# Dhanashree (13) asks “Why is sound a longitudinal wave?”

Dhanashree, thank you for the question. I asked my team about the question and they arrived at the following answer….which I was not happy with …….

There are two types of ways in which waves manage to travel ……. Longitudinal and Transverse. I can see the way the waves they develop BUT need time to think about how to explain them. Longitudinal waves travel along straight lines away from the source …  I am trying to understand how the particles associated with the travelling disturbance (sound) travel in each of these ways.

How do the particles of air (in longitudinal movement) make further particles of air move in the same way…if they are doing forward and back vibrations? The vibrating air particles (or particles of any gas) must be moving forwards as well as moving back and …thus moving the wave forward.

So …to answer your question …..sound is created by things vibrating, the drum skin vibrates, up and down when it is hit…….the air above the drum skin follows this vibration, and the air next to the vibrating air , starts to vibrate and so on. This is your longitudinal vibration.

With Transverse vibrations the particles move up and down. Like waves …which stay in the same place.  More thinking about this later.

# Pete (10) asks “What is the pitch of a sound?”

Thank you team.  Lets first look at the motion of the particles. You suggest that the motion of the particles is in the form of waves. I think that is quite difficult to imagine but I think I have an example that will help illustrate this type of motion. Look at what happens when you drop a pebble in a pond. The pebble, when it hits the water, it creates  one vibration.

In this image the sound is produced by the piston creating the waves. Notice the wavelength, that is important. If we can manipulate the piston, make it go slower or faster  we can change the wavelength. Changing the wavelength is changing the pitch, think about that.  How does the sound change?

Try blowing over, or tapping, some bottles.

Pitch and Frequency ..Test your hearing a little bit more …….

In the video below you can see and hear how the pitch of the sound and the wavelength change together.  We measure wavelength in units called Hertz. 1 Hertz is one cycle per second. In the image above imagine that it takes one second to get from the flute to the ear. Then there are 8 cycles in the top sound is so frequency is 8 hertz and there are 3 cycles in the bottom sound so the  wavelength is 3 Hertz.

You can now test your hearing.    Take Care….make sure you have control of the volume.

Thanks to
Orion Lawlor, for the water ripples video, Published on 9 Jan 2011
Earmaster at https://www.earmaster.com/music-theory-online/
The ISVR from the Institute of Sound and Vibration Research, University of Southampton.
The Sound Video, unknown but thanks.

Thank you team. Some of the things in the image above make sounds without tapping or blowing. Can you find them? Miwah asked a similiar question where he investigated how the sound a drum made was created. Can you try an make a sound using a ruler. Try holding one end of it on the desk and then bending it slightly by pushing down on the end which is not resting on the desk.

Now let go ….. and here come the questions.  Did you hear anything? What did the end of the ruler do? If there was a sound when did it stop?

Another question. If there was a sound what part of your body detected it? So how did the sound get to you? What is between your bodies sound detection and the ruler?

Now for a little bit of information. How did the ruler move when you let go of it? The word to describe this movement is ‘vibration’. A vibration is a continuous small movement up and down, or side to side. This is what Miwah discovered in the drum investigation. Below is a small video of a plastic ruler being pushed downwards and then allowed to vibrate.

Investigate making other objects making sounds and see if you can ‘feel’ the vibrations.

Any more thoughts , then please make a comment or ask another question.

# Miwah (7) asked ” I hit a drum and it produced a banging sound. How did it do that?”

Miwah, what a great question. It really needs some thoughts from my team.

So Miwah what are your thoughts? What happens to the rice when the sound  occurs?

Is there anything around the rice? If there is will that behave in the same way?