Understanding Power Ratings
Finding the true power potential from a given audio component is not as simple as it may seem. In their attempt to make commission, salesmen often overwhelm customers with all sorts of make believe power ratings, while some manufactures do little to help the situation. The solution? We tidy up all the terms and reveal the truth.
Related terms
There are three terms that regularly accommodate a power rating, these are Total Harmonic Distortion (THD), Voltage (Volts or just V) and Ohms. Ohms are a complicated subject and so have been given their own technical article. That article can be found here.
Voltage
The voltage received by an amplifier has a great impact on the power it can produce. When a car is running it will normally provide approximately 13.8 volts, when switched off the battery will provide only about 12Volts. When an amplifier is operating it draws current (amps) from the vehicles electrical system. When this drain of current exceeds the output of the electrical system, the voltage supplied to all of the car’s electrical devices drops. This is what causes headlights to dim and also reduces the power output of the amplifier during a heavy drain period i.e. a significant bass note.
Total Harmonic Distortion (THD)
Power produced by amplifiers is measured at a given Total Harmonic Distortion (THD). As an amplifier increases the power of a signal, distortion of that signal starts to occur. This distortion can be noticed by the human ear at levels above 1% THD.
Power terms
There are four terms most commonly used for rating power output and input.
- Max power
- Peak Maximum Power Output (PMPO)
- Root Means Squared (RMS)
- Nominal Power
Max Power and PMPO
Max power and PMPO are essentially the same thing. This rating is the absolute maximum power given perfect operating conditions. For a speaker this may literally mean sitting in a freezing environment, playing at its optimum frequency, a reading then taken half a second before it rips itself to pieces.
Some manufactures will take these max power ratings at levels as high as 10% THD and above.
Because of different testing conditions and distortion levels, comparison of these ratings between brand names isn’t feasible. Some may adhere to fairly strict testing conditions while others may exaggerate already ridiculous claims. Many of the better quality brands claim max power ratings that may actually be achievable while others refuse to even give a maximum power rating and instead choose a more reliable rating type.
RMS and Nominal Power
RMS and nominal power are also regarded as the same although nominal power is only used to measure speaker input while RMS can be used for amplifier output or speaker input.
To obtain an RMS or nominal power figure, a speaker is driven at a continuously equal power level over its entire intended frequency range for a 24-hour period. After this period the speaker must remain undamaged.
An amplifier RMS rating must be given at a certain THD and input voltage. The RMS rating is then the average power output over the amps entire frequency range, in several different conditions.
These ratings are used by all good manufactures. When purchasing after market audio such as amplifiers and speakers, RMS and nominal power figures are the only ratings that should be considered.
Because there is no standard to measure max power, no accurate conversion exists although it’s commonly about 2.5 times larger then RMS values.
NOTE: Head unit amplifiers are normally rated in max power, RMS power for these units is normally only provided in their manuals
Ratings accuracy
Every amplifier made by every manufacturer will make a slightly different amount of power. Many of the top quality brands will therefore understate the power that their amplifiers can output. While some of the lower end brands will find the highest rated amplifier from a batch and use that rating.
Some manufactures also underrate their amplifiers in order to fit it to competition bracket’s separated into power classes eg. 301 - 600 watt class.
An example of extreme underrating is the Phoenix Gold Tantrum 500.2 2 channel amplifier, rated at 50Watts RMS x 2 @ 12.5 volts. This same amp is tested at 550 Watts RMS x 1 at 14.4V. A more common scale of underrating is the Alpine MRV-1005 which is rated at 600W but tested at an actual 660W RMS.
Many amplifiers are supplied new with birth sheets. These sheets provide results of the actual tested results of the particular amplifier that they accommodate.
Happy hunting!
