# Sound

Almost everything you need to know about sound is in this web page http://www.acoustics.salford.ac.uk/schools/index1.htm

some nice sound experiments here

http://members.impulse.net/~science/sound.htm

OP40 show that sound is a form of energy, and understand that sound is produced by vibrations

show that sound is a form of energy

Take a small bowl, cover the opening with cling film, spinkle some grains of rice on the surface, make a large noise (or signal generator), observe what happens.

The book 'exploring science' thinks you should place a table tennis ball in front of a speaker, It would want to be a very loud speaker, with a certain frequency, possibly defined by the length of the string .... try 330 hz for a meter long string

A candle flame in front of a speaker might work as it does in the video above.

OP40 understand that sound is produced by vibrations

sound is a set of oscillating pressure waves.

Take a 'shatterproof' ruler, hold it down on your desk and twang it. Change the length twang again ... and again ....

The ruler vibrates and

Watch this video and it shows that common or at least sounds you are familiar with are actually objects that vibrate at high speeds.

Sounds are made by vibrations, you can check out some of these vibrations and then make music by following the link

http://jwgreynolds.co.uk/index.php?/the-elastic-band/

To further show you that sounds are made by vibrations take a ruler, lay some of it off the desk holding it near the edge of the desk, twang it!

What do you notice ?

Change the length of ruler that is off the desk and repeat the twanging!

What has changed ?

Repeat until you can conclude something to do with what you are changing (the length) and what is changing due to that (the sound)

So what changes between these different sounds ?

Frequency

what is frequency, frequency is the number of times something occurs in a certain amount of time. In the case of sound we say it is the number of compressions that pass a point per second. Frequency is measured in Hertz (Hz), a thousand Hertz is often wrote as kHz, the k standing for kilo.

The frequency of a wave is the count of how many full waves pass a point per second.

In the diagram below imagine this is a snapshot of waves passing a point per second, how could you calculate the frequency of the waves?

Frequency = how often

Amplitude = how strong

Phase = in step or out of step

Frequency is used to compare other things that happen in a unit of time. Use bus timetables to find the frequency of different routes per week, timetables can be found at www.dublinbus.ie

For example the 40D has a frequency of 87 buses per week.

Pitch is a word also used for frequency

http://onlinetonegenerator.com/

Vibrations can be set up

more

The Amplitude is the power of the wave, a measure of the energy transferred

The wavelength is the distance between 2 peaks or 2 troughs in a wave

The speed of the wave is constant = frequency x wavelength

Sound travels at 330 m/s (usually)

So if a lightning bolt is seen and 7 s later the thunder is heard how far away is the storm ?

Multiply the speed by the time = distance

330m/s x 7s = 2310 m

A dolphin swimming in water, makes a call to its mammy, the mammy replies back the instant she hears the young dolphins call, the time taken for this signal to be heard by the kid is 4 seconds after the original call. If the speed of sound in water is 1500m/s how far away is mammy?

speed x time = distance

1500m/s x 4s = 6000m

however this is the distance to the mammy and back again, we only need half that distance

6000m / 2 = 3000m