How to See Your Voice

One day Lenya came home from the construction site shouting:

"Look what they gave me at the construction site!" In his hands he held several strips of glass wrapped in paper so he wouldn't cut his hands. Lenya put the glass on the table, everyone gathered around, and they started thinking about what could be made from this glass. Suddenly Tanya said:

"I think I know. You, Lenya, bring a tin can and a key, and you, Irishka, bring paper and glue."

Lenya brought a tin can without a lid, and Tanya cut out the bottom with the key. The result was a wide, short tin tube. After that, Tanya cut out a circle of thin paper and glued it in place of the bottom. When the glue dried, Tanya sprinkled the paper with water. She punched two holes in the sides of the can with a nail and inserted thick wire in the shape of the letter "P" into these holes. By this time, the wet paper bottom had dried and stretched tight, like on a drum. Tanya dropped a drop of office glue onto the center of the paper, quickly attached a narrow strip of tin with a pointed tip to this drop - it looked like a bent needle - and left it to dry. Then Tanya lit a candle and began moving a strip of glass over the flame. Soon the glass turned black from soot.

Tanya placed the strip on the table with the sooted side up and set up the tin can so that the tin needle touched the sooted glass with its tip.

"So what?" asked Lenya, examining this strange machine.

"Here's what," said Tanya. "Now I'll pull the glass strip, and you shout. Get ready..." Tanya quickly pulled the glass strip, and Lenya shouted:

"Ta-a-nya!"

Tanya lifted the glass strip and turned it toward the light. On the sooted glass, the pointed tin had scratched a transparent line. At first the line was straight, but starting from the middle it became wavy.

"Well, there you have it," said Tanya. "While Lenya was quiet, the line was straight, but when he shouted, it became wavy. Why?"

"I know," said Irishka. "When Lenya shouted, the paper vibrated from the shout, and the tin needle vibrated, and the line came out with curves."

Lenya took the glass strip and also looked at it.

"So this is a drawing of my voice?" he asked.

"Yes!" Tanya laughed.

"Let's draw your voices too," Lenya suggested.

"Let's do it," Tanya and Irishka agreed.

They placed the glass on the table and set the tip of the tin needle on the sooted glass. Tanya pulled the strip, and Irishka shouted:

"Le-e-nya!"

"Now you," the children said to Tanya.

Tanya approached the can and got ready. Irishka pulled the glass strip, and Tanya shouted:

"I-i-ra!"

Look at these drawings more carefully, read the story again, and try to record your own voice on sooted glass using a tin can, a paper bottom, and a tin needle.

---

The device for recording sound on sooted glass is not simple to make, but the result you achieve will justify your efforts.

Another design of the device for recording sound is clear from the drawing.

A membrane made from a piece of paper can be glued to a metal can using office glue. Instead of a regular tin can, you can use a small cardboard box (for example, from face powder) or simply make one from cardboard or thick paper. A very important narrow strip of tin is attached to the membrane with office glue and small supplementary paper pieces. It's very important that the membrane is well stretched before gluing the needle - to do this, you need to sprinkle it with water and let it dry.

In order for the needle to precisely follow the vibrations of the membrane, you need to give it the shape shown in the drawing.

For the recording to be successful, the pressure of the needle on the glass should be minimal. To achieve this, you need to balance the can by attaching pieces of plasticine to its opposite edge.

Since you'll have to shout quite loudly to record sound, the pitch will be high. Let's assume, approximately, that the frequency of vibrations will be 600 vibrations per second. This means that if you move the glass strip at a speed of 600 mm/sec, then the length of one wave in the recording will be 1 mm. Although 600 mm/sec is quite a high speed, we won't manage to record more than one short track from one path. However, there's nothing to write down except a short shout. If you have a gramophone or phonograph (with mechanical sound reproduction) at home, you can do the following: set the needle on the last spiral track of a record and shout loudly into the tube of the gramophone or into the wide opening on the top of the phonograph box. After that, you'll hear a faint squeak - an echo of the sound of your voice on the record. Discuss with the child how the recording turned out.