Similarly, when electrons scattered from crystals exhibited interference, their wave nature was confirmed to be exactly as predicted by symmetry with certain wave characteristics of light. The wave nature of light was established by experiments showing interference. Interference is such a fundamental aspect of waves that observing interference is proof that something is a wave.

Although completely destructive interference is possible only under the simplest conditions, it is possible to reduce noise levels by 30 dB or more using this technique. Sound waves in fluids are pressure waves and consistent with Pascal’s principle pressures from two different sources add and subtract like simple numbers that is, positive and negative gauge pressures add to a much smaller pressure, producing a lower-intensity sound. To obtain destructive interference, a fast electronic analysis is performed, and a second sound is introduced with its maxima and minima exactly reversed from the incoming noise. Larger-scale applications of active noise reduction by destructive interference are contemplated for entire passenger compartments in commercial aircraft. (Figure) shows a clever use of sound interference to cancel noise.

So, sound being a wave, we expect it to exhibit interference we have already mentioned a few such effects, such as the beats from two similar notes played simultaneously. In fact, one way to prove something “is a wave” is to observe interference effects. Interference is the hallmark of waves, all of which exhibit constructive and destructive interference exactly analogous to that seen for water waves. Some types of headphones use the phenomena of constructive and destructive interference to cancel out outside noises.

How to use resonance