<prompt> Frequency of 1st hea sound10-15 Hz20-25 Hz25-45 Hz50 Hz  <response>: The first sound has a duration of about 0.15 s and a frequency of 25-45 Hz. It is soft when the hea rate is low because the ventricles are well filled with blood and the leaflets of the AV valves float together before systole.Ref: Ganong;s Review of medical physiology 25th edition. Page: 542 