Aren't Watts Watts?
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Aren't Watts Watts?
from the FAQ:
What's the difference between Watts, Peak Watts, Watts RMS, and Continuous Watts? Aren't Watts Watts? Why is this so confusing? How can I possibly know what I need to buy?
Marketing is what makes it confusing and combined with the fact that most people want to avoid technical specifications, it leaves a lot of room for uncertainty. Hopefully, this portion of the FAQ will help clear up some of the confusion. More than likely, it will leave you with quite a few questions as well.
First, let's define what power is: Power is performing work over some defined period of time.
In our cases, when we're talking about stereo gear, the work that is being performed is the movement of speaker cones, which creates sound waves, which is the music, noise, speech, etc. that we hear.
Now, one way to calculate electrical power is using the formula Power = (Voltage * Voltage) / Resistance. Briefly written, this is P = E^2/R. ***
In the case of car stereos, the Voltage is provided by the amplifier, and the speaker is our resistor.
The amplifier provides current (measured in amps), which moves through the voice coil, and creates an electromagnetic field, which moves the voice coil (and thus speaker cone) inside the permanent magnetic field.
Going back to our formula for power, we know that the resistance of the speaker is generally chosen and fixed when we purchase it, so the power output changes based upon the voltage+++. Now, music isn't usually a single tone or group of tones playing at a constant level. Music is dynamic and varies quite a bit from moment to moment, and can vary dramatically from song to song, artist to artist, etc. These rapid (sometimes instantaneous) changes in intensity, tone, and rhythm generally have rapid changes in power and (thus) voltage that are associated with them. So, if we take a snapshot of what's going on power wise at any given moment, it can fluctuate dramatically from previous or later moments. And the voltage will vary accordingly with these changes in power. Keep the fact that the voltage is constantly changing and shifting (and thus changing the power) in mind. Ok, hold that thought.
Alright, let's make a brief aside here: RMS is the abbreviation for Root Mean Squared. Mathematically, what is happening when you perform a RMS is that you are taking a group of data and taking the squares of all the data points. These squared values are all added together. Then the average (mean) is calculated of the squared values. Then square root of the average (mean) value is calculated.
Now, back to the main topic: Voltage is a very measurable component, whereas power is generally a calculated value. Since these voltages are generally inducing an alternating + to - in order to create an alternating current, the typical musical signal (if graphed over time) is a stacking of waves. The simplest way to envision this change is by thinking of the voltage changing like a sin wave over time. Now, considering that we have these waves (a sin wave in simplest terms), there are 3 ways to measure the voltage. Since there are peak positive voltages, peak negative voltages, in the middle there is a reference voltage.
One way to measure the voltage is to measure 'Peak Voltage', which is the measurement of the difference in voltage from a peak (+ or -) to the reference.
A second way to measure the voltage is to measure the Peak to Peak voltage, which is the measurement from the lowest peak to the highest peak.
A third way is to measure the voltage over a period of time, and calculate the RMS of the voltage values. This is the MOST common way to measure voltage. And it's generally the most useful. While you have peaks that jump up and down, the RMS will remain fairly steady, given a constant and consistent musical signal.
Now, to actually answer the question at hand: Which power values should I look for when shopping?
Well, peak power ratings can be very misleading, and should generally be ignored all of the time. If I can build an amp and trigger a single massive voltage spike for a instant, but cannot sustain that voltage or fry the amp in the process, then I will obtain a single extremely high power figure. It looks great on a sticker on the side of the amp or in print in an advertisement or on the box, but for all practical purposes, it's a lie. Ever see a $100 amp that can make 2,000 peak Watts? Bingo. Chances are that if you ever made those 2,000 Watts, you'll have a rather smelly pile of burning junk at the end. Still, the marketing folks love it, and so the BS thrives.
In the same vein of myth, legend, and BS -- the general term watts is often thrown around and will usually represent the same lie of peak watts. So, you can't trust it either.
A wattage RMS is a bit more believable, as it should be the wattage as derived from the RMS Voltage measurement. But even this isn't perfect, as the interval of time for which they sampled the voltage (and thus calculated the RMS value) isn't specified. So, the RMS interval could potentially be during an optimistically short time or during just the right circumstances. Even though this is true, watts RMS is the most commonly used value, and generally reflects a realistic output of the amp.(*)
One of the best ways to find a good reliable power figure is to look for continuous RMS power values. This value implies that not only are they determining the power based upon Voltage RMS, but that the amp can continuously (and indefinitely) supply this voltage and power. Thus this is the most honest value you'll generally find.
-So, when you're shopping, you'll want to look for the RMS Continuous or at least RMS power of the amp, and compare that value to the RMS power handling of the speaker you're considering.
Now, peak power can generally be related to power RMS, but the situation generally only works one way. The peak power an amp can produce is generally the RMS power x 2. The same goes for the peak power handling of speakers. It would be nice if we could readily take the peak power and divide by 2 to get RMS, but generally it does not work that way, as the marketing folks keep that from being possible.
Let's look at how the speaker resistance enters into this equation. Remember back above that P = E^2/R. Since the resistance of the speaker enters into this equation, that is why when you refer to power, you should always look to see at what resistance the power is made. Such as 100 Watts RMS @ 4 ohms. The voltage in this situation would be 20V. Now, if we cut the resistance in half, and connect a 2 ohm speaker, you can see that the Power will double (assuming the amp is capable of a 2 ohm load, and that the amp has no losses). This works the same way with peak power.
[marketing speak on]
So, how does that little $100 20 Wrms x 4 channels @ 4 ohms make 600 watts power? Well, if I am a marketing type and I want to make the power look huge, I'll first start by connecting that amp to the lowest resistance possible. Let's say we can get it to remain stable at 1.5 ohms for a few moments. Now our amp is making 60 Wrms per channel. But peak is 2 times the RMS value. SO, we are (in theory) making 120 peak watts per channel. But if we spike that little amp hard, maybe we can get it to momentarily make 150 peak. And if we consider that we have 4 channels, the 150 x 4 = 600 Watts. *print sticker*
[marketing speak off]
The same kind of routine is practiced when they figure out that your HU will make 60 Watts! When in reality, it's probably making closer to 15 Wrms x 4 @ 4 ohms. Generally HU amps aren't 2 ohm stable, so a 4 ohm rating is the best they can do.
Now, the last topic, which is sort of an aside. I mentioned bridging in a question above in the FAQ. A more technical explanation of how bridging works is this:
When you bridge an amp, instead of harnessing the peak or RMS voltage to the center reference, you harness the peak/RMS voltage to the other peak/RMS voltage. This essentially doubles your voltage for the signal. Again, looking at our first Power equation we can see that P = E^2/R. If we double the voltage, that value is squared. So, P = (2*E)^2 / R = P = 4*(E^2)/R. Thus, by bridging the amp with a constant resistance, the power is multiplied by a factor of 4 in theory. In practice, amps are generally not perfectly efficient, so some value less than 4 times the non bridged power will be actually attained.
***(Yes, voltage is defined as E, not V. Don't ask me why, as I don't know or care. Having earned both a math and engineering degree, where every letter in the alphabet (both standard and Greek) is defined in some way for different variables, meanings, constants, etc., and common letters can often mean two different things depending on the subject/class you're in, well, you stop caring and just accept.
)
+++(Actually, this isn't 100% true. Speakers do not present a constant single number resistance to an amplifier. Their 'resistance' is actually more accurately termed as impedance, and the Resistance you see in a catalog or online is generally a nominal or average impedance. In reality the impedance can vary depending on the frequency of the AC (alternating current) signal. So, there are some fluctations in voltage that will alter the power and be affected depending on the frequency of the signal. Look here for a lot more info: http://www.bcae1.com/resvsimp.htm To keep this answer relatively simple however, we'll pretend that the impedance is constant regardless of frequency.)
(*)(Actually for the purist out there, Watts aren't ever measured in RMS, so the whole concept of RMS wattage (or RMS power) is a joke and a misnomer. What RMS wattage does represent is the wattage calculated based upon RMS voltage (or RMS current). The real way to define this concept would be via Average Power, which is the RMS voltage multiplied by the RMS current. One of the other formulas used to calculate power is P = E * I, where I is current.)
For more detailed explanations of all of this check out Basic Car Audio Electronics and more specifically the page on Voltages
What's the difference between Watts, Peak Watts, Watts RMS, and Continuous Watts? Aren't Watts Watts? Why is this so confusing? How can I possibly know what I need to buy?
Marketing is what makes it confusing and combined with the fact that most people want to avoid technical specifications, it leaves a lot of room for uncertainty. Hopefully, this portion of the FAQ will help clear up some of the confusion. More than likely, it will leave you with quite a few questions as well.
First, let's define what power is: Power is performing work over some defined period of time.
In our cases, when we're talking about stereo gear, the work that is being performed is the movement of speaker cones, which creates sound waves, which is the music, noise, speech, etc. that we hear.
Now, one way to calculate electrical power is using the formula Power = (Voltage * Voltage) / Resistance. Briefly written, this is P = E^2/R. ***
In the case of car stereos, the Voltage is provided by the amplifier, and the speaker is our resistor.
The amplifier provides current (measured in amps), which moves through the voice coil, and creates an electromagnetic field, which moves the voice coil (and thus speaker cone) inside the permanent magnetic field.
Going back to our formula for power, we know that the resistance of the speaker is generally chosen and fixed when we purchase it, so the power output changes based upon the voltage+++. Now, music isn't usually a single tone or group of tones playing at a constant level. Music is dynamic and varies quite a bit from moment to moment, and can vary dramatically from song to song, artist to artist, etc. These rapid (sometimes instantaneous) changes in intensity, tone, and rhythm generally have rapid changes in power and (thus) voltage that are associated with them. So, if we take a snapshot of what's going on power wise at any given moment, it can fluctuate dramatically from previous or later moments. And the voltage will vary accordingly with these changes in power. Keep the fact that the voltage is constantly changing and shifting (and thus changing the power) in mind. Ok, hold that thought.
Alright, let's make a brief aside here: RMS is the abbreviation for Root Mean Squared. Mathematically, what is happening when you perform a RMS is that you are taking a group of data and taking the squares of all the data points. These squared values are all added together. Then the average (mean) is calculated of the squared values. Then square root of the average (mean) value is calculated.
Now, back to the main topic: Voltage is a very measurable component, whereas power is generally a calculated value. Since these voltages are generally inducing an alternating + to - in order to create an alternating current, the typical musical signal (if graphed over time) is a stacking of waves. The simplest way to envision this change is by thinking of the voltage changing like a sin wave over time. Now, considering that we have these waves (a sin wave in simplest terms), there are 3 ways to measure the voltage. Since there are peak positive voltages, peak negative voltages, in the middle there is a reference voltage.
One way to measure the voltage is to measure 'Peak Voltage', which is the measurement of the difference in voltage from a peak (+ or -) to the reference.
A second way to measure the voltage is to measure the Peak to Peak voltage, which is the measurement from the lowest peak to the highest peak.
A third way is to measure the voltage over a period of time, and calculate the RMS of the voltage values. This is the MOST common way to measure voltage. And it's generally the most useful. While you have peaks that jump up and down, the RMS will remain fairly steady, given a constant and consistent musical signal.
Now, to actually answer the question at hand: Which power values should I look for when shopping?
Well, peak power ratings can be very misleading, and should generally be ignored all of the time. If I can build an amp and trigger a single massive voltage spike for a instant, but cannot sustain that voltage or fry the amp in the process, then I will obtain a single extremely high power figure. It looks great on a sticker on the side of the amp or in print in an advertisement or on the box, but for all practical purposes, it's a lie. Ever see a $100 amp that can make 2,000 peak Watts? Bingo. Chances are that if you ever made those 2,000 Watts, you'll have a rather smelly pile of burning junk at the end. Still, the marketing folks love it, and so the BS thrives.
In the same vein of myth, legend, and BS -- the general term watts is often thrown around and will usually represent the same lie of peak watts. So, you can't trust it either.
A wattage RMS is a bit more believable, as it should be the wattage as derived from the RMS Voltage measurement. But even this isn't perfect, as the interval of time for which they sampled the voltage (and thus calculated the RMS value) isn't specified. So, the RMS interval could potentially be during an optimistically short time or during just the right circumstances. Even though this is true, watts RMS is the most commonly used value, and generally reflects a realistic output of the amp.(*)
One of the best ways to find a good reliable power figure is to look for continuous RMS power values. This value implies that not only are they determining the power based upon Voltage RMS, but that the amp can continuously (and indefinitely) supply this voltage and power. Thus this is the most honest value you'll generally find.
-So, when you're shopping, you'll want to look for the RMS Continuous or at least RMS power of the amp, and compare that value to the RMS power handling of the speaker you're considering.
Now, peak power can generally be related to power RMS, but the situation generally only works one way. The peak power an amp can produce is generally the RMS power x 2. The same goes for the peak power handling of speakers. It would be nice if we could readily take the peak power and divide by 2 to get RMS, but generally it does not work that way, as the marketing folks keep that from being possible.
Let's look at how the speaker resistance enters into this equation. Remember back above that P = E^2/R. Since the resistance of the speaker enters into this equation, that is why when you refer to power, you should always look to see at what resistance the power is made. Such as 100 Watts RMS @ 4 ohms. The voltage in this situation would be 20V. Now, if we cut the resistance in half, and connect a 2 ohm speaker, you can see that the Power will double (assuming the amp is capable of a 2 ohm load, and that the amp has no losses). This works the same way with peak power.
[marketing speak on]
So, how does that little $100 20 Wrms x 4 channels @ 4 ohms make 600 watts power? Well, if I am a marketing type and I want to make the power look huge, I'll first start by connecting that amp to the lowest resistance possible. Let's say we can get it to remain stable at 1.5 ohms for a few moments. Now our amp is making 60 Wrms per channel. But peak is 2 times the RMS value. SO, we are (in theory) making 120 peak watts per channel. But if we spike that little amp hard, maybe we can get it to momentarily make 150 peak. And if we consider that we have 4 channels, the 150 x 4 = 600 Watts. *print sticker*
[marketing speak off]
The same kind of routine is practiced when they figure out that your HU will make 60 Watts! When in reality, it's probably making closer to 15 Wrms x 4 @ 4 ohms. Generally HU amps aren't 2 ohm stable, so a 4 ohm rating is the best they can do.
Now, the last topic, which is sort of an aside. I mentioned bridging in a question above in the FAQ. A more technical explanation of how bridging works is this:
When you bridge an amp, instead of harnessing the peak or RMS voltage to the center reference, you harness the peak/RMS voltage to the other peak/RMS voltage. This essentially doubles your voltage for the signal. Again, looking at our first Power equation we can see that P = E^2/R. If we double the voltage, that value is squared. So, P = (2*E)^2 / R = P = 4*(E^2)/R. Thus, by bridging the amp with a constant resistance, the power is multiplied by a factor of 4 in theory. In practice, amps are generally not perfectly efficient, so some value less than 4 times the non bridged power will be actually attained.
***(Yes, voltage is defined as E, not V. Don't ask me why, as I don't know or care. Having earned both a math and engineering degree, where every letter in the alphabet (both standard and Greek) is defined in some way for different variables, meanings, constants, etc., and common letters can often mean two different things depending on the subject/class you're in, well, you stop caring and just accept.
)+++(Actually, this isn't 100% true. Speakers do not present a constant single number resistance to an amplifier. Their 'resistance' is actually more accurately termed as impedance, and the Resistance you see in a catalog or online is generally a nominal or average impedance. In reality the impedance can vary depending on the frequency of the AC (alternating current) signal. So, there are some fluctations in voltage that will alter the power and be affected depending on the frequency of the signal. Look here for a lot more info: http://www.bcae1.com/resvsimp.htm To keep this answer relatively simple however, we'll pretend that the impedance is constant regardless of frequency.)
(*)(Actually for the purist out there, Watts aren't ever measured in RMS, so the whole concept of RMS wattage (or RMS power) is a joke and a misnomer. What RMS wattage does represent is the wattage calculated based upon RMS voltage (or RMS current). The real way to define this concept would be via Average Power, which is the RMS voltage multiplied by the RMS current. One of the other formulas used to calculate power is P = E * I, where I is current.)
For more detailed explanations of all of this check out Basic Car Audio Electronics and more specifically the page on Voltages
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