diff --git a/notebooks/ch04_Python-SP.ipynb b/notebooks/ch04_Python-SP.ipynb
index d8ceacd..6bc9e0d 100644
--- a/notebooks/ch04_Python-SP.ipynb
+++ b/notebooks/ch04_Python-SP.ipynb
@@ -481,7 +481,7 @@
     "logX = 20*np.log10(X)\n",
     "\n",
     "fig, ax = plt.subplots(1, 2, figsize=(10, 4), sharex=True)\n",
-    "freq = np.arange(len(X)) * sr / 2 / len(X)\n",
+    "freq = np.arange(len(X)) / 2 / len(X) * sr\n",
     "ax[0].plot(freq, X)\n",
     "ax[0].set_title(\"Amplitude spectrum\")\n",
     "ax[0].set_xlim(0, sr // 2)\n",