Can you hear me ... now?

June 22, 2006
Mobile phones rely on acoustic filters to make speech and sound come through loud and clear.

Acoustic fabrics can be combined with one or several adhesive layers for quickly attaching filters to any housing.

Acoustic filters in front of loudspeakers or micro-phones absorb unwanted peaks in frequency and, thus improve speech quality.

Precision woven fabrics from Sefar Filtration Inc., Depew, N.Y., are one option phone designers have to regulate sound transmission and promote electromagnetic compatibility. Proprietary hydrophobic surface treatments and choice of inherently hydrophobic fabrics such as fluorocarbons also protect against moisture uptake and contamination.

The acoustic fabrics are put in front of a speaker or micro-phone to absorb unwanted frequency peaks, smoothing it and significantly improving sound quality. Sefar woven fabrics are thin, flexible, dimensionally stable, strong, and easy to handle.

The key to effectively filtering unwanted noise is spec'ing the fabric with the right acoustic resistance. Fabric mesh is also important. Precise mesh openings and controlled air permeability guarantee reliable acoustic attenuation and reportedly provide more consistent sound quality than possible with nonwoven or paper filters. The mesh must be woven tight enough to protect micro-phones from turbulent, fast-moving particle airflow but also permeable enough to let in acoustics with slow particle velocity. Depending on mesh opening and fabric thickness airflow resistance ranges from approximately 20 to 200 RaylMKS.

The best filter depends on technical requirements of the phone as well as the housing design. Sefar has launched a new Acoustic product range that covers a wide range of Rayl values, fabric thickness, and colors. Different filter types offer various degrees of dust and water protection so that each manufacturer can choose the optimal filter for their mobile phone type. Filter-material selection is also important.

Polyester precision woven fabrics, for example, don't absorb much water and stand up to solvents, acids, alkalines. They, however, have poor light stability and limited abrasion resistance. Their nylon 6 counterparts have limited resistance to acids and also poor light stability but do resist abrasions. Material selection may also hinge on phone assembly, i.e., how filters will be installed in the phone (ultrasonically welded, adhesively bonded, or hot staked).

Sefar's woven fabric portfolio also includes conductive shielding materials. These metallized fabrics prevent electric charge build-up, protect cell-phone users from electromagnetic waves, and shield cables and instrument panels from EMI. They also provide architectural shielding, lightning strike protection, and electric grounding. A metal-lic-coated Sefar fabric, for example, can improve charge dissipation and shield 99.9% of the emitted radio-magnetic waves from reaching the loudspeaker. All Sefar Acoustic fabrics comply with EU 2002/96/EG Directives WEEE and RoHS.

Sefar Filtration Inc., (800) 995-0531,

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