sentences:  Resonating absorbers vary in construction, with one type of membrane absorber using a springy sheet of wood that attaches to the enclosure only along the edges/corners, and another using a more floppy sheet of thin material stretched like a drumhead.A Helmholtz resonator can have one port tuned to a single frequency, or several ports tuned to either a single or to multiple frequencies, with round port, slotted port, or even perforated construction.Resonating absorbers often incorporate porous absorption internally to simultaneously lower the resonant frequency and broaden the spectrum of absorption.