Misconceptions about Sound and Waves
From children to college students, the concept of sound can be a slippery one. Here is a list of common misunderstandings of sound and waves [1]:
  1. Sounds can be produced without using any material objects.
  2. Hitting an object harder changes the pitch of the sound produced.
  3. Human voice sounds are produced by a large number of vocal cords that all produce different sounds.
  4. Loudness and pitch of sounds are the same things.
  5. You can see and hear a distinct event at the same moment.
  6. Sounds can travel through empty space (a vacuum).
  7. Sounds cannot travel through liquids and solids.
  8. Sounds made by vehicles (like the whistle of a train) change as the vehicles move past the listener because something (like the train engineer) purposely changes the pitch of the sound.
  9. In wind instruments, the instrument itself vibrates (not the internal air column).
  10. Sound waves are transverse waves (like water and light waves).
  11. Matter moves along with waves as the waves move through the medium (such as water or air)
  12. When waves interact with a solid surface, the waves are destroyed.
  13. In actual telephones, sounds (rather than electrical impulses) are carried through the wires.
  14. Ultrasounds are extremely loud sounds.
  15. Sound travels in a beam, in one direction, like light from a flashlight

Matter and Sound
A common misconception about sound and waves in general is that the sound comes from the specific air molecules moving from the source to the receiver (i.e. human ear). The common misconception is that waves carry matter with them as they propagate along (see “B” in figure 1). In some studies it’s been found that some students think that there is an actual “sound particle” that is created by the source and travels through the air, between the air particles, to the receiver (D, figure 1).
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Figure 1 - Incorrect views of sound travel [2]


How It Really Is
There are no “sound particles” as yet discovered nor any proof of such. When a sound is created, there is no significant net movement of matter from the source to the receiver, meaning that whatever the medium the sound is traveling through doesn’t get transferred from point to point. If this were so, a very loud or sustained sound (in air) could end up moving all matter away from it, creating a vacuum while cutting off its own sound.

As matter vibrates, it might bump into more matter, passing the energy of the vibrations. This could be anything from swinging a stick through the air from to stomping your feet on the ground to a vibrating guitar string. On earth, it’s hard not to bump into other matter, especially air. It’s all around. And although they are flying around freely, there are lots of molecules packed around each other. When something moves very rapidly, such as the vibrating guitar string, it pushes on the air around it. The air right next to the guitar gets pushed away into the air right behind it and bounces back. The air it bumps does the same, bumping into the air right next to it, then back and forth. This would be like someone getting shoved while standing in a tightly packed group of people. They will bump into the person in front or to the sides, push away and fall back where they were. The persons he bumped would bump the people around them and so on it a ripple effect. People are not good at transmitting this “disturbance” because they tend to stop it. Air, however, as well as water, bricks, concrete, and other kinds of matter don’t mind getting bumped around and will pass on these disturbances that we call sound. If what ever cause the air to bump and vibrate does so at a very fast rate (for instance, 400 times per second) then our ears, which are created to sense vibrations from the air, will detect it. The faster the vibrations, the higher the pitch (which is different than loudness but that’s another misconception…). We are limited, though. If whatever it is vibrates too slowly or too quickly, it still makes sound but our ears won’t hear it.

What is important is that although the air is vibrating back and forth, it is still roughly in the same spot; the sound is not from air flying from the source to your ear. Just like how the person who got shoved isn’t flying through the crowd, air molecules bump other air molecules. This causes some areas to briefly have air bunched up a little more than normal in one spot while thinning out a little more than normal nearby (Figure 2).


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Figure 2 - And interpretation of sound wave travel


Where Do These Ideas Originate?
It’s hard to say exactly why and how these misconceptions come about. I’ve looked at a few studies but mostly they were done to show the existence of certain misconceptions but weren’t created to find the cause.

What could be the cause of the misconception that sound pushes air along (or waves in the water pushing water along, etc.) is simply intuitive thinking from early observations in life. We notice that things have to travel through the air, or something, to get to us, like a ball or someone walking towards us or even mail getting carried along. We also tend to forget that air is actually made of something that is moved around by stuff that moves in it. It is a fluid, just like water or other liquids and gases. It seems rather likely that sound, too, must be something that is “shot off” (i.e. from someone’s mouth) and reaches your ear. We know it travels quickly because we usually hear sounds almost instantaneously (I say "almost" because here again is another misconception that making and hearing sound is an immediate event).

It does take some instruction and thinking, and perhaps some creative study and observation, to understand that sound is a wave (longitudinal) that does not move the air along, only the “disturbance” and the energy from some kind of vibration is transferred. Teachers giving instruction need to be aware of phrases they may say that could be misunderstood, such as "sound waves travel through air". Sentences like this, if not carefully explained, can leave a student to draw their own interpretation, correct or not.








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Sources:

[1] http://homepage.mac.com/vtalsma/syllabi/2943/handouts/misconcept.html#sound

[2] Cristina Periago, Arcadi Pejuan, Xavier Jaén, Xavier Bohigas; Misconceptions about the Propagation of Sound Waves. Departament de Física i Enginyeria Nuclear
Universitat Politècnica de Catalunya
Avda Diagonal, 647. 08028 Barcelona. Spain
Cristina.Periago@upc.edu