Photoacoustic spectroscopy employs acoustic resonators for signal amplification. Resonators are usually closed, however, in some applications, open resonators are preferred. The opening deteriorates the photoacoustic signal hence reducing the sensitivity of the photoacoustic measurement. We present two new approaches for simulating the photoacoustic signal in open resonators using finite element modelling. The approaches are based on the amplitude mode expansion model and the viscothermal model with the opening modelled using perfectly matched layers and the boundary element method respectively. Additionally, the performance of the viscothermal model using perfectly matched layers for simulating open resonators is extended to the ultrasound region. The simulation results are verified by comparing them to photoacoustic measurements. The approaches provide an accurate basis for designing and optimizing open resonators with high sensitivity.