diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -87,7 +87,9 @@ The lower nibble (bits 0 to 3) of the status can be modified from the applicatio
 
 ## Sound output
 
-ESOP supports monophonic sound output with a beeper controllable via `06` port. As long as the port value is non-zero, the beeper must emit a sound wave of the specified frequency. The frequency is specified in semitones relative to A4 (440 Hz), with A4 itself being located at the value `30`. Setting the port value to `00` must halt the sound output.
+ESOP supports monophonic sound output with a beeper controllable via `06` port. As long as the port value is non-zero, the beeper must emit a sound wave of the specified frequency. The frequency is specified in semitones relative to A4 (440 Hz), with A4 itself being located at the value `30`. Setting the port value to `00` must stop the sound.
+
+Note: this specification does not enforce any particular waveform to be used for playing the notes, but in emulated ESOP environments, a square wave is recommended as the closest option for phone beeper emulation.
 
 ## System calls