What is resonance?
If two sources are at the same frequency and they are entrained, a physics phenomenon called “resonance” occurs where the overall amplitude or volume is relatively increased. In other words, the combined output is much stronger than when each source is not at resonance with each other. Both sources boost or amplify each other to create a much louder or higher output.
In the image title “In Phase”, notice how the two frequency waves that are in phase are combined to create a wave that is much louder or higher in amplitude. We indicate a louder or higher amplitude with peaks being much higher on a vertical scale.
A resonant frequency is what you are hearing or experiencing as a result of two or more vibrating sources being in resonance with each other.
Example of combined resonance and entrainment:
Let’s say you are holding two 440hz tuning forks and you strike each fork at slightly different times. Then you hold both tuning forks in front of your head so they are near each other and equal distance from your right and left ears.
The frequency of each tuning fork is the same so the individual and combined tones will sound the same. After a certain amount of time, the tuning forks will entrain to each other and the overall volume will increase once they reach synchronicity. They did not technically increase their individual volume, but their combined volume created a strengthening phenomenon which caused them to sound louder.
One plus one does not equal two
This resonance phenomenon is considered a “relative” increase in volume or amplitude because there was no physical increase in volume by either of the two sources. The only way to actually make the tuning forks louder would be to hit them harder to increase the amplitude. The increase in volume is only because of the two frequencies being in phase with each other. This is the most important concept to understand when dealing with entrainment and resonance.