Loudness

Loudness is the subjective perception of sound pressure. It describes how we order sounds on a scale from quiet to loud. Many people confuse loudness with physical measures like sound intensity or sound power. However, loudness is a psychological experience rather than just a physical one. This study is part of a field called psychoacoustics. Scientists use methods of psychophysics to understand how our minds process sound. Different industries use various measurement standards to define loudness. For example, the ITU-R BS.1770 standard focuses on electronic sounds in cinema or broadcasting. Other standards, like ISO 532A, characterize environmental noise.

To understand how loudness works, we must look at three factors. These are sound pressure level (SPL), frequency content, and duration. The relationship between SPL and the loudness of a single tone follows Stevens's power law. This law uses an exponent of 0.67 to show how loudness grows. A more complex model is the Inflected Exponential function. This model shows that loudness increases faster at very low or very high levels. At moderate levels, the growth rate is actually lower. This means the way we perceive volume changes depending on how loud the sound already is.

The human ear does not respond to all frequencies equally. Our sensitivity changes based on the frequency of the sound. This is shown on an equal-loudness graph. This graph shows the SPL required for different frequencies to sound equally loud. Humans with normal hearing are most sensitive to sounds between 2 and 4 kHz. Our sensitivity drops off if the sound is much higher or much lower. A complete model of loudness must integrate the SPL across all these different frequencies.

Measuring loudness has changed significantly over time. Historically, scientists used an ear-balancing method. A person used an audiometer to adjust the amplitude of a sine wave. They would change the wave until the perceived loudness felt equal. Modern standards are much more technical. They rely on the summation of energy in critical bands. Standards like ISO 532B and DIN 45631 are used to characterize noise. Newer standards like Nordtest ACOU112 also consider the onset rate and spectral masking.

History shows how technology has forced better measurement standards. In 2001, work began on the ITU-R BS.1770 standard. This was necessary because digital converters and lossy codecs caused 0 dBFS+ level distortion. In 2003, Gilbert Soulodre proposed a metric called Leq(RLB). Many groups, including Dolby and CBC, helped test this through subjective listening. The EBU improved this system in 2008 for multi-channel applications. This helped make sound more consistent across different genres. Later, the standard was updated to include 22.2 surround sound.

Loudness perception can change due to sensorineural hearing loss. This occurs when there is damage to the cochlea or the brain. Two main theories explain this change. The first is loudness recruitment, which is the classical explanation. It suggests that loudness grows much more rapidly for these listeners. The second theory is called softness imperception. This term was coined by Mary Florentine around 2002. It suggests that the softest audible sounds are simply louder for these listeners than for others.

We use various units to quantify these experiences. The units sone and phon are used to measure loudness and loudness level. A-weighting is a filter that mimics human sensitivity. It is often used for moderate speech levels around 40 phons. In professional production, engineers use LKFS units. Loudness normalization is another important tool. It equalizes perceived levels so commercials do not sound louder than programs. Systems like ReplayGain or Spotify use these methods.

Technology also helps us compensate for our ear's natural limits. Some consumer stereos have a loudness compensation feature. This feature alters the frequency response to match the ear's equal-loudness characteristic. It is designed to make music sound natural at low volumes. It does this by boosting low frequencies. This is helpful because the ear is less sensitive to low sounds at low pressure levels. This ensures the music stays balanced even when you turn the volume down.