The science of volume and the Fletcher Munson curves

Continuing on the theme of the human ear I will write about the study that led to our understanding of that subjectivity I have been talking about in other posts

The science of volume and the Fletcher Munson curves

You must have noticed that music seems to lose its weight when we turn down the volume of the radio or sound system significantly. The bass disappears and the brightness of higher pitched instruments fades away leaving only the voice and mid range sounds in the spotlight. It is common to blame the speaker quality for this but the real responsibility lies with how the human brain processes audio.

To understand this phenomenon it is necessary to differentiate physical volume from psychoacoustic perception. The technical concept of volume usually refers to sound pressure measured in decibels and sound behaves linearly for a microphone or a decibel meter. This means a certain amount of decibels of a low sound has exactly the same physical energy as the same amount of decibels of a high pitched sound because the machine is objective and does not judge frequencies

The subjectivity of hearing and the study of curves

The human ear works differently and is quite subjective because we do not hear all frequencies with the same ease. Our hearing evolved to prioritize the human voice frequency and alert sounds which sit in the mid range. The consequence is that we are naturally not very sensitive to very deep lows and very high highs unless they are at high volumes.

Researchers Harvey Fletcher and Wilden Munson decided to map this human characteristic in 1933 and conducted tests to discover how much physical volume was needed at different frequencies for the listener to perceive the sound with the same intensity. The result of this study became known as the Fletcher Munson Curves and the resulting graph teaches us that the mid range is our auditory reference where we hear easily even at low volumes.

Low sounds require much more energy and one must apply much more sound pressure for you to perceive a low sound with the same intensity as a voice. The sensitivity curve for bass drops drastically at low volumes and our ear practically stops perceiving the weight of the music but the response becomes flatter as we increase the overall volume and that is when bass and treble appear in a balanced way

The impact on equipment and music production

This knowledge changes how we deal with audio equipment daily and the most practical example is the Loudness button present on many amplifiers and car stereos. Many people believe this button serves to increase power but its real function is corrective for low volume listening. The circuit applies an equalization that artificially boosts bass and treble when activated and this compensates for the natural flaw described by the Fletcher Munson curves making the music sound full and balanced even when playing softly.

This concept is also fundamental for audio professionals because an engineer who mixes a song at a very low volume tends to exaggerate the bass and drums to hear them. The problem is that the low frequencies will become deafening when that song is played at normal volume by someone else and that is why professional studios work with standardized volume levels. Volume is not just an intensity control but a component that alters timbre perception. Music heard at low volume is a different experience from the same music heard at high volume for our brain and understanding this relation helps us configure our equipment better to ensure a rich sonic experience regardless of the loudness

The next topic regarding the human ear is something that interests me greatly and influences how I approach my mixes and how I listen in general which is how our ear localizes sounds