# Sound

Sound waves are longitudinal vibrations which produce pressure waves. In dense materials, sound travels faster than in less dense materials, because the vibrating molecules push against others closer to them, so energy is transmitted more quickly. We usually hear sound through the air. In air, speed increases with increasing temperature, as shown.Frequency is the number of waves every second. Pitch is what we actually hear, or the sensation of frequency. An octave is the name we give to a frequency ratio of 2:1 ( example 256Hz to 512Hz). Timbre is the quality or the musical aspects of the note we hear, so we can tell the difference between the same note played on a violin or a trumpet.

These three waves show the idea.

Each is playing the same note and the waveform is recorded and displayed on an oscilloscope. e) is a French horn, f) is a clarinet and g) is a violin.

Notice that the spacing of each wave is the same  – the same frequency – and their loudness  (amplitudes) are the same as well. It’s just that they ‘sound different’.

Now for some biology, This link shows how the energy of the vibrating air particles gets converted into sound which we can hear. Our ears are sensitive to a range of frequencies between 20Hz and a maximum of 20,000Hz (20kHz) I am very old, and can only manage about20Hz to 11kHz, but my baby grandson can hear right up to 20kHz. My dog can hear up to 35kHz. If I had a pet bat (which I haven’t!) it could hear a sound at a frequency of 150kHz.

Is there a sort of ‘rule’ here? Do big animals hear at lower frequencies than little ones. Perhaps. Have a look at this… You know what… maybe not. Also, we as humans don’t hear all frequencies equally well.

Here’s a graph from a diving site. Look at the black airborne one first. We need very much higher pressure waves  at low frequencies to hear them well, and much lower pressure (or smaller amplitude, quieter sound) at about 3-4kHz, so we hear these best.

Devices that make a noise to attract our attention make sounds in this sort of range so we don’t miss them.  My washing machine plays me a little high-pitched song to tell me when it’s done.

Also, look at the blue part of the graph. Notice that we hear better underwater, perhaps because sound travels almost five times faster than in air.

Finally, listen to some humpback whales  communicating with each other (Right click and open the tab in a new window works best)