in musical instruments - when a reflected wave travels back through itself causing fixed points of Nodes (Deconstructive Interference) and Antinodes (Constructive Interference) due to the fractional relationship between the wavelength (𝜆) of the wave and the length (L) of the resonating chamber

Harmonic	String	Double Open Ended Pipe	Closed Ended Pipe
1st	𝜆 = 2L f = f1st	𝜆 = 2L f = f1st	𝜆 = 4L f = f1st
2nd	𝜆 = L f = 2f1st	𝜆 = L f = 2f1st
3rd	𝜆 = ⅔ L f = 3f1st	𝜆 = ⅔ L f = 3f1st	𝜆 = 4/3 L f = 3f1st
4th	𝜆 = ½ L f = 4f1st	𝜆 = ½ L f = 4f1st
5th	𝜆 = ⅖ L f = 5f1st	𝜆 = ⅖ L f = 5f1st	𝜆 = ⅘ L f = 5f1st

Standing Waves Part I: Demonstration

Standing Waves Part II: Explanation

Standing Waves

Making standing waves

Singing plates - Standing Waves on Chladni plates

Acoustic Levitation in ULTRA SLOW MOTION

Beats

two waves of a similar frequency (similar wavelength) superimpose to come in and out of phase causing constructive and deconstructive interference respectively. This causes a warbling/beat sound of frequency (fb) equal to the difference in respective frequencies

fb = | f1 - f2 |

Wave Beats