Reverberation and noise have a significant effect on the intelligibility of speech in rooms. Detection of clear speech in highly reverberantand noisy enclosures is an extremely difficult problem. In this work we take the approach of spatial filtering by spherical microphone array to overcome noise and reverberation. In order to investigate speech intelligibility, an acoustic model was developed. The model is based on U50, which was shown to have good correlation with speech intelligibility, while combining the effect of both reverberation and noise. This paper presents the array beamforming design for optimal speech intelligibility performance. The beamformer is designed to enhance the speech signal while minimizing noise power and maintaining robustness at a wide frequency range. Having such a model allows the prediction of the required array order for detecting speech clearly.