Acoustic and dielectric properties of nanocellulose

Otto Soidinsalo | March 13, 2018

Nanocellulose has been a hot topic for several years and numerous applications have been proposed, some of them more potential than the others. The major limitation for the wider use of nanocellulose has been the limited commercial availability. The term nanocellulose, however, covers several different types of nano- and microfibrillated and fibrillar cellulose products. One of those is bacterial cellulose which is also more commonly referred as bio-cellulose. It might come as a surprise for many of us, but bio-cellulose is in fact present in several commercially available products. One of those were the legendary Sony MDR-R10 headphones which were introduced already as early as 1988.

We have previously discussed about the use of bacterial cellulose in wound care. In this post, we will discuss about the role of bacterial cellulose in acoustics and take a look of the commercially available products.


The role of nanocellulose in acoustic diaphragm

The speaker diaphragm is one of the major elements of the loudspeaker and responsible for producing the sound waves. Ideally, diaphragm should be lightweight but at the same time rigid. In addition, the material should be self damping and have high propagation velocity. Speaker diaphragms have been traditionally fabricated from paper which mechanical properties have been adjusted by beating. Other materials used are polypropylene, aluminum and mixtures of polymers and fibers.


Bacterial cellulose has suitable mechanical and acoustic properties for speakers

Yamanaka et al. studied the mechanical properties of bacterial cellulose based sheet already almost 30 years ago. The sheets were prepared from bacterial cellulose gel membranes by pressing and drying at various temperatures. The resulting sheet had Young`s modulus over 15 GPa and tensile strength over 200 MPa. Bacterial cellulose sheets were lightweight and rigid, just as it is needed for acoustic diaphragm. Interestingly, they observed that when the bacterial cellulose gel membrane was disintegrated with lab pulp-disintegrator prior the sheet formation, the mechanical properties were only one-third compared to the non-disintegrated. For a comparison, the mechanical properties of sheet prepared from microfibrillated cellulose are around 13 GPa / 200 MPa (Exilva, Borregaard).

In addition to the mechanical properties, also the acoustic properties of bacterial cellulose have been studied. Indrati et al. studied the internal loss, density and sound velocity of diaphragm, prepared from sodium hypochlorite treated bacterial cellulose membranes. The Young`s modulus was 23.5 GPa combined with acoustic absorption property of 0.02 (tan δ), sound velocity of 4522.67 m/s and density between 1-1.5 g/cm3. Markiewicz et al. studied the dielectric and acoustic response of bacterial cellulose with diaphragms which were prepared from bacterial cellulose by hot-pressing and coated with gold sputtered electrodes. The diaphragm with optimum acoustic properties had permittivity value (ε`) of 10 and Young`s modulus 10 GPa.

Based on the above-mentioned research, it`s easy to understand why so many speaker manufactures are using bacterial cellulose to manufacture high quality headphones.


Nanocellulose based headphones on the market

Several international manufacturers, such as Acer, Audioquest, Creative, Klipsch and Panasonic have commercial products that are equipped with a bacterial cellulose diaphragm. According to Audioquest, their headphone drivers achieve true pistonic motion through the implementation of an ultra-rigid bio-cellulose diaphragm. Moreover, Creative states that their bio-cellulose diaphragms can deliver the same sound velocity as aluminum or titanium diaphragms while adding the warmer and more delicate sound of paper diaphragms.


Opportunities for cellulose fibrils

The commercial product utilizing bacterial cellulose are limited to headphones even if the achieved properties are excellent. Reason for this might be the limited availability and physical form of bacterial cellulose. Bacterial cellulose is delivered mainly in gel sheets not bigger than A4 and the manufacturing of it is scattered around Asian countries with several small manufacturers. Both these factors make the use of bacterial cellulose challenging. Wood based cellulose fibrils instead can be delivered as suspension which is easy to form to continuous films or mold to different sizes of cones.

This enables the use of cellulose fibrils in bigger speakers than headphones, for instance loudspeakers or portable speakers for your living room. Thus, this is an exciting, new area for cellulose fibrils to be explored!

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Written by:

Otto Soidinsalo

Dr. Otto Soidinsalo holds a Ph.D. in organic chemistry from the University of Helsinki. Currently he works as a Technical Application Manager at Borregaard. Before joining Borregaard, Otto was working in various positions in pharma and cellulose derivatives related industries for 8 years, ranging from R&D to product and application development and technical services. Currently Otto is focusing on CASE as well as on paper & packaging applications, especially on barrier coatings. Otto possesses a deep understanding of the natural polymers and their derivatives and their impact on rheology as well as mechanical properties of various applications.