While it is relatively easy to grasp and control sound at higher frequencies, low-frequency sound in normal living conditions can cause considerable problems. Standing waves can cause considerable drop outs in bass frequencies on the one hand or heavy amplification on the other hand and these cause the room to resonate. What can be done to prevent this resonance? Simply applying fabric or foam coatings to the walls in an attempt to control these resonant sounds cannot work. These methods can only damp the higher frequencies - an ideal way to damp those frequencies, by the way. However, lower frequencies are not influenced in the slightest by these methods. Damping only works where the sound particle velocity is at its maximum which, paradoxically, is where the sound is inaudible. This point is always somewhere near the middle of the room but never near the wall. There is, nevertheless, an effective method, a complex one, though, as used in concert halls. Here, thin wooden baffles with differing surface areas are suspended from the ceiling / wall close together, only a few centimetres apart. The baffles are only held at one edge and are allowed to swing freely. They will absorb the energy within the range of their own self-resonance from the sound field, and this will prevent resonance in the room spreading to the same extent. It is important, however, to ensure that the self-resonant frequency of the baffles is close to that of the room's own resonant frequency. Avoiding parallel surfaces also reduces standing waves. It is often possible to improve the acoustics of a room by trying out various positions for the speakers.