1. Is the speaker exciting the supporting surfaces creating dissonant sounds?
2. Are structure borne noises creating anomalies within the listening area and disturbing your neighbors?
3. Is the interaction between the supporting surface and the base of the speaker causing the speaker energy to reflect internally and resulting in smear?
The first two points are commonly understood and are typically the first things that come to mind when thinking of speaker isolation. When a speaker is resting on a surface like a flimsy desktop, it is easy to imagine the impact that the speaker can have on the surface. The speaker is vibrating and exciting the supporting surface which is creating dissonant sounds. The second point describes a situation where the energy resonates through the supporting surface and carries into other rooms or excites other objects within the listening area. In response to this, high-end pro studios isolate or “float” their control rooms in order to eliminate structure borne energy transfer into the room and also out of the room. The solid materials are in fact better conductors of sound compared to air which is illustrated in figure 1. For example, a train can be heard through the structure borne noise in the tracks well before the soundwaves can he heard.
Internal reflections are another consequence of speaker placement which can result in a loss of sound clarity and can cause sounds to become less three dimensional and spacious. Vibrations are inherent in the speaker, but any vibrations reflecting off the supporting surface and returning through any mechanical connection are a cause of internal reflections. Imagine, for example, bolting a pipe to a wall and hitting it with a hammer. The vibrations go down the pipe, hit the wall and then come back. These internal reflections cause smear which results in a loss of sound clarity and openness. Any artefacts (smear) that are replicated in the two channels are perceived to be in the middle, causing the sound stage to collapse.