Light, heat and sound are all forms of energy that travel as waves. The waves created by strumming a guitar string, for example, vibrate molecules in the air — and in our ears. That’s why we hear the guitar. But not all sounds are audible. For instance, ultrasound is not. The difference between the guitar’s sound and ultrasound is the frequency at which the wave vibrates. Musicians call that frequency the pitch.
Scientists measure a wave’s frequency in cycles per second, or hertz. Human ears can hear anything between 20 and 20,000 hertz. Waves with a higher frequency are known as ultrasound. Those higher pitches are beyond human hearing. (Sounds below human hearing are known as infrasound.)
In air, sound travels at a constant speed. If the wave’s frequency changes, so does its wavelength. That wavelength is the distance from the peak of one wave to the peak of the next. Long waves sound low and short waves sound high.
Waves of the same frequency can differ in the amount of energy they carry. That energy is measured by the height of a wave’s peak. Sound waves with higher peaks, or bigger vibrations, carry more energy than those with lower peaks. (Within the range of human hearing, for instance, we perceive sound waves with higher peaks as louder.)
Medical applications of ultrasound
For many decades, medicine has relied on ultrasound to picture soft tissues inside the body. This type of diagnostic ultrasound uses low-energy waves. That makes it safe for checking the health of unborn babies in the womb. Doctors also use it to scan for diseases in children and adults. More recently, researchers have been studying how ultrasound with higher energy might be used to treat certain diseases. This is called therapeutic ultrasound.
Ultrasound imaging with low-energy waves uses a transducer. This device converts one form of energy into another. One part of the transducer converts electrical energy into short ultrasound pulses. It sends those pulses into the body. Another part of the transducer receives the echoes that bounce back from the tissues those pulses hit inside the body. A computer analyzes those echoes to create an image. The echoes from different tissue types show up as darker and brighter parts in the image.
Doctors already use therapeutic ultrasound to treat kidney stones. These are small, hard mineral deposits in the kidneys that are painful to pass out along with urine. The high-energy sound waves break kidney stones into tiny pieces. That makes it easier for the body to flush them out.
Researchers are studying other possible therapeutic uses for ultrasound, too. For example, they would like to destroy cancer cells without harming neighboring cells. They also have shown that ultrasound can trigger brain cells to release signaling chemicals. This might one day lead to treatments for brain diseases and mood disorders.
Some researchers have even shown that ultrasound can make certain cells release insulin. That’s a hormone that keeps blood sugar at healthy levels. One day, it might be possible to use pulses of ultrasound to manage diabetes.
audible Something that can be heard, usually with ears or other sound-sensing structures.
cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
diabetes A disease where the body either makes too little of the hormone insulin (known as type 1 disease) or ignores the presence of too much insulin when it is present (known as type 2 diabetes).
disorder (in medicine) A condition where the body does not work appropriately, leading to what might be viewed as an illness. This term can sometimes be used interchangeably with disease.
hertz The frequency with which something (such as a wavelength) occurs, measured in the number of times the cycle repeats during each second of time.
infrasound Sound waves with frequencies below the lower limit of human hearing.
insulin A hormone produced in the pancreas (an organ that is part of the digestive system) that helps the body use glucose as fuel.
kidney Each in a pair of organs in mammals that filters blood and produces urine.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).
mineral deposit A natural concentration of a specific mineral or metal
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
pitch (in acoustics) The word musicians use for sound frequency. It describes how high or low a sound is, which will be determined by the vibrations that created that sound.
prospect (n.) The vista (as in what’s in view) or the future of something (such as whether it’s going to be successful).
software The mathematical instructions that direct a computer’s hardware, including its processor, to perform certain operations.
sonar A system for the detection of objects and for measuring the depth of water. It works by emitting sound pulses and measuring how long it takes the echoes to return.
threshold A lower limit; or the lowest level at which something occurs.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
trough (in physics) the bottom or low point in a wave.
ultrasound (adj. ultrasonic) Sounds at frequencies above the range that can be detected by the human ear. Also the name given to a medical procedure that uses ultrasound to “see” within the body.
vibrate To rhythmically shake or to move continuously and rapidly back and forth.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
womb Another name for the uterus, the organ in mammals in which a fetus grows and matures in preparation for birth.