Sound travels as waves at about 340 m/s in air and if you can’t remember some of the terminology about waves you might look here at a previous post. You could also look at this excellent page to play with an animation exploring amplitude, frequency and wavelength. (Don’t forget that you can also use the Focus e-Learning software by logging into your RM Unify account – you will find some stuff about Sound in the Physics section.)
Sound waves are not quite like waves in the sea nor the waves drawn to illustrate them, they are pulses of vibrating molecules. They consist of dense peaks and less dense troughs so they are really variations in air pressure. It is these variations that our ears detect allowing us to interpret them as sounds. There are some interesting illustrations of this here.
The closer together the peaks are, the higher the frequency and therefore the higher the pitch. If you double the frequency of a sound then its pitch increases by one octave. The reference note to which orchestras tune is an A at 440 Hz. Some orchestras tune to slightly higher (about 442 Hz) to give themselves a brighter sound. As long as all the instruments are tuned to the same reference then the instruments will work together as an ensemble.
An octave is divided into 12 equal notes called semitones. You can get from one note to the next one above by multiplying the frequency by 1.05946. Try it with the note A; start with 220 Hz and work your way up to the octave above in semitone intervals e.g. 220 Hz × 1.05946 = 233.1 Hz (A#) × 1.05946 = 246.9 Hz (B) × 1.05946 = 261.6 Hz (C) etc. until you get to an A (440 Hz) one octave up from your starting position. You can see it all here with buttons to push and everything!
Sound will not travel through a vacuum because there are no particles to transmit the waves. Sound does travel well through solids, liquids and gases. The speed of sound through steel is roughly 6100 m/s. The utterly fabulous machine in the video below is called a Chladni plate with salt shaken over it. We now have one of these in the department.
If you have ever made an annoying sound from a wine glass by rubbing a damp finger around its rim then you might consider stepping up to this challenge.
This is the video from Twig about the speed of sound that we watched in class.
Questions…
- What property of a sound is governed by the amplitude of the wave?
- Roughly how long does it take a sound wave to pass through a 1 km length of steel?
- \Roughly how long would it take for a similar wave to cover 1 km in air?
- What property does steel have that allows it to transmit sounds more quickly than air?
- Does sound travel faster through hot water or cold water?
- Explain your answer to Q5.
This is a somewhat more sophisticated video about the way sound travels through different materials but it is well explained so might be worth a view.
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