diff --git a/examples/piano.cr b/examples/piano.cr
index 0631362..2f2b18d 100644
--- a/examples/piano.cr
+++ b/examples/piano.cr
@@ -18,13 +18,14 @@ module PF
     @white_keys = [] of Tuple(Vector2(Int32), Vector2(Int32), String)
     @black_keys = [] of Tuple(Vector2(Int32), Vector2(Int32), String)
 
-    @instruments : Array(Instrument) = [RetroVoice.new, PianoVoice.new, Flute.new, KickDrum.new, SnareDrum.new, Harmonica.new]
+    @instruments : Array(Instrument) = [RetroVoice.new, SineVoice.new, PianoVoice.new, Flute.new, KickDrum.new, SnareDrum.new, Harmonica.new]
 
     def initialize(*args, **kwargs)
       super
 
       @text_color = Pixel.new(127, 127, 127)
       @controller = PF::Controller(Keys).new({
+        Keys::E     => "echo",
         Keys::UP    => "octave up",
         Keys::DOWN  => "octave down",
         Keys::LEFT  => "prev inst",
@@ -52,18 +53,27 @@ module PF
 
       @sounds = [] of Sound
       @keysdown = {} of String => Tuple(Instrument, UInt32)
+      @echo = false
+
+      echo_effect = EchoEffect.new(44100 // 3)
 
       # Initialize an audio handler
       # - the given Proc will be called at the sample rate/freq param (44.1Khz is standard)
       # - the channel variable describes which speaker the sample is for
       @audio = Audio.new(channels: 1) do |time, channel|
         value = 0.0
+
         @instruments.each do |instrument|
           instrument.sounds.each do |sound|
             value += sound.sample(time)
           end
         end
-        value
+
+        if @echo
+          echo_effect.apply(value)
+        else
+          value
+        end
       end
 
       @key_size = height // 2 - 25
@@ -115,6 +125,10 @@ module PF
       @base_note += 12 if @controller.pressed?("octave up") && @base_note <= 112
       @base_note -= 12 if @controller.pressed?("octave down") && @base_note >= 21 + 12
 
+      if @controller.pressed?("echo")
+        @echo = !@echo
+      end
+
       if @controller.pressed?("next inst")
         @instrument = (@instrument + 1) % @instruments.size
       end
@@ -134,9 +148,11 @@ module PF
         end
 
         if @controller.released?(name)
-          instrument, note_id = @keysdown[name]
-          instrument.off(note_id, @audio.time)
-          @keysdown.delete(name)
+          if tuple = @keysdown.[name]?
+            instrument, note_id = tuple
+            instrument.off(note_id, @audio.time)
+            @keysdown.delete(name)
+          end
         end
       end
     end
@@ -147,9 +163,9 @@ module PF
       draw_string(<<-TEXT, 5, 5, @text_color)
         Press up/down to change octave, Bottom row of keyboard plays notes
         #{@instruments.map(&.name).join(", ")}
-        Octave: #{@base_note // 12 - 1}, Voice : #{@instruments[@instrument].name}
+        Octave: #{@base_note // 12 - 1}, Voice: #{@instruments[@instrument].name}, Echo: #{@echo ? "on" : "off"}
         #{@instruments[@instrument].sounds.map { |s| s.hertz.round(2) }}
-        TEXT
+      TEXT
 
       @white_keys.each do |key|
         top_left, bottom_right, name = key
diff --git a/src/audio.cr b/src/audio.cr
index b23be68..3ea047e 100644
--- a/src/audio.cr
+++ b/src/audio.cr
@@ -1,5 +1,6 @@
 module PF
   class Audio
+    alias Callback = Float64, UInt8 -> Float64
     # stored as a class variable to avoid garbage collection, since it's passed to a C function
     @@box : Pointer(Void)?
     @spec : LibSDL::AudioSpec
@@ -12,7 +13,7 @@ module PF
     getter time : Float64 = 0.0
     @channel : UInt8 = 0u8
 
-    def initialize(freq : Int32 = 44100, channels : UInt8 = 2, samples : UInt16 = 512, &callback : Float64, UInt8 -> Float64)
+    def initialize(freq : Int32 = 44100, channels : UInt8 = 2, samples : UInt16 = 512, &callback : Callback)
       # Information to be passed to the audio callback
       boxed_data = Box.box({
         callback,
diff --git a/src/audio/echo_effect.cr b/src/audio/echo_effect.cr
new file mode 100644
index 0000000..67fab2d
--- /dev/null
+++ b/src/audio/echo_effect.cr
@@ -0,0 +1,24 @@
+struct EchoEffect
+  @cursor : Int32 = 0
+
+  def initialize(frames : Int32)
+    @buffer = Slice(Float64).new(frames, 0.0)
+    @filter = LowPassFilter.new(440.0, 0.7, 44100)
+  end
+
+  def read
+    @buffer[@cursor]
+  end
+
+  def write(sample : Float64)
+    @buffer[@cursor] = sample
+    @cursor += 1
+    @cursor = 0 if @cursor >= @buffer.size
+  end
+
+  def apply(sample : Float64, strength = 0.5)
+    sample += @filter.apply(read * strength)
+    write(sample)
+    sample
+  end
+end
diff --git a/src/audio/instrument.cr b/src/audio/instrument.cr
index e8d4aa9..43f430d 100644
--- a/src/audio/instrument.cr
+++ b/src/audio/instrument.cr
@@ -41,7 +41,20 @@ module PF
         sustain: Envelope::Stage.new(Float64::INFINITY, 0.5, 0.5),
         release: Envelope::Stage.new(0.5, 1.0, 0.0)
       )
-      @wave = Sound.saw_wave(7.0, 0.001)
+      @wave = Sound.saw_wave(10.0, 0.005)
+    end
+  end
+
+  class SineVoice < Instrument
+    def initialize
+      @name = "Sine"
+      @envelope = Envelope.new(
+        attack: Envelope::Stage.new(0.01, 0.0, 1.0),
+        decay: Envelope::Stage.new(0.1, 1.0, 0.5),
+        sustain: Envelope::Stage.new(Float64::INFINITY, 0.5, 0.5),
+        release: Envelope::Stage.new(0.5, 1.0, 0.0)
+      )
+      @wave = Sound.sin_wave(5.0, 0.001)
     end
   end
 
@@ -62,17 +75,25 @@ module PF
         release: Envelope::Stage.new(0.3, 1.0, 0.0)
       )
 
+      harmonics = [
+        {1.0, 0.0},
+        {0.5, 0.0},
+        {0.25, 0.0},
+      ]
+
       @wave = ->(time : Float64, hertz : Float64) do
         # https://www.desmos.com/calculator/mnxargxllk
-        av = 2 * Math::PI * hertz * time
-        y = (Math.sin(Math::PI * (av / Math::PI)) ** 3) + Math.sin(Math::PI * ((av / Math::PI) + (2 / 3)))
-        y = (Math.sin(av) ** 3) + Math.sin(av + 0.6666)
-        y += y / 2
-        y += y / 4
-        y += y / 8
-        y += y / 16
-        y += y / 32
-        y /= 5
+        y = 0.0
+
+        harmonics.each do |amplitude, phase|
+          av = 2 * Math::PI * (hertz / 2.0) * time + phase
+          y += amplitude * (
+            (Math.sin(Math::PI * (av / Math::PI)) ** 3) +
+            (Math.sin(Math::PI * ((av / Math::PI) + (2 / 3))))
+          )
+        end
+
+        y
       end
     end
   end
diff --git a/src/audio/low_pass_filter.cr b/src/audio/low_pass_filter.cr
new file mode 100644
index 0000000..9c5468b
--- /dev/null
+++ b/src/audio/low_pass_filter.cr
@@ -0,0 +1,53 @@
+# first-order low-pass filter
+class LowPassFilter
+  @x1 : Float64
+  @x2 : Float64
+  @y1 : Float64
+  @y2 : Float64
+  @b0 : Float64
+  @b1 : Float64
+  @b2 : Float64
+  @a1 : Float64
+  @a2 : Float64
+
+  def initialize(cutoff_frequency : Float64, quality_factor : Float64, sample_rate : Float64)
+    # Compute the filter coefficients
+    omega = 2.0 * Math::PI * cutoff_frequency / sample_rate
+    sin_omega = Math.sin(omega)
+    cos_omega = Math.cos(omega)
+    alpha = sin_omega / (2.0 * quality_factor)
+
+    b0 = (1.0 - cos_omega) / 2.0
+    b1 = 1.0 - cos_omega
+    b2 = b0
+    a0 = 1.0 + alpha
+    a1 = -2.0 * cos_omega
+    a2 = 1.0 - alpha
+
+    # Initialize the filter state
+    @x1 = 0.0
+    @x2 = 0.0
+    @y1 = 0.0
+    @y2 = 0.0
+
+    # Store the filter coefficients
+    @b0 = b0 / a0
+    @b1 = b1 / a0
+    @b2 = b2 / a0
+    @a1 = a1 / a0
+    @a2 = a2 / a0
+  end
+
+  def apply(sample : Float64) : Float64
+    # Apply the filter to the sample
+    output_sample = @b0 * sample + @b1 * @x1 + @b2 * @x2 - @a1 * @y1 - @a2 * @y2
+
+    # Update the filter state variables
+    @x2 = @x1
+    @x1 = sample
+    @y2 = @y1
+    @y1 = output_sample
+
+    output_sample
+  end
+end