The present invention relates to an apparatus for estimating the air humidity within an oven cavity (14) by using ultrasound waves with at least two different frequencies (f1, f2). Said apparatus comprises at least one ultrasound transmitter (10) for generating the ultrasound waves, at least one ultrasound receiver (12) for receiving the ultrasound waves and at least one phase detecting device for detecting the phase (φ1, φ2) of the ultrasound wave at the ultrasound receiver (12) relative to the same ultrasound wave with the same frequency (f1, f2) at the ultrasound transmitter (10). Said apparatus comprises further at least one evaluation unit for calculating the velocity (V) of the ultrasound waves on the basis of the phases (φ1, φ2) and frequencies (f1, f2) of the ultrasound waves with the two different frequencies (f1, f2), at least one temperature sensor (32) for detecting the temperature (T) in the oven cavity (14) and at least one estimation unit for estimating the humidity in the oven cavity (14) on the basis of the temperature (T) and the velocity (V) of the ultrasound waves. Further, the present invention relates to a corresponding method for estimating the air humidity within an oven cavity (14).
