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But the example fails to take into account that the materials have vastly different compressibility, which more than makes up for the differences in density, which would slow wave speeds in the denser materials. An illustrative example of the two effects is that sound travels only 4.3 times faster in water than air, despite enormous differences ...
Sound is defined as "(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation.
The behaviour of sound waves encountering a different medium is dictated by the differing acoustic impedances. As with electrical impedances, there are matches and mismatches and energy will be transferred for certain frequencies (up to nearly 100%) whereas for others it could be mostly reflected (again, up to very large percentages).
The way that sound behaves in a room can be broken up into four different frequency zones: The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone, sound behaves very much like changes in static air pressure.
Due to the exclusion principle and other fundamental interactions, some "point particles" known as fermions (quarks, leptons), and many composites and atoms, are effectively forced to keep a distance from other particles under everyday conditions; this creates the property of matter which appears to us as matter taking up space.
According to the Standard Model of particle physics, the particles that make up an atom—quarks and electrons—are point particles: they do not take up space. What makes an atom nevertheless take up space is not any spatially extended "stuff" that "occupies space", and that might be cut into smaller and smaller pieces, but the indeterminacy ...
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All the above-mentioned microphone arrays take up considerable space, making them quite ineffective for field recordings. In this respect, the double mid-side (MS) technique is quite advantageous. This array uses back-to-back cardioid microphones, one facing forward, the other backward, combined with either one or two figure-eight microphones.