The sound of rotating helicopter blades Why do helicopter blades make this pulsing, oscilating, slapping(?) sound?
Since their movement is smooth, shouldn't the sound be a similar, constant shush, perhaps increasing or decreasing in frequency as the speed of the blades changes?
update: Won't the sound heard depend on the location of the listener? Perhaps a person standing below (or above;) the helicopter would hear a more constant and uniform sound wave, while a listener standing to the side will hear the mentioned pulsing sound?
 A: First, assume a spherical helicopter...
A helicopter isn't a sphere, or even close to one.  Consequently, you get complicated acoustic effects where the pressure waves from the blades reflect off other parts of the helicopter, such as the tail boom.  You also get intermittent reflection of the tail rotor sound off the main rotor blades, intermittent reflection of the engine noise off the main rotor blades, and waves from all these sources interfering with each other.  You may also have secondary sound sources such as the tail boom vibrating in response to the periodic pressure changes of the main rotor's blades passing over it.
A spherical helicopter produces a uniform, unchanging sound field.  A real-world helicopter is a very complex sonic environment.
A: Actually you do hear a constant whoosh (although often drowned out by engine noise) when you are directly below the helicopter.
The tips of the blades cause a wave to propagate outward at the speed of sound.  This wave does not have the same strength in all directions.  If you are a distance away laterally, then you hear these waves in succession produced by the succession of blades that passed towards your direction.  From underneath, you receive the waves equally regardless of where they were emitted in the rotation of each blade.  As a result, you don't hear the usual helicopter whap-whap-whap-whap.  You just hear a sort of steady rumble.
A: In start-up and hover each blade produces more or less constant sound. But the sound is attenuated by distance and may not be the same in all directions. Therefore you hear it differently depending on the blade's position relative to you. So as the blades rotate, the sound you hear pulsates because the blades alternately get to positions where you hear them more or less strongly. In this video showing helicopter start-up from cockpit you can clearly hear the swish of each blade as it passes overhead with the pulsing increasing in frequency as the rotor spins up.
The blade tips also move quite fast, often more than half of speed of sound, so Doppler effect is adding more variation to the sound if you are standing to the side.
In cruise flight additionally the advancing blade moves faster relative to air than the retreating one, so even the generated sound changes as the rotor turns.
This effect increases as the helicopter accelerates. If it overspeeds, blade tips on the advancing side may (depending on helicopter type) get close to the speed of sound and shockwaves start to form on that side that add even more pulsating sound.
In some cases (turns at high speed, descent) the blades may also be hitting the wake vortex shed by the previous blade resulting in sharp increase in the puslating sound called "blade slapping". The reason is the blades only hits the vortex when it passes one particular place on the rotor disk, usually on the advancing side. Apparently it is rather complex; I found there is a paper about it (not read it; it is behind paywall).
A: It is generally caused by poor piloting technique. Blade-slap occurs when the helicopter is allowed to slowly drift downward while the pilot is still applying significant power. When blade-slap occurs, the pilot should either stop the descent, or lower the collective to enter a more positive descent. 
A: The movement of air around the blades of the rotor produces a "white noise" due to turbulence.  However, the "white noise" is actually "colored" and a certain band of frequencies predominates.  
As the blade approaches you there is a Doppler shift toward a higher frequency, and as it turns and goes away from you the Doppler shift reverses.  It is this frequency shift you hear as a sort of "swish/swash/swish/swash"
A: Sound is different speeds of air coming to your ear resulting in different air pressure. When a blade is coming towards you it generates a higher air pressure, than when it is going away from you. Therefore the flapping sound.
Imagine if you could be exactly above or under the rotor. Then the different air pressures will equalise each other and you would hear the smooth sound. Same concept as in a 3phase Electric motors magnetic field.
