surface.rb

class Surface
  #...

  SNARE_OPTIONS = { left: -51, right: 0, note: 38 }
  KICK_OPTIONS  = { left: 0, right: 100, note: 36 }
  HAT_OPTIONS   = { left: -100, right: -50, note: 57 }

  def initialize(drum_type=nil, *opts)
    case drum_type
    when :snare
      opts = SNARE_OPTIONS
    when :kick
      opts = KICK_OPTIONS
    when :hat
      opts = HAT_OPTIONS
    end

    if opts.present?
      @left_boundary = opts[:left]
      @right_boundary = opts[:right]
      @drum_note = opts[:note]
    end
  end

  #...

end