Nominal watt
Encyclopedia
Nominal wattage is used to simplify the measurement of the efficiency of a loudspeaker
.
The impedance
of a loudspeaker varies with frequency
. This means that if different sine wave
tones are fed into the loudspeaker at the same voltage
(or the same current), the amount of electric power
consumed will vary.
By convention, loudspeakers are designed to generate the same sound pressure level (SPL) at the listener for the same voltage at varying frequencies - regardless of the variation in electric power. This permits a loudspeaker to be used with an amplifier
having a low internal impedance and a flat frequency response
is realized for the combined amplifier/loudspeaker system.
However, an amplifier with a low internal impedance delivers more electrical output power when the load impedance reduces (until the impedances become approximately matched
). Such high power levels could cause damage to either the amplifier or the amplifier's power supply, or the circuit connected to the amplifier's output (including the loudspeaker).
Therefore, an additional convention exists whereby loudspeaker manufacturers specify a conservative estimate of the average impedance that the loudspeaker will present while playing typical music. This is called the nominal impedance. Amplifiers can therefore be safely specified to operate into a load that has this nominal impedance (or higher, but not lower).
Typical nominal impedances for speakers include 4, 6, 8 and 16Ω (ohm
s), with 4Ω being most common in in-car loudspeakers, and 8Ω being most common elsewhere. A loudspeaker with an 8Ω nominal impedance may exhibit actual impedances ranging from approximately 5 to 100Ω depending on frequency.
In this context, the nominal wattage is the theoretical electric power that would be transferred from amplifier to speaker if the loudspeaker was actually exhibiting its nominal impedance. The actual electric power may vary from about twice the nominal power down to less than one tenth.
Loudspeaker efficiency is measured with respect to nominal power in order to emulate the situation outlined above where a low internal impedance amplifier is used with a loudspeaker. The convention is to supply one nominal watt during testing. If the nominal impedance is 4 ohms, the voltage would be 2 volts. If the nominal impedance is 8Ω, the voltage would be 2.83 volts.
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
.
The impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of a loudspeaker varies with frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. This means that if different sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
tones are fed into the loudspeaker at the same voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
(or the same current), the amount of electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
consumed will vary.
By convention, loudspeakers are designed to generate the same sound pressure level (SPL) at the listener for the same voltage at varying frequencies - regardless of the variation in electric power. This permits a loudspeaker to be used with an amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
having a low internal impedance and a flat frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
is realized for the combined amplifier/loudspeaker system.
However, an amplifier with a low internal impedance delivers more electrical output power when the load impedance reduces (until the impedances become approximately matched
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
). Such high power levels could cause damage to either the amplifier or the amplifier's power supply, or the circuit connected to the amplifier's output (including the loudspeaker).
Therefore, an additional convention exists whereby loudspeaker manufacturers specify a conservative estimate of the average impedance that the loudspeaker will present while playing typical music. This is called the nominal impedance. Amplifiers can therefore be safely specified to operate into a load that has this nominal impedance (or higher, but not lower).
Typical nominal impedances for speakers include 4, 6, 8 and 16Ω (ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s), with 4Ω being most common in in-car loudspeakers, and 8Ω being most common elsewhere. A loudspeaker with an 8Ω nominal impedance may exhibit actual impedances ranging from approximately 5 to 100Ω depending on frequency.
In this context, the nominal wattage is the theoretical electric power that would be transferred from amplifier to speaker if the loudspeaker was actually exhibiting its nominal impedance. The actual electric power may vary from about twice the nominal power down to less than one tenth.
Loudspeaker efficiency is measured with respect to nominal power in order to emulate the situation outlined above where a low internal impedance amplifier is used with a loudspeaker. The convention is to supply one nominal watt during testing. If the nominal impedance is 4 ohms, the voltage would be 2 volts. If the nominal impedance is 8Ω, the voltage would be 2.83 volts.