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.
But when multiple critical frequency ranges are stimulated, the brain integrates the signals from each range to produce a perception of loudness. As a result, equal-loudness curves based on noise bands show an upward slope above 1 kHz and a downward slope below 1 kHz compared to those derived from single tones.