Sound level is a quantity expressed in decibels that quantifies the intensity of a sound perceived by the human ear on a logarithmic scale.
Discover FizziQ
How to measure it in class
With FizziQ, students can measure the sound level of their environment and study how it varies with distance.
Steps:
- Open FizziQ and select the Sound Level (dB) instrument. Place the smartphone at a fixed location in the classroom.
- Measure the sound level in different situations: silence, whispers, normal conversation, applause. Record each value in the experiment notebook.
- Place a sound source (another smartphone emitting a continuous sound) and measure the sound level at different distances: 0.5 m, 1 m, 2 m, 4 m.
- Plot the sound level curve as a function of distance. Observe that the level decreases by approximately 6 dB when the distance is doubled. Relate this result to the inverse square law of distance.
Scientific activities on this topic
Several experiments easily achievable with a smartphone, tablet, or computer allow measuring sound level and studying its properties.
1: Chloe at the Concert - Relationship between sound level and distance from the source: https://www.fizziq.org/en/activities/chloe-at-the-concert/
2: Sound Wave Addition - What is the increase in sound level when adding two waves of the same intensity?: https://www.fizziq.org/en/activities/addition-of-sound-waves/
3: White Noise - What frequencies make up white noise?: https://www.fizziq.org/en/activities/white-noise/
Learn more
A bit of history
The decibel unit pays tribute to Alexander Graham Bell, inventor of the telephone. The bel, too large for practical use, was replaced by the decibel (one-tenth of a bel). This logarithmic scale was adopted by telecommunications engineers in the 1920s.
Orders of magnitude
Rustling leaves correspond to approximately 20 dB, a quiet office to 40 dB, urban traffic to 80 dB, a jackhammer to 100 dB, a rock concert to 110 dB. The noise record produced by humans is that of the Saturn V rocket takeoff: approximately 204 dB.
A-weighting and hearing health
Noise measurements often use A-weighting (dBA), which accounts for the ear’s sensitivity to different frequencies. The ear is less sensitive to low and very high frequencies. A-weighting corrects measurements to reflect actual perception.
Additional experiments
The experiment Chloe at the Concert studies the variation of sound level with distance and opens a discussion about hearing risks.
Formula
L = 10 x log10(I / I0)
Meaning:
L: sound level (dB)
I: measured sound intensity (W/m^2)
I0: reference sound intensity = 10^-12 W/m^2 (threshold of hearing)
log10: decimal logarithm
Practical rule: when intensity is doubled, sound level increases by 3 dB.
Application examples
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A whisper corresponds to approximately 30 dB, a conversation to 60 dB, a concert to 100 dB
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Workplace regulations require hearing protection above 85 dB
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Sound level meter apps on smartphones allow checking the sound level of a room
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The noise of an airplane at takeoff reaches 130 dB nearby
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Legislation limits the volume of earbuds and headphones to protect hearing
FAQ
Q: Why are decibels used instead of watts? A: The human ear perceives sounds over an immense intensity range (10^12 W/m^2). The logarithmic decibel scale compresses this range into a practical interval of 0 to 130 dB.
Q: How many decibels are gained by doubling the sound power? A: Doubling the sound power adds exactly 3 dB. That’s why two identical speakers playing together produce a sound 3 dB louder than one alone.
Q: At what level does sound become dangerous? A: Prolonged exposure above 85 dB can damage hearing. Above 120 dB, pain occurs. Acoustic trauma can cause irreversible damage.
Q: Does the smartphone measure decibels accurately? A: Smartphones give a good estimate, sufficient for educational experiments. However, their microphone is not calibrated like a professional sound level meter. Values may vary by a few dB.
Q: What is the difference between sound level and sound intensity? A: Sound intensity is the power transported per unit area (W/m^2). Sound level is the translation of this intensity into decibels on a logarithmic scale.
Related concepts
Decibel - Sound intensity - Amplitude - Frequency - Threshold of hearing - Pain threshold - Hearing risks - Logarithmic scale