If 70 volt line 100 volt line and high-impedance is new to you, you're
likely to find on the back of that something called tappings. So the
tappings they've figured it out with the math there's a transformer in there and
for that voltage line they tap it at what they call a wattage down. It's not
really a wattage tap, it's tapping the transformer for a specific impedance. So
it tells that transformer at say a 70 volt line voltage squared divided by
watts equals impedance, so 70 volt 70.7 volts it's 4998.49
divided by say a watt. Call it 5000
watts you know we're friends, if I tapped it at 1 watt that's really
tapping it at 5000 ish ohms impedance. If I tap it at 2 watts it's
tapping that transformer 2500 ohms impedance, the less impedance we have the
more wattage goes out. So if I keep lowering the impedance I get more
wattage and if I raise the impedance I get less wattage. So it's really not a watt tap but it's an impedance
tab but instead of having everyone to do the math they just call it a watt. Why are
we going to use that, let's use the ceiling for example ceiling speakers we've got
20 ceiling speakers in the ceiling all with the same wattage output but
effectively I don't want the same output in different zones. Well it might be
different ceiling heights, so we would use a calculation for the amount of
wattage we need at that loudspeaker at that height to get a certain sound level
at the listening area. So say if we're sitting on a chair a normal chair not
one of these raised bar chairs, our ears are going to be about 1.2 meters above
the floor so I want my sound right at 1.2 meters
to a specific level. The loudspeaker is going to put out a specific sound
pressure level per watts that I give it. So say at 1 watt at 1 meter, it's gonna
make 91 DB SPL. So if I have a 3 meter ceiling and at 1 meter with 1 watt I
have 91 DB SPL, if I went down to 2 meters closer to our listening level now
I just took 6 DB SPL off. If I go 91 DB SPL take away 6 that's 85 DB SPL so 85
DB 4 1 1 that should be good. But I need some headroom with that loudspeaker so
for program audio I want 20 DB of headroom so I need to get up to 105
watts. Now I have to figure out how many watts I put in the speaker to get a 105
at 90 of 1 watt is 91 I double that to 2 watts that's 94. I double that to 4
Watts that's 97, I double that to 8 watts that's a 100. I'd double that to 16
watts it's 103. Okay I'm going to tap it at 103 watts now I'll get a good
constant 85 when I need it, but I have the headroom in program for explosions
and Peaks and whatnot. And now I have to calculate that loudspeaker at 16 watts
at a 70 volt is probably going to give me about
532 ohms impedance. So I want to make sure I tap all the speakers, do all the
math, figure out how many watts I'm using total and the final impedance on the
line. I want to make sure my amplifier is not going to go below that, I don't want
to kill it. So I have enough headroom, I have enough watts the impedance is a
good match and bing-bang-boom Bob's your uncle and I got a good system. Now if I
lower or raise those that will change the amount of wattage I need. So if I say I
have a different low spot in the speaker I might only need to tap it for 8 to
get the same result. Plenty more information on audio
installation, low impedance, high impedance, 70 or 100 volt line
available on HowToAV.TV. If you've got a specific question leave it in the
comment section below, we'll get right back to you.