# can someone explain what ohm and rms is?

Discussion in 'Amps and Cabs [BG]' started by melvin, May 7, 2001.

1. ### melvin

Apr 28, 2001
im stupid when it comes to amps so can someone tell me what ohm and rms is?

2. ### throbbinnut

An ohm is a measurement unit for electrical resistance, how hard it is for the electrical current to pass through the device. Like describing how small a pipe is. A garden hose would have a higher resistance to flowing water than a fire hose. Just like an 8 ohm speaker would have a higher resistance than a 4 ohm speaker. (Actually impedance instead of resistance, but they are very similar. It "impedes" the flow, or "resists" the flow this much.)

RMS stands for Root Mean Square, and it really means the average value as it applies to power. 100 Volts DC has an RMS value of 100 V. A sine wave that has peak value of 141V and a negative peak value of -141V has an RMS value of 100V. Why is this necessary? Because if you hooked up a resistor across that 100VDC, it would heat up a certain amount, and if you hooked up that same resistor to the sine wave described above, it would heat up the exact same amount because it would dissipate the same amount of power. That's why the sine wave has an RMS value of 100, even though it has peaks of 141. The mathematical relationship from sine wave to RMS is (peak value)/1.414.

RMS just lets you call a voltage by the same number that would dissipate the same amount of power if it were DC, which is like a constant amount of pressure. That way it gives you an idea of how much power it can supply. If you ran the calculation with 141V, that would give you Peak power, and since you square the voltage, it would result in 2 times the power. So for a sine wave, the peak is twice the RMS. (square root of 2 is 1.414)

Clear as mud?

Chris

3. ### melvin

Apr 28, 2001
so an ohm is how hard the amp has to work? and rms is pretty much watts? and if theres a bass amp that says 4 ohms could you hook up an ext cab that has 8 ohms?

4. ### Joris

Yep! And a second one too -> total load impedance will be 4 ohms.

5. ### Ziggy

May 9, 2001
Orange County, CA
Just to clarify...

RMS - in layman's terms, is an average output ie: 175 watts RMS and 350 watts peak. (typically, RMS, is about half the amount of peak power)

OHMs - a 4ohm head is rated at just that, 4 ohms. As you add cabinets that rating will change. Additionally, adding an 'extension' cabinet can have one of two effects... if the cabinet is 'added' in "series" it's ohm / impedance is an additional load. If it's in "parallel", the total amount of impedance is 'halfed'.

You keep this in mind when adding cabinets to your head... dropping the load too far will burn out the head. Pushing it too high, will put a 'harder' burden on the head and force you to increase the volume to obtain the same output. Neither of which is good for the head.... ziggy