An acoustical bass absorber which reduces peak and dip frequency response errors caused by interference from naturally occurring axial standing waves in rectangular rooms. The apparatus uses two forms of simple harmonic resonance: pistonic diaphragm resonance and Helmholtz cavity resonance. The pistonic diaphragm resonance is achieved by attaching a rigid planar membrane to metal springs. The Helmholtz cavity resonance is achieved by constructing an enclosed chamber attached to an open cylindrical tube. Coupling these two dissipation devices leads to several-fold improvement in absorption and total room mode attenuation.