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Sound Waves and Pressure

Investigation 3 – Concept Day








Sound Waves and Pressure: Investigation 3

Concept Day


  • This slide depicts a pressure wave that is caused by the vibrating tines of a tuning fork. If we could actually see air molecules and slow down the vibrating tuning fork enough to observe its movements, we would see that as a tine vibrates outward, it pushes air molecules away with a burst of mechanical energy. The tine then returns to its original position and there is a momentary reduction in pressure and mechanical force. The pressure wave that is created interacts with adjacent air molecules and the wave is propagated as shown in the upper part of the slide.
  • It is important to remember that the air molecules are not themselves moving along with the wave, but rather vibrating when they are impacted by it and then transferring that energy to the next air molecules down the line, and so on. It is the kinetic energy that moves through the air in a wave, not the air molecules.
  • On the lower half of the slide, the pressure wave from the vibrating air molecules above is represented as a typical sine wave. Notice that increasing pressure is in the upward direction. Each upward peak of the sine wave coincides with high pressure, while each peak below the centerline coincides with a low-pressure zone.



  • The upper part of this slide depicts the relationship of molecules to each other in different mediums. In a vacuum, there are no molecules to interact with each other. In a gas, molecules move rather quickly and collide with each other in a random manner. There is essentially no organization or sustained interactions between the gas molecules.
  • In a liquid and solid, the organization of their constituent molecules is increasingly firm and ridged compared to the gas. The gas is much more compressible than either the liquid or solid. The molecules in the liquid are more mobile in relation to each other than in the solid, where molecules are much more rigidly associated with each other through chemical bonds.
  • In the lower part of this slide, we have a plot of the relative speed of sound through the different mediums above. Notice that there is no sound at all carried through the vacuum. This is because, in order for sound to propagate, molecules much vibrate with kinetic energy from the source of the sound. Without any molecules, the vacuum is totally incapable of transmitting sound.
  • The speed of sound increases from the gas to the liquid to the solid. Let’s consider why. The propagation of a sound wave requires the transfer of kinetic energy from one molecule to the next. The distance between the molecules in the gas, liquid, and solid-phase points to why sound travels differentially between these mediums. In order for sound to travel through the gas, a vibrating molecule must wait to come in contact with another gas molecule before it can transfer its kinetic energy to it. In the case of the liquid, the molecules are more closely packed and the vibrations of one molecule will much more rapidly be transferred to another molecule than in the case of the gas. In the solid, molecules are even more tightly packed than in the liquid and sound energy can quickly be transferred from one molecule to the next. Therefore, sound waves can travel very rapidly through most solids.



  • In Investigation 3 Lab, you will model the speed of sound through gas, liquid, and solids. You will once again use the formula:

Speed = Frequency X Wavelength

  • This formula is rearranged to solve for wavelength as:

Wavelength = Speed / Frequency

Note: You can use the chart to the right in this slide to obtain the speed of sound through various materials.

  • Notice in the list of materials on the chart that some materials, like rubber and polyethylene, are not good at transporting sound and as such are often used in soundproofing rooms. A picture of polyethylene tile material is included for discussion. This is the type of material commonly used in recording studios. Cork, another material sometimes used for soundproofing, has a speed of sound of about 400m/s.



  • This slide is the first in a series discussing sonar. Sonar stands for SOund Navigation And Ranging. In sonar, a sound wave is dispersed into the water through a sender/receiver and travels through the liquid medium as a pressure wave. When the original wave comes in contact with an object (ocean floor, submarine, shipwreck, etc.), it is reflected back to the sender/receiver. Sonar calculates the time between the original signal and its echo and is able to determine the distance of the object.


  • This slide shows a sonar device being towed by a boat to chart the surface features of the ocean floor.



  • This slide shows the detailed images that surface sonar can produce. Both of the pictures depict shipwrecks. The image on the right uses Synthetic Aperture Sonar, a process that can give augmented resolution to submerged objects.



  • This slide shows Orca whales (Killer Whales) and begins a series of three slides that discuss how this animal and other whales and dolphins use sonar to find prey.



  • This slide shows Orca anatomy. Important structures for sonar are the nasal passages, the melon organ, the jawbone, inner ear, and the brain. The melon is a fat-filled structure in the head that helps modulate the sound waves sent out by the animal.
  • When sound is reflected back to the Orca, the jawbone absorbs it and the vibrations are transferred to the whale’s inner ear where it is transduced into nerve impulses and communicated to the brain. The brain is capable of interpreting the incoming echo signals to determine the precise distance and location of prey items. Apparently, Orcas are so good at this type of sonar that they are able to distinguish between fish species entirely by sound!



  • This final slide simply shows the events that occur underwater as an Orca uses its sonar to locate prey.