# Mechanical waves compressions and rarefactions

sophiecentaur
Gold Member
2020 Award
I would agree with the above.
It may help to concentrate on one particular wave - of your choice - and then see how the 'displacement' idea, in that context, can apply to all waves in general.

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?

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 ?

sophiecentaur
Gold Member
2020 Award
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 ?
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?
You CAN measure pressure so that's how we choose to describe a sound wave. Likewise, density is something you can measure so it's a good parameter to discuss.

nasu
Gold Member
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.
Without the disturbance, the pressure, let say, is the same everywhere (Po).
A disturbance means that somewhere the pressure is slightly increased or decreased to a value po + (or - ) p(t). p(t) measures the value of the disturbance. The maximum value of p(t) is the amplitude of the disturbance. The maximum value of p(t) is a pressure so it has units of pressure.
Similar for density.