Introduction

Explorations to do inside the sound column exhibit at the Exploratorium. The sound column is a cement tube 20 m, 60 ft, high. There is a xylophone-like set of aluminum bars inside the sound column tuned to resonant frequencies of the column. Twenty people can stand in the column.

To do and Notice/ What’s going on?

Resonance

Hit the bars with a mallet, Listen to the long sustain.
It’s in-tune today
“How do you know that?” Tap the bars gently with a finger tip. Listen to the growth of the sound. When the sound wave in air travels up the column and back in exactly the same time a bar vibrates up and back then the sound is made louder at each pass. The sound starts off soft and grows louder. When it’s out of tune the sound builds and then decays as the bars and the sound wave move in and out of phase, or the sound just never builds.

The bars are tuned by moving the steel bars on their ends in and out.
The tuning of the column changes because the speed of sound changes, the speed of sound in air is a function of the temperature.

Vibration

Strike the largest plate, put a coin on the plate and watch it bounce.
In the middle of the plate the coin bounces a lot, this is an antinode. At the suspension the coin bounces a little.

Feeling sound

Place your hand near the top edge of a vibrating plate and feel the air vibrations which comprise the sound. You can feel loud, low frequency sounds as vibrations.

Hearing sound at nodes

Bend down and put your ear near the floor. The floor is a node of motion for the sound wave. You can hear the sound quite clearly. raise your head off the floor, at some point the sound will fade away almost completely, this point is an antinode, i.e. at that point the air molecules are moving back and forth with their greatest amplitude. This shows that your ear is an instrument which picks up pressure changes not the displacement of the air.

A deep Philosophical Question

Play two adjacent bars listen to the beat note. Move your ear up and down from the pressure node of the sound from one bar to the pressure node of the other. With your ear at the node you can only hear one note at a time. If you play three notes at a time and move your head just right you can play a tune. This raises the question, is the tune in the air of the sound column or not?

Math Root

Note the tile pattern on the wall. The pattern shows the standing wave of pressure produced by each bar. The patterns are color coded, so are the bars.
How many of the longest waves are there in the column? Note that only a half wavelength is portrayed. There are four half-waves in the column so the lowest note is the 4’th harmonic of the column. Six half-waves of the second note fit into the column so the second note is the 6’th harmonic. The fundamental frequency of the column is 7.5 hertz. The fourth harmonic is 30 Hz, just barely within the range of human hearing which extends down to 20 Hz.

Beats

The beat frequency which you hear when you play two adjacent bars simultaneously is the difference in the frequency of the two notes. The difference in frequency between two adjacent bars is two times the fundamental frequency of the column, 15 Hz. You do not hear 15 Hz as a tone, you hear it as a beat.

The Ultimate Shower Stall

Sing a note in the column
This is the ultimate shower stall experience because the “good” notes are resonantly amplified, the bad ones just hang there without getting louder. Good notes are those which resonate, these resonances have a melodious musical relationship with each other. The notes also last a long time because the sound waves in the column cannot spread out, and there is little in the column except human bodies to turn the sound waves into heat.

You see why it might be fun to sing in a cathedral, although the staff of most cathedrals frown on people who just drop by to explore the resonances.
I search out long concrete pedestrian underpasses. Sounds will resonates inside these concrete tunnels by bouncing off the open ends.

A Sound Laser

The sound inside the sound column builds up in a resonance similar to the way light resonates between the mirrors at the end of a laser.

Return to Day 11

Scientific Explorations with Paul Doherty

© 1999

25 May 2000