A broadband absorber is a wall treatment that acts like an absorber across a wide range of frequencies, rather than focusing on a specific frequency range, e.g. A bass trap. If you are looking for a manufactured product, broadband wall treatments are available from a number of different companies, like GIK Acoustics, ATS Acoustics, and RealTraps. Panels are usually 4'x2', and you should choose one that has at least 4" of absorption material.
Thanks I will look into that!