# Do different frequencies move at the same speed with a given medium?

According to this website, it talks about plate reverbs with the characteristics of their sound. However, I have highlighted the section that confuses me as it states that:

So, my question is:

Aren’t different frequencies of sound supposed to have different speeds? (To my understanding all frequencies are supposed to travel at the same speed with a given medium)

• "To my understanding all frequencies are supposed to travel at the same speed with a given medium." What makes you think that?
– noah
Jun 9, 2021 at 19:48
• @noah Supposedly, I go to a concert and notice the music played by a band. Therefore, the high-frequency sounds should arrive to my ears at the same time as the low-frequency sounds, that’s why I hear all frequencies at the same time. So they must travel at the same speed. Am I correct? Please suggest. Jun 9, 2021 at 19:57
• @user506602 Consider that different wavelengths of light travel at different speeds through a medium. That's how prisms make rainbows. Yet you see everything at the same time, do you not? Our senses aren't precise enough to discern the true nature of things sometimes. Jun 9, 2021 at 20:09

Typically, pressure waves of different wavelengths travel at different speeds in any medium, which we call dispersion, as is described in the text you posted. But, in most everyday situations, the differences are so small that we don't notice them.

One example where it can be noticed, however, is thunder. When lightning strikes, it produces sound at a lot of frequencies at the same time, in the same place. Yet, most of the time you can hear a distinct, high pitched clap before a more low pitched rumble, because the high frequency waves travel faster than the low frequency ones. Additionally, the further you are away from the site of the lightning strike, the longer the thunder lasts, because the time from the first (fast) waves reaching you to the last (slow) waves becomes longer the further they must travel.

• I'm not sure how much of your question was directed at why this happens, so I focused on how to notice this effect in the real world.
– noah
Jun 9, 2021 at 20:19
• My interpretation was that OP's question was merely: "Does this happen or not?" Jun 9, 2021 at 20:28
• @DKNguyen Yes, that’s correct! At first I was confused as I read the text from the site because the relationship of the speed of sound, its frequency, and wavelength is the same as for all waves according to this equation $v_w=f\lambda$. That’s why I didn’t quite understand in the first place. Jun 9, 2021 at 20:52
• @user506602 Ah I see, the misleading part in this equation is that $f$ and $\lambda$ depend on each other. So it would be more correct to write $v_w = f(\lambda) \lambda$ or $v_w = f \lambda(f)$.
– noah
Jun 9, 2021 at 21:04
• I wonder whether the dispersion that we notice with thunder is especially large because of the very large amplitude of the waves near their source. In other words, might it be that $v'=f(A)$ in which $v'=\frac{dv}{df}$, $v$ being the speed of sound of frequency $f$ in air and $A$, its amplitude. Jun 10, 2021 at 2:41