We have to distinguish between

  • the source (origin of the sound),
  • the transfert path (the way the sound has travel between source and receiver) and
  • the receiver (which in general is the person listening to the sound, or the measurment microphone picking up the sound).

Means to influence the sound can in principle be taken at all of the three levels above.

Up to the receiver we deal with pure physical signals: sounds are mechanical oscillations. They can either propagate in air (so called air-borne sound which we can directly hear) or in  materials (structure-borne sound). These signals can be measured with well-known traditional maesurement tools like microphones (for air-borne sounds) and acceleration meters (structure-borne sounds).

If the receiver is human, we have to leave the area of directly physically measurable signals. We receive sound waves by our ears and transfer them into physiological signals which are sent via complex paths to our brain. At this level we finally have our perceipts.