Question about sound generation If I have a membrane, and I bring it into vibration, than it emits a sound. 
I thought, that the sound frequency is determined by the velocity of the vibrations and the loudness (soundpressure) is determined by the deflection of the membrane.
But if the soundpressure (amplitude) is determined by the deflection of the mebrane, why does a single slow deflection (deflect the membrane with a finger forcefully) does not create a sound? I‘m new in acoustics.
 A: Frequency (pitch) of emitted sound is determined by the number of deflections of the membrane per second, not by the velocity of deflection.  We can hear sound frequencies ranging from about 20 cycles per second to about 20,000 cycles per second. 
Sound amplitude or loudness is related to the pressure difference between the high and low pressure parts of the sound wave.  If the membrane is vibrating (deflecting) at, say 100 cycles per second, that says nothing about the pressure differences in the sound wave.  However, if you know that the membrane is vibrating at 100 cycles per second with a deflection range of, say 1 millimeter, then you can calculate the velocity of the membrane (e.g., an average of ~200 millimeters per second in each direction), from which the pressure difference can be calculated (e.g., roughly the pressure the membrane must exert against the air to move at that speed).
If you press on the membrane with your finger, you are moving it relatively slowly, so both the frequency and the amplitude of any emitted sound will be very low.  Tap on it, like tapping on a drum, and you will make a sound because it will vibrate at its natural resonant frequency (or frequencies), with an initial deflection amplitude depending on the force of your tap.
