If you are asking about conventional single element transducers as opposed to phased arrays, you will find the equations in section 2(d) of the technical notes portion of the Panametrics-NDT transducer catalog, which you can download at
The basic theory of beam forming is easy to find. For example in Wikipedia, but also at many other places in the web and in different books and papers, describing medical or other ultrasonic devices.
The exact description is very complicated, because it must include also the real function of each transducer element and the propagation of sound in the medium. There are many papers dealing with this problem.
I've done some research on the similar topic before and I have a short summary on how to calculate the time delay (to focus and steer the sound beam) for linear array. Shot me an email if this is what you are looking for.