Conventionally ultrasound uses unfocused and focused beams to gather image information in tissue. However limitation in the acquisition speed has been reach and new techniques need to be developed to overcome these. Negative focus has in few cases shown potential to solve this by providing a wider beam thus enhancing the parallel focusing capability in ultrasound. There is however still a lack of knowledge for the negative focus compared to the unfocused and focused beam. This project has determine an algebraic model bringing together the influential factors aperture size, negative focal distance and range to determine the beam width. The model was validate with good results and shows how the beam width is a projection of the aperture. This model is based on a newly developed technique to simulated ultrasound beams and validated with measurements of the beam width from two transducers. The model has the potential to combine the separate theories of the three beams into one.