# What is the frequency of the sound?

Can someone tell me what is the frequency of the sound waves? Is it the number of compression or rarefaction going through in a second or the number of vibrations of the particles of the medium through which the sound travels per second? Are they the same thing? If so, how? I'm in 10th grade so please use simple language and less technical terms

• One compression and one rarefaction is one cycle. So if you have ten of each per second, the frequency is ten Hz. Mar 3, 2022 at 9:29
• @AakashGarain Related: see also my answer here (especially the nice animation I found, near the bottom), and check out the other answers as well. Mar 3, 2022 at 22:58

When sound travels through air, the frequency refers to the compression and rarefaction, not to a vibration of individual molecules.

The particles in the air are moving and colliding with each other at random. Typically an individual molecule might move at around 500 metres per second and collide several times per microsecond, so the motion is fast and chaotic. You can think of the motion due to sound to be a superimposed movement on top of the random movement of individual molecules, much as the current in a river is an overall movement quite distinct from the random motion of the individual water molecules.

• Oh my god,I can't thank you enough,you saved my life and me breaking my devices because I wasn't able to find a answer to this,and yeah thanks a lot Mar 3, 2022 at 9:24
• And also,the hearing range of humans is 20-20,000 hertz frequency,so,this means that the sound waves whose compression or rarefactions move 20 to 20,000 times within one second are audible to us,right?NOT the 20-20,000 vibrations of one air particle,but,20 to 20,000 compression or rarefaction travelling in one second,right? Mar 3, 2022 at 9:27
• Yes, you now understand it perfectly! Mar 3, 2022 at 9:47
• Hope you achieve all of your dreams and ambitions Mar 3, 2022 at 9:48

I'm trying a slightly different angle here.

Marco Ocram is correct that air molecules in a room randomly bounce around at pretty high speeds (depending on temperature). That has nothing to do with sound and any sound that happens is simply overlaid over this basic movement.

Sound DOES involve air movement though. Sound is basically small pockets of air wiggling back and forth. When adjacent pockets move toward each other the pressure increases, which in turn pushes them apart again. A change in pressure creates movement and a change in movement creates pressure. This back and forth is what makes a wave happen.

The velocity of these air pockets is well defined. For sound pressure of 90 dB the air pockets move with a speed of about 1.5mm/s (in free air without any surfaces nearby).

Frequency is simply how often per second they wiggle back and forth. If they move back and forth 1000 times per second, the frequency is 1000 Hz or 1 kHz. The frequency is always the same for the wiggling and the compression/refraction cycle.

Many textbooks actually ascribe the movements to the individual air molecules, which is technically not correct since the movement of an individual molecule is dominated by its thermal motion. However, the image of "air particle moving back and for" is a very useful description of what's happening. See for example: https://opentextbc.ca/universityphysicsv1openstax/chapter/17-1-sound-waves/

• What you are saying as sound does have air movement is the vibration of a set or layer of air molecules right?like I know they vibrate back and forth colliding with the adjacent layers of air molecules and in the process gaining kinetic energy and transferring it,and if we make a displacement-distance or displacement-time graph of an individual air molecule,the graph's frequency will be how many vibrations the air molecules does per second,am I right? Mar 3, 2022 at 12:05
• Correct. Here is a useful animation: resource.isvr.soton.ac.uk/spcg/tutorial/tutorial/Tutorial_files/… Mar 3, 2022 at 12:14
• So, everything I said in the previous paragraph was correct? Mar 3, 2022 at 13:00
• Mostly. Depends how deep you want to go. There are two forms of energy: kinetic energy and potential energy. High velocity = high kinetic energy; High pressure = high potential energy. That's pretty normal for waves: the total energy bounces back and forth between two different types of energy . Mar 3, 2022 at 13:08
• Hmm,maybe not so deep,so,at a basic level, everything else is fine,right? Mar 3, 2022 at 13:18

Imagine a beach. Frequency is the number of waves that come in on that beach per second.

• So,it is not the number of times a particular air molecule vibrates or oscillates in one second, rather,how many compression or rarefaction travel in one second,right? Mar 3, 2022 at 20:23
• That is correct. Individual Molecules can all be doing their own thing: en.wikipedia.org/wiki/… Mar 4, 2022 at 13:50