All arbitrators must meet together and hear all of the allegations and evidence of the parties pertinent or material to the cause; but the parties may mutually waive, in writing, the appearance of all of the arbitrators named in the articles of submission and consent for those present to proceed, or they may, in like manner, substitute other persons for the absent one. An award made, and every other act done, by a majority of the arbitrators shall be valid, unless the concurrence of all or a certain number of the arbitrators to the award or acts be expressly required in the submission.