Sound Absorption and Anechoic chambers

1. Sound Absorption and Anechoic chambers Kris Gill

2. What is Sound Absorption? Any technique used to manage the reflection of sound off of a surface causing sound energy to be dissipated. Absorption Diffusion Reflection

3. Two types of sound absorption • Pressure Activated • Energy below 100Hz • Really long oscillating waves (L) • Excites air space between walls • Creates distortions, room modes • Requires a pressure related treatment • Diaphragmatic, Helmholtz, Membranes • Velocity/Air Movement (Molecular Movement) • Energy above 100Hz • Middle and High frequencies • Rays • Requires Foam and Limp Mass Materials • Rate and Level (R&L)

4. Sound Absorption Quality • Noise Reduction Coefficient (NRC) • Scalar representation of the amount of sound energy absorbed upon striking a particular surface • Provides a single-number rating for sound absorption (higher values are better). • Average of 4 spot frequencies, gives equal weighting across the frequency range • Not as accurate as α • Sound Absorption Coefficient (α) • Defined as the ratio of sound energy absorbed by its surface to total sound energy on the surface • 6 spot frequencies • α is dependent upon material as well as frequency of sound

5. Types of reflected Sound

6. Reverberation time (RT) • Reverberation: Amount of sound in a room that persists over a definite period of time after the source has stopped. • RT = Amount of time sound bounces around before absorption • RT-60: Time it takes for energy to decay 60dB • Can be controlled precisely • Volume of the room • Type of materials • Surface area of materials • Affects how well one can hear and understand speech • Can change how music sounds • Optimal RT for general auditoriums ~2s • Shorter = More clarity “Longest Echo”

7. Anechoic Chambers • Space in which there are no echoes or reverberations from the walls, ceiling, or floor. • Engineered specifically to eat sound. • Surfaces absorb all sound, reflect none. • 99.5% of radiated sound energy absorbed • Constructed as a room within a room • Majority = 3 walls, Microsoft’s Building 87 has 6 • Chamber floats on an independent foundation • Decoupling it from building vibrations

8. How do they work? what are they used for? • The panels deaden sound by absorbing soundwaves • Panels are wire mesh containing porous material • Energy squishes material and dissipates as heat • Angles of the foam • Bouncing soundwaves = no reflected energy out • Free-field Conditions • Used for psychoacoustic work relating to loudness • Testing loudness of equipment • Loudspeaker and Microphone directivity and frequency response functions • Astronauts – space adaptation • Concert/Auditorium acoustics simulations

9. Ambient noise at the Quietest Place on Earth • Quieter than complete silence • Normal Conversation +60 dB • Human whisper +30 dB • Calm breathing +10 dB • Dead Silence +0 dB • Building 87 -20.6 dB • Brownian Motion, the noise produced by colliding air molecules at room temperature is ~ -23 dB • Vacuum of Space Quieter than dead silence “We are used to every sound producing a small echo from the world around us,” he points out. “In these chambers, there is just dead sound. It is just like going into a dark room, at first you cannot see anything but over time your eyes adapt.” Suedfeld

10. This month: Researchers develop 'acoustic metamaterial' that cancels sound • Silences noise using an open, ring-like structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow. • Transversely placed bilayer medium, • Large degrees of contrast layers' acoustic properties • Asymmetric transmission • High-performance sound silencing • Large degree of open area – air permeable • Shape is completely customizable - cube or hexagon • Demonstrates a reduction in the transmitted acoustic energy of up to 94% • Possible applications • Smart sound barriers, fan or engine noise reduction, etc. https://phys.org/news/2019-03-acoustic-metamaterial-cancels.html

11. Citations: ISO-354, “Measurement of sound absorption in a reverberation room”. Reza Ghaffarivardavagh et al, Ultra-open acoustic metamaterial silencer based on Fano-like interference, Physical Review B (2019). DOI: 10.1103/PhysRevB.99.024302 Ressl, Marc S. S, and Pablo E. E Wundes. Proceedings of the 11th WSEAS International Conference on Acoustics and Music: Theory and Applications, AMTA '10, 2010, pp. 18–23. Cox, Trevor J, and Allan Kilpatrick. “A Record ‘Longest Echo’ within the Inchindown Oil Despository.” The Journal of the Acoustical Society of America, vol. 137, no. 3, 2015, pp. 1602–4. Xu, Qian, et al. “Building a Better Anechoic Chamber: A Geometric Optics-Based Systematic Solution, Simulated and Verified [Measurements Corner].” IEEE Antennas and Propagation Magazine, vol. 58, no. 2, 2016, pp. 94–119. Ellingson, Roger M. M, and Patrick V. V Helt. Proceedings of Meetings on Acoustics, vol. 19, no. 1, 2013, pp. . Any questions? Thank you.