To cite signal in publications use:

signal developers (2013). signal: Signal processing. URL: http://r-forge.r-project.org/projects/signal/.

Corresponding BibTeX entry:

  @Manual{,
    title = {{signal}: Signal processing},
    author = {{signal developers}},
    year = {2013},
    url = {http://r-forge.r-project.org/projects/signal/},
  }