diff --git a/effect_modulated_delay.cpp b/effect_modulated_delay.cpp
index 3358d32..b06a3d5 100644
--- a/effect_modulated_delay.cpp
+++ b/effect_modulated_delay.cpp
@@ -59,8 +59,9 @@ boolean AudioEffectModulatedDelay::begin(short *delayline, int d_length)
   _delay_length = d_length;
 
   // modulator filter
-  // coefficients calculated with "IOWA Hills IIR Filter Designer 6.5"
+  // coefficients calculated with "IOWA Hills IIR Filter Designer 6.5", http://www.iowahills.com/8DownloadPage.html
   // OmegaC = 0.5, SR = 44117.64706, Fc = 11 kHz, N=2
+  // Example: https://web.fhnw.ch/technik/projekte/eit/Fruehling2016/MuelZum/html/parametric_equalizer_example_8c-example.html
   modulator_filter_coeffs[0] = 0.291938819295525787;  // b0
   modulator_filter_coeffs[1] = 0.582700575839973478;  // b1
   modulator_filter_coeffs[2] = 0.291938819295525787;  // b2