LESSON 80

Loudness: phons and sones

Name: Loudness: phons and sones
Uploaded: 2026-11-04T03:15:00+03:00
Duration: 11 min
Description: Intensity is physical; loudness is perceptual. In this lesson we look at equal-loudness contours, the phon and sone units, why 40 phons equal 1 sone, why a rise of about 10 phons doubles loudness, and what this means in the clinic.

Intensity is physical; loudness is perceptual. In this lesson we look at equal-loudness contours, the phon and sone units, why 40 phons equal 1 sone, why a rise of about 10 phons doubles loudness, and what this means in the clinic.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn how loud a sound is heard, that is, the perception of loudness.
We will see equal-loudness contours, the phon and sone units, and what they mean in the clinic.
Intensity is the physical size of a sound; it relates to sound pressure or the energy the sound carries.
Intensity is measured with a sound level meter and is usually given in decibels SPL.
The same decibel SPL value describes the same physical size, whatever the frequency.
Loudness, on the other hand, is how strong the listener hears the sound to be; it is a perceptual quantity.
No device can measure loudness; we measure it by asking the listener, that is, with psychoacoustic methods.
Loudness rises with intensity, but it also depends on frequency, duration and the structure of the sound.
The human ear works over a very wide range of intensities.
At the most sensitive frequencies, the hearing threshold is around zero decibels SPL; a whisper is a little above that.
Everyday speech is around sixty decibels SPL.
Loudness varies over a range of about one hundred twenty decibels, up to the threshold of pain.
Within this range, perception is not linear; doubling the intensity does not make a sound twice as loud.
To understand this relationship, we need two units: the phon and the sone.
We find out how loudness depends on frequency with matching experiments.
The listener is played a one thousand hertz tone as a reference.
Then a tone at another frequency is adjusted until the listener hears it as equally loud as the reference.
When these equally loud levels are joined, we get an equal-loudness contour.
Fletcher and Munson published the first full contours in nineteen thirty three; today's contours are in the ISO 226 standard.
The standard contours are based on the average judgments of young, normal-hearing listeners in a free field.
The phon value of a sound is the decibel SPL value of a one thousand hertz tone heard as equally loud.
So the phon is the unit of loudness level, and it uses one thousand hertz as a common ruler.
By this definition, phons and decibels SPL are equal at one thousand hertz; forty decibels SPL is forty phons.
Each equal-loudness contour is named by a phon value; for example, the forty-phon contour.
All tones on that contour are heard as equally loud, even though their frequencies and decibel values differ.
The phon tells us which sounds are heard as equal, but not how many times louder one sound is than another.
Let us look at a few values taken from the ISO 226 contours.
For a loudness of forty phons, about sixty four decibels SPL is needed at one hundred hertz.
At five hundred hertz, this value drops to about forty three decibels SPL.
At one thousand hertz, by definition, it is forty decibels SPL.
The ear is most sensitive between three and four thousand hertz; less intensity is enough for the same loudness.
At eight thousand hertz, more intensity is needed again.
At ninety phons, the gap between one hundred and one thousand hertz shrinks to about nine decibels, from about twenty four at forty phons.
The first lesson from the contours: at low levels, the ear is much less sensitive to low-pitched sounds.
The most sensitive region is between about three and four thousand hertz; the resonance of the ear canal plays a part in this.
As the level rises, the contours flatten; the loudness of low-pitched sounds grows faster with intensity.
That is why, when we listen to music at a low volume, the bass seems to disappear, and it comes back when we turn it up.
A-weighting in sound level meters also mimics this low-level sensitivity of the ear.
A-weighting looks like an inverted forty-phon contour; it attenuates low frequencies in the measurement.
The threshold curve is like the lowest equal-loudness contour; drawn in decibels SPL, it rises at low frequencies.
The audiometer takes the average threshold of normal listeners at each frequency as zero decibels HL, which corrects this curve.
So the audiogram of a normal-hearing person appears not curved but as a flat line around zero decibels HL.
But this correction applies only to threshold; above threshold, loudness still grows differently across frequencies.
To say how many times louder one sound is than another, we use the sone, a unit proposed by Stevens.
One sone is the loudness of a one thousand hertz tone at forty decibels SPL, that is, a sound of forty phons.
A sound heard twice as loud is two sones, and a sound heard half as loud is half a sone.
Above forty phons, loudness doubles for about every ten phons: fifty phons is two, sixty is four, seventy is eight sones.
A sound of eighty phons is heard sixteen times louder than a sound of forty phons.
In short, a tenfold rise in intensity, that is, ten decibels, creates only a twofold rise in perceived loudness.
According to Stevens's power law, loudness grows in proportion to a power of intensity.
For a one thousand hertz tone, this exponent is about zero point three, in terms of intensity.
Ten decibels means a tenfold rise in intensity; ten to the power of zero point three is about two, so loudness doubles.
Above forty phons, this lets us compute sones as two to the power of phons minus forty, divided by ten.
Near threshold, however, loudness changes faster than this law predicts.
This rule is also the basis for understanding how loudness grows differently in an ear with cochlear damage.
Loudness is not set by intensity and frequency alone.
Even if the total intensity stays the same, a sound is heard as louder if its energy spreads over a range wider than the critical band.
This is called loudness summation; we will cover the critical band in detail in the masking lesson.
Very short sounds are heard as softer; loudness grows with duration up to about two hundred milliseconds.
The same sound is also heard as louder with two ears than with one; this is called binaural summation.
In the clinic, loudness is often assessed with categorical scaling.
The patient is given a card with ordered, defined categories, starting from very soft.
Sounds are raised step by step, and the patient shows the category each sound falls into; this is how the comfortable range is found.
Methods such as the Contour test by Cox and colleagues standardize this measurement with seven categories.
The top category is uncomfortably loud; this level is recorded as the uncomfortable loudness level, or UCL.
This shows how loudness grows from threshold to discomfort, in what is called the loudness growth curve.
Loudness matters most in measuring the MCL, the most comfortable level, and the UCL, the uncomfortable loudness level.
The range between threshold and UCL is called the dynamic range; usable hearing lies within it.
In an ear with cochlear damage, threshold rises but UCL often changes little, so loudness grows faster.
The maximum output of a hearing aid should be set so that sound does not exceed the UCL.
Compression in hearing aids is also used to fit a wide range of sounds into this narrowed range.
A tip from the clinic: loudness measurements are very sensitive to the instructions given to the patient.
When measuring UCL, say clearly that you mean a sound too uncomfortable to listen to for long, not a sound that causes pain.
If the instructions stay vague, results in the same patient can differ markedly.
Use the category card and instructions the same way with every patient, so comparisons between visits are reliable.
Intensity is physical and loudness is perceptual; loudness also depends on frequency, duration and bandwidth.
The phon is the decibel SPL value of an equally loud one thousand hertz tone; equal-loudness contours are named in phons.
The sone is a ratio scale; a sound of forty phons is defined as one sone.
Above forty phons, a rise of about ten phons doubles loudness.
Test yourself: how many sones is a sound of sixty phons?
The correct answer is C: four sones.
Forty phons is one sone; fifty phons is two and sixty phons is four sones, because every ten phons doubles loudness.
In the next lesson we move from loudness to pitch and learn how the ear discriminates frequency.
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Prerequisites

LESSON 79 Psychoacoustics: Absolute and Difference Thresholds Coming soon