Why Do We Only Hear the Bass Through the Walls?
We’ve all heard it. A house party next door or a car passing by blasting music. But why can we only hear the bass?
Low-frequency sound waves penetrate the walls better than higher frequency waves. This happens because higher frequencies lose energy quicker than low frequencies.
But why does that happen?
Energy Loss and Frequency
It all boils down to the frequency and the energy loss depending on the frequency.
Even though a sound wave is a longitudinal wave, let’s first consider transverse waves to more clearly understand waves in the first place:
Consider the figure above. We see that high-frequency waves are going up and down more often than low-frequency waves. The reason why high-frequency waves attenuate quicker than low-frequency waves is that they have to move up and down more frequently, thus, using (and losing) more energy.
Let’s have a little real-life thought experiment to further help to understand:
Imagine you and your mate doing squats next to one another. If you squat 2 times quicker than your mate you will naturally get exhausted quicker, thus, slow down and stop sooner.
This is analogous to the waves attenuating with frequency. Over distance, low-frequency waves have more energy left because they’ve not gone up and down as many times as the high-frequency waves. This makes low frequencies carry on stronger than the high frequencies. Thus the bassy frequencies can be heard further away from the sound origin.
Previously, we examined what happens with transverse waves. But sound waves are actually longitudinal waves so we need to make a little correction as to how they actually move.
Instead of moving up and down, longitudinal sound waves form regions of increased pressure followed by a region of decreased pressure as depicted below:
The higher the frequency, the tighter the gaps between low and high-pressure areas. The distance between the gaps is called the wavelength.
The high-pressure area of the longitudinal sound wave compresses the medium. The compression generates heat due to the resistance of the material to change its shape (viscosity). This heat is generated at the expense of the energy of the sound wave. As already mentioned, the energy loss is seen as the attenuation of the wave over distance.
The higher the frequency, the tighter the gap between the lower and higher pressure areas, and the more energy is lost as the pressure goes up and down more frequently. Just like in the previous example of transverse waves where the wave itself is going up and down.
So the bass is heard better because it doesn’t lose energy as much as the higher tones.
Sound wave attenuation is related to the frequency and the energy of the wave. In the case of a high pitch sound, the energy of the wave is lost over distance quicker than in the case of a low pitch sound. This happens because a higher pitch makes the material’s pressure go up and down quickly, spending the energy of the wave. The same happens with lower frequencies as well but more slowly. This is why bassy frequencies are heard better through a material.
Thanks, I hope you learned something new today!
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