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Sound Waves and Pressure

Investigation 1 – Concept Day








Sound Waves and Pressure: Investigation 1

Concept Day


Note: In this Investigation, we will discuss sound as a vibration. Vibration is a motion, therefore sound is an example of kinetic energy. It is the kinetic energy of air molecules vibrating and interacting with adjacent air molecules, thus transferring kinetic energy from the original source of the vibration (a tree falling in the woods, for example) to our ears, where the vibrations are perceived and sent to our brains for processing and interpretation. We also want to introduce the concept that sound travels through the air as a series of pressure waves or sound waves. It will make sense that, since sound is generated by a source of vibrations of a specific frequency, the “waves” of kinetic energy transferring those vibrations through the air or other medium will have the same frequency. If we can get this fundamental concept through, you will find sound a readily understandably and very useful physical concept.



  • This is the first slide in a sequence devoted to explaining how human speech produces sound and how the human ear perceives it.
  • The two small inserts on the upper left simply show where the vocal cords are located in the human body.

Note: You can talk or hum while placing a hand on your own (or a partner’s) throat to find the exact location of the larynx. If you like, try to hum at very low and very high pitches and see if you can detect the difference in the speed (frequency) of the vibrating vocal cords.

  • A stroboscope is shown in the upper right of the slide. While the patient in this picture is smiling, having one’s tongue held with gauze and a metal tube inserted to the back of one’s throat may not be an entirely pleasant experience for everyone! The reason the instrument is called a stroboscope is that the light that shines on the vocal cords is pulsating like any other strobe light. In practice, the vocal cords vibrate almost too rapidly to be clearly observed by the unaided eye. The pulsating strobe has the effect of seeing only rhythmically selected vibrations, thereby “slowing down” the movements. Somewhat like a dancer appears to move in slow motion when illuminated by a strobe light.



  • When the vibrating sound waves reach the human ear, they cause the tympanic membrane or “eardrum” to vibrate at the same frequency as the incoming wave.
  • This kinetic energy of vibration is then picked up by stereocilia in the cochlea of the inner ear. As shown in the left section of the pair of black and white micrographs on the lower right, stereocilia are ridge rods. They vibrate at the frequency of the incoming sound waves from the eardrum through the closed liquid-fill inner ear. The stereocilia are connected to “hair cells”, which convert the vibrations into nerve signals. These are transported by the Vestibular and Cochlear nerves (shown in the drawing), which form the auditory nerve, to the brain: specifically to the primary auditory cortex of the cerebrum.
  • Returning to the two micrographs at the lower right of the slide, we see the results of an experiment in which stereocilia are subjected to intense noise of a high decibel level (“After Noise”). This demonstrates that stereocilia can be damaged by too high a decibel exposure. While rock musicians have taken to wearing earplugs to protect their stereocilia, their audience typically does not.



Note: Now that you understand how our vocal cords generate the kinetic energy vibrations of sound and our stereocilia perceive the vibrations, we must now discuss how the vibrations from the speaker to the listener occurs.

  • This slide draws an analogy between sound waves and a drop of water producing waves on the surface of a pool or basin. The similarities are many. For example, if sound is not directed in some way, it will spread in all directions. This is why we cup our hands to our mouths to direct our voices in a specific direction. Also, the more energy used to produce the waves, the more waves are produced and the further they travel. This is why we exert more force and yell across a soccer field when we want to be heard.
  • In addition to such analogies, we may also point out here that sound waves themselves can travel through a liquid medium like water. The inner ear is filled with water, for example, and the sound is carried as pressure waves to the stereocilia-bearing hair cells. In fact, sound travels very well in liquid indeed. If you submerge yourself in a community pool, you are able to hear splashing and voices emanating from quite some distance. We will see later that sound also travels very well through solids as well. How else could we hear the pounding beat of rap music coming from a car passing by on the road or a baby crying in its bedroom through a closed door?



  • This is the first of a pair of slides pointing specifically at the transfer of kinetic energy through the medium of air from the vocal cords of one individual to the stereocilia of another. The gray dots represent air molecules vibrating in response to the kinetic energy they receive from adjacent air molecules, originally derived from the vibrating vocal cords of the speaker. As they transfer the kinetic energy from molecule to molecule, the wave of sound moves from left to right in this picture.

  • Notice that the young lady on the left has cupped her hands to her mouth to direct the kinetic energy, and therefore sound waves, toward her intended target. On the other side, notice that the blonde girl has cupped her hand to her ear, thus collecting more of the kinetic energy set her way from her friend and, at the same time perhaps, shielding other extraneous sound waves from interfering with those she is interested in.
  • Each peep of sound has its own loudness (we will learn later that this is referred to as amplitude) and pitch (frequency of vibration). Each utterances sends out a unique wave. In this slide we see the waves coming from a single peep. In the real world, it would be followed immediately by other bursts of kinetic energy with their own combinations of loudness and pitch.
  • The human ear is a magnificent instrument. It senses sound from a wide range of frequencies and amplitudes. However, these signals would be of limited use if the process of hearing stopped there.
  • As mentioned above, the signals are collected in the cochlea and sent to the brain for “processing”. It is in the brain, not the ear that we interpret sound. The brain compares the flow of nervous impulses reaching it to patterns of impulses that it has encountered previously and allows us to identify the sound as that of our friend’s voice, for example. When a door slams down the hallway, students interpret the combination of sound waves as a door slamming without seeing it almost instantaneously.
  • Understanding how to produce the intricacies of human speech on the one hand, and interpreting the complex patterns of sound waves produced by it on the other, is among the most spectacular achievements in biology.



  • In the second slide of this pair, we have simply introduced a wave to the illustration. Notice how the peak of each part of the wave overlaps the compressed air molecules as both the wave and the vibration of air molecules travel from mouth to ear. We will continue a discussion of waves on the next slide.



  • In this slide, we set up an analogy of a jump rope and a wave. The two children holding the rope represent the nonmoving nodes. The extreme “peak” of the rope when moving is the antinode. The entire rope is symbolic of the standing wave. That is, the standing wave doesn’t go anywhere. It stays between the two nodes. It moves up and down when viewed from the side.
  • Anyone who has jumped rope knows that as the twirling rope rushes past our heads, we can feel a breeze from it. If we are nearby watching and the rope is being twirled with enough kinetic energy, we may feel the breeze as well. Pause and think about this for a moment. What are we actually feeling? We are feeling a rush of air molecules that have been pushed by the rope! This is a perfect illustration of how a standing wave, like the rope in this case or a guitar string, which stays firmly attached between its nodes, forms a pressure wave that can move away from the standing wave by transferring kinetic energy through the air.

Note: Before leaving this slide, let’s consider the jump rope analogy further.

  • You may have noticed that a spinning jump rope may make a sound as it buzzes by our ears, particularly when we hold the rope ourselves.

  • Have you ever noticed a difference in the pitch of that sound, which is how high or low a pitch or note it makes, as you increase or decrease the speed at which the rope is twirled? It turns out that the faster the rope is twirled, that is the number of times it makes one complete cycle per second, the higher the pitch!



  • A sound wave looks more like this, as if the two rope twirlers whipped the rope up and down while holding onto the ends. The standing wave (solid black rope) has a node at each end and one in the middle. When the rope is whipped like this, it has two antinodes where it reaches its peak above and below.
  • The blue dashed line shows the pressure wave caused by the kinetic motion of the rope. It looks just like the standing wave but just in an opposite direction. The complete standing wave extends from one of the children (end node) to the other. For sound waves, this distance is known as the wavelength. Notice that the resulting pressure wave has the exact same wavelength as the standing wave.
  • On the next slide, we will see how the wavelength of a sound wave is related to its frequency and pitch.



  • This final slide shows two sound waves. Each sound wave consists of an upper and lower peak. The faster an object vibrates, a guitar string, for example, the shorter its sound wave. Shorter sound waves are said to have a higher frequency. The frequency is defined quite simply as the number of vibrations per second.
  • In the upper example of a wave, one complete wave cycle takes only 1/880th of a second. That means that the object that caused the wave was vibrating 880 times per second. By definition, this means it has a frequency of 880Hz (Hertz).
  • The lower sound wave has a lower frequency. In one second, it only completes an entire cycle 440 times, half as many times as the upper wave. This means that the object that caused the lower wave was vibrating at only 440Hz.
  • The frequency of the vibration and the resulting pressure wave (sound wave) are related to the pitch we hear. Higher frequencies have a higher pitch than lower frequencies. When we hear music, we hear a rich mixture of frequencies and pitches caused by the many vibrations of strings of instruments and the vocal cords of singers.

Note: We have said little regarding the important concept of wavelength in this Investigation other than that it is determined by the distance between the two end nodes of a vibrating standing wave or the pressure wave it produces. In the next Investigation, we will explore how wavelength is related to frequency and how we can use this relationship to actually calculate the speed of sound in the Lab.