Physics 11 Waves 1 - Vibrations and Waves Mr. Jean

The plan: n Video Clip of the day oo oo n Introduction to waves n Ways to transmit & transfer energy n Wave interference

Picture of a Transverse Wave Crest Trough Wavelength A A - Amplitude

is true for all waves. Note: v is dictated by the medium. (must change medium to change v) f is dictated by the source. (must change the source to change f ) Demo - Complete Bell Wave Machine

5.TRANSVERSE WAVES Demonstration: Transverse Waves Examples: string musical instruments ripples on water electromagnetic waves

6.LONGITUDINAL WAVES Video - Slinky Longitudinal Waves Parameters Rarefactions are regions of low density. Compressions (condensations) are regions of high density.  is the distance between successive rarefactions or successive compressions.

7.INTERFERENCE Video - Superposition of Waves SlideSlide - Interference Slide

Constructive interference occurs when waves are in phase, that is when crests are superimposed and troughs are superimposed.

Destructive interference occurs when waves are out of phase, that is when crests are superimposed with troughs.

Interference is a characteristic of all waves. 1) Demonstrations with Audacity 2) BYU Physics Demonstrations

Standing Waves n n When two sets of waves of equal amplitude and wavelength pass through each other in opposite directions, it is possible to create an interference pattern that looks like a wave that is “standing still.” It is a changing interference pattern.

n There is no vibration at a node. n n There is maximum vibration at an antinode.  is twice the distance between successive nodes or successive antinodes.

Sound Waves: n The disturbance which travels through air is the compression of air molecules – they are squeezed together and pulled apart. Sound is a series of traveling high pressure and low pressure fronts.

Pressure vs. Time

Wave Videos: Bright Storm: EXCELLENT PHYSICS VIDEOS n 1B F9D 1B F9D 1B F9D

Resonant Air Columns: n Have you ever blown into a pop bottle and gotten the thing to make a nice, deep, melodious sound? u Bottles can do this because they will resonate. u When you blow across the top of the bottle, you create turbulence – bubbles of air – which occur at a broad band of frequencies. u This is called the edge effect. One of those frequencies is the bottle’s resonant frequency.

Air Columns: n A standing wave forms in the bottle’s interior. As energy is fed in from the blowing thing, the standing wave gains energy until it is loud enough to hear.

Closed Ended Pipes: n The reason that the bottle resonates is that a standing wave forms in it. The wavelength of the standing wave has to "fit the bottle", so only the one frequency (or its harmonics) will resonate and be heard. The other frequencies aren't loud enough to be audible. u The closed end of the pipe is a displacement node because the wall does not allow for the longitudinal displacement of the air molecules. u As a result, the reflected sound pulse from the closed end is 180  out of phase with the incident wave. The closed end corresponds to a pressure antinode.

Reflection to fixed ends (Rigid): n For fixed end reflection think of the medium as being constrained in its motion. n In the picture to the left you see a string that is securely fixed to the wall. n The string (the old medium) is free to move up and down, but at the boundary where it meets the new medium (the wall) it is constrained – the string can’t really move up and down like it could before. n In fixed end reflection, the wave that is reflected back is out of phase by 180 .

Reflection for non-fixed (not rigid… “open”): n In free end reflection, the medium is free to move at the boundary. The reflected wave will be in phase. In the drawing on the right, you see an erect pulse traveling into the boundary being reflected with no phase change. The pulse went in erect and came out erect. Water waves reflecting off a solid wall are a good example of free end reflection.

Closed End Pipes: n The open end of the pipe is, for all practical purposes, a displacement antinode and a pressure node. u The reflected wave pulse from an open end of the pipe is reflected in phase.

Harmonics: n The first one is a quarter of a wave. This is the lowest resonant frequency that can form a standing wave in the tube. Note that the closed end reflects the sound wave out of phase - like a fix-ended wave is reflected.

Closed Ended Pipes: n Here fn is the harmonic frequency that resonates in the pipe, n v is the speed of sound, n L is the length of the pipe, and n n is an integer for the harmonic that you want.

Closed Ended Pipes in general: n This should go on your formula sheet n The wavelength for any harmonic would be:

Open Ended Pipes: n Open Ended Pipes: Open-ended pipes can also resonate. At both ends of the pipe, the wave is reflected in phase. The fundamental wave and associated harmonics would look like this:

Open Ended Pipes: n Put this on your formula sheet

Musical Instrument Specific Frequencies:

Superposition of waves: n The graph above shows the intensity of the different harmonics for the same instruments. This is why they each sound different to our ears.

Speed of Sound: n The speed of a sound wave refers to how fast the disturbance is passed from particle to particle; speed refers to the distance in meters which the disturbance travels per unit of time in seconds.

n The speed of a sound wave in air depends upon the properties of the air, namely the temperature and the pressure. v = 331 m/s + (0.6 m/s/C)T n where T is the temperature of the air in degrees Celsius. Using this equation to determine the speed of a sound wave in air at a temperature of 20 degrees Celsius yields the following solution.

Calculations for 20 degrees:

For Thursday/Friday: 4 people per group. u All group members are responsible for a completed lab. u Staple all labs together and marking top lab only.