Function doppler_effect_kernel 

pub fn doppler_effect_kernel<R>(
    freq_source: &Frequency<R>,
    wave_speed: &Speed<R>,
    obs_speed: &Speed<R>,
    src_speed: &Speed<R>,
) -> Result<Frequency<R>, PhysicsError>
where
    R: RealField,

Calculates the observed frequency due to the Doppler effect for longitudinal motion.

Formula: $f_{obs} = f_{src} \frac{v + v_o}{v - v_s}$