SLP 409 (1)

1. Sound; Pressure (waves)

  • Sound is a mechanical vibration that travels through a medium (like air) as a series of pressure changes. The air normally has ambient pressure (Pam). When a force (such as a vocal fold vibration) acts on a group of air molecules, it creates a localized increase in pressure that then propagates outward as a wave. In this way, sound is essentially energy passed along by molecules compressing and then returning to equilibrium rather than the molecules themselves traveling long distances. 

    • The air around you has an ambient pressure, Pam


2.Describe sound as a pressure wave using the terms compression/peak, rarefaction/trough.

  • compression/peak: the molecules are forced closer together, creating a region of higher pressure. On a pressure–time graph, this appears as a peak

    • When a force pushes on a group of air molecules, they move closer to the molecules next to them (compression)

  • rarefaction/trough: as the compressed molecules rebound (due to elastic forces), they create a region where the molecules are more spread out, leading to lower pressure. This is seen as a trough on the graph. 

    • The pressure from the first group of molecules causes the second group to move away

    • At the same time, the first group of molecules heads back towards its rest position due to elastic forces

    • This creates a region of low pressure (rarefaction)

  • This alternating pattern of compression (peaks) and rarefactions (troughs) constitutes a pressure wave that we perceive as sound

    • The second group of molecules, in motion due to inertia, will press on the molecules next to them (creating compression), then snap back (creating rarefaction).


3.“When I produce a speech sound, air molecules from my lungs travel to your ear.” Is this statement accurate? Why or why not?

  • While producing speech does create a pressure wave in the air:

  • Reality: The air molecules themselves do not travel all the way from the speaker’s lungs to the listener’s ear. Instead, each group of molecules oscillates around it equilibrium position

  • Mechanism: energy is transmitted as each molecule passes on its motion to adjacent molecules in a chain reaction. This is possible because of the elasticity of the air, which helps restore molecules to their resting state

  • Therefore, it is the energy (the pressure wave) that travels, not the individual air molecules making a long journey

    • Note: Due to elasticity, the molecules that transmit sound to the eardrum do not travel all the way from the sound source to your ear

    • But by passing the energy to many sets of molecules in succession, the wave as a whole can be transmitted over a long distance

  • This statement is not accurate because when you speak, your vocal cords create pressure changes in the air. However, the individual air molecules do not make a long journey from your lungs to someone else’s ear. Instead, each molecules vibrates back and forth around its resting spot. They pass on the energy of the sound to nearby molecules. Its like a “wave in a stadium where people stand up and down in sequence—the energy moves around, but the people (or in our case, air molecules) mostly stay in place


4.Describe the oscillatory movement of particles in terms of the interaction of inertia and elastic forces.

  • When air molecules are displaced by a sound source://when a sound wave is created, air molecules are pushed away from their normal positions:

  • Inertia: keeps the molecules moving in the direction of the initial push

  • Intertia: this is the natural tendency of a moving object (or molecule) to keep moving. Once a molecule starts moving, it wants to continue in that direction

  • Elastic Forces: act to pull the molecules back toward their equilibrium position

  • Elastic Forces: think of a rubber band that wants to snap back to its original shape. In the air, elastic forces act like this–they pull the molecule back to its normal resting position

  • Because of intertia, a molecule overshoots its resting position, and the elastic (restoring) forces then pull it back. This interplay casues the molecules to oscillate(back-and-forth movement) back and forth. As one group of molecules compresses and then rebounds, it creates a new compression in the neighboring group, which in turn repeats the cycle–thus propagating[generate/spread] the sound wave


5.Define or draw a diagram to illustrate the following properties of waves: frequency, period, amplitude. What is the relationship between frequency and period?

  •  

    • The period of a wave is the time it takes to complete one full cycle of pressure changes (seconds per cycle)

    • A more common measure is frequency, which is the number of complete cycles occurring per unit ot time (cycles per second = Hertz/Hz)

    • Frequency and period have a reciprocal relationship → F=1/t ; t=1/F

    • So if you know the frequency, you know the period, and vice versa

    • Amplitude is the magnitude of the maximum change in pressure associated with the wave 

  •  DIAGRAM??!!  → written in paper notes


6.Identify the perceptual counterpart of each of the following properties of sound waves: frequency, intensity/amplitude.

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    • The perceptual counterpart of Frequency & Period is pitch

    • The perceptual counterpart of amplitude/intensity is loudness


7.Define constructive and destructive interference. Explain how the phase relationship of waves meeting each other leads to constructive or destructive interference.

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    • Phase (relative timing of peaks and troughs) dictates what happens when waves meet or interfere with one another

    • If a peak meets a peak, the resulting wave has greater amplitude = constructive interference

    • If a trough meets a peak, the resulting wave has lower amplitude = destructive interference


8.Define: periodic wave, aperiodic wave. Explain how periodic and aperiodic waves are perceived in terms of sound quality (include speech sound examples).

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    • A periodic wave is made up if identical repeating cycles of equal duration (period) → periodic sound waves have a pitch. Speech example: Vowels 

    • In an aperiodic wave, there is no repeating pattern of period or waveform shape → aperiodic waves sound like noise rather than musical pitch. Peech example: Fricatives


9.State the difference between simple and complex waveforms.

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    • A simple waveform has only one frequency

    • A complex waveform is made up of two or more different frequencies. Can be periodic or aperiodic