The Vibrational Nature of Sound Cary M. Seidman

Transcription

1 SCIENCE EXPERIMENTS ON FILE Revised Edition The Vibrational Nature of Sound Cary M. Seidman Topic Sound frequency, pitch, and conduction Time hours! Safety Please click on the safety icon to view the safety precautions. Adult supervision is required. Care must be taken when using any glasses, particularly when striking them. Materials four tuning forks of different frequencies sounding block paper cups string Ping-Pong ball eight drinking glasses or soda bottles of the same size Procedure You will need a partner for this experiment. PART A: STRING TELEPHONE 1. Take two paper cups and poke a small hole in the bottom of each with a pencil. (Make sure the pencil is sharpened and that the hole is just large enough to pass the string through. 2. Pass one end of string (2 3 m in length) through each hole, and tie a knot on the inside of the cup so that the string runs between them from bottom to bottom (figure 1). Figure 1

2 SCIENCE EXPERIMENTS ON FILE Revised Edition 3. Your partner and you should each take a cup and stretch the string out between you, making sure the string is taut. Take turns, with one partner talking quietly and the other one listening. 4. After each partner has had a turn talking and listening, experiment by pinching the string at various points while talking. Notice what happens to the sound. Try loosening and tightening the string and observe the effect this has. PART B: TUNING FORK EXPERIMENTS Tuning forks are delicate instruments that should be struck only on a rubber sounding block and held only by the handle. Try to avoid bending or dropping them. 1. Repeat steps 1 to 4 from Part A with each tuning fork. 2. Strike a tuning fork on the rubber sounding block. Hold the vibrating end to your ear. Try to correlate the differences you hear between the different tuning forks and the frequency values (numbers) on them. 3. Touch the handle of a vibrating tuning fork to the top of an empty desk or table. 4. Touch the vibrating end of a tuning fork to a Ping-Pong ball suspended from a string (figure 2). Figure 2 Ping-Pong ball tuning fork 5. Gently place the vibrating end of a tuning fork in a beaker of water. Observe what happens to the water and to the pitch and duration of the sound. PART C: MUSICAL GLASSES Gather eight drinking glasses or glass soda bottles of the same size and fill them with different amounts of water. Strike each glass gently with a piece of wooden doweling or a pencil (be careful not to break the glass). Note what happens. Try adjusting the amounts of water in each glass to produce a musical scale. If you are using soda bottles, try blowing air across the top of each bottle. Observe what effect this has.

3 SCIENCE EXPERIMENTS ON FILE Revised Edition ABOUT PART A 1. Which materials conducted the sound more effectively, the string and cup or the air in the room? How do you know? 2. The most important vibrating components are the bottoms of the cups. Can you think of a musical instrument that also vibrates very effectively with the same general configuration as the cup bottom? 3. What happened when you pinched the string? Why did this happen? 4. Describe the series of different materials that vibrate to make the telephone carry sound. ABOUT PART B 1. How do the numbers printed on the tuning forks relate to how high or low the pitches of their sounds are? How do the numbers relate to the physical size of the forks, and why should that make a difference? 2. What happened when you touched the tuning fork to the desktop and why? 3. What happened when you placed the tuning fork in the water, and why do you think this happened? ABOUT PART C 1. What is the relationship between the amount of water in the glass and its pitch when you strike it? 2. Why does blowing on a bottle filled with water create a different pitch than striking it with the dowel? What s Going On About Part A: String Telephone Answer to step 1: the string and cup, because you can hear your partner s words more clearly through the telephone than through the air in the room. Many solids and liquids conduct sound waves better than air. Step 2: A drum. Step 3: The sound stops. When you pinch the string, the vibrations are stopped, preventing them from being transmitted along the string any further. Step 4: When you speak, your vocal cords vibrate and cause the air in front of your mouth to vibrate. This starts the air in the cup vibrating. The cup bottom takes the vibrations of the words spoken into it and, vibrating at the same rate, amplifies them. Then the string transmits (carries) these vibrations along its length to the other cup. This starts the bottom of the cup vibrating and reamplifies the words, setting up vibrations in the air in the cup, so that your eardrum can be vibrated by them and you hear the words your partner spoke. About Part B: Tuning Forks Answer to step 1: The numbers on the tuning fork tell you the rate at which the fork vibrates. This is called its frequency value or pitch. The numbers vary according to the pitch. Forks with lower numbers make lowerpitched (frequency) sounds; higher-numbered forks have higher pitches (frequencies). Size is related to how fast or slowly a tuning fork vibrates. The larger it is, the slower the rate of its vibration, so larger tuning forks have smaller numbers. This tells us that they vibrate at a slower rate, with fewer vibrations per second. Step 2: When you touch the handle of the tuning fork to the desktop you can hear its sound clearly. The desk amplifies and transmits the vibrations of the tuning fork. Step 3: When you place the vibrating end of the tuning fork into a beaker of water the pitch is lowered. This is because the water lowers the frequency of the vibrations. If you watch carefully you can see the vibrations making little waves in the water.

4 SCIENCE EXPERIMENTS ON FILE Revised Edition About Part C: Musical Glasses Answer to step 1: The more water in the glass the lower the pitch you hear when you strike it. This is similar to the different pitches produced by the different-sized tuning forks. Step 2: When you blow into a bottle, the column of air inside it vibrates, not the water and glass as occurs when you struck the glasses. There is an inverse proportion shown here: As an object gets larger, its rate of vibration gets slower. Connections A vibrating object creates disturbances in the air. These disturbances, or waves, move through the air, eventually reaching our eardrums, which vibrate in response, causing us to hear. This is sound. Sound waves cannot travel through empty space; the vibrations must be conducted by some material: a gas (such as air), a liquid, or a solid. Some materials are better conductors of sound than others. In this experiment you explored different characteristics of sound as it is conducted by a variety of media.

5 Safety Precautions READ AND COPY BEFORE STARTING ANY EXPERIMENT Experimental science can be dangerous. Events can happen very quickly while you are performing an experiment. Things can spill, break, even catch fire. Basic safety procedures help prevent serious accidents. Be sure to follow additional safety precautions and adult supervision requirements for each experiment. If you are working in a lab or in the field, do not work alone. This book assumes that you will read the safety precautions that follow, as well as those at the start of each experiment you perform, and that you will remember them. These precautions will not always be repeated in the instructions for the procedures. It is up to you to use good judgment and pay attention when performing potentially dangerous procedures. Just because the book does not always say be careful with hot liquids or don t cut yourself with the knife does not mean that you should be careless when simmering water or stripping an electrical wire. It does mean that when you see a special note to be careful, it is extremely important that you pay attention to it. If you ever have a question about whether a procedure or material is dangerous, stop to find out for sure that it is safe before continuing the experiment. To avoid accidents, always pay close attention to your work, take your time, and practice the general safety procedures listed below. PREPARE Clear all surfaces before beginning work. Read through the whole experiment before you start. Identify hazardous procedures and anticipate dangers. PROTECT YOURSELF Follow all directions step by step; do only one procedure at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure that there is adequate ventilation. Do not horseplay. Wear an apron and goggles. Do not wear contact lenses, open shoes, and loose clothing; do not wear your hair loose. Keep floor and work space neat, clean, and dry. Clean up spills immediately. Never eat, drink, or smoke in the laboratory or near the work space. Do not taste any substances tested unless expressly permitted to do so by a science teacher in charge. USE EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives and other sharp or pointed instruments with caution; always cut away from yourself and others. Pull plugs, not cords, when inserting and removing electrical plugs. Don t use your mouth to pipette; use a suction bulb. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges. Clean up broken glassware immediately. v

6 vi Safety SCIENCE EXPERIMENTS ON FILE REVISED EDITION Do not use reflected sunlight to illuminate your microscope. Do not touch metal conductors. Use only low-voltage and low-current materials. Be careful when using stepstools, chairs, and ladders. USING CHEMICALS Never taste or inhale chemicals. Label all bottles and apparatus containing chemicals. Read all labels carefully. Avoid chemical contact with skin and eyes (wear goggles, apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING INSTRUCTIONS Use goggles, apron, and gloves when boiling liquids. Keep your face away from test tubes and beakers. Never leave heating apparatus unattended. Use safety tongs and heat-resistant mittens. Turn off hot plates, bunsen burners, and gas when you are done. Keep flammable substances away from heat. Have a fire extinguisher on hand. WORKING WITH MICROORGANISMS Assume that all microorganisms are infectious; handle them with care. Sterilize all equipment being used to handle microorganisms. GOING ON FIELD TRIPS Do not go on a field trip by yourself. Tell a responsible adult where you are going, and maintain that route. Know the area and its potential hazards, such as poisonous plants, deep water, and rapids. Dress for terrain and weather conditions (prepare for exposure to sun as well as to cold). Bring along a first-aid kit. Do not drink water or eat plants found in the wild. Use the buddy system; do not experiment outdoors alone. FINISHING UP Thoroughly clean your work area and glassware. Be careful not to return chemicals or contaminated reagents to the wrong containers. Don t dispose of materials in the sink unless instructed to do so. Wash your hands thoroughly. Clean up all residue, and containerize it for proper disposal. Dispose of all chemicals according to local, state, and federal laws. BE SAFETY-CONSCIOUS AT ALL TIMES