The "wop wop", usually known as blade slap, is heard when the tip of the blade passes through the vortex created by the previous one.
It can be avoided. The most common flight regime when this happens is a shallow descent but still with quite a lot of power - e.g. fast and shallow. The vortex starts to move down as soon as it leaves the tip of the blade so in level flight, the following blade passes over it. In a shallow descent with high pitch angle, the following blade can literally "slap" into the previous vortex. The vortices of the two blades now interact and can cause local, transient supersonic flow. To avoid it, simply lower the pitch to establish a more positive descent or pull back on the cyclic to increase disc loading and flatten the attitude.
The blade tips do not go supersonic. In fact, in nearly all helicopter designs, the rotor rotates within a very narrow range of speeds, typically between 90% and 110% of the normal speed. In most flight regimes, the rotor is rotating at 100%, +/- a few percent, whether you are climbing, descending or cruising. Only during auto-rotation and aggressive manoeuvring does the range vary by 10% or more. It depends on the type of helicopter but absolute limits would be something like 85% (panic time, risk of complete stall) and 115% (smaller panic, risk of damage to the machine, especially in the tail rotor drive shaft).
In normal operations, and design aims to achieve this, the rotor tips do not go supersonic since when they do, there is a sudden and large decrease in performance with more power required, higher blade loads, vibration and noise.
Think about a helicopter flying forwards. The advancing blade at its most perpendicular position experiences a relative airflow which is equal (ignoring all kinds of minor side effects) to the forward speed plus the speed of the blade. The retreating blade is experiencing a relative airflow equal to the speed of the blade minus the speed of the helicopter.
If the blades rotate so fast that the tips are supersonic, then the main lift generating part of the retreating blade, the outer two thirds of the span, would experience such a low airspeed, for some of the span it will even be negative, that the blades will stall causing a catastrophic roll into that side. It is this phenomenon which ultimately limits the rotational speed of the blades and the maximum speed of the helicopter.
Let's look at the R22 as an example. The following figures are approximate.
The rotor tip speed is about 670 fps (feet per second). The speed of sound at ground level on a standard day is about 1100 fps. The R22 is flying at close to VNE, let's say 100kts which is about 170 fps.
The tip on the advancing side, at it's fastest, is therefore flying relative to the airflow at 840 fps and on the retreating side, at it's slowest, at 500 fps.
The blade length is about 11 feet, so the middle portion of the blade on the retreating side is only flying at 190 fps (half of 670 minus the airspeed). When you get to about 4 feet out from the blade root, it's now only 50 fps and not much further in from that, becomes zero, then negative.
Remember that lift is proportional to the square of the speed. You can now see the huge discrepancy between lift on either side as airspeed increases.
To answer your question directly, the R22 would need to fly at 530 fps to approach supersonic tip speed which equates to about 330 kts which it can't get anywhere near achieving.
PS. The R22 POH speaks in Imperial measures. When I get some time, I'll redo the figures in metric which I, and most of the world, prefer.
