Last Updated: September 26, 2017
You’ve probably heard of ultrasound within the context of medical diagnostics and pregnancy. But what exactly is ultrasound, anyway? The term “ultrasound” actually applies to sound waves spanning a range of frequencies that are above what is audible to the human ear.
Explore this page to learn more about ultrasound, the properties of sound waves in general, and the fascinating applications of ultrasound in our lives.
Let’s say that I’m holding a tuning fork. When a mechanism produces sound - for example, when I strike the tuning fork - it sends a vibration through a compressible medium, like air in this case. This vibration through the medium forms a wave as it travels away from the sound source. The sound from the tuning fork is essentially a wave that passes through the medium.
The sound wave travels at a certain speed, depending upon the medium through which it travels. Sound travels faster through both water and solids than through air, for instance. Why? It’s because the movement of sound is the movement of a vibration from particle to particle in a medium. If the particles in the medium are closer together (as the particles in water are closer together than those in air), then the vibration can transfer more rapidly from particle to particle. What we often reference as the speed of sound - 343.59 meters per second (or 768 miles per hour) - is the speed of sound traveling through air under particular temperature and humidity conditions.
The wave created by the vibrating sound source has certain characteristics aside from speed, such as wavelength, amplitude and frequency. Frequency is our term for the number of wave cycles that occur in a period of time. We measure frequency in hertz (Hz). If a particular sound has a frequency of 1 Hz, its frequency is one wave cycle per second. Among the characteristics of sound waves, frequency affects pitch the most.
You could think of wavelength as the distance that the wave travels from the start of one cycle to the start of the next one (or, if you want to imagine the sound wave as static and unmoving for a moment, then wavelength would be the measurement of the distance between wave repetitions). Within a medium like air, wavelength has an inverse relationship to frequency, because speed of sound is basically consistent within a particular medium under consistent conditions. Consequently, higher frequency sound has smaller wavelength, and vice versa.
The range of sound frequency audible to us (known as ‘acoustic’) stretches from about 20 Hz in the bass register to 20,000 Hz (or 20 kHz), on the high end. What we call ‘sound’, when we hear it, is the result of an incredible process. The sound wave enters our auditory canal and vibrates our tympanic membrane (eardrum). These vibrations are intricately sent to the inner ear to be converted into electrical signals, which the brain interprets as sound. (Learn more about the anatomy behind hearing.)
Naturally, we carve up and define the range of sound frequencies based on what we can hear ourselves. The upper limit frequency for a typical human ear is 20,000 Hz. Frequencies above 20,000 Hz are called ultrasound, while those below 20 Hz (the lower limit for humans) are called infrasound (not unlike how we define the ultraviolet and infrared portions of the electromagnetic spectrum).
While we humans can’t hear infrasound, the world is filled with it, across the land and throughout the seas - sounds made by a variety of animals as well as sound that we produce, but only they can hear. Baleen whales like the blue whale can produce sound with a frequency of 10 Hz, while the endangered Sumatran rhinoceros has produced recorded sound as low as 3 Hz in frequency.
So in terms of the physics, ultrasound is no different than the sounds we can hear, except that its frequencies are far too high for us to hear. Many animals, such as bats and dolphins for instance, produce ultrasound in order to interact with the natural world. Ultrasound has many applications for humans. If you hear people using the term “ultrasonics”, they’re talking about how ultrasound is applied to do various things for us.
In medicine, ultrasound allows for safe, effective and usually non-invasive imaging within our bodies. Medical ultrasound (often called ultrasonography or just sonography) usually relies on sound waves of frequencies between 1,000,000 Hz and 18,000,000 Hz (1-18 megahertz) - quite a bit higher than the upper limit of 20,000 Hz that we can hear, and even higher in frequency than what bats can produce (bats produce and detect sounds upwards of 100,000 Hz, but nowhere near 1 gigahertz).
Highly skilled professionals, such as ultrasound technicians and technologists, use special devices called transducers, which both emit and receive sound pulses at these high frequencies. A well-positioned transducer sends sound into the human body, where it scatters or bounces back (echoes) from objects in the body. The transducer registers those echoed waves as vibrations, which it converts into electrical signals that it sends to the scanner. The scanner turns those signals into an image, relying mainly on a couple of key qualities about the echoed waves:
The strength of the echo allows the machine to delineate tissues and objects of different densities in the body; denser structures will cause a stronger echo. The time elapsed between sending the signal and receiving the echo imparts information about the depth of objects; one central assumption of the equipment (and also a cause of some blurring in the imaging) is that the speed of the ultrasound waves will remain constant within the body, even though the waves pass through tissues of differing densities (affecting the actual speed of the waves).
The radiologist - a physician who specializes in medical imaging and subsequent interventions - may then assess these images and, working with the referring physician, order additional diagnostic work or arrive at a diagnosis.
So in principle, the basics of ultrasonography are not all that different from sonar as it’s used to detect and define objects underwater, though sonar employs frequencies ranging from infrasound all the way up to the lower frequencies of medical ultrasound. One other major difference is that medical ultrasound has been demonstrated to be safe and non-disruptive to the medium in which it is deployed (the human body).
Here are some of the branches of medicine that rely on ultrasound and just a few examples of how medical professionals apply it:
Medical ultrasound technology has evolved significantly since early sonography and continues to evolve, with applications spanning practically all specialties within medicine. Today, the medical community uses over a half dozen modes of ultrasound in order to visualize the inner mechanics of our bodies like never before.
On the positive side, ultrasound carries many benefits as a diagnostic tool.
Limitations of ultrasound include the following:
In cases where ultrasound is inadvisable, a medical team often turns to computed tomography (CT) or magnetic resonance imaging (MRI).
We’ve already touched upon one such application of ultrasound - sonar (Sound Navigation and Ranging). The higher frequencies of sonar (which amount to the lower frequencies of medical ultrasonography) are indeed ultrasound waves, useful when distances are relatively small and when there’s a need for greater accuracy than could be achieved from lower frequency sonar. Here are some of the many other applications of ultrasound in our lives.