A monitoring apparatus comprising a multichannel pressure sensing sensor for measuring a ballistocardiographic signal of a human body is disclosed. The monitoring apparatus also comprises means for selecting a time window for heart inter beat interval including two consecutive heart beats to be estimated, defining a spectrum for the ballistocardiographic signal averaging between at least two measurement channels of the multichannel pressure sensing sensor, a cepstrum from the logarithm of a spectrum, and a heart inter beat interval. The invention relates to a method for defining a heart inter beat interval, where a ballistocardiographic signal of a body is measured with a multichannel pressure sensing sensor, a time window for heart inter beat interval Including two consecutive heart beats to be estimated is selected, a spectrum for the ballistocardiographic signal averaging between at least two measurement channels of the multichannel pressure sensing sensor, a cepstrum from the logarithm of a spectrum, and a heart inter beat interval are defined.