Apparatus comprising: a container having an aerated fluid therein; an array of acoustic transmitters that is arranged in relation to the container having aerated fluid therein being processed and configured to generate acoustic signals that travel through the aerated fluid; an array of acoustic receivers that is arranged in the container and configured to receive the acoustic signals and provide a signal containing information about the acoustic signals generated by the array of acoustic transmitters, the array of acoustic receivers and the array of acoustic transmitters being separated by a known distance; a signal processor configured to: receive the signal; determine the gas volume fraction of the aerated fluid based at least partly on upon a speed of sound measurement of the acoustic signals that travel through the aerated fluid in the container, the signal processor being configured to determine the speed of sound measurement based at least partly on an elapsed or propagation time between generation and detection of the acoustic signals and the known distance between the array of acoustic transmitters and the array of acoustic receivers, wherein the signal processor is configured to receive signals containing information about the acoustic signals that are generated by the array of acoustic transmitters, that travel through the aerated fluid in the container, having each acoustic transmitter selectively encoded with a different frequency, slice of spectrum, chirp/modulation characteristic allowing each channel to be individually analyzed, that travel through the aerated fluid in the container, and that are received by the array of acoustic receivers; and provide an output signal containing information about the gas volume fraction of the aerated fluid to control the entrained air in the aerated fluid being processed in the container, based upon the signal received, including for providing a chemical additive to the aerated fluid being processed in the container. a container having an aerated fluid therein; an array of acoustic transmitters that is arranged in relation to the container having aerated fluid therein being processed and configured to generate acoustic signals that travel through the aerated fluid; an array of acoustic receivers that is arranged in the container and configured to receive the acoustic signals and provide a signal containing information about the acoustic signals generated by the array of acoustic transmitters, the array of acoustic receivers and the array of acoustic transmitters being separated by a known distance; a signal processor configured to: receive the signal; determine the gas volume fraction of the aerated fluid based at least partly on upon a speed of sound measurement of the acoustic signals that travel through the aerated fluid in the container, the signal processor being configured to determine the speed of sound measurement based at least partly on an elapsed or propagation time between generation and detection of the acoustic signals and the known distance between the array of acoustic transmitters and the array of acoustic receivers, wherein the signal processor is configured to receive signals containing information about the acoustic signals that are generated by the array of acoustic transmitters, that travel through the aerated fluid in the container, having each acoustic transmitter selectively encoded with a different frequency, slice of spectrum, chirp/modulation characteristic allowing each channel to be individually analyzed, that travel through the aerated fluid in the container, and that are received by the array of acoustic receivers; and provide an output signal containing information about the gas volume fraction of the aerated fluid to control the entrained air in the aerated fluid being processed in the container, based upon the signal received, including for providing a chemical additive to the aerated fluid being processed in the container. receive the signal; determine the gas volume fraction of the aerated fluid based at least partly on upon a speed of sound measurement of the acoustic signals that travel through the aerated fluid in the container, the signal processor being configured to determine the speed of sound measurement based at least partly on an elapsed or propagation time between generation and detection of the acoustic signals and the known distance between the array of acoustic transmitters and the array of acoustic receivers, wherein the signal processor is configured to receive signals containing information about the acoustic signals that are generated by the array of acoustic transmitters, that travel through the aerated fluid in the container, having each acoustic transmitter selectively encoded with a different frequency, slice of spectrum, chirp/modulation characteristic allowing each channel to be individually analyzed, that travel through the aerated fluid in the container, and that are received by the array of acoustic receivers; and provide an output signal containing information about the gas volume fraction of the aerated fluid to control the entrained air in the aerated fluid being processed in the container, based upon the signal received, including for providing a chemical additive to the aerated fluid being processed in the container.