Find the frequency ν of sound vibrations in steel if the distance between the nearest points of the sound wave

Find the frequency ν of sound vibrations in steel if the distance between the nearest points of the sound wave, differing in phase ∆φ = 90о, is l = 1.54 m. The speed of sound in steel is v = 5000 m / s.

The distance between the nearest points of the sound wave, differing in phase by 90 °, is a quarter of the wavelength.
Oscillation frequency:
ν = u / λ = u / (4 l);
v = 5000 / (4 * 1.54) = 811.6883116883117 = 812 Hz;
Thus, l is the distance between the nearest points of the sound wave = 1.54 m.
Answer: 1.54 m.