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rishch
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Are all mechanical waves composed of compressions and rarefactions ? Apart from sound waves what other mechanical waves are there ?
Are all mechanical waves composed of compressions and rarefactions ? Apart from sound waves what other mechanical waves are there ?
Could you answer my other questions too ?
Studiot said:Sure, ask away.
1)
'Displacement' is a general term for the quantity that varies in the wave as time goes on. It could be distance but need not be.
If you understand graphs, we plot time (and sometimes space) along the x axis. This is the primary or independant variable.
The Displacement is the quantity plotted on the y-axis and is also known as the dependent variable, because its variation depends upon its x (time) coordinate. The mean value is the value it has undisturbed (=without the wave).
So a plot of pressure variation above and below mean atmospheric pressure gives a sound wave.
A plot of voltage with time gives an electric wave such as the AC mains.
The maximum 'displacement' (= the maximum or minimum value the plotted quantity reaches) is called the amplitude.
2)
Take a guitar string plucked and vibnrating in transverse mode. Every element of the string is still part of the string and therefore still mechanically coupled to its neighbour. So any motion it makes has an effect on its neighbour. The transfer of (some of) its motion to its neighbour and from neighbour to neighbour is what makes the wave.
Every transverse wave has such a mechanism.
Does this help?
'Displacement' is a general term for the quantity that varies in the wave as time goes on. It could be distance but need not be......
If I say that by displacement they were not referring to the actual motion of the particles, they were referring to the density/pressure, is that correct?
And you missed my question on transverse waves in water molecules.
How is sound 'energy'
How can you measure the amount of sound energy ?
In my book they initially say that if a sound wave has greater amplitude then it will be louder i.e more amplitude=louder sound and then later on in the chapter they say that sometimes we use the terms 'loudness' and 'intensity' interchangeably but this is wrong because loudness is actually a measure of response of the ear to the sound. This means that 2 identical sound waves having same amplitude need not have same loudness (because we may here one wave better) which contradicts what they said earlier.
rishch said:Does that mean amplitude and intensity are the same thing ? My book says intensity is the amount if sound energy passing each second through a unit area. Also,in my book when they said that loudness is proportional to amplitude did they mean intensity instead of loudness ?
To "travel well" is a little fuzzy.Studiot said:Water is a (nearly) incompressible fluid so compression waves do not travel well in it.
The speed of longitudinal sound in water is almost 5 times larger than in air. Attenuation is about 50-70 times lower in water than in air (at 1 kHz).
nasu said:To "travel well" is a little fuzzy.
However considering speed of sound and attenuation, it may appear that compression (longitudinal) waves travel better in water than in air. The speed of longitudinal sound in water is almost 5 times larger than in air. Attenuation is about 50-70 times lower in water than in air (at 1 kHz). The whales and dolphins know this very well, I suppose. As the submarine sonar operators.
As a more general observation, the effect of decreasing compressibility is to increase the speed of longitudinal waves.
The effect on attenuation is not so straightforward. The solids with their low compressibility (lower than for water) are pretty good at transmitting longitudinal waves (they can travel across the whole earth).
sophiecentaur said:And the modulus of the mantle is so high that the speed takes a P seismic wave to the antipodes in about 20 minutes!
Of course there are excellent animations around but, along with the ever-popular Simulations, they may or may not deliver the right message when used in isolation. They never constitute 'proof'. Just look at what Pixar would have you believe.Bobbywhy said:The referenced website in post # 19 is not by Pixar.
"Acoustics and Vibration Animations
Dan Russell, Ph.D., Professor of Acoustics & Director of Distance Education
Graduate Program in Acoustics, The Pennsylvania State University
rishch said:Sorry, but I couldn't understand your answer.Also,here are a few more questions I have -
1)My textbook says that- "the magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.For sound its unit will be that of density or pressure." How ? If amplitude is displacement shouldn't its unit SI unit be meter.But why is it of density of displacement ? Is it because in a sound wave amplitude is proportional to maximum density ?
2)If in transverse waves the particles moves up and down ( if the direction of the wave is horizontal ), then how is this up and down motion transferred from one particle to another ?
If you understand graphs, we plot time (and sometimes space) along the x axis. This is the primary or independant variable.
The Displacement is the quantity plotted on the y-axis and is also known as the dependent variable, because its variation depends upon its x (time) coordinate. The mean value is the value it has undisturbed (=without the wave).
So a plot of pressure variation above and below mean atmospheric pressure gives a sound wave.
A plot of voltage with time gives an electric wave such as the AC mains.
The maximum 'displacement' (= the maximum or minimum value the plotted quantity reaches) is called the amplitude
Studiot said:Hello rishch, you seem to be having trouble with this idea
When you plot voltage or pressure or force or some other quantity on a piece of paper you obtain a graph of the variation of that quantity.
I'm sure you know that in a graph the quantity is represented by distance on the paper.
A posh term for the y value at any point would be 'the excursion from the mean'. You could use that if you like, many Victorians did. Other expressions you might come across would be 'the instantaneous voltage', the 'instantaneous pressure' , the deviatory pressure'.
I'd warrant that most people would understand and prefer a single term to cover all of these and I used displacement as logical since this is what you actually measure on paper.
Does this help?
There's an easy answer to that. The displacement of the molecules is different for each molecule. They are in violent random thermal motion so how would you measure it?rishch said:No, I understood what 'displacement' on a graph is.I'll tell you my doubt again.My textbook says that- "the magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.For sound its unit will be that of density or pressure." My doubt is- Why is the unit for 'maximum disturbance',density or pressure ? In other words if amplitude is the maximum disturbance, then why is it measured in units of density or pressure ?
Because they measure the disturbance in pressure or density.rishch said:No, I understood what 'displacement' on a graph is.I'll tell you my doubt again.My textbook says that- "the magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.For sound its unit will be that of density or pressure." My doubt is- Why is the unit for 'maximum disturbance',density or pressure ? In other words if amplitude is the maximum disturbance, then why is it measured in units of density or pressure ?
Mechanical waves are disturbances that travel through a medium, such as air, water, or solids. Compressions are regions where particles of the medium are closer together, while rarefactions are regions where particles are farther apart.
Mechanical waves are created when a source, such as a vibrating object, transfers energy to the surrounding medium. This energy causes the particles in the medium to vibrate and create the wave.
The difference between compressions and rarefactions is caused by the movement of the particles in the medium. When a wave passes through, the particles are pushed closer together in compressions and pulled farther apart in rarefactions.
No, mechanical waves can only travel through mediums that have particles that can move. For example, they cannot travel through a vacuum because there are no particles to transmit the wave.
Some examples of mechanical waves include sound waves, water waves, and seismic waves. These waves all require a medium to travel through and can be categorized as either transverse or longitudinal waves.