Abstract: A sootblower wallbox assembly for decreasing noise emissions during the cleaning of a heat exchanger apparatus. In particular, the invention relates to a wallbox assembly containing a number of reverberant chambers exhibiting varying attenuation characteristics. When a cleaning lance is extended centrally through the wallbox assembly, a baffle portion of each chamber is in close fit relation with the exterior surface of the lance. Each reverberant chamber has a different axial length and therefore exhibits a different frequency range where it achieves its best sound attenuation. The wallbox assembly can also be provided with control systems to prevent combustion products from escaping the heat exchanger through the assembly.