Ultrasound loudness Assuming no absorption, what is the change in sound intensity level, in dB, in travelling from a position 1.0 cm from a small ultrasound source, to a position 9.0 cm from the source. Assume this to be a point source that emits ultrasound waves equally in all forward directions and give thought to the sign of your answer. Solving: (2A/ STRAN SA SA 2 Surface area, A = r where we define SA for a sphere = 47r and A for any cone x r. So, A/A = r2/r2. Note also, I x P/A so 1/1 = r/r Ans. -19 dB as a 'solid angle". 13:50