- Why is the threshold curve shaped like a 'U'? Why are we less sensitive to low and high pitches?
- The shape is primarily due to the biomechanics of the inner ear (cochlea) and the resonance of the outer ear canal. The cochlea is most efficient at translating mechanical vibrations into nerve signals for mid-range frequencies, which correspond to important speech sounds. Low frequencies require more energy to excite the cochlear fluid, while very high frequencies are damped by the mass and stiffness of the auditory structures.
- Does 0 dB mean no sound at all?
- No. 0 dB SPL is a defined reference point, not the absence of sound. It represents the average threshold of hearing for a 1 kHz tone—the faintest sound a young, healthy ear can detect. Sounds below this curve are inaudible. Absolute silence would correspond to a sound pressure of 0 Pa, which is a physical impossibility in our environment and is far below the 20 µPa reference.
- If two different frequency tones are played at the same decibel level, will they sound equally loud?
- Not necessarily. The decibel level measures physical intensity (SPL), not perceived loudness. Due to the ear's frequency sensitivity, a 100 Hz tone at 60 dB SPL will sound much quieter than a 1 kHz tone also at 60 dB SPL. To make them sound equally loud, you would need to increase the SPL of the 100 Hz tone significantly, following an equal-loudness contour.
- How does this relate to volume controls on music players or hearing loss?
- A simple volume control increases the SPL uniformly across all frequencies. Because our hearing is less sensitive to bass, boosting the overall volume makes low frequencies more perceptible, which is why music sounds fuller when turned up. Hearing loss, especially age-related, often starts with reduced sensitivity at high frequencies, effectively steepening the right side of the threshold 'U' shape.