Playing with sound

Dana Mandíková, Mensa gymnázium, KDF MFF UK, Prague, Czech Republic

dana.mandikova@mff.cuni.cz How is sound created - Tuning-fork and bead

Tap into the tuning-fork and make it sound. Vibration of the arms of tuning-fork is not visible. We make the vib-ration visible with help of a bead on the thread. Hang the bead on the thread on the stand so that it reaches a little below the upper edge of the tuning-fork arm and lightly touches it. Chime the tuning-fork again and watch how the bead bounces. Note that sometimes it bounces more, sometimes less.

Resonance – two tuning-forks and a bead

Place two tuning-forks next to each other, point the openings of the re-sonance boxes against each other. Hang the bead on the thread on the stand so that it reaches a little below the upper edge of the tuning-fork arm. Approach the bead to the arm of one of the tuning-forks to lightly touch it. Tap into the second tuning-fork and chime it. After a while, the bead starts to bounce from the tuning-fork though we haven't touch it. If we mute the sound of the tuning-fork by hand, we hear the sound of a tuning-fork with the bead.

Beats – detuned tuning-forks Place two tuning-forks next to each other, point the openings of the resonance boxes against each other. Attach a weight to the arm of one of the tuning forks to make it a little detuned. Hang the bead on the thread on the stand so that it reaches a little below the upper edge of the second tuning-fork arm. Tap into the tuning-fork with the weight and make it sound. The bead will not start to bounce. Put the bead aside, chime both tuning-forks and listen to the resul-ting fluctuating sound.

Why does the tuning-fork have a box Remove the tuning-fork from the resonance box and chime it. The tuning-fork emits only a quiet sound. If we put the tuning-fork on the resonancebox, the sound becomes louder. Then remove the tuning-fork from the resonance box again, make it sound and place it on a table or chal-kboard, for example. The sound of the tuning-fork be-comes louder again. Try to attach the tuning-fork to your head. Bell from the fork Tie the metal fork to the thread. Strike it against the edge of the table. Then put the ends of the thread on the ears, as shown in the picture, and repeat the strike on the table. Compare the two sounds we've heard.

Playing bars Grip the bar with two fingers in the middle. Rap on one end of the bar first in the direction of the bar and listen as it sounds. Then rap on the end of the bar perpen-dicular to it and listen again how it sounds. Compare the pitches.

Alarm clock under vacuum pump Put a piece of foam rubber on the bottom of the vacuum pump and put a mobile on it. Close the pump and call the mobile. Listen how it rings. Then start pumping air out of the pump and listen how the ring volume changes.

Whistle Cut out the "L" shape from the lid of the compote and shape it into a whistle. The procedure is shown in the figure.

In the folded part is necessary to leave the hole and do not squeeze the sheet completely together. The edge of the twisted part has to be against the hole, forming a reed of the whistle. Grasp the whistle with fingers from the sides, as we can see in the picture, thus blocking the holes and try to whistle.

Trumpet from straw Make a simple trumpet from a straw. At one end, cut the straw into the tip as shown. Gently trim the cut end with teeth, put between lips and blow into straw. Playing the trumpet requires a little training. After we succeed to play, we can try to cut the other end of the straw gradually and listen how the pitch changes.

„Bagpipes“ String the end of the surgical glove on one end of the hard pa-per roll and fasten it well with tape to prevent it from blowing through. Stick the middle finger to the other end of the roll. Cut off the tip of the thumb on the glove, insert the end of the straw into the hole and stick it together. Blow into straw and "play".

Stick harmonica

Take the wide rubber band and stretch it length-wise around one craft stick. Cut two pieces of straw. Place one piece of the straw under the rubber band near to the edge on one end. The second piece of straw place on the rubber band on the opposite end. Take the second craft stick and place it directly on the top of craft stick with the rubber band. Secure them together at the ends with help of the small rubber bands. Blow in the center of harmo-nica.You can move the straw pieces and listen to different pitches.

Guitar made out of the box Stretch the rubber band over the box or book and underly it with two pencils as shown in the picture. Stumble on the rubber band and listen to the sound it emits. Then change the distance of the pencils and listen how the pit-ches changes.

Furthermore, we can test how the pitch de-pends on the rubber band tension and thic-kness.

Drum from the cup Cut the bottom of the balloon and stretch it onto the cup or can. If the balloon's rubber membrane does not hold on the cup, fasten it with rubber band or thread. Use finger or pen-cil to tap it gently. Test how the drum sound changes when we change the membrane tension.

Growling tube

Put the balloon on the end of the tube and inflate it. Then, at the point of deployment, grip the end of the balloon with both hands between the toes and rele-ase air from it through the tube.

Sounding serrated pipes

Grab the pipe and spin it overhead. The pipe emits sound.

Cosmic sounds

Make a small hole in the middle of the bottom of the cans. Then first use one can and fasten the spring in the hole in its bottom. Stretch the spring and release it or tap it into the floor. The can creates interesting sounds.

Then connect both cans with a spring. Stretch the spring and tinkle it at one end. Send the tinkle each other. There is a swish like cosmic science fiction sounds.

Crowing cup, mooing bucket

Pierce the bottom of the yogurt cup and stretch the string through it. Hold the cup in your hand, take a wet cloth into your fingers, press the string and move it down. Then do the same with a larger cup, a bucket and a big bucket. In-teresting sounds will be heard.

Why do we have two ears

Can we orient ourselves in space with help of hearing? If we hear on both ears well then yes. Both ears receive sound, and after processing in the brain, we get information for our spatial orientation. If we have a problem with one ear, there is also some problem in spatial orientation.

We do the experiment in a couple. The one who tests his/her hearing takes the hose, leaves it behind his/her back and puts it to his/her ears. The other of the pair taps with a pencil in different places into the hose. The first of the pair signaled by raising a fin-ger to see if the tap was closer to the left or right ear.

Sound reproduction – gramophone tone arm

The tone arm we make from a roll of toilet paper. Cut a circle of diameter of the roll from the tracking paper and stick it to one end. Glue the needle on the tracking paper so that it laps over a piece (see picture) and let it dry well.

Then carefully place the needle tip into the groove of the rotating disc. Altho-ugh reproduction is not perfect, it resembles the voices of the first gramo-phones.

The principle of the tape recorder

Attach a coil with 600 turns to the voltmeter. Set a small range on the voltmeter (e.g. 10 mV with zero in the middle). Connect the second coil with 1200 turns to a 12 V AC source. This coil is a erase head in our model. Pull out a meter long piece of metal tapeline, in-sert it into a „erase head“ (coil connected to an AC power source) and pull the meter a few times through the coil. This will eliminate the previous magnetization of the tapeline. Now make a record. With one pole of the magnet cross several times, for example mark 10 cm on the tapeline, the 30 cm mark cross with the oppo-site pole of the magnet, turn the magnet again and cross the 50 cm mark. Stretch the magneti-zed tapeline through the coil attached to the volt-meter. We observe the throw of pointer at the mo-ment of the passage of the magnetized marks. In this way, we demonstrate how a signal can be re-corded on the tape and played back. If we want to delete the record, we will stretch the tapeline through the „erase head“.

Measuring the speed of sound in the air

Fill the cylindrical vessel with water. String two rubber bands on the tube. Chime the tuning-fork with freque-ncy 440 Hz and bring it near the tube edge (see picture). At the same time, pull the tube out of the water and look for a place where the tuning-fork's sound gets louder. Mark the place with a rubber band. Then chime the tuning-fork again and pull out the tube away until we find another place where the louder sound occurs. Mark it with a second rubber band.

When the tuning-fork is attached, longitudinal stan-ding waves are created in the air in the tube (the transverse wave is drawn for illustration). There is a node in the place of the water level, there were two antinodes in places marked with rubber bands. The distance of the rubber bands d thus corresponds to half the wavelength. So we know the wavelength λ and the frequency f and we can determine the speed of sound in the air v.

v = λf = 2df (Speed of sound in the air at 20 °C is about 343 m/s.)