Glossary definition of 'Dynamic range'The ratio of the maximum signal level to the level of noise expressed in dB. This can apply to a signal or a device. In the former case, it is also used to mean the ratio of the loudest signal to the lowest signal in a recording or transmission, and in the latter it is used to refer to the ratio of the loudest signal that can be produced without distortion to the output noise floor.
Although it sounds simple, the measurement of dynamic range is done in enough different ways to make it hard to compare results from different manufacturers. For a start, how do you define what the loudest signal is that a device can reproduce? Then, when you measure the noise level, over what bandwidth do you measure and do you apply any weighting filters? Do you measured the output muted? Unless these things are specified, the measurement of dynamic range is meaningless.
In order to clarify these questions, the AES17 standard specifies the maximum output amplitude of a device as its highest undistorted output and defines an undistorted output as one with less than 1% THD+N and less than 0.3dB amplitude compression.
In order to answer the question of whether the device is measured muted or not, the AES17 standard specifies that the device be measured with a sinusoidal signal at 997Hz with amplitude 60dB below the maximum signal amplitude. This is in order to ensure that the channel is not muted and the noise is measured in the presence of a signal. The tone itself is removed by using a band reject filter with a Q of between 1 and 5. The signal that remains is weighted using an ITU-R 468 2k filter and measured relative to the level of the -60dB tone. Adding 60dB to this measurement gives the dynamic range of the device.
When referring to signal levels, it has become a common practice to compress audio signals in order to give maximum loudness on radio transmissions. This has the effect of increasing the level of the lowest signals relative to the highest or reducing the dynamic range.