PulseAudio  5.0
Volume Control

Overview

Sinks, sources, sink inputs and samples can all have their own volumes. To deal with these, The PulseAudio library contains a number of functions that ease handling.

The basic volume type in PulseAudio is the pa_volume_t type. Most of the time, applications will use the aggregated pa_cvolume structure that can store the volume of all channels at once.

Volumes commonly span between muted (0%), and normal (100%). It is possible to set volumes to higher than 100%, but clipping might occur.

There is no single well-defined meaning attached to the 100% volume for a sink input. In fact, it depends on the server configuration. With flat volumes enabled (the default in most Linux distributions), it means the maximum volume that the sound hardware is capable of, which is usually so high that you absolutely must not set sink input volume to 100% unless the the user explicitly requests that (note that usually you shouldn't set the volume anyway if the user doesn't explicitly request it, instead, let PulseAudio decide the volume for the sink input). With flat volumes disabled (the default in Ubuntu), the sink input volume is relative to the sink volume, so 100% sink input volume means that the sink input is played at the current sink volume level. In this case 100% is often a good default volume for a sink input, although you still should let PulseAudio decide the default volume. It is possible to figure out whether flat volume mode is in effect for a given sink by calling pa_context_get_sink_info_by_name().

Calculations

The volumes in PulseAudio are logarithmic in nature and applications shouldn't perform calculations with them directly. Instead, they should be converted to and from either dB or a linear scale:

For simple multiplication, pa_sw_volume_multiply() and pa_sw_cvolume_multiply() can be used.

Calculations can only be reliably performed on software volumes as it is commonly unknown what scale hardware volumes relate to.

The functions described above are only valid when used with software volumes. Hence it is usually a better idea to treat all volume values as opaque with a range from PA_VOLUME_MUTED (0%) to PA_VOLUME_NORM (100%) and to refrain from any calculations with them.

Convenience Functions

To handle the pa_cvolume structure, the PulseAudio library provides a number of convenience functions: