An empty room with hard walls can sound bright and echoey, while a furnished bedroom sounds much quieter. The difference is caused by what happens when sound waves reach surfaces. Hard surfaces such as glass, plaster and concrete reflect a large fraction of the sound. Reflected waves can reach your ears after the direct sound, producing reverberation or a distinct echo if the delay is long enough. Curtains, carpets, bedding and soft furniture absorb more sound energy and scatter reflections. That reduces the strength of the returning waves. Recording studios use the same physics deliberately, choosing materials and shapes to control reflection, absorption and reverberation.

The same physics matters in theatres, classrooms, studios and hospitals. Designers choose shapes and materials to control reflections and unwanted sound, while engineers use microphones and signal processing to measure or deliberately alter waves.

Waves transfer energy and information without transferring matter from source to receiver overall. Frequency tells us how many oscillations pass each second, wavelength is the spacing between matching points, and wave speed links them through v = fλ. Different frequencies interact with materials differently, which is why engineers can choose particular parts of the electromagnetic spectrum—or particular sound frequencies—for sensing, communication, heating or imaging.