The basic relationship between frequency, wavelength and speed of a wave is given by:
[tex]f= \frac{v}{\lambda} [/tex]
where
f is the frequency
v is the wave speed
[tex]\lambda[/tex] is the wavelength
The ultrasound wave in our problem has wavelength of
[tex]\lambda = 0.19 mm = 0.19 \cdot 10^{-3}m[/tex]
and speed of
[tex]v=1.5 km/s = 1.5 \cdot 10^3 m/s[/tex]
So its frequency is
[tex]f= \frac{v}{\lambda}= \frac{1.5 \cdot 10^3 m/s}{0.19 \cdot 10^{-3} m}=7.89 \cdot 10^6 Hz [/tex]
