<prompt> Which of the following is true about fouh hea sound S4Can be heard by the unaided earFrequency is greater than 20 HzHeard during ventricular filling phaseHeard during ventricular ejection phase  <response>: 4th hea sound cannot be heard even by a stethoscope since its frequency is below the audible limit 20 Hz. It is produced during last rapid filling of ventricles, which occurs due to atrial systole. Hence, it is also called atrial sound. S1: Closure of A.V. valves 25-45 Hz and S2: closure of semilunar valves 50 Hz. They can be heard by stethoscope. 