Multi-functional Materials

Our long-term goal is to develop multi-functional materials for challenging applications involving combined acoustic, mechanical, and thermal functions. As the first step towards this goal, we are developing a new class of heat sinks based on periodic porous materials that are able to dissipate heat, absorb noise, as well as have high mechanical strength. This work is done in collaboration with Prof. Shankar Krishnan, IIT Bombay.

Thermal management in many systems involve fan-mounted heat sinks which results in in fan-generated noise as an undesired by-product . Further, as fan speed increases, thermal resistance decreases but noise level increases (panel (A) in the figure above). Many of these applications require noise reduction and attempt to reduce noise using separate devices. However, space for separate heat sinking and noise-reducing functionalities may be limited in complex systems such as consumer electronics cooling, motors, automotive, aviation, and data center thermal and acoustics management.

To address this challenge, we are developing new type of porous materials that can perform both heat sinking and noise reduction in the same functional volume. We also include mechanical strength functionality for some applications.

Designed periodic unit cells are repeated in a controlled form to generate three-dimensional materials that can perform multiple functions. Examples of two unit cells are shown in panel (C). A heat sink of simple block-shape, where a selected unit cell is repeated uniformly in all three directions, and attached to a base plate, is shown in panel (B). The principles involved in multi-functional performance, and the methods involved in designing these unit cells, design of the macro-scale shape, and their resulting multi-functional performances are discussed in the publications listed below. Computational, analytical, and experimental methods are adopted in this development. Improving the multi-functional materials is an ongoing effort in our lab.

Using unit-cell approach, we designed periodic foams, predicted and experimentally benchmarked their acoustic absorption and mechanical strength.

Evaluation of combined noise reduction and heat dissipation performance of parallel plate heat sinks, stochastic foams, and designed periodic foams for the same weight showed that periodic foams can be designed to achieve better combined thermal-acoustic performance for noise-reducing heat sinks.

Journal publications relevant to this work:

  • Ramamoorthy, S. and Krishnan, S., "Towards thermal-acoustic co-design of noise reducing heat sinks, IEEE Transactions on Components, Packaging, and Manufacturing Technology, 8(8), pages 1411-1419, 2018.

  • Deshmukh, S., Ronge, H., Ramamoorthy, S., "Design of periodic foam structures for acoustic applications: concept, parametric study, and experimental validation", Materials & Design, 175, p. 107830, 2019.

  • Choudhary, R., Sachan, S., Krishnan, S., and Ramamoorthy, S., “Design and parametric study of macro-structure of foams for combined high absorption and low pressure drop.”. Applied Acoustics, 166, 107358, 2020.

  • Ronge, H., Krishnan, S., and Ramamoorthy, S. Evaluation of stochastic and periodic cellular materials for combined heat dissipation and noise reduction: Experiments and modeling. IEEE Transactions on Components, Packaging and Manufacturing Technology, 2020.

  • Deshmukh, S., Borkar, A., Alankar, A., Krishnan, S., and Ramamoorthy, S., “A priori determination of the elastic and acoustic responses of periodic poroelastic materials.”, Applied Acoustics, 169, 107455, 2020.

  • Sachan, S., Kumar, S., Krishnan, S., and Ramamoorthy, S., "Impact of entry-exit loss on the measurement of flow-resistivity of porous materials, AIP Advances, 10 (10), 105031, 2020.

Active Noise Cancellation

We are also integrating active feedback control in some applications involving these multifunctional materials. Our prior work on active feedback noise cancellation, unrelated to multifunctional materials, can be seen in the 'Past work' tab.

Active noise cancellation demonstrated in an acoustic system in our lab using DSPACE hardware is shown here (figure shows closed-loop sensitivity in decibels as a function of frequency, measured vs. predicted using a computational model):

Hearing Research

Deafness is the most prevalent sensory disability across nations. The sense of hearing has been least understood owing to the challenge in accessing the inner ear deeply positioned inside the head. In addition to the challenging physiological environment, experiments in the live mammalian cochlea continue to demand better methods and innovative sensors. Our long term goal is to develop better understanding and effective diagnosis of hearing loss by developing novel instrumentation geared towards live intra-cochlear measurements, as well as to develop better understanding of the active vibro-acoustics in the inner ear using computational modeling.

Computational modeling of sound processing in the human ear

Sound entering the outer ear vibrates the middle ear bones and launches vibroacoustic waves inside the inner ear organ, cochlea. Complex processing of sound inside the cochlea eventually leads to signals that provide input to the auditory nerve.

In our 2007 JASA paper, titled "A mechano-electrical-acoustic model of the cochlea: response to acoustic excitation", we developed a multi-scale model of the cochlea integrating mechanical, electrical, and acoustic domains. This 'MEA' model is able to predict the data obtained from several physiological experiments in live guinea pig cochlea. A description of the MEA model can be seen in the "Past work" tab.

In our recent publication in 2020, we showed that there is a fine balance between the local electromechanical and vibroacoustic properties of the cochlea, and that this balance is important for the active feedback amplification of low level sounds processed by our ear. The relevant journal publication is:

  • Agarwal, N., and Ramamoorthy, S., “Balance in the feedback loop components of the mammalian cochlear amplifier.”, Journal of Applied Physics, 128(3), 034701, 2020.

We are now integrating nonlinear mechanics of the cochlea into the MEA model and combining it with a model for the middle and outer ear to predict otoacoustic emissions in humans. The relevant conference abstract is:

  • Agarwal, N., and Ramamoorthy, S.,Investigation of distortion product otoacoustic emissions in humans using a nonlinear mechano-electro-acoustic model of the cochlea.”, Journal of the Acoustical Society of America 149(4), A76-A76, 2021.

Development of new sensors for auditory research experiments

We are developing new sensors and methods for auditory research experiments. Our first sensor for this purpose is built upon optical coherence tomography (OCT). Using OCT, we have developed a sensor for simultaneous measurement of the velocity of a vibrating structure and the fluid pressure in the vicinity of the structure. To the best of our knowledge, there is no alternative method in the literature for simultaneous vibration and pressure detection. Spectral-domain OCT can simultaneously measure the vibrations of multi-layered structures in the axial direction (that is along the OCT light). By inserting a designed miniature vibroacoustic sensor whose diaphragm is placed in the vicinity of the vibrating diaphragm of interest and measuring the vibrations of both diaphragms simultaneously using spectral-domain OCT, the proposed method is demonstrated. This measurement method therefore provides simultaneous measurement of both velocity and pressure inside any micro-channel, including the mammalian cochlea as illustrated in the right panel.

The sensor is described in the following journal publication:

  • Ramdas, R., Agarwal, N., Atpadikar, M., and Ramamoorthy, S., “Simultaneous measurement of vibration and pressure in vibroacoustic microchannels.” Applied Acoustics, 169, 107489, 2020.

This sensor could be used in intra-cochlear experiments to help determine how otoacoustic emissions, which are sounds generated inside the cochlea, propagate backwards to exit the ear. Simultaneous vibration and pressure detection is also useful for other applications such as active noise control.

We are continuing to develop new sensors and methods for auditory research.

Non-invasive auditory measurements in humans

We have just initiated this effort. Our broad objective is to combine our extensive computational modeling expertise with non-invasive auditory measurements to help understand nuances of human hearing. The non-invasive auditory measurements will include psychoacoustic experiments as well as otoacoustic emissions.