Fletcher Munson Curves

Fletcher Munson Curves are sometimes referred to as the Equal Loudness Contours, although these are two slightly different standards (see figure 1).

The Human Ear

The human ear operates by sensing pressure variations above and below atmospheric pressure. The process is as followed:

1. A sound wave entering the ear canal exerts a fluctuating pressure on one side of the eardrum; the air on the other side of the eardrum is at atmospheric pressure
2. The pressure difference on the two sides sets the drum into motion which in turn oscillates three tiny bones in the middle ear called the ossicles
3. This oscillation is finally transmitted to the fluid-filled inner ear; the motion of the fluid disturbs hair cells within the inner ear, which transmit nerve impulses to the brain with the information that a sound is present.

Sound Not Equally Sensitive

An important factor in the understanding of sound is that the ear is not equally sensitive to all frequencies in the audible range. A sound at one frequency may seem louder than one of equal pressure amplitude at a different frequency.

Notice in the graph in figure 1 how the low-frequency curved lines flatten as the overall loudness level increases.

The low-frequency lines flatten out because at higher sound pressure levels the ear is more sensitive to lower frequencies. Notice how the ear is less sensitive to the frequencies above 6,000 Hz.

The Fletcher Munson Curves help explain why quieter music seems to sound less rich and full than louder music.

The louder music is, the more we perceive the lower frequencies, and thus it becomes more full and rich. Many stereo systems have a loudness switch which boosts the low and high frequencies of the sound.

Summary Points

• The ear is less sensitive to low frequencies at low volumes
• The ear is most sensitive to the mid-range/upper mid-range frequencies
• The ear is slightly less sensitive to higher frequencies compared to mid-range frequencies at the same volume.