Hearing sensitivity peaks near 4 kHz, so noise must be frequency-shaped to be inaudible

Evidence

As discussed in Chapter 2, our threshold of hearing is not constant with frequency, as shown in Figure 2.14. We are much more sensitive to sounds around 4 kHz than at the extremes of the frequency range. Therefore a uni­ form quantization spectrum may be more inaudible at low and high fre­ quencies and yet still be heard if it is above the threshold of hearing in the most sensitive part of our hearing range. Ideally we need to have more noise where we are less sensitive to it and less noise where we are more sensitive. This is possible via a technique known as “noise shaping.” — David Howard, Jamie Angus, coustics and Psychoacoustics, 2664

Categorization

• DEFINE: Introduces ‘noise shaping’ as the technique of redistributing quantization noise to match the ear’s uneven sensitivity.
• XREF: Directly cites the equal-loudness threshold curves from Chapter 2, grounding the whole psychoacoustic-coding discussion in the earlier threshold-of-hearing chapter.