Month: January 1997

What types of sound can humans hear? What types of materials are soundproof? How…

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What types of sound can humans hear? What types of materials are soundproof? How is the volume of a sound changed? Is the speed of sound the same in all types of media, such as water or air? — JM, Fairfax, VA

In air, sounds are disturbances that consist of compressions and rarefactions—the air molecules are packed either more tightly or less tightly than normal. These regions of too high or too low pressure and density move through the air at about 330 meters per second—the speed of sound and when they pass our ears, we may hear them as sound. As a particular sound passes our ears, the air pressure rises and falls and then rises again, over and over. The number of full cycles—a pressure rise then a pressure fall—that pass our ears each second determines the pitch of the sound we hear. The lowest pitch that our ears are sensitive to is about 20 cycles per second and the highest pitch that we can detect is about 20,000 cycles per second. While other pitches are possible, we simply can’t hear them with our ears.

A sound’s volume is determined by the extent to which the air pressure fluctuates as the sound passes. A loud sound involves a stronger pressure fluctuation than a soft sound. Soundproof materials are ones that decrease the volume of the sound passing through them by weakening the pressure fluctuations. There are two ways to decrease the volume of sound passing through a material: by absorbing the sound or by reflecting it. Soft materials such as carpet or foam rubber absorb sound by allowing the sound’s pressure fluctuations to waste their energies bending the materials. The sound’s energy is converted into thermal energy. Hard, dense materials reflect sounds by making the sounds change speed. Sound travels quickly through most solids and liquids—typically about 5 to 10 kilometers per second. Whenever a wave changes speed in passing from one medium to another, part of that wave is reflected. Thus as sound speeds up in entering a hard surface from the air and as that sound slows down when reentering the air, much of the sound reflects.

What are gas permeable contact lenses made from and what do they use to pigment …

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What are gas permeable contact lenses made from and what do they use to pigment them? — TG, Tulsa, OK

A gas permeable contact lens is one that allows oxygen to diffuse through it to the cornea of the wear’s eye. While conventional hard lenses were made almost entirely of a plastic known as poly(methyl methacrylate) or PMMA, commonly known as Plexiglas or Lucite, gas permeable hard or semirigid lenses are copolymers containing both methacrylate and siloxane molecular units. The polymers used in soft lenses are made only of siloxane molecular units and are commonly known as silicon rubbers. The molecules in silicon rubbers are mobile at remarkably low temperatures, giving silicon rubber its flexibility. In fact, these molecules are so mobile that they must be linked together or “vulcanized” to keep them from flowing as a liquid at room temperature. Even when they have been linked together, portions of these molecules are very mobile, so that gas atoms and molecules can diffuse easily through them. I’m not sure what chemicals are used to color contact lenses, but I expect that the dye molecules are permanently linked to the polymer molecules to keep them in place.

How does the auto-focusing system on a camera work?

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How does the auto-focusing system on a camera work? — RM, Lititz, PA

There are several different systems for autofocusing. I think that the three most popular systems are optical contrast, rangefinder overlap, and acoustic distancing. The optical contrast scheme places a sophisticated light sensitive surface in the focal plane of the camera’s lens. This sensor recognizes when sharp focus is achieved by looking for the moment of maximum contrast in the image. When the lens is out of focus, the image is fuzzy and has little contrast. But when the lens is focused properly, the image is sharp and the sensor detects the strong spatial variations in darkness and brightness. The camera automatically scans the focus of its lens until it detects maximum image contrast.

The rangefinder overlap system observes the scene in front of the camera through two auxiliary lenses that are separated by a few inches. It uses mirrors to overlap the images from these two lenses and can determine the distance to the objects in the picture by the angles of the mirrors. The camera uses this distance measurement to set the focus of its main lens.

The acoustic distancing system bounces sound waves from the objects in front of the camera to determine how far away they are. The camera then adjusts its main lens for that distance. While this acoustic scheme has the advantage of working even in complete darkness, it’s confused by clear surfaces—if you take a picture through a window, it will focus on the window. The optical schemes will focus on the objects rather than the window, but they will only work when there is light coming from the objects. That’s why many autofocus cameras that use optical autofocus schemes have built in lights to illuminate the objects during the autofocusing process.

What is light?

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What is light? — KB, Winnipeg, MB

Light consists of electromagnetic waves. An electromagnetic wave is a self-sustaining disturbance in the electric and magnetic fields that can exist even in empty space. You have probably seen two electrically charged objects push or pull on one another, such as when a sock clings to a shirt as you pull the two from the clothes dryer. You have probably also seen two magnetically poled objects push or pull on one another, such as when a magnet pulls itself toward a refrigerator door. These electric and magnetic forces are mediated by electric and magnetic fields respectively and, while those fields certainly exist in the space between the sock and shirt or between the magnet and refrigerator, they can also exist all by themselves. In an electromagnetic wave, the electric field creates the magnetic field and the magnetic field creates the electric field so that these two fields go on creating one another indefinitely as the wave travels through space at an enormous speed—the speed of light. Electromagnetic waves are distinguished by their frequencies or wavelengths, characteristics that are familiar to anyone who has watched water waves approaching the beach. But only a certain group of electromagnetic waves are visible to our eyes—those with frequencies between about 4.0*1014 cycles per second and 7.5*1014 cycles per second (wavelengths between about 750 nanometers and 400 nanometers). Outside of this range are infrared light at the low frequency end and ultraviolet light at the high frequency end.

Don’t microwaves change the molecular structure and composition of food, by ejec…

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Don’t microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?

Microwaves don’t affect the molecular structure of the food, except through the thermal effects we associate with normal cooking (e.g., denaturing of proteins with heat and caramelizing of sugars). That’s because, like all electromagnetic waves, microwaves are emitted and absorbed as particles called “photons.” The energy in a microwave photon is so tiny that it can’t cause any chemical rearrangement in a molecule. Instead, it can only add a tiny amount of heat to a water molecule. During the microwave cooking process, microwave photons stream into the food and heat it up. But millions of them would have to work together in order to cause non-thermal chemical changes in the food molecules and they don’t normally do that. The photons can only work together if there is a conducting material, such as a metal wire, inside the oven. In that case, the photons can accelerate mobile electric charges along the conducting paths and create sparks. Such sparks can cause chemical damage, but nothing worse than the chemical damage caused by scorching food with a flame or broiler. Even if your microwave is full of sparks for some reason, I doubt that the food will be any worse for you than it would be if you cooked it over an open flame or barbecue.

I recently visited an audio store where I saw electrostatic speakers. These spea…

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I recently visited an audio store where I saw electrostatic speakers. These speakers have no moving parts like conventional speakers and are more expensive. How do they produce sound? — BC, Ottawa, Canada

Electrostatic speakers uses the forces between electric charges (so called “electrostatic forces”) to move a thin metal diaphragm back and forth rapidly. The motions of this diaphragm compress and rarefy the air in front of it, producing sound. On each side of the diaphragm is a rigid metallic grill that can hold electric charges. When the speaker is silent, the diaphragm has a large positive electric charge on it and both the metal grills have large negative charges on them (it could be the other way around, depending the speaker’s exact design). The diaphragm is then attracted equally toward both grills and the electrostatic forces cancel perfectly. The diaphragm doesn’t undergo any acceleration. To make the speaker produce sound, the electric charges on the two grills are changed so that the electrostatic forces on the diaphragm don’t cancel. Instead, the diaphragm is pulled strongly toward whichever grill has more negative charge on it (or less positive charge). The charges on the grills fluctuate as the music plays and the diaphragm accelerates back and forth between the grills. It pushes on the air as it does and produces sound. You’ll notice that the diaphragm is a moving part, so the claim that the speaker has “no moving parts” is misleading. The speaker cone of a conventional speaker only moves back and forth, too, so it has an equal claim to having “no moving parts.” The relative expense of an electrostatic speaker comes from the requirement of careful construction and the need for a high voltage adapter to match an amplifier to the speaker.

Do whales drink salted water?

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Do whales drink salted water? — GR, Montreal, Quebec

No. Marine mammals rely on water obtained from their food. Because they don’t sweat, they only lose water through their urine, which they concentrate to minimize the loss of water. What little water these animals do need comes from eating foods that are already relatively low in salt. Most of the lower sea animals, including fish, have active systems—ones that consume ordered energy—for eliminating salt so that when a sea mammal eats one of the lower animals, it inherits that animal’s relatively salt-free water. Moreover, metabolizing fats and carbohydrates produces water as a byproduct.

I have a gas steam heating system and the second floor radiators don’t heat well…

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I have a gas steam heating system and the second floor radiators don’t heat well. How does this system work and how can I balance the system so that the upstairs radiators warm at the same rate as the first floor radiators?

In a steam heating system, steam rises upward from a boiler in the basement and condenses in the radiators. As the steam transforms into water, it releases an enormous amount of heat and this heat is transferred to the air in the rooms. The condensed water than descends back to the boiler to be reheated. The beauty of this system is that the rising steam and the descending water can both pass through the same pipes, propelled by gravity alone. The low-density steam is lifted upward by the high-density water.

However, there are a few potential problems with this system. If there is air trapped in the pipes, the steam will have trouble reaching the radiators. Even though steam is lighter than air, it will diffuse slowly through the trapped air. That’s why each steam radiator has a small bleeder valve. When the steam pressure exceeds atmospheric pressure, it should push the air in the pipes out the bleeder valves of the radiators. You ought to be able to hear the air leaving and the valves may continue to sputter a bit even when the pipes and radiators are essentially full of steam. I suspect that the bleeder valves on your upstairs radiators aren’t functioning well so that steam isn’t reaching them.

What are the relative efficiencies and reasons for power losses in sprocket and …

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What are the relative efficiencies and reasons for power losses in sprocket and chain drives, rubber cogged belt drives, pulley drives, and gear drives? — RA, Montreal, Quebec

The only power loss mechanisms I can think of in each case are sliding friction and vibration. The drive system most likely to experience substantial sliding friction is a pulley (or smooth belt) drive. If the belt slips as the pulleys turn, the belt will do work against the force of sliding friction and that work will be converted into thermal energy. But, as one of my readers points out, if the belt is properly tightened, has an adequate coefficient of friction to prevent slipping, and has a high tensile strength so that it doesn’t creep across the pulley surface, then it can operate with very little power loss.

In the other drive systems, there is no possibility of slippage so that any power loss that occurs must be due to internal sliding friction within the components, or from vibrations. Flexing a chain involves some internal sliding friction and wastes some power. I suppose this could be minimized with careful chain construction and I wouldn’t be surprised if large change drive systems placed bearings in the chain links to eliminate sliding friction altogether. Flexing a rubber-cogged belt also involves some molecular friction within the belt material so it wastes some power. I’m not sure which system is more efficient, the chain drive or the cogged belt drive. Finally, the gear drive is the least likely to waste significant energy. The only sliding friction that occurs is between the gear teeth. If the teeth are designed well and cut carefully, they should slide very little. In that case, the only significant power loss would be through vibrations. If everything is carefully mounted to prevent vibrations, there should be very little power loss in a gear drive.