Using a subwoofer in the right place is crucial for the low-frequency sound arriving at the listening position. Placed in the wrong location, a subwoofer will excite unwanted resonances of a room. These resonances, or better, modes, are formed when long waves (i.e. waves with a low frequency) are reflected from walls that are hard on sound. In practice, this means that the low-frequency sound is perceived at different volumes at different points in the room.
Ideally, the subwoofer should now be positioned so that it is a quarter of the respective room length away from the nearest walls, e.g. in a 5x6 m room (front x side) 1.50 m away from the front wall and 1.25 m away from the left sidewall.
It is important to note that this information only refers to cuboid rooms - in more complicated rooms, a calculation tool (various programs can be found free of charge on the Internet) is necessary for calculating the optimal installation location.
Does the subwoofer always have to be placed vertically or can it also be placed horizontally without hesitation?
It is indeed the case that some manufacturers in the HiFi sector install internal speakers horizontally. An upright standing cabinet is loaded transversely to the voice coil with gravity and can thus be easily decentered. In the case of subwoofers, this can hardly be determined by measurement due to the often multiple centring. If the cabinet is now placed horizontally, the centring remains 100% and the coil sits where it should. Therefore, it's always up to the customer to decide whether to set up the subwoofer vertically or horizontally.
Even more, information can be found here ->Instruction.