DEVELOPMENT OF ACOUSTIC ATTENUATOR USING HELMHOLTZ RESONATOR WITH MEMBRANE BASED ON PERIODIC SYSTEM APPROACH

• Main Author: DESENDRA (NIM : 23317016), GRADI
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/27574
• Institution: Institut Teknologi Bandung

<p align="justify"> Helmholtz resonator is a commonly used conventional acoustic attenuator. Although Helmholtz resonators have the ability to control low-frequency noises, they have narrow attenuation bandwidth by their nature. Problems arise in some noise control cases which need more broadband attenuation. <br /> <br /> Numerous works and studies has been conducted to increase this narrow bandwidth attenuation, one of which is by coupling the Helmholtz resonators periodically in a waveguide. This resonator-mounted-pipe can be treated as a single 1 dimensional crystal cell, which can be paired with its kind to create a periodic system which may induce bandgap effect from Bloch theorem. Interaction between the crystals cells induce atomic like interaction where forbidden frequency bands in which waves could not propagate are formed.The dispersion relation of the wave can show where these band gaps occur from imaginary part and the strength of attenuation from real part. <br /> <br /> In this study, membrane is used in the middle of the resonator’s volume to further increase the attenuation effect. The sample consists of PLA ducts made using 3D printer, and the insertion loss (IL)is evaluated using a single source and a single receiver microphone in an anechoic chamber to meet the required boundary conditions. Single cell resonator may induce up to 10 dB attenuation around 500 Hz and 40 dB for 5 adjacent cells. This result is comparable to the analytical model, and shows positive increase in bandwidth. <br /> <br /> The addition of membrane induces additional attenuation in the system. The measurement of one modified unit cell yields two narrow-band attenuations with different amplitude and frequency because of acoustical energy conversion to circular mode mechanical vibration which is correlated to membrane density and also from acoustical energy to Helmholtz resonance. The system is not able to produce a broadband attenuation as the two mechanisms could not be combined linearly, but can be made broadband using periodic system. <br /> <p align="justify"> <br />