Two source sound wave interference

This links with the ripple tank investigation where you used a double slit to produce two sets of circular waves spreading out.  In that investigation you noticed ‘dead spots’ in the water, where a peak of a wave met a trough of another wave and, when added together, the two cancelled each other out; this is called destructive interference.  You also noticed strong wave patterns in the water, where a peak and a peak would meet or a trough and a trough would meet and make a larger peak or trough; this is called constructive interference.

a) Theory research

Read this page until you get to the heading ‘Musical Beats and Interference’.  This links the ripple tank to the practical you are about to write up.  For an interactive simulation of two-source interference, click here.  Have a play and see what effect changing the spacing or wavelength has on the dead spot patterns.

b) Practical investigation

Write up the investigation you did into interference between two sound sources.  If you tried more than one method, mention it (see evaluation).  Include photos and a diagram of the setup (two speakers connected to a signal generator, one metre apart from one another, Sound Level (SPL) meter being moved in a straight line across at a distance of your choice).

Include a full set of data and a graph showing the patterns in the SPLs.  Draw conclusions – remember that ANY fluctuation in the SPLs is evidence that the waves from the two speakers are interfering with one another.

Evaluate your investigation –  why was one method superior?  How could you improve the better method further and what things would be difficult to improve?  Think about matching speakers, room reflections, background noise, wind, and so on.

c) Apply to Sound Systems

Read this page from the line ‘A final application of two-point source interference …’ until it starts talking about light bulbs, and the section on Interference on this page.  This page (in the Nodes and Anti-nodes section) talks about designing venues with rough surfaces and baffles – try to find out how they help.

Can you explain why some audience members might think the sound is too loud, while others can hardly hear it (no, not because they’re old)?  What is so important about room design if you want a good experience for every audience member?