Definition
Sound is produced by a vibrating object and travels as a longitudinal wave, needing a medium (solid, liquid or gas) — it cannot travel through a vacuum, since there are no particles to vibrate and pass the disturbance on.
The bell-jar experiment proves this: an electric bell inside a sealed jar is heard clearly with air present, but as air is pumped out, the sound fades and eventually disappears, even though the bell is still visibly ringing.
Rule
Sound travels fastest in solids, slower in liquids, and slowest in gases — particles are packed closest and most strongly bonded in solids, so a vibration passes from particle to particle with the least delay. In air, sound travels at about m/s at room temperature.
Remember
A tuning fork held in the air produces a faint sound but grows much louder when its stem is pressed onto a wooden table — the table's large flat surface vibrates along with the fork and pushes far more air, so surface area (not just the vibration itself) matters for how much sound reaches your ear.
Definition
A normal human ear detects frequencies roughly between Hz and Hz ( kHz) — this range is called the audible range.
Infrasound: frequency below Hz (too low to hear) — e.g. some earthquake and elephant-communication waves.
Ultrasound: frequency above kHz (too high to hear) — used in medical imaging and cleaning, since it reflects off internal boundaries (e.g. organ/tissue edges) without harming tissue.
Example 1 — frequency from speed and wavelength
Speed of sound in air is m/s. A sound wave has wavelength cm m. Find its frequency, and say whether it's in the audible range.
- .
- Hz.
- Compare with the audible range – Hz.
Answer: Hz — yes, within the audible range
Example 2 — frequency and period of a repeating chime
A clock chimes times in minute. Find the frequency and time period of the chimes.
- chimes per second.
- Hz.
- .
Answer: Hz, s
Example 3 — frequency of a car horn
A car horn has wavelength m; speed of sound in air is m/s. Find its frequency.
- .
Answer: Hz
Example 4 — time period from speed and wavelength
A wave travels at m/s with wavelength m. Find its time period.
- Hz.
- .
Answer: s
Definition
Sound reflects off hard surfaces just like light reflects off a mirror. An echo is a reflected sound heard separately from the original, because the human ear can only distinguish two sounds as separate if they arrive at least about s apart.
Rule
Minimum distance for a clear echo: since sound must travel to the surface and back in at least s, and travels at about m/s, the reflecting surface must be at least m away — closer than this, the reflected sound overlaps with the original and no separate echo is heard.
Example 5 — depth of the sea from an echo
A research boat sends a sound pulse straight down and receives its echo s later. Speed of sound in seawater is m/s. Find the depth.
- The pulse travels down and back in s, so time for one-way travel s.
- Depth .
Answer: m
Example 6 — distance to a mountain from an echo
A person claps and hears the echo after s. Speed of sound is m/s. Find the distance to the mountain.
- Round-trip time s, so one-way time s.
- Distance .
Answer: m
Remember
Soft, uneven materials (carpets, curtains, acoustic foam) absorb sound rather than reflecting it cleanly, so rooms lined with them have fewer echoes and clearer acoustics — this is exactly why concert halls and recording studios use such materials, and why acoustic protection matters in building design near busy roads or airports.
Definition
Loudness depends on amplitude — a larger amplitude vibration pushes air particles harder, carrying more energy and sounding louder, at the same frequency.
Pitch depends on frequency — a higher frequency sounds higher-pitched. Men's voices sound deeper than women's or children's because their vocal cords are typically longer and thicker, vibrating at a lower frequency for the same loudness.
Quick Recap
- ●Sound: longitudinal wave, needs a medium; fastest in solids, slowest in gases; m/s in air
- ●, ; audible range – Hz; infrasound below, ultrasound above
- ●Echo needs a reflecting surface m away (round trip s) to be heard separately
- ●Loudness amplitude; pitch frequency
- ●Soft materials absorb sound and reduce echoes; ultrasound used in medical imaging
