From left to right: Late Afternoon Near Lower Cathedral Lake and Lopsided Oak, Ohlone Wilderness. © 1995 and 1993 Ben Hubbell.
Many of the units used to measure sound and electricity belong to the meter-kilogram-second nomenclature and are defined in relation to each other. A coulomb is a unit of electrical charge. An ampere is a unit of energy transfer at the rate of 1 coulomb per second. Current is measured in amperes. An ohm is a measurement of how much of the available energy is not transferred. A volt is a unit of both pressure differential and acceleration. 1 volt represents how much pressure it takes to move 1 coulomb across 1 ohm when the current is 1 ampere.
Let me draw an analogy with a 10 speed bicycle. How hard you press against the pedals represents voltage. How much resistance the pedals provide represents ohms. And how fast you pedal represents current. Therefore, low gear is equivalent to low voltage/high current electricity, and high gear is equivalent to high voltage/low current electricity. As we all know, high gear has low torque. This analogy demonstrates that current supplies torque and that voltage provides acceleration.
It should be understood that current does not move through circuits. The current contains the charge and the charge moves through the current, just as sound moves through air without generating wind.
Watts are a measurement of power. Electrical watts are defined as voltage multiplied by current, or as current squared multiplied by ohms, or as voltage squared divided by ohms.
When the performance of audio equipment is measured, it is a common practice to only measure the voltage and ignore the amount of current. The efficiency of speakers is sometimes rated in the manner of “90 dB at 1 Watt at 1 meter,” and sometimes in the manner of “90 dB at 2.83 volts at 1 meter.” Both of these are intended to mean the same thing. As previously stated, the number of watts can be calculated as voltage squared divided by the impedance in ohms. So in the case of an 8 ohm speaker we have:
2.832 = 8.0089 8.0089 ÷ 8 = 1.001
For a 4 ohm speaker 2.83 volts would be 2.002 watts. The line level inputs of consumer audio equipment are usually rated at 10,000 ohms, and the line level inputs of professional audio equipment are usually rated at 600 ohms. Therefore, the line level power of 2.83 volts with consumer equipment will usually be 0.8 milliwatt and with professional equipment it will usually be 13.3 milliwatts.
Let us consider the voltage and current consumption of an 8 ohm speaker when it is fed the following numbers of watts.
As previously stated, when the performance of audio equipment is analyzed the measurements are usually taken in volts, and current is usually ignored. However, current in a power amplifier is very important, and this has been common knowledge among amplifier designers since the mid 1980s. A 64 watt amplifier will sound better at 4 watts output than a 16 watt amplifier, because the more powerful amplifier can back up the same voltage with more current. More available current means more torque from the power supply, even if only 5.67 volts is applied to the speaker.
In translating electrical measurements into acoustic properties, it is helpful to think of voltage as the intensity of fluctuation between above normal and below normal local barometric pressure, and of current as inverse to the density of the air. For example, when I go backpacking at an altitude of 11,000 feet above sea level, the sound transmission qualities of the air are different than at low altitude. Distant sounds seem less muffled and more immediate, but just as far away. This is because the air is thinner. Thinner air means less friction diminishing the vibrations. More current from a power amplifier has a similar acoustic effect to thinner air.
Please note that a decibel—dB— is not an absolute value; it is a power ratio. When measuring watts, 3 dB is a 2:1 power ratio, 10 dB is a 10:1 power ratio, and 20 dB is a 100:1 power ratio. On the other hand, when measuring only volts, 6 dB is a 2:1 voltage ratio and 20 dB is a 10:1 voltage ratio. The human sense of hearing perceives a 10 dB change in loudness as a 2:1 ratio and a 20 dB change in loudness as a 4:1 ratio. The frequency response of audio equipment is generally determined by the bandwidth within a ±3 dB range.
Where decibels are used to describe the loudness of sounds, there is an international standard for the average threshold of human hearing at the frequency of 1 kHz, which is used as a reference point for the ratio. Decibels of sound pressure are determined by the ratio of sound pressure at the moment to the sound pressure of this reference point.