Posts Tagged ‘demonstrations’

See the unseen

To promote their new vibration analyzer, measurement instrumentation company Fluke commissioned some amazing high-speed video of—you guessed it—things vibrating!  Putting the fascinating physics of vibrating plates and cylinders aside, you will have to admit that it’s interesting how a cymbal deforms at 1,000 frames per second.  (And if that doesn’t do it for you, the shaking basset hound at the end is pretty nice too!)

Listen up, kids

The Acoustical Society of America, an international scientific society, has unveiled a new educational website aimed at kids (and their parents and teachers):

Explore Sound

Aimed at developing an early interest in “the science of sound”, the Explore Sound site features information on the science of acoustics, online demonstrations, project ideas, and curriculum materials for teachers (including a free series of posters, available on request in any of six languages).  The site even details some of the things that we acousticians do for a living, and what could be more interesting than that!

Catch a wave

You may know that the sounds you hear travel through the air as waves, but the invisibility of air makes this concept a tricky one to visualize.  For those who like physics demonstrations (and who doesn’t), we recently came across this video of a series of pendulums—and the pendulum is perhaps the most accessible form of wave motion we witness in everyday life.

A pendulum’s length determines its frequency, just as sound waves in air have a frequency that corresponds to pitch.  The demonstration superimposes different frequencies to illustrate traveling waves, standing waves, beats, and “random” noise, which are all phenomena that come from mixing different frequencies together.

Visualizing modes

The Graves on SOHO VoIP blog tipped us off to this cool video showing the vibration of a square plate at different frequencies. By covering the plate with salt, we can see the areas where the plate vibrates a lot (the salt rolls away) and the areas where it doesn’t vibrate at all (the salt collects). These spots and lines of little or no movement are called nodes. As the driving frequency (and sound) gets higher and higher, the patterns (called mode shapes) get more and more complex (and cool looking!)

This behavior is a great example of why we don’t use big speakers (woofers) to generate high-frequency (treble) sound. Instead of moving in unison like a piston, the speaker cone resonates internally at high frequencies, and different parts of the cone are moving in different ways, like a group of uncoordinated smaller speakers. This causes a poor frequency response, with lots of peaks and dips — often referred to as “breakup”. Smaller drivers (tweeters) are more at home with high-frequency sound, since their resonant “breakup” occurs at frequencies near or beyond the limit of hearing.

This is also a great way to visualize the analogous effect of room modes. Instead of a vibrating plate, a room is full of vibrating air, and at certain frequencies there will be points or areas in a room’s volume — nodes — with very little sound. This is most pronounced in hard, reverberant rooms, and at low frequencies. Still, even though the effect is more subtle in normally absorptive rooms, it can wreak havoc with the reproduction and recording of low-frequency sound, so identifying and managing room modes is a common task in the design of recording studios and listening rooms.