Voltage from amplifier output to loudspeakers?
In a typical home hi-fi system what is the potential difference across the amplifier terminals that feed a loudspeaker? What is the typical current the loudspeaker wires carry? What is the typical resistance of a loudspeaker?
Loudspeakers are typically 4 or 8 ohm, though there are other impedances.
Note it’s Impedance, not Resistance - as they have a wound coil (effectively an electromagnet), the actual resistance is low at low frequencies and higher at high frequencies.
As an example, assuming an 8 ohm speaker (calculated as a resistance for simplicity) with ten watts RMS power in to it from the amp.
The RMS voltage (think of it as the equivalent DC to give the same power) would be 8.94V
The current would be 1.12 Amps
As the amp signal is AC, the actual voltage & current would be peaking at about 1.4x those figures and dropping to zero at each half cycle.
Power out is ’square law’ ratio to voltage or current, so eg. at 40W RMS, both the voltage and current would be doubled. (Watts are Volts times Amps, so doubling both gives four times the product).
Loudspeakers are typically 4 or 8 ohm, though there are other impedances.
Note it’s Impedance, not Resistance - as they have a wound coil (effectively an electromagnet), the actual resistance is low at low frequencies and higher at high frequencies.
As an example, assuming an 8 ohm speaker (calculated as a resistance for simplicity) with ten watts RMS power in to it from the amp.
The RMS voltage (think of it as the equivalent DC to give the same power) would be 8.94V
The current would be 1.12 Amps
As the amp signal is AC, the actual voltage & current would be peaking at about 1.4x those figures and dropping to zero at each half cycle.
Power out is ’square law’ ratio to voltage or current, so eg. at 40W RMS, both the voltage and current would be doubled. (Watts are Volts times Amps, so doubling both gives four times the product).
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Robert’s answer sums it up nicely, without going into too much detail.
Take careful note of the point about impedance compared to resistance. If you put a standard "ohm" meter across loudspeaker terminals you would be measuring resistance, but when the alternating current of an audio signal is applied it will be encountering impedance. The two figures will not be the same.
Also, because of a phenomenon known as the maximum power transfer theorem, you will only get the best performance from a sound system if the loudspeaker exactly impedance matches the given output impedance of the amplifier.
The impedance will vary according to the frequency of the signal. Because audio signals contain a vast range of frequencies (typically 90Hz-19kHz) the manufacturers tend to quote impedances for a single frequency of 1kHz. This makes it easy for consumers to mix and match equipment. The performance of loudspeakers either side of the 1kHz tone varies according to the design and maker. This is why some loudspeakers cost considerably more than others (for the same notional power handling) and they sound very different too.
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Audio amplifier enlarges input signal from about 10mV peak to peak (mic. input) into about 10 V peak to peak with current capacity about 3 amperes AC rms for rated 10 watts output audio power. To match with this amplifier,we choose 10W speaker . Choose any lower than 10 watts speaker could result burning of speaker at maximum output. Choose higher than 10 watts speaker ( like 50W ) could also result less audio output (lower efficiency). Signal feeds to loud speaker is AC power with variable frequencies from 100 c/s to 14000 c/s ( for symphony ) and variable voltage peak to peak value. The best way to understand it by using a scope to look how music displays in sound waves.
Loud speaker has typical impedance ( reactant Xl + coil resistance R ) 3.2 ohm ; 4 ohm ; 6 ohm ; 8 ohm ; 10 ohm ; 16 ohm ; 32 ohm and 40 ohm. They were used in different application and device.
Speaker wattage varies from 250mW to 500W.
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