posted
Can someone remind me what the formula is for figuring watts? My amps are doing about 70 volts of output... just kinda curious how you can determine how many watts that is?
posted
To determine watts going into a speaker you have to know the current. the formula is Watts = I*I*R I = current R = the resistance of the speaker (use a ohm meter) If you only know the voltage the amp is putting out you must use a resistor and the formula for watts into a resistor is W = V*V / R.
what most people dont know is that a speaker is not a resistor and this makes power calculations more complex.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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quote:Originally posted by mr.dank: To determine watts going into a speaker you have to know the current. the formula is Watts = I*I*R I = current R = the resistance of the speaker (use a ohm meter)
Do I need to know the current into the speakers or the amp draw of the amp? Because my amps draw 125 Amps of current, and that would also give me some insanely high number.
quote:Originally posted by mr.dank: If you only know the voltage the amp is putting out you must use a resistor and the formula for watts into a resistor is W = V*V / R.
What value resistor? The same as the sub? And are both V's the measured output of the amp while using a resistor?
posted
Your amps are not 100% effecient. Only 50-75% of that power is actually put out to the speakers. And that is power, not the actual current that is going to the speakers. So, 125 amps x 12.5 volts input = 1562.5 watts taken from battery. Say you are using class D amps and have 70% effeciency, 1562.5 x .7 = 1093.75 watts output to speakers. If the voltage is higher you will get more, at 14.5 volts with same effeciency, you'll have 1268.75 watts total output. Those are close to the actual output, but not nearly exact.
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Posts: 3956 | From: State College, PA | Registered: Sep 2000
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posted
Hey Scottie, you need someone with LMS or PCRTA to take an in box impedence measurement of your speakers. Then you could use the voltage accurately to figure.
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posted
Why does everyone refuse my method of measuring the voltage and the amperage of the speaker outputs????
if you measure the voltage of the speaker leads with a volt meter and the amperage with an amp clamp there you go there is voltage and amperage and you dont have to be a math genius just punch it into an ohms law calculator and it will tell you the resistance at that given measurement...
for instance my us amps did 80 volts and 23 amps at what i believed to be 2 ohms....
in my horn enclosure which has a high acoustical impedance at my burp frequency, according to ohms law and the measurements i took from the amp at my burp frequency, i know that my load presented to my amp was 3.48 ohms....
that come up to 1840 watts through ohms law...
now if you stop there, you can compare amps tit for tat.
but youre gonna hear all kinds of stuff of how to convert this to real watts and talk of apparent power and this and that but really thats a real measurement you can transfer from test to test...
now if your using it to know what your amps putting out if i was guessing multiply it times .707 for rms....
that would have made my amp putting out 1301 watts... but let me remind you this wasnt at 2 ohms either....
------------------ Single 15+ Horn Enclosure+ 3000+ Watts= 155.8 DB/ USAC OUTLAW 169.9 PORT WARS
posted
Brad B, Sorry man, but the voltage * current is only for DC or purely resistive circuits. In the case of the speaker the current and voltage are out of phase with each other. If you do this you will get what us engineers refer to as apparent power which is the magnitude of the real and imaginary power.
Based on the info you gave in your post, the power your amp is putting out is 1058Watts.
You can not use your formula to compare different systems. Depending on the phase angle, which is dependent on frequency big time, you will get totally different answers.
Just do it correctly, measure your current, and measure the resistance of your sub(s), Then use I*I*R = watts. You’ll be correct every time.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
[This message has been edited by mr.dank (edited 06-10-2001).]
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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