However, when more than one critical band is stimulated, the outputs of the various bands are summed by the brain to produce an impression of loudness. For these reasons equal-loudness curves derived using noise bands show an upwards tilt above 1 kHz and a downward tilt below 1 kHz when compared to the curves derived using pure tones.
The brain combines signals from multiple frequency ranges to perceive loudness, rather than relying on a single tone. This integration affects the shape of equal-loudness curves, which differ between noise bands and pure tones.