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What is a low voltage system?
Basically, no matter how many devices you hang on to
the load of the line, the voltage remains constant. This
gives us the ability to hang a large number of speakers
off of one amp and to run a fair distance. Yes, there are
25 and 100 volt systems out there (in some schools or
over extremely long runs), but for our review we will
reference the most common; 70 volt.
DETERMINE SPEAKER COVERAGE
Each speaker has a dispersion pattern providing what
circle of coverage can be provided before the level
begins to drop off at the edges. If you know that, along
with the ceiling height and the height if the listener, you
can quickly figure out what the size of that "foot-print"
coverage area is.
Now, we need to open the calulator on your PC and go to
the View pull-down. Select scientific mode, because with
dB in our math, there is a log as well as a tangent
function. Don't run out on me because you squeaked by
in Algebra like me. Here is the first equation.
D= 2 x (H-E) x TAN (A/2)
A is the coverage pattern at the -6dB down point as
stated on the manufacturer spec sheet. H is ceiling
height. E is ear height (4 foot seated).
Let's say we're
using a speaker
with a 110 degree
pattern. We divide
by two for 55. Hit
the TAN button to
get 1.428. Let's
say the ceiling
height is 12 feet
and the listeners
are seated at 4 feet. So, the 8 feet difference times 2,
times 1.48, yeilds a 22.85' diameter of coverage area at
ear height
Determine speaker coverage
A distributed sound system of ceiling speakers or even
down-firing horns (as in a factory or warehouse) serve
the basic concept of blanketing sound across an area
evenly for everyone in the area to hear clearly. The
amount level changes across that area can be mea-
sured in plus or minus X dB deviation. Generally, you do
not want more than a 3 dB deviation. That is achieved
when the coverage pattern of each speaker is edge-to-
edge with the adjadcent pattern. A 2dB variation is
achieved when an 50% overlap is arranged. When the
patterns are edge-to-center, the greatest consistency in
coverage is achieved with less than 1 dB of change
across the area.
Determine the needed sound level
Generally speaking, you need about a 25 dB level above
what is considered the ambient noise of the room. This
may be considered the base Noise Crtieria (NC) of the
room empty or you may want to consider what the
ambient level is with people murmuring or even talking
loudly, as in a crowded ballroom.
So if our NC is 45 and we add the 25 dB factor, we get
over the ambient level for a 70 dB target level at the
listener.
Determine the needed power
We've chosen our speaker. We know the coverage
pattern and the speaker overlap configuration. We know
the distance of the speaker to the listener. Now we can
use two equations to figure power. This first one tells us
the dB loss from the speaker to the listener position.
dB= 20 log(D1/D2)
The D1 is the distance from the speaker to the listener of
8 ft. Convert 8 feet into metric, because the D2 is
sensitivity and that is measured in 1W per meter. So, 8
times .3048 is 2.4 meters. The log of 2.4 is .38.
Multiplied that by 20 and you get a 7 dB loss.
The target level is 70 dB plus a Crest Factor (this allows
for headroom from RMS -or average signal- to peaks; we
will say 12 dB for this example). The difference from the
91 dB sensitivity and target level of 82 is 9 dB. The loss
was 7dB, resulting in a need for 2 dB total gain make up.
A second equation figures power. dB= 10 log(P1/P2)
The P1 is unknown power we need. P2 is the 1 watt
sensitivity. P1 divided by one is P2. We know the dB
needed as 2. So, divide that by 10 for .2=logP2. Find the
antilog* of .2, which is 1.58 or 2 watts.
Now since you have laid out your speakers and know
how many are in the room (or in zones, which we'll cover
next edition), mutliply the number of speakers times the
watts per speaker. In our case, it is 18 speakers. Siince
the EVID C8.2 has that as a 1.88 W tap. if we have 18
speakers, our total speaker wattage is 36 (18*2). But
wait! We're not done. We need to be sure the amplifier
power has another 20% margin of safety.
Amp Power = Total Speakers x Watts X 1.2
So, our 36 watts needed times 1.2 is 44 watts.
Phone: 219.929.4127
Fax: 219.929.4377
E-mail: techs@starin.biz
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