Sound waves are vibrations that travel through matter, creating the sensation of hearing. This section covers their core properties (pitch, loudness, and speed), how they behave in different mediums, and acoustic phenomena like harmonics, resonance, and echoes. You'll also learn about sounds outside human hearing range, including ultrasound and infrasound.
Properties of Sound
Characteristics of Sound Waves
Sound waves are longitudinal waves, meaning the particles in the medium vibrate back and forth in the same direction the wave travels. As the wave moves through, it creates regions of compression (where particles are pushed closer together) and rarefaction (where particles spread apart). Think of it like a slinky being pushed and pulled from one end.
A few key facts about sound waves:
- They require a material medium (solid, liquid, or gas) to travel. Sound cannot move through a vacuum because there are no particles to vibrate.
- The frequency of a sound wave determines the pitch you hear.
- The amplitude of a sound wave determines how loud it sounds.
Pitch and Loudness
Pitch is how high or low a sound seems, and it's directly tied to frequency, measured in Hertz (Hz). Higher frequency means higher pitch; lower frequency means lower pitch. The typical human hearing range spans from 20 Hz to 20,000 Hz. A bass guitar playing a low note might produce frequencies around 40–100 Hz, while a piccolo can reach above 4,000 Hz.
Loudness depends on the amplitude of the sound wave. Greater amplitude means a louder sound. Loudness is measured on the decibel (dB) scale, which is logarithmic. That means each 10 dB increase represents a tenfold increase in sound intensity (though your ears perceive it as roughly twice as loud).
Some reference points on the decibel scale:
- 0 dB: threshold of human hearing
- 30 dB: a whisper
- 60 dB: normal conversation
- 110 dB: a rock concert
- 120 dB: the pain threshold for human ears
Speed of Sound
The speed of sound depends on the medium it's traveling through. Sound moves fastest through solids, slower through liquids, and slowest through gases. That's because particles in solids are packed more tightly, so vibrations pass between them more quickly.
- Air (at 20°C): approximately 343 m/s
- Water: about 1,480 m/s
- Steel: approximately 5,120 m/s
Temperature also matters. In air, the speed of sound increases by about 0.6 m/s for every 1°C rise in temperature. Warmer air has more energetic particles, so they transmit vibrations faster.
Acoustic Phenomena
Harmonics and Overtones
When a string or air column vibrates, it doesn't just vibrate at one frequency. It produces a fundamental frequency (the lowest frequency, also called the first harmonic) along with higher frequencies called harmonics.
- The first harmonic is the fundamental frequency.
- The second harmonic vibrates at twice the fundamental frequency.
- The third harmonic vibrates at three times the fundamental, and so on.
So if a guitar string has a fundamental frequency of 200 Hz, its second harmonic is 400 Hz, its third is 600 Hz, and the pattern continues.
Overtones are all the frequencies above the fundamental. The first overtone is the second harmonic, the second overtone is the third harmonic, and so on. The specific mix of harmonics and overtones a sound contains is what gives it its timbre (pronounced "TAM-ber"). Timbre is the reason a piano and a violin sound different even when playing the same note at the same volume.
Resonance and Standing Waves
Resonance happens when an object is driven to vibrate at its natural frequency by an external force. When the driving frequency matches the object's natural frequency, the vibrations build up and the amplitude increases dramatically. A classic example: pushing someone on a swing. If you push at just the right rhythm (matching the swing's natural frequency), the swing goes higher and higher with each push.
Standing waves form when waves reflect back and forth in a confined space and interfere with each other. They create a stable pattern with:
- Nodes: points that stay still (zero amplitude)
- Antinodes: points that vibrate the most (maximum amplitude)
Musical instruments rely on resonance and standing waves to produce sound:
- String instruments like guitars and violins use vibrating strings and resonating body cavities to amplify specific frequencies.
- Wind instruments like flutes and trumpets use resonating air columns of specific lengths to produce their pitches. A longer air column produces a lower pitch, which is why a tuba sounds much deeper than a flute.
Resonance isn't always helpful, though. It can cause dangerous vibrations in structures like bridges and buildings if the driving frequency matches the structure's natural frequency.
Echo and Reverberation
An echo occurs when a sound wave bounces off a distant surface and returns to the listener as a separate, distinct sound. For you to perceive an echo (rather than just a continuation of the original sound), the reflecting surface needs to be at least about 17 meters away. At that distance, the round trip takes about 0.1 seconds, which is long enough for your brain to register two separate sounds.
Reverberation is what happens when sound reflects many times off surfaces in an enclosed space, creating a lingering, blended effect rather than distinct echoes. Concert halls are designed with controlled reverberation to make music sound rich and full. Too much reverberation in a classroom or lecture hall, though, makes speech hard to understand because words blur together.
To manage echoes and reverberation, architects use:
- Sound-absorbing materials like carpets, curtains, and acoustic panels to soak up sound energy
- Diffusers that scatter sound waves in many directions to prevent focused reflections
Animals like bats and dolphins use echoes for echolocation, sending out sound pulses and listening for the reflections to map their surroundings and locate prey.
Sound Beyond Human Hearing
Ultrasound and Its Applications
Ultrasound refers to sound waves with frequencies above 20,000 Hz, beyond the upper limit of human hearing. It has a wide range of practical applications:
- Medical imaging: Sonograms and echocardiograms use ultrasound to create images of internal body structures without surgery or radiation.
- Therapeutic treatments: Ultrasound is used in physical therapy to promote tissue healing and to break up kidney stones (a procedure called lithotripsy).
- Industrial uses: Non-destructive testing checks materials for internal flaws, and ultrasonic cleaners use high-frequency vibrations to clean delicate items like jewelry and surgical instruments.
Several animals can hear or produce ultrasound. Bats use ultrasonic echolocation to navigate and hunt insects in the dark. Dogs can hear frequencies up to about 45,000 Hz, which is why dog whistles (typically 23,000–54,000 Hz) are inaudible to humans but easily get a dog's attention.
Infrasound and Natural Phenomena
Infrasound refers to sound waves with frequencies below 20 Hz, below the lower limit of human hearing. These very low-frequency waves are often produced by large-scale natural events:
- Earthquakes generate infrasound that can be detected over hundreds of kilometers.
- Volcanic eruptions produce infrasound waves that scientists use to monitor volcanic activity remotely.
- Large machinery like wind turbines and heavy vehicles can also generate infrasound.
Some animals have evolved to use infrasound for communication. Elephants produce infrasonic calls (as low as 14 Hz) that travel long distances across the savanna, allowing herds to coordinate over several kilometers. Whales similarly use low-frequency sounds to communicate across vast stretches of ocean.
The effects of infrasound on humans are still being studied. Some people report feelings of unease or discomfort when exposed to strong infrasound. On the practical side, infrasound monitoring stations are part of early warning systems for natural disasters like tsunamis and volcanic eruptions.