diff --git a/lib/webvtt.rb b/lib/webvtt.rb
index 29664bd..18d51df 100644
--- a/lib/webvtt.rb
+++ b/lib/webvtt.rb
@@ -1,4 +1,5 @@
 # encoding: UTF-8
+# frozen_string_literal: true
 
 if defined?(Encoding)
   Encoding.default_internal = Encoding.default_external = "UTF-8"
diff --git a/lib/webvtt/parser.rb b/lib/webvtt/parser.rb
index 388345a..295c19a 100644
--- a/lib/webvtt/parser.rb
+++ b/lib/webvtt/parser.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module WebVTT
 
   def self.read(file)
@@ -118,7 +120,7 @@ def self.parse(cue)
     end
 
     def to_webvtt
-      res = ""
+      res = String.new
       if @identifier
         res << "#{@identifier}\n"
       end
diff --git a/lib/webvtt/segmenter.rb b/lib/webvtt/segmenter.rb
index 211ee0d..bb60e69 100644
--- a/lib/webvtt/segmenter.rb
+++ b/lib/webvtt/segmenter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'fileutils'
 
 module WebVTT
diff --git a/tests/parser.rb b/tests/parser.rb
index 75d9008..316ef2f 100644
--- a/tests/parser.rb
+++ b/tests/parser.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 $LOAD_PATH << "lib/"
 require "minitest/autorun"
 require "webvtt"
@@ -257,7 +259,7 @@ def test_build_cue
     cue.start = WebVTT::Timestamp.new 0
     cue.end = WebVTT::Timestamp.new 12
     cue.text = "Built from scratch"
-    output = ""
+    output = String.new
     output << "00:00:00.000 --> 00:00:12.000\n"
     output << "Built from scratch"
     assert_equal output, cue.to_webvtt
diff --git a/tests/segmenter.rb b/tests/segmenter.rb
index b6ba8ea..949a48d 100644
--- a/tests/segmenter.rb
+++ b/tests/segmenter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 $LOAD_PATH << "lib/"
 require "minitest/autorun"
 require "webvtt"
@@ -53,9 +55,11 @@ def test_shortcut_method
     res = WebVTT.segment("tests/subtitles/test.vtt")
     assert_instance_of Array, res
     assert_equal 2, res.size
-    assert_equal 35, res[1].size
+    subs = res[1]
+    assert_equal 35, subs.size
 
     # clean up
+    subs.each {|f| FileUtils.rm(f.filename)}
     FileUtils.rm("prog_index.m3u8")
   end