# An Experimental Study of the Performance of a Crossed Rib Diffuser in Room Acoustic Control
###### Abstract  
This paper presents a crossed rib diffuser (CRD) as an effective tool for room acoustic control. We performed an experimental investigation of its effectiveness using a specimen manufactured for this trial. The CRD is constructed by overlapping two one-dimensional (1D) periodic rib diffusers with different specifications so that they are crossed at non-right angles. The CRD achieves a higher scattering coefficient than 1D periodic rib diffusers in a wide band while maintaining the simple and friendly design of 1D periodic rib diffusers applicable to various architectural spaces. Moreover, inserting an absorbing layer between upper and lower ribs of the CRD, (CRD-A) yields a high broadband absorption coefficient. We first evaluated the random-incidence scattering coefficient of CRD using a 1/5 scaled model in comparison with those of 1D periodic diffusers assessed with a numerical method. Then, absorption coefficients for the CRD and the CRD-A were measured using a reverberation room. Subsequently, an experiment on a small meeting room with a 1D periodic rib diffuser, the CRD and the CRD-A was conducted to present performance of the CRD in room acoustic control. Impulse response measurements and evaluations of reverberation parameters (\(T_{20}\) and \(EDT\)) and speech clarity (\(D_{50}\)) were conducted. Additionally, we present differences in structure of reflected sounds found for the flat wall, the CRD and the CRD-A visually using a four-channel sound field microphone. acoustic diffuser; architectural acoustic; meeting room; reverberation; room acoustic control; scattering coefficient; speech clarity +
Footnote †: journal: Medical  
A one-dimensional (1D) periodic rib-type acoustic diffuser is the simplest and most widely used acoustic diffuser. The 1D periodic rib diffuser is constructed with a pattern of ribs and voids, presenting a simple and friendly design well suited to various architectural spaces. However, its simple periodic structure simultaneously produces strong sound incident-angle dependence [15]; i.e., a frequency bandwidth acquiring high scattering performance is narrow like a peak of sound absorption coefficient characteristic of a simple Helmholtz resonator without porous material. Consequently, to use the 1D periodic rib diffuser effectively for room acoustic control, more elaborate investigations related to characteristics of room acoustics, rib configurations and locations at which diffusers are set are needed for comparisons with diffusers with higher scattering coefficients in wide-band applications. The present study was conducted to increase the acoustical performance of a 1D periodic rib diffuser while maintaining its convenient design. The scattering performance of a 1D periodic rib diffuser is likely to be improved with ease by overlapping two 1D periodic rib diffusers with different configurations so that they are mutually crossing at non-right angles, because a superposition of two periodic rib structures yields two-dimensional and more complex periodic structures than a 1D periodic rib diffuser. The crossed rib diffuser maintains the simple and intimate design of a 1D periodic rib diffuser. Moreover, it is easier to manufacture than high-performance diffusers such as [16; 17]. Additionally, inserting a thin absorbing layer between the upper and lower ribs can transform the crossed rib diffuser into a broadband absorber like a quadratic residue diffuser with a fabric cover [18], extending the applicability of the crossed rib diffuser. From those features, some versatile activity of the crossed rib diffuser in architectural spaces is expected, but no study has examined acoustical properties of such crossed rib diffusers or their applications in rooms. This paper describes a manufactured crossed rib diffuser (CRD) and an experimental investigation of its applicability to room acoustic control.