Caroline, a piano tuner, suspects that a piano's B4 key is out of tune. Normally, she would play the key along with her B4 tuning fork and tune the piano to match, but her B4 tuning fork is missing! Instead, she plays the errant key along with her A4 tuning fork (which has a frequency of 440.0 Hz), displays the resulting waveform on a handheld oscilloscope, and measures a beat frequency of 15.9 Hz. Then, she plays the errant key along with her C5 tuning fork (which has a frequency of 523.3 Hz) and measures a beat frequency of 67.4 Hz. What frequency is being played by the out-of-tune key? If the B4 key is supposed to produce a frequency of 493.9, is the frequency of the key lower than it should be ("flat") or higher than it should be ("sharp")?

This question deals with the phenomenon of "Beat Frequency".

The frequency being played by the B4 key is "455.9 Hz", which is "lower (flat) than it should be".

When two waves with a slightly different frequency interfere with each other, they produce a pattern known as beat frequency. The beat frequency is the difference between the frequencies of the interfering wave. Since the frequency of B4 key produces beat frequency with both the C5 tuning fork (which has a frequency of 523.3 Hz) and the A4 (which has a frequency of 440 Hz). Hence, the frequency of the B4 key must be between the range of these frequencies, that is, 440 Hz to 523.3 Hz.

ANALYZING THE BEAT FREQUENCY WITH A4 TUNING FORK:

Beat Frequency with A4 tuning fork = Frequency of B4 - Frequency of A4

Frequency of B4 = Frequency of A4 + Beat Frequency with A4 tuning fork

Frequency of B4 = 440 Hz + 15.9 Hz

Frequency of B4 = 455.9 Hz

ANALYZING THE BEAT FREQUENCY WITH C5 TUNING FORK:

Beat Frequency with C5 tuning fork = Frequency of C5 - Frequency of C5

Frequency of B4 = Frequency of C5 - Beat Frequency with C5 tuning fork

Frequency of B4 = 440 Hz - 67.4 Hz

Frequency of B4 = 455.9 Hz

Both the answers validate each other.

Since the expected frequency of the B4 key is 493.9 Hz, which is greater than the actual frequency produced by the B4 key. Hence, we can say that "the frequency of the key lower than it should be ("flat")"

The attached picture shows the general formula of the beat frequency.

Learn more about beat frequency here:

link

455.9 Hz, Flat

Explanation:

The beat frequency is basically the difference between the frequency of the B4 note and the frequency of the A4 tuning fork. This means the B4 note is 15.9 Hz off of 440Hz, and 67.4 Hz off of 525.3 Hz. As B4 is between the two notes, it would make sense to find its frequency by adding 15.9 to 440 and subtracting 67.4 from 523.3, both if which give us a frequency 455.9 Hz for the B4 key. This is because the note doesn't change for the different turning forks so both differences should result in the same frequency. Because the note should be 493.9 frequency but instead has a frequency of 455.9 Hz, it is flat because the frequency is lower than it is supposed to be.

Hope this helped!