When we talk about sound, one of the most important characteristics we notice is how loud or soft it is. Loudness refers to how strong or intense a sound feels to our ears. But have you ever wondered, what is the unit of loudness of sound? While sound intensity is measured in decibels (dB), loudness itself is measured in phons and sones. These units are designed to match how our ears perceive sound at different frequencies and intensities.
In this topic, we will explain what loudness is, how it is measured, the units of loudness of sound, and why these measurements are important in daily life.
What Is Loudness of Sound?
Loudness is the way we perceive the strength or power of a sound. Unlike sound intensity, which is a physical property, loudness is a subjective experience. It depends not only on the actual sound energy but also on the sensitivity of the human ear at different frequencies.
For example, two sounds may have the same intensity level but can be perceived differently if one is at a low frequency and the other at a higher frequency. This is why loudness is more complex to measure than simple sound intensity.
The Main Unit of Loudness: The Phon
The official and most widely used unit of loudness is the phon. The phon scale is based on comparing the loudness of any sound to a pure tone of 1,000 Hz frequency.
How does it work?
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A sound’s loudness in phons is the same as the decibel level of a 1,000 Hz tone that sounds equally loud.
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For example, if a sound seems as loud as a 70 dB tone at 1,000 Hz, it has a loudness of 70 phons.
Key point:
1 phon equals the loudness of a 1,000 Hz tone at a specific decibel level.
This allows researchers, engineers, and professionals to measure loudness in a way that reflects human hearing perception.
Another Unit of Loudness: The Sone
While the phon measures loudness relative to a 1,000 Hz reference tone, the sone is used to express loudness on a linear scale.
What is a sone?
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1 sone is the loudness of a sound that is perceived to be twice as loud as a 40 phon tone.
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Each time the loudness increases by 10 phons, the perceived loudness doubles in sones.
Formula for sones:
Loudness (in sones) = 2^((phon level – 40) / 10)
Examples:
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40 phons = 1 sone
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50 phons = 2 sones
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60 phons = 4 sones
The sone scale is useful because it matches how our brains interpret increases in loudness.
Loudness vs. Sound Intensity
It’s important to understand the difference between loudness and sound intensity:
| Aspect | Loudness | Sound Intensity |
|---|---|---|
| Nature | Subjective (depends on human perception) | Objective (measured physical quantity) |
| Unit | Phon and sone | Decibel (dB) and watts per square meter (W/m²) |
| Frequency influence | Yes, varies with frequency | No, it is frequency-independent |
| Measurement basis | Equal-loudness contours and human hearing | Sound pressure levels measured by instruments |
Sound intensity tells us the amount of energy in a sound wave, while loudness describes how that energy is perceived by the ear.
The Role of Frequency in Loudness Perception
Human ears do not respond equally to all frequencies. We are most sensitive to frequencies between 2,000 Hz and 5,000 Hz. Sounds outside this range often need to be louder to be perceived at the same level.
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Low-frequency sounds (like bass notes) must have higher intensity to sound equally loud compared to mid-range frequencies.
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High-frequency sounds are more easily heard, even at lower intensities.
This frequency-dependent perception is why loudness measurements use standardized curves known as equal-loudness contours.
What Are Equal-Loudness Contours?
Equal-loudness contours are graphical lines that show how loud a sound at any frequency needs to be in decibels to be perceived as equally loud as a 1,000 Hz tone.
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At low frequencies, higher decibel levels are required for the sound to feel as loud as a 1,000 Hz reference tone.
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At mid-frequencies, less intensity is needed for sounds to feel loud.
These contours are used by audio professionals and researchers to adjust and calibrate sound systems so that sound feels balanced and natural.
How Loudness Is Measured
Loudness is measured with a sound level meter equipped with special filters to account for the ear’s frequency sensitivity.
Steps involved in measuring loudness:
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Measure sound intensity in decibels.
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Apply frequency weighting (commonly A-weighting) to simulate human ear response.
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Convert the weighted decibel levels to phons and, if needed, to sones.
This ensures that loudness measurements reflect the way people actually experience sound.
Why Understanding Loudness Units Matters
1. Audio Engineering
Audio professionals use loudness measurements to balance recordings. A song with uneven loudness levels can sound uncomfortable. Knowing how to use phons and sones helps create better audio experiences.
2. Hearing Safety
Understanding loudness levels is crucial for protecting hearing. Exposure to sounds over 85 dB (which correlates to about 85 phons for certain frequencies) for prolonged periods can cause permanent hearing damage.
3. Acoustic Design
Architects and sound engineers rely on loudness measurements to design spaces like theaters, conference rooms, and public spaces where sound quality matters.
4. Consumer Products
Manufacturers of headphones, loudspeakers, and audio devices use loudness measurements to ensure their products deliver clear, balanced, and safe sound.
Common Loudness Levels in Everyday Sounds
| Sound Source | Approximate Loudness (Phons) |
|---|---|
| Whisper | 30 – 40 phons |
| Normal conversation | 50 – 60 phons |
| Busy traffic | 70 – 80 phons |
| Vacuum cleaner | 80 – 85 phons |
| Rock concert | 110 phons |
| Jet engine at close distance | 120 – 130 phons |
Being aware of these levels helps individuals understand when sound might become harmful.
Health Risks of High Loudness
Excessive exposure to loud sounds can have serious health consequences:
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Hearing loss: Continuous exposure to loud environments can permanently damage the inner ear.
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Tinnitus: Loud sounds can cause ringing or buzzing in the ears.
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Stress and anxiety: Loud noise increases stress levels and impacts mental health.
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Sleep disturbance: High noise levels reduce sleep quality and can affect long-term well-being.
Knowing the loudness units and the safe limits helps prevent these health issues.
Tips for Managing Loudness Exposure
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Limit time spent in loud environments.
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Wear ear protection in noisy places, such as concerts or construction sites.
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Keep volume levels moderate when using headphones or audio devices.
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Take regular breaks from continuous exposure to loud noise.
The unit of loudness of sound is primarily measured in phons, with sones used to express perceived loudness on a more intuitive scale. Loudness is different from sound intensity, as it considers both intensity and the human ear’s frequency response.
Understanding these units helps in various fields, from audio engineering and product design to public health and safety. Loudness measurements ensure sound is not only enjoyable but also safe. Whether you are adjusting the volume on your favorite music, working in a noisy environment, or simply enjoying the sounds around you, knowing how loudness is measured helps you make informed and healthy decisions.