Abstract: A method for assessing the particle size of a bulk particulate material (1), such as powdered or granular material includes transmitting sound energy (as herein defined) through the particulate material (1) from a source (2) to a detector (3), and assessing the particle size from the time taken to pass through the material from the source (2) to detector (3) or signal velocity, through the material (1). Typically the signal is a frequency or frequencies in the range about 20 Hz to 20 kHz (but higher frequencies are envisaged), and the particulate material may be in a moving production stream of the particulate material (1). Apparatus is also claimed.