diff --git a/CH01/CH01_SEC04_2_Cement.ipynb b/CH01/CH01_SEC04_2_Cement.ipynb
index 41c43f4..3118afc 100644
--- a/CH01/CH01_SEC04_2_Cement.ipynb
+++ b/CH01/CH01_SEC04_2_Cement.ipynb
@@ -51,7 +51,7 @@
     "# x = regress(b,A)\n",
     "\n",
     "# Alternative 2:\n",
-    "x = np.linalg.pinv(A)*b"
+    "x = np.linalg.pinv(A)@b"
    ]
   },
   {
diff --git a/CH02/CH02_SEC04_2_SpectrogramBeethoven.ipynb b/CH02/CH02_SEC04_2_SpectrogramBeethoven.ipynb
new file mode 100644
index 0000000..940a075
--- /dev/null
+++ b/CH02/CH02_SEC04_2_SpectrogramBeethoven.ipynb
@@ -0,0 +1,108 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "5d998359",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import scipy\n",
+    "import scipy.io\n",
+    "import IPython.display\n",
+    "import numpy as np\n",
+    "import librosa\n",
+    "\n",
+    "import matplotlib.pyplot as plt"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "1f7443a8",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "#Example to load mat file (first 40 seconds)\n",
+    "#file = scipy.io.loadmat('../DATA/beethoven_40sec.mat')\n",
+    "#sound = file[\"y\"][0] # data\n",
+    "#T = int(file[\"T\"][0][0]) # length in seconds\n",
+    "#FS = int(file[\"FS\"][0][0]) #sample frequency\n",
+    "#samples = T*FS\n",
+    "#t = np.arange(samples)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "db4ff4da",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "#original mp3 file\n",
+    "FS = 24000\n",
+    "y, sr = librosa.load('../DATA/beethoven.mp3', sr=FS)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "510e4bd3",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "plt.figure(figsize=(20,5))\n",
+    "plt.plot(y)\n",
+    "\n",
+    "#for \"mat\" file use command\n",
+    "#plt.plot(t, sound)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "697e8139",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "IPython.display.Audio(y, rate=FS)\n",
+    "\n",
+    "#For mat file use command \n",
+    "#IPython.display.Audio(sound, rate=FS)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "72d27077",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "plt.rcParams[\"figure.figsize\"] = (20,5)\n",
+    "plt.specgram(y, NFFT=8192, Fs=FS, noverlap=2048)\n",
+    "plt.colorbar()"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.8.12"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}