http://translate.google.de/translate?hl=de&sl=de&tl=en&u=http://www.lautsprecherbau.de/Magazin/Lautsprecherbau-Magazin-2012/Ausgabe-Februar-2012/_Saschas-Duetta-%25C3%25A1-la-Martin_8636,de,900674,2789&sandbox=0&usg=ALkJrhhRtQz2eo3CWB69rex0tKi4gZOSRA

http://www.parts-express.com/resources-build-a-speaker

http://www.popularmechanics.com/technology/audio/how-to/a3485/build-your-own-speakers/

• "Liquid bubbles stop sound." Nature 508.7497 (2014): 435. Academic OneFile. Web. 23 Oct. 2015.
• Liquid foam can block the transmission of ultrasound, report Valentin Leroy at Paris Diderot University and his colleagues.
  
  The French team created the foam by bubbling an insoluble gas, perfluorohexane, into water containing a surfactant, and then sent ultrasound through the foam.
  
  The elastic, thin films at the edges of adjacent foam bubbles reflected the sound waves. The exact frequencies that were blocked varied with the average bubble size, the biggest of which measured up to 50 micrometres in radius.
  
  The findings show that foams can act as acoustic metamaterials--materials that block sound waves because of their unique structure.