- Phon and Sone
Music is everybody’s possession. It’s only publishers who think that people own it – John Lennon
The above is a section of the song Black or White by Michael Jackson from three different releases. Same song, but mixed to sound louder with each passing release.
Our perception of sound amplitude is Loudness or Volume.
Sound Pressure Level readings are a good indicator of the amplitude of sound, but cannot be said to match perfectly with our perception of it. An easy example of this is heavy metal music, which to some are a pleasurable experience. Its loudness is not perceived as loud while the listener enjoys it.
Another example is the ticking of a clock in a quiet room. The ticking is perceived to be louder than it really is on the decibel scale.
So can loudness be measured? People have certainly tried!
The above graph shows the frequency of the human ear on the x-axis, which is 20 Hz to 20 KHz as we have seen. Within this range, the human ear is most sensitive between 2 KHz and 5 KHz.
What the scientists do is ask many people to listen to tones at various SPL levels (decibels), and have them match what they perceive using a reference tone, and all this information is averaged to find the graph above. It’s far from perfect, and some of the information (dotted lines) is extrapolated.
The graph shows clearly why ‘deep’ or low frequency sounds are more tolerable at higher decibel values, while ‘shrill’ and high-pitched sounds become unbearable at relatively lower decibels.
So, the perception of loudness varies according to SPL and frequency. Is that all? No.
The perception of loudness always varies according to duration. The human auditory system averages the effects of SPL over a 600–1,000 ms window. If a constant SPL of different duration is played back, the perception of loudness will change depending on its duration.
For example, a constant SPL will be perceived to increase in loudness as 20, 50, 100, 200 ms samples are played, up to a maximum of approximately 1 second (1,000 ms) at which point the perception of loudness will ‘stabilize’.
For sounds greater than 1 second, the moment by moment perception of loudness will be based on the integration of the preceding 600–1,000 ms. It’s relative. The longer the sound plays the less loud it sounds.
Precise measures show that loudness grows more rapidly at low and high SPL levels and less rapidly at moderate SPL levels.
All in all, you can appreciate how measuring loudness directly is certainly not easy, if not impossible. In spite of this, there are two units that attempt to measure loudness – the Phon and the Sone.
1 Phon is equal to 1 dB (SPL) at a frequency of 1 kHz. Note that the frequency is between our ‘best range’ and the SPL is just above the absolute threshold of hearing.
It is a simple way to look at loudness but it has one big disadvantage, potentially fatal to its universal acceptance – the equal-loudness contours (the graph above) on which the phon is based apply only to the perception of pure steady tones. Complex tones cannot be measured this way.
1 Sone is equivalent to the loudness of a signal at 40 phons. This corresponds to the loudness level of a 1 kHz tone at 40 dB SPL. Each 10 phon increase (or 10 dB at 1 kHz) produces almost exactly a doubling of the loudness in sones – so it’s not a linear relationship:
- Our perception of sound amplitude is Loudness or Volume.
- Loudness depends on SPL, frequency and duration, among other factors.
- Two units (both non-SI) used to measure loudness are the Phon and the Sone.
Links for further study: