Addition of sound waves
Addition of sound waves - experiment with a smartphone
Author:
Title 4
Learning objectives :
This experiment allows us to discover that the addition of two identical sound sources does not lead to a doubling of the perceived intensity but to a lower gain. It introduces the notion of logarithmic decibel scale.
Concepts covered
Superposition of waves, Logarithmic scale of decibels, Difference between pure tones and complex noises, Constructive and destructive interference, Sound power and acoustic intensity
What students will do :
The student uses three smartphones: one to measure the sound level and two others to emit white noise. After calibrating each transmitter smartphone to 60 dB, the student measures the sound level when the two sources transmit simultaneously. The experiment is repeated with different types of sounds to analyze whether the sound increase remains constant and to understand the differences observed.
What is required :
Three smartphones with the FizziQ application, A quiet space with few echoes, A tape measure to measure distances
Scientific background :
When two sound sources of the same intensity are combined, the increase in sound level is not 100% but approximately 3 decibels. This counterintuitive property is explained by the logarithmic nature of the decibel scale. The sound level in decibels is calculated according to the formula: L = 10 × log(I/I₀), where I is the measured sound intensity and I₀ is the reference intensity. When two identical, uncorrelated sounds are added together, their intensities (not their amplitudes) are added. Thus, doubling the sound intensity corresponds to an increase of 3 dB. Behavior may vary depending on the nature of the sounds. For white noise (containing all frequencies at equal intensity), the addition generally follows this 3 dB rule. On the other hand, for pure sounds of the same frequency, interference phenomena can occur: if the waves are in phase, the increase can reach 6 dB (constructive interference), while waves in opposition to phase can cancel each other out (destructive interference).