Loudness, pitch and timbre

Amplitude is how far the density swings away from average in a compression or rarefaction: the "size" of the wave. A bigger amplitude carries more energy: strike the plate harder and the grains jump higher.

Intensity is the sound energy passing through one square metre each second. One bulb lights your room brightly but is lost in a cricket stadium: the same energy spread over a bigger area. Sound is the same: as it spreads out, its intensity falls with distance.

How our ears hear these:

• Pitch is how we hear frequency. A whistle or siren is high pitch (high frequency); thunder is low pitch. Say "Mummy, paani lao!" in a thin voice, then a deep voice: same words, different pitch.
• Loudness is how we hear amplitude. It also depends on the listener. It is measured in decibels (dB): a conversation is about 60 dB; firecrackers can be over 100 dB.

What we can hear. Humans hear from about 20 Hz to 20 kHz. This varies from person to person and shrinks with age.

• Below 20 Hz is infrasonic: elephants can detect it.
• Above 20 kHz is ultrasonic: dogs, cats, bats and dolphins can detect it.

Protect your hearing. Unwanted, unpleasant sound is noise. Hearing loud sound for a long time, including through earphones turned up high, can cause hearing loss. Hearing is tested with an audiogram. A hearing aid has a microphone, an amplifier and a speaker.

How you hear. Sound makes your eardrum vibrate. Three tiny bones make the vibration bigger. The cochlea turns it into electrical signals and sends them to your brain.

Why a flute and a tabla sound different on the same note. A tone has just one frequency. A musical note mixes a main frequency (the fundamental) with higher overtones. That mix gives each instrument its own quality, its timbre: a flute, an ektara and a tabla all sound different. An octave is a note at double the frequency (200 Hz and 400 Hz).

C. V. Raman studied the tabla and mridangam. Their black patch, the *syaahi*, gives them their rich, musical sound.