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2. Howard, D.M., Murphy, D.T.: Voice Science, Acoustics, and Recording. Plural Publishing, Inc. (2007)
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4. Examples of state-of-the-art products: http://www.axistive.com/voice-amplifier-by-falck-vital-as.html , http://www.cordlessworkz.com/index.asp?PageAction=VIEWPROD&ProdID=1860 , http://www.brucemedical.com/adkitplfepro.html (last access 01|31|2010)
5. CN000101188637A (DAPTISNET-Recherche, http://depatisnet.dpma.de/DepatisNet/depatisnet?window=1&space=main&content=treffer&action=treffer&so=asc&sf=vn&firstdoc=1 (last access 04/03/2010)