Volume Controls: Relative vs. Absolute & Linear vs. Logarithmic

Logarithmic Volume Control vs. Linear Volume Control

As you can see, from the graph to the left, a logarithmic volume scale follows an almost exponential like curve.

A linear scale, on the other hand, follows a straight line that changes in equivalent, consistent output relative to the distance the pot is moved.

While your initial reaction may be to think a linear control is ideal, that is not the case. This is because we do not perceive loudness linearly.

For example, if we perceived loudness linearly, then doubling the output would double the loudness. Realistically, a signal needs to have 10 times the power output (or a 10dB increase) to be perceived as twice as loud.

As you can see, graphed to the right, a linear control is clearly not suitable for the audio world. This is because the amount of travel, of the volume control, does not match the amount of change in loudness or perceived volume in any easily understandable way.

A linear control does not operate or function appropriately for the way we hear. Compound this with the fact that one end of a linear control provides minuscule changes over a vast amount of wiper travel, while the other end of the control provides extreme changes in loudness with minimal movement in wiper travel*, and you can see that a logarithmic based volume control is needed for audio based applications.

*This is actually why when you use older equipment, with a linear control, you barely had to turn up the volume control. Linear based potentiometers provide massive power increases when barely touching the control, at the lower end of the listening range, and barely any adjustment near the max end of the range. 

A logarithmic control would require you to rotate the volume control significantly more. It isn't that you don't have as much power output capabilities. Rather, it is that the logarithmic control provides more finite, even incremental changes across the whole range of the volume control where as the linear control provides massive increases in output with minimal adjustment/movement of the volume control at the lower end and barely any change in output, with the same amount of adjustement, at the higher end of the volume control.

Relative Volume Scale

The unit of measurement for a relative volume scale is dB Full Scale (dBFS). This is short for Decibels relative to Full Scale. dBFS is traditionally used in digital systems and lets you know much lower your signal, or level, is compared to the maximum possible output of the digital system. 0dBFS is used to designate the maximum output of the system. Normal operating values (volume levels) for a relative volume scale usually range from -99dBFS to 0dBFS - although this can fluctuate between manufacturers and the IC (integrated circuit) chip they use for their volume control. This is because these type of systems usually use a shaft encoder which sends pulses to an IC chip letting it know to adjust the volume up or down accordingly.

Again, this is called a relative scale because the volume is relative to the maximum output of the system and how much gain you have left to apply.

Consider this: let's say you have a stereo receiver capable of outputting 120W into an 8 ohm load and your friend has a stereo receiver that does 60W into an 8 ohm load. If both receivers are set to -40dBFS, they are NOT supplying the same amount of output.

Absolute Volume Scale

This volume scale does not have a unit of measurement. This can range from your vintage gear with a 0-10 scale, or your more modern AVR's with a 0-99 scale. This type of scale does not give you an idea of how much of a loudness change is applied with each step or where you need to be on the scale to double the output.

An absolute scale can utilize either a linear or logarithmic potentiometer.