Amplitude and Volume
The volume (loudness) of a sound depends on its amplitude: how big the vibrations are. A small ripple in a pond makes a quiet splash; a huge stone makes a big splash. The bigger the air vibrations, the louder the sound. Loudness is usually measured in decibels (dB), an unusual scale that runs from a whisper at 30 dB up to a jet engine at 140 dB. The decibel scale is logarithmic, which means a 10-dB increase actually means the sound is 10 times more intense. That is why even short bursts of loud sound can permanently damage your hearing.
- AmplitudeSize of vibrationSets the volume
- Unit of loudnessDecibel (dB)Logarithmic scale
- Threshold of hearing0 dBQuietest you can detect
- Quiet conversation50 dBComfortable level
- Rock concert120 dBPain threshold approaches
- Hearing damageAbove 85 dB long termOr above 120 dB suddenly
What is amplitude?
Amplitude is the maximum size of the disturbance in a wave. For a sound wave, it is how far the air molecules are pushed back and forth. Big pushes mean big amplitudes mean loud sounds. Small pushes mean small amplitudes mean quiet sounds.
Amplitude is NOT the same as frequency. A high-pitched sound can be quiet; a low-pitched sound can be loud. Volume and pitch are independent qualities of a sound. (See frequency and pitch.)
The decibel scale
The decibel scale is logarithmic. Each 10 dB step means the sound is 10 times more intense. That makes the scale work over an enormous range:
- 0 dB: the quietest sound a healthy young ear can detect.
- 10 dB: a leaf rustling.
- 30 dB: a whisper.
- 50 dB: a quiet conversation.
- 60 dB: a normal conversation.
- 70 dB: city traffic (heard from inside a car).
- 85 dB: starting to be hearing-damaging if heard for hours.
- 100 dB: a chainsaw or motorbike close up.
- 120 dB: a rock concert near the speakers; pain threshold.
- 140 dB: a jet engine 30 metres away.
- 180 dB: a rocket launch.
- 194 dB: the theoretical maximum for sound in normal air (any louder and the wave becomes a shock wave or explosion).
What is "loud" and "quiet"?
Loudness is partly objective (a real measurement of sound intensity) and partly subjective (how loud your brain decides it is). Two sounds with the same decibel level can feel very different depending on their pitch, your distance, the background and your mood.
Your ears are actually most sensitive to sounds around 2,000 to 4,000 Hz, the range of human speech. Sounds at very low or very high frequencies need more amplitude to seem equally loud.
How sound gets louder or quieter
- Louder source: bigger vibrations at the start mean bigger amplitude.
- Distance: sound spreads outward like ripples on a pond. As it spreads, the energy gets spread over a larger surface, so it sounds quieter the further you are. Doubling the distance from the source roughly quarters the intensity.
- Absorbing materials: soft surfaces like carpets, curtains and foam soak up sound, making rooms quieter. Hard surfaces like concrete and tile reflect sound, making rooms echo and feel louder.
- Walls and barriers: blocking the direct path of sound reduces its intensity. This is why double glazing helps with road noise.
Hearing damage
Very loud sounds can permanently damage the sensitive hair cells in your inner ear. Once dead, those hair cells do not grow back. The damage adds up over a lifetime, leading to gradually worse hearing and (often) constant ringing in the ears (tinnitus).
Rules of thumb:
- Sounds under 70 dB are safe for long periods.
- Sounds 85 dB or higher can damage hearing if you are exposed for hours at a time (factories, building sites, loud concerts).
- Sounds over 120 dB can cause immediate damage even from a brief exposure.
- Personal music players, especially with earbuds, can easily reach 100 dB or more. Keep the volume low.
Cancelling sound
Modern noise-cancelling headphones use a clever trick called destructive interference. A tiny microphone on the outside listens to the background noise. A small computer chip then plays the exact opposite of that sound through the speakers in the headphones. When opposite waves meet, they cancel out. Background hum from aircraft, trains and offices can drop by 20 to 30 dB this way, letting you hear music or speech much more clearly.
Deeper dive: why no sound on Earth can be louder than 194 decibels
The decibel scale has no upper limit in theory, but for sound waves in normal Earth air, there is a physical maximum of about 194 dB. Above that, the disturbance is no longer a sound wave: it is a shock wave or explosion.
The reason is simple. A sound wave is a back-and-forth pressure change in the air around the resting atmospheric pressure (101 kilopascals at sea level). The pressure increases as the air is compressed, and decreases as the air expands.
At about 194 dB, the wave is so powerful that its low-pressure part reaches zero pressure: a perfect vacuum, briefly. The air cannot stretch any further without breaking apart. Anything louder is no longer a smooth oscillating sound wave but a one-way shock wave: a sudden, violent compression front that pushes the air ahead of it.
The atomic bomb explosions of Hiroshima and Nagasaki produced shock waves with peak pressures equivalent to roughly 250 dB at close range. The Tunguska meteor explosion of 1908 in Siberia is estimated to have produced around 300 dB at ground zero. The asteroid impact that killed the dinosaurs may have been louder still.
Outside the atmosphere, in interstellar space, you can have shock waves with arbitrarily high "loudness" by terrestrial standards, although nobody is there to listen. In the dense gases near supernovae, scientists estimate sound-like waves can reach the equivalent of 400 to 500 dB. Thankfully, we have never had to hear one in person.
For more, see what is sound and frequency and pitch.