Key concepts
Sound
Acoustics
Siren
Physics

Introduction
You probably hear them almost every day: sirens. Police cars, ambulances, fire trucks—they all can come blaring. Their wailing sound is piercingly loud and pretty effective at clearing the road in front of them. But have you ever thought about how this loud noise is generated? Make your own simple disk siren in this activity and find out for yourself.

Background
To understand how sirens work one must first look at the physics of sound, which is basically just the movement of air. By moving or displacing air particles, you generate a sound wave, which consists of a repeating pattern of high- and low-pressure regions that travels through the air in the form of a pressure wave. The air particles vibrate in a back-and-forth motion and the rate at which this vibration occurs is called frequency.

The simplest version of a siren is a mechanical device that pushes an air stream against a rotating perforated disk. As the disk spins, the airflow will be alternately interrupted and allowed to pass. The ensuing fluctuation in air pressure produces a series of regular pressure waves that we perceive as sound. The number of holes in the disk determines its frequency. The disk’s rotation speed also has an impact on the sound’s tone, which is its pitch and is heard as the characteristic wailing siren sound. You can calculate the tone’s precise frequency by multiplying the number of rotations per second by the number of holes in the disk.

Today electronic sirens are slowly replacing the mechanical siren design. Many of the sirens on fire trucks, however, are still based on the same principle as the disk siren. In this design an electric motor turns a fan that spins inside a slotted drum. The fan sucks in air from the front and expels it through the sides. When the fan openings align with the slots in the drum, an airburst is generated. Similar to the perforated disk, the fan basically chops the air stream into individual airbursts and their frequency determines the siren’s pitch.

That was a lot of information. Now it is time to make some noise using a self-made perforated disk siren!

Materials

• Protractor
• Heavy construction paper
• Double-sided foam tape
• Scissors
• Pencil
• (Math) compass
• Round-hole puncher
• Cordless electric screwdriver (You should only use it with adult assistance.)
• Small hexagonal bit
• Straw
• Paper clip or needle
• Paper towel

Preparation

• Draw a circle with the compass that has an eight-centimeter radius on the cardboard. Within the same circle draw two smaller circles with seven- and six-centimeter radii.
• Cut out the biggest circle.
• Use the protractor to draw lines through the center of the circle in equal 15-degree intervals.
• Take the hole puncher, and on the seven-centimeter circle punch a hole at every intersection of the lines and the circle.
• On the innermost six-centimeter circle punch holes at every second intersection of the lines and the circle.
• You should now have a disk with two concentric rings of holes in which the outermost ring has double the amount of holes compared with the innermost ring.
• Cut a small piece of double-sided foam tape and stick it on the center of one side of the cardboard disk.
• With a paper clip or needle make a small hole through the center of the circle and foam tape.
• Put a small hexagonal bit in your cordless screwdriver.
• Take the perforated cardboard disk and push the hexagonal bit through the small center hole of the circle so that the foam tape faces toward the electric screwdriver. Use the foam tape to adhere the cardboard disk to the cordless screwdriver so that it sits firmly on the screwdriver.
• Set the cordless screwdriver to its highest setting.

Procedure

• Take the straw and hold your hand in front of it while blowing into it. Can you feel the air flowing through the straw? Listen carefully. Does the air make any sound?
• Ask your adult helper to hold the cordless electric screwdriver with the cardboard disk attached. Do not turn it on when anyone else is nearby. Now have your adult helper run the cordless screwdriver on the highest setting. Is there any sound coming from the disk? Do you hear anything besides the noise of the cordless screwdriver?
• Next, have your adult helper power the cordless screwdriver again and keep the disk spinning on the highest setting. Make sure to keep the rotation speed of the disk the same. While the disk is spinning take the straw and place it about one to two centimeters in front of, or perpendicular to, the outer ring of holes and blow through it. Be sure you do not let the straw touch the disk. What happens this time? Listen again; does your siren make a sound? Do you hear a high- or low-pitch tone?
• While the disk is still spinning at the same speed move your straw in line with the inner circle of holes and blow air through the straw against the disk. Can you hear a sound again? How does this tone differ from the previous one? Why do you think the sound changed?
• Stop the cordless screwdriver, and thus the disk’s rotation. Ask your adult helper to slowly power it again, but this time pay attention to the disk speed. Is the rotation speed always the same? Can you make the disk spin at different rates?
• Once you and your helper have figured out how to create different disk rotation speeds, take the straw again and blow against the outer ring of holes on the disk. While blowing, change the speed of the cordless screwdriver so that the disk spins at different rates. What happens to the sound of the siren when you change the speed? How does the tone sound with a high-speed setting compared with low speed? Can you explain your observations?
• Finally, play around with blowing at the two rings of holes at different rotation speeds. Can you create a realistic wailing siren sound?
• Extra: Create more disks with different hole patterns and listen to how they sound. What happens if the holes are not equally spaced? How does it sound if you have a series of holes followed by a gap?
• Extra: Take a lightweight paper, such as a paper towel, and have your adult helper hold it closely behind the spinning disk (between their body and the disk) while you blow through your straw. Observe the paper towel. What happens to the paper towel? Does it move at all? Can you see a movement pattern? (It helps if you spin the disk relatively slowly for this part of the activity.)

Observations and results
Did you get your siren to make some wailing sounds? Just blowing through the straw on its own probably did not produce a siren sound. You only created airflow, as you probably felt with your hand, but no pressure wave. Also, if you just spin the disk with the cordless screwdriver, the only noise that you should hear is the cordless screwdriver itself. Once you push an air stream against the perforated disk with your straw, however, a sound wave is generated that you can hear. This is because the air you blow through the straw alternately puffs through the holes and is cut off as the wheel spins. If you did the extra activity with the paper towel, you probably actually saw the wave pattern created by the on-and-off airflow. You should have seen the paper towel "flapping" back and forth, depending on if the air was able to pass through the hole or not. Each of these airbursts acts like a single sound wave and many of them together create a continuous sound. The sound might not be very pure but you probably were able to detect a smooth note.

When you blew air through the inner circle—with half the amount of the holes—you should have noticed that the sound’s pitch decreased. Specifically, the tone should have been one octave lower than the tone from the outer circle, meaning that the two circles create two musical pitches with half or double frequency. The reason for this is that with the number of holes, you increase the number of created air puffs, thus increasing the sound wave’s frequency. The higher a frequency, the higher a note’s pitch. Thus, if you halve the number of holes, the created tone should have half the frequency. Finally, if you change the rotation speed of the disk while you are blowing against the disk, the pitch of the sound also changes. As the disk spins faster, again the puffs’ frequency increases and their sound rises in pitch. By alternating the rotation speed back and forth from low to high, you can create the characteristic wailing of a siren. Every hole pattern you generate on the disk will result in a specific sound—sometimes a smooth musical tone, sometimes a rough noise. Give it a try!

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