The ability to understand how the powerful sonic boom of an exploding ocean wave finds its way to your ears or how a hushed secret is conveyed silently across a quiet room is quite interesting. That is a beautiful demonstration of science, physics, vibrations and a medium for the transmission of those vibrations. In simplest terms, sound is transmitted via waves of compression and expansion. Those waves require a medium to pass through (whether it be air, water or a solid); without a medium, there is no sound. Therefore, as silent as it may seem, space is completely empty of all types of sound; although there are many explosions and massive cosmic events occurring throughout the universe.
Fundamental Properties of Sound Waves
There is nothing magical about sound. Sound is energy in motion. Anytime something vibrates, it generates vibrations in the area surrounding it. Take a drum as an example. When you strike the drum head, it begins vibrating rapidly back and forth. With this vibration, it transmits pressure to nearby air molecules causing the molecules to become compressed forming a zone of greater pressure. Conversely, once the drum head reverses direction and stops vibrating, the adjacent air molecules expand and form a zone of lesser pressure. As the drum head continues vibrating, the series of compressions and expansions continue traveling away from the source similar to ripples on a pond.
Individual air molecules do not travel from the drum to your ear. Rather, they bump into neighboring molecules and transmit energy. They return to nearly their initial positions. The manner in which particles vibrate in relation to the direction of wave propagation is known as a longitudinal wave.
Other Key Features of Sound Waves
Some features that help characterize a sound wave are:
- Amplitude: This relates to the strength or size of the wave. The louder the sound, the larger the amplitude. Amplitude is also related to the magnitude of pressure change.
- Wave Length: Wave length is defined as the distance between two successive compressions or two successive rarefactions.
- Frequency: Frequency represents the total number of cycles (successive compressions and rarefactions) that traverse a point in one second, expressed in Hertz (Hz). High frequency indicates a higher pitch. Humans are able to hear frequencies ranging from 20 Hz to 20 kHz.
- Speed: Speed represents how fast a sound wave propagates through a medium. Speed is dependent upon the characteristics of the medium itself.
Why Does the Medium Matter?
To allow sound to propagate, there has to be a medium. That medium can be anything composed of molecules capable of being both compressed and expanded. Additionally, the properties of the medium including density and rigidity greatly influence how quickly sound travels.
Properties of Sound in Varying States of Matter
Gas (air)
Gases represent the typical medium for sound we experience every day. Molecules in gaseous states are separated by great distances compared to liquids and are also mobile. When a sound wave passes through the gas state, the molecules collide with one another to transmit energy. Under average temperature and atmospheric conditions (about 20 C / 68 F) in dry air, sound travels approximately 343 meters/second (1125 ft/sec).
Liquid (water)
Water contains much closer spaced molecules than does air. Also, water molecules are not as readily compressible as those found in air. Because of these factors, water serves as a more effective transmitter of sound than does air. As such, sound travels at approximately 4.5 times faster in water than in air, or approximately 1500 m/s. This fact explains why whales and dolphins are able to communicate with one another over large distances through the ocean.
Solid (metal or wood)
Solid materials contain molecules packed extremely close and are fixed in place. Strong bonds between these molecules provide an excellent conduit for rapid transmission of vibrations. Generally speaking, solid materials allow sound to travel at the greatest rate possible. An example of this can be seen when comparing steel to air; steel has been shown to support sound propagation rates of up to approximately 5100 meters/second, which is over 15 times faster than that supported by air. This is why you often hear trains passing at a distance when placing your ear against a rail bed.
Density and Stiffness Determine Speed
Generally speaking, the more dense and stiff a medium is, the faster sound will travel through it. Temperature however plays a part in speed as well. In air for example, sound will travel faster at higher temperatures since molecules will be moving more quickly and therefore collide more frequently and rapidly transfer energy.
How Your Ear Translates Pressure Waves into Sound
Once these pressure waves reach your ear, an incredible biological process occurs converting them into something your brain recognizes as sound. This begins with the outer ear that collects sound waves and directs them down the ear canal to your eardrum.
Your eardrum is a thin membrane that vibrates as struck by pressure waves. Vibrations generated by your eardrum then pass on through three small bones located in the middle ear: the malleus (hammer), incus (anvil) and stapes (stirrup). These bones magnify the vibrations and send them onward to your inner ear, particularly to an organ shaped like a snail shell named the cochlea.
Fluid inside your cochlea responds to vibrations caused by incoming pressure waves, which stimulates hundreds of thousands of fine hairs located inside your cochlea. These hairs convert vibrations into electrical impulses which are carried to your brain by the auditory nerve. Your brain then identifies these electrical impulses as sounds that you hear; whether music, conversation or rustling leaves.
Vacuum & The Transmission Of Sound
An additional critical aspect of comprehending how sound travels is realizing that it cannot travel through a vacuum. By definition, a vacuum contains little to no molecules; therefore there are little to no molecules available to transmit the necessary vibrations required for sound waves to propagate.
Therefore, for reasons explained above, space is absolutely silent. Although numerous astronomical events such as star explosions, planet collisions etc., continuously occur within our universe; no one would be able to hear them from within a vacuum. For example astronauts aboard the International Space Station need radio equipment to communicate with each other or with earth because there are no molecules present in space through which sound can travel.
Above And Below Human Hearing Range
Humans are capable of detecting certain ranges of frequencies; however, sound itself exists outside those ranges.
Ultrasound
Sounds produced by waves having frequencies greater than human hearing range (>20kHz). We utilize ultrasound extensively in medical imaging techniques to view internal organs or an unborn child; underwater sonar devices to map ocean floors; and industrial applications to clean parts or test materials’ durability. Bats and dolphins utilize ultrasound to navigate and hunt prey.
Infrasound
Sounds generated by waves with frequencies <20Hz. Examples of sources of infrasound are earthquakes, volcanic eruptions, landslides and windmills. Certain animals such as elephants employ infrasound to communicate with one another over long distances.
Although ultrasonic and infrasound represent different frequency ranges than those perceived by human hearing, both represent longitudinal waves propagating through a medium containing molecules.
Reflections & Refractions & Absorption
When sound waves travel through an environment they interact with that environment in several manners.
Reflection
When sound waves impact a surface they can rebound off that surface; similar to light. Reflections produce echoes. Hard surfaces that are flat and smooth serve as better reflection mediums while softer surfaces that are irregular or porous tend to act as absorption mediums for sound.
Refraction
When sound waves travel through a medium and enter into another medium or undergo changes in either temperature or density in that medium they can bend. Bending due to changing media is referred to as refraction. Refraction allows sound to “bend” around physical barriers or “ride” on top of warm layers of water as temperature varies within the air layer above.
Absorption
Once sound waves contact a material some portion of their energy is lost as heat energy and therefore converted into alternative forms of energy. Porous materials such as foam, fabric and/or insulation are good examples of materials that serve as absorption mediums for sound and therefore reduce reverberation levels in recording studios and concert venues.
Understanding how reflections, refractions and absorption interact with sound provides knowledge essential for controlling acoustic environments in building design; managing excessive noise in industrial settings; developing advanced sonar technologies etc.
Frequently Asked Questions Regarding Sound Propagation
Which state(s) of matter supports propagation speeds faster than others for sound?
Sound travels fastest through solids; next fastest through liquid states; lastly through gas states. The reason behind this order lies in how closely packed molecules are; rigid connections between solid-state molecules allow for quick passage of vibrations thus providing an ideal conduit for energy transfer.
Can sound travel in space?
No, sound cannot travel in space; space is essentially free from all types of matter; therefore there exist no molecules through which sound waves can travel.
What is the speed at which sound travels through air?
Approximately 343 m/s (1125 ft/sec) at room temperature (or about 20C / 68F) and sea level pressures in dry air.
Do all frequencies of sound travel at equal speeds?
Yes; provided that they all travel through identical media under constant conditions (such as temperature), all frequencies of sound travel at equal speeds. Thus, a high-pitched whistler and a low-pitched rumble will arrive simultaneously at your ear if both originated from the same distance from your location.
Conclusion: From thunder claps to gentle breezes; sound is an amazing natural phenomenon based on vibrations and some sort of medium supporting energy flow between a source and an observer’s ear and ultimately translating into perception by our brains regarding our world around us.
Sources
- What Is Sound? — NASA
- Sound Physics — Encyclopædia Britannica
- How Does Sound Travel? — Scientific American
- Sound — National Geographic Society
- Anatomy, Head and Neck, Ear Tympanic Membrane — National Center for Biotechnology Information (NCBI)
