"Box rise" has become the latest catch-phrase to sound smarter without actually understanding it. I personally don't like the phrase, because it's not electrically accurate, and because everyone regards it as some mythical phenomenon that can't be measured, calculated, or understood, but you should just buy a bigger amp because it's real. I think "box rise" should be replaced with "variable impedance", or simply "impedance", which is NOT the same as resistance, despite using the same unit of measurement.
I'm not an expert, but I'll do my best to explain the difference. Please correct me if I say anything wrong (as if I have to tell this group that)!
Ever wonder why your 1 ohm voice coil doesn't measure 1 ohm on an ohmmeter? It's because ohmmeters can only measure resistance, but voice coils are rated in impedance. Resistance resists alternating current and direct current equally. Impedance is a combination of resistance and reactance, both capacitive and inductive. In a circuit with both resistance and reactance, direct current is only affected by the resistance, whereas alternating current is affected by both the resistance and the reactance. (Tangent: Inductive and capacitive reactance are the foundation of crossovers and high- and low-pass filters, because inductors attenuate higher frequencies, and capacitors attenuate lower frequencies.)
Impedance is a function of frequency, which is why different frequencies result in different impedances (which is where the phrase "box rise" likely comes from). Impedance cannot be measured directly. It has to be calculated with ohm's law: Z = E / I. Z being impedance, E being voltage, and I being current. From there it's a matter of algebra to deduce the coil's inductance, or L, which is measured in henries. Once you know the inductance of the coil, you can use it to calculate your impedance at any frequency. First you calculate the inductive reactance of the coil using the formula X = 2 x pi x F x L, where X is inductive reactance, pi is ~3.14, F is frequency in hertz, and L is inductance in henries. Then you calculate the impedance of the coil at that frequency using the formula Z = √(R^2 + X^2). You can directly measure inductance of the coil to eliminate some of the algebra if your multimeter has that function.
Anyway, my point is that "box rise" is an incorrect term for what is happening. "Impedance rise" would be more correct, but still falls short. Honestly, the word "impedance" is sufficient, as the definition of the word already identifies that it will change with frequency. However, I think "variable impedance" is best for conveying the message that people are going for when they use "box rise".
I hope that clears up some misconceptions regarding resistance and impedance. I have no doubt that the box plays some role in output, but it isn’t solely responsible for impedance variation at different frequencies. And no, I don't expect a single person to change their terminology because of this, but at least they will have a better understanding of what's actually occurring.
- Joe
FLAME ON!
I'm not an expert, but I'll do my best to explain the difference. Please correct me if I say anything wrong (as if I have to tell this group that)!
Ever wonder why your 1 ohm voice coil doesn't measure 1 ohm on an ohmmeter? It's because ohmmeters can only measure resistance, but voice coils are rated in impedance. Resistance resists alternating current and direct current equally. Impedance is a combination of resistance and reactance, both capacitive and inductive. In a circuit with both resistance and reactance, direct current is only affected by the resistance, whereas alternating current is affected by both the resistance and the reactance. (Tangent: Inductive and capacitive reactance are the foundation of crossovers and high- and low-pass filters, because inductors attenuate higher frequencies, and capacitors attenuate lower frequencies.)
Impedance is a function of frequency, which is why different frequencies result in different impedances (which is where the phrase "box rise" likely comes from). Impedance cannot be measured directly. It has to be calculated with ohm's law: Z = E / I. Z being impedance, E being voltage, and I being current. From there it's a matter of algebra to deduce the coil's inductance, or L, which is measured in henries. Once you know the inductance of the coil, you can use it to calculate your impedance at any frequency. First you calculate the inductive reactance of the coil using the formula X = 2 x pi x F x L, where X is inductive reactance, pi is ~3.14, F is frequency in hertz, and L is inductance in henries. Then you calculate the impedance of the coil at that frequency using the formula Z = √(R^2 + X^2). You can directly measure inductance of the coil to eliminate some of the algebra if your multimeter has that function.
Anyway, my point is that "box rise" is an incorrect term for what is happening. "Impedance rise" would be more correct, but still falls short. Honestly, the word "impedance" is sufficient, as the definition of the word already identifies that it will change with frequency. However, I think "variable impedance" is best for conveying the message that people are going for when they use "box rise".
I hope that clears up some misconceptions regarding resistance and impedance. I have no doubt that the box plays some role in output, but it isn’t solely responsible for impedance variation at different frequencies. And no, I don't expect a single person to change their terminology because of this, but at least they will have a better understanding of what's actually occurring.
- Joe
FLAME ON!
