High and low sounds
A tiny bird chirps with a high, squeaky sound. A big dog growls with a deep, low sound. Pitch is how high or low a sound seems, and it depends on frequency: how many times per second something vibrates.
| High pitch | Low pitch |
|---|---|
| High frequency (fast vibrations) | Low frequency (slow vibrations) |
| Short wavelength | Long wavelength |
| Whistles, flutes, violins, birds | Tubas, bass drums, cellos, thunder |
Frequency is measured in hertz (Hz). Humans hear from about 20 Hz to 20 000 Hz. As people age, they usually lose the ability to hear the highest frequencies.
What changes pitch?
For string instruments, three things control pitch:
| Factor | Higher pitch | Lower pitch |
|---|---|---|
| Length | Shorter string | Longer string |
| Tightness (tension) | Tighter string | Looser string |
| Thickness | Thinner string | Thicker string |
That’s why a guitarist presses strings against frets (shortening them) and tightens tuning pegs to raise pitch.
For wind instruments, the length of the air column matters. A short tube (like a piccolo) plays high notes; a long tube (like a tuba, which has about 5.5 m of tubing coiled up) plays low notes.
Musical notes and octaves
Each musical note has a specific frequency. The note A above middle C is 440 Hz, the standard orchestras tune to. When you double a frequency, you get the same note one octave higher: 880 Hz is also an A.
The Doppler effect
When an ambulance races toward you, its siren sounds higher. As it passes and moves away, the pitch drops. This is the Doppler effect.
- As the source moves toward you, sound waves get bunched up (shorter wavelength, higher frequency).
- As it moves away, the waves stretch out (longer wavelength, lower frequency).
Austrian scientist Christian Doppler described this in 1842. Today, it’s used in weather radar, police speed guns, medical ultrasound of blood flow, and astronomy (to tell whether stars and galaxies are moving toward or away from us).
Acoustics: sound in spaces
Acoustics is the science of how sound behaves, especially in rooms and buildings.
Echoes
An echo is a reflected sound you hear separately from the original. You need a hard surface at least about 17 m away for your brain to hear the echo as separate, because sound takes about 0.1 s to travel there and back.
Reverberation
In a smaller room, reflections arrive too quickly to hear separately. They blend together, making the sound linger. This is reverberation.
| Space | Reverberation | Why |
|---|---|---|
| Empty gym or cathedral | Long | Hard, flat surfaces reflect sound |
| Bedroom with carpet and curtains | Short | Soft materials absorb sound |
| Recording studio | Very short | Foam panels absorb echoes |
| Concert hall | Carefully designed | Enough to sound rich, not muddy |
Designing for sound
Acoustic engineers shape spaces using:
- Absorbers (carpet, curtains, foam, acoustic tiles) to reduce echoes
- Diffusers (bumpy or angled surfaces) to scatter sound evenly
- Reflectors (curved panels) to direct sound toward the audience
- Sound insulation (thick walls, double doors) to keep outside noise out
Resonance
Every object has a natural frequency at which it vibrates most easily. If something pushes it at that frequency, it vibrates strongly. This is resonance.
- A guitar’s hollow body resonates, making the strings much louder.
- Blowing across a bottle makes the air inside resonate.
- A singer can shatter a wine glass by singing its natural frequency loudly.
- Pushing a swing at just the right moment makes it go higher.
Noise pollution
Too much unwanted sound, from traffic, construction or airplanes, is noise pollution. It can cause stress, sleep problems and hearing loss. Cities use sound barriers along highways and noise bylaws to reduce it.
Common mistakes
- Confusing pitch and loudness. Pitch is frequency; loudness is amplitude.
- Thinking the siren itself changes. Only the sound you hear changes with the Doppler effect.
- Thinking echoes and reverberation are the same. Echoes are heard separately; reverberation blends together.
Try it yourself
- Fill glass bottles with different amounts of water. Blow across the tops, then tap them. Compare which is highest in each case. (Hint: blowing makes the air vibrate; tapping makes the glass and water vibrate.)
- Clap in a bathroom, then in a bedroom with carpet. Which has more reverberation?
- Hold a ruler over the edge of a desk and twang it. Change how much sticks out. What happens to the pitch?
Quick recap
- Pitch depends on frequency; higher frequency means higher pitch.
- Shorter, tighter and thinner strings make higher notes.
- The Doppler effect changes pitch when a source moves.
- Acoustics studies echoes, reverberation and room design.
- Resonance makes objects vibrate strongly at their natural frequency.