Overview

As mentioned in section "Audio Chain/Network Technologies", the audio that is ultimately delivered to a pair of speakers or headphones passes through a number of processing functions and/or appliances. Some of processing functions include a volume control, and Bluetooth playback devices typically have a volume control.

A volume control on the Bluetooth source device (DUT1), such as on a smartphone often controls the level of the audio sent over the Bluetooth channel. Indirectly, as a result, it also effectively controls the listening level.

In contrast, a volume control on the Bluetooth sink device (DUT2) often controls only the volume level delivered to the speakers. The result is that changing the volume setting may affect only the level delivered to the speakers and has no effect on the level sent over-the air in the Bluetooth channel.

As an example, shown in "Example 1" is an illustration of a smartphone playing a stored audio file to a Bluetooth speaker system. Both phone and speakers have their (vertical slider) volume controls set to maximum. This represents the loudest (but probably distorted) volume the complete system is capable of delivering.

Shown in "Example 2" is second example with the volume control of the speakers set to half volume, and this results in the level to the listener being reduced to a desirable level. However, note that in this example the volume control of the phone has been left at maximum, thus the over-the-air volume through the Bluetooth channel remains at maximum and may be distorted.

Shown in "Example 3" is a third example. In this case, in order to reduce the listening volume the slider on the phone has been reduced while the slider on the speakers has been left at maximum. This achieves the same objective as example #2 as far as the listener is concerned. However, reducing the volume at the source (DUT1) also reduces the volume level sent over-the-air through the Bluetooth channel. In effect this reduces the dynamic range that the audio channel is normally capable of accommodating.

Since the Frontline’s Audio Expert System only “sees” the over-the-air audio, it is important that the over-the-air volume be adjusted to a sufficiently high level, but not so high that the signal is distorted. Too much reduction of volume through the over-the-air channel can result in audio levels falling below required Measurement Threshold levels (see "Low Volume Alarm Event" and "High Volume Alarm Event"), and thus making useful audio analysis measurements are not possible.