一條簡單的物理問題

The velocity of sound wave in air depends only on the temperature. It doesn't depend on the frequency of the wave.

Hence, sound wave of frequency 2f travels at the same speed as sound wave with frequency f.

Just imagine if the speed of sound wave depended on frequency, you would not be able to enjoy a nice music performance in a concert, because high frequency and low frequency waves from the music would reach your ears at different times. This is, of course, not realistic in daily life.

The change in frequency will have no effect to the velocity of sound. The velocity of sound only depends on the density, pressure and temperature of the medium which the sound wave is passing through. Thus, even if the frequency of the sound wave has been doubled, its velocity remains the same as before.

A little remainder for you: do not be confused by the sinusoidal wave formula ( speed = wavelength X frequency). This formula can only be applied if the wave is sinusoidal and traveling at a constant speed. Sound wave travels in longitudinal wave form.

The velocity of a sound wave changes only when the medium that the sound
wave travelling changes.
Since the frequency will not affect the velocity of sound wave,the velocity of the sound wave of frequency 2f will be the same as that of the sound wave of
frequency f (i.e. v ) under the same condition.