things sound reasonable....

Regards, Dan.

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Audiophiles use phono leads because they are unbalanced people!

DariusMyszka

Joined: 20/07/12
Posts: 4
Re: How to calculate amount of speakers needed [Re: dmills]
#999008 - 20/07/12 05:34 PM
ahh yes sorry that was a typo, my calculations were stating the double is a 3db increase.

And so basically, to figure out how many speakers you would need for a gig you would find the level you wish to reach at the end of the area and how wide the crowd area is to cover everywhere. Of course everything with a minimum of 3db headroom (more if its in your preferences).

I was told that people absorb the energy and thus more people = more required power
Also the doubling of distance is a decrease by 6db was for outdoor venues, indoor was 3db for each double (this is what i was taught)

dmills

Joined: 25/08/06
Posts: 2129
Re: How to calculate amount of speakers needed [Re: DariusMyszka]
#999086 - 21/07/12 10:38 AM
Quote DariusMyszka:

Also the doubling of distance is a decrease by 6db was for outdoor venues, indoor was 3db for each double (this is what i was taught)

I would be demanding your tuition fees back in that case, because that is crap.

Within the critical distance a point source is -6dB with each doubling of distance, (area of the surface of a sphere rises as the square of the radius, so power flux density drops as the square of the distance.
For a line source array, the wave front is approximately cylindrical out to a reasonable distance so the area is proportional to the distance, hence a line source will appear to have a 3dB fall off with distance, but that is irrespective of whether the thing is used indoors or out.

What differs is that the critical distance is usually much greater outdoors then in as the diffuse reverb levels are usually very much lower.

As to people absorbing power, true, but of limited importance, the HF boxes should be high enough that the crowd have a direct path to the high frequency horns, and a person is small compared to the wavelength of the bass anyway. The major effect of adding people is to reduce the level of the reverberant field which increases the critical distance and that is only a good thing.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

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