diff --git a/pytta/_plot.py b/pytta/_plot.py
index c0a0970..f980b38 100644
--- a/pytta/_plot.py
+++ b/pytta/_plot.py
@@ -821,7 +821,7 @@ def spectrogram(sigObjs, winType, winSize,
             window size in samples
 
         * overlap (), (float):
-            window overlap in %
+            overlapping fraction of consecutive windows, 0 to 1
 
         * xLabel ('Time [s]'), (str):
             x axis label.
@@ -868,7 +868,7 @@ def spectrogram(sigObjs, winType, winSize,
         ax.set_snap(False)
 
         _spectrogram, _specTime, _specFreq = \
-            _calc_spectrogram(data['timeSignal'], data['timeVector'],
+            _calc_spectrogram(data['timeSignal'],
                               data['samplingRate'], overlap, winType,
                               winSize, data['dBRef'])
 
@@ -956,30 +956,18 @@ def _curve_data_extractor_spectrogram(sigObjs):
     return curveData
 
 
-def _calc_spectrogram(timeSignal, timeVector, samplingRate, overlap, winType,
-                      winSize, dBRef):
-    window = eval('ss.windows.' + winType)(winSize)
-    nextIdx = int(winSize*overlap)
-    rng = int(timeSignal.shape[0]/winSize/overlap - 1)
-    _spectrogram = np.zeros((winSize//2 + 1, rng))
-    _specFreq = np.linspace(0, samplingRate//2, winSize//2 + 1)
-    _specTime = np.linspace(0, timeVector[-1], rng)
-
-    for N in range(rng):
-        try:
-            strIdx = N*nextIdx
-            endIdx = winSize + N*nextIdx
-            sliceAudio = window*timeSignal[strIdx:endIdx]
-            sliceFFT = np.fft.rfft(sliceAudio, axis=0)
-            sliceMag = np.absolute(sliceFFT) * (2/sliceFFT.size)
-            _spectrogram[:, N] = 20*np.log10(sliceMag/dBRef)
-
-        except IndexError:
-            sliceAudio = timeSignal[-winSize:]
-            sliceFFT = np.fft.rfft(sliceAudio, axis=0)
-            sliceMag = np.absolute(sliceFFT) * (2/sliceFFT.size)
-            _spectrogram[:, N] = 20*np.log10(sliceMag)
-
+def _calc_spectrogram(timeSignal, samplingRate, overlap, winType, winSize,
+                      dBRef):
+    """Spectrogram in dB of peak amplitude re dBRef. `overlap` is the
+    overlapping fraction of consecutive windows; each column is at the
+    centre of its window."""
+    window = getattr(ss.windows, winType)(winSize)
+    _specFreq, _specTime, magnitude = ss.spectrogram(
+        timeSignal, samplingRate, window=window,
+        noverlap=int(winSize * overlap), detrend=False, mode='magnitude',
+        scaling='spectrum')  # |X| / sum(window): half the peak amplitude
+    with np.errstate(divide='ignore'):
+        _spectrogram = 20 * np.log10(2 * magnitude / dBRef)
     return _spectrogram, _specTime, _specFreq
 
 
diff --git a/pytta/classes/signal.py b/pytta/classes/signal.py
index 79151c3..91e63a3 100644
--- a/pytta/classes/signal.py
+++ b/pytta/classes/signal.py
@@ -753,7 +753,7 @@ def plot_spectrogram(self, winType:str='hann', winSize:int=1024,
                 window size in samples
 
             * overlap (0.5), (float):
-                window overlap in %
+                overlapping fraction of consecutive windows, 0 to 1
 
             * xLabel ('Time [s]'), (str):
                 x axis label.
diff --git a/pytta/functions.py b/pytta/functions.py
index 9b39239..da0d0d4 100644
--- a/pytta/functions.py
+++ b/pytta/functions.py
@@ -550,7 +550,7 @@ def plot_spectrogram(*sigObjs, winType:str='hann', winSize:int=1024,
             window size in samples
 
         * overlap (0.5), (float):
-            window overlap in %
+            overlapping fraction of consecutive windows, 0 to 1
 
         * xLabel ('Time [s]'), (str):
             x axis label.
diff --git a/tests/test_spectrogram.py b/tests/test_spectrogram.py
new file mode 100644
index 0000000..fe674e3
--- /dev/null
+++ b/tests/test_spectrogram.py
@@ -0,0 +1,63 @@
+"""
+Tests for the plain spectrogram of pytta.plot_spectrogram
+(pytta._plot._calc_spectrogram).
+
+Each test pins one rule: `overlap` is the overlapping fraction of
+consecutive windows, each column sits at the centre of its window, and a
+sine reads at its peak amplitude in dB whatever the window.
+"""
+import os
+import unittest
+
+os.environ.setdefault('MPLBACKEND', 'Agg')
+
+import numpy as np
+
+from pytta import _plot
+
+FS = 8000
+W = 1024
+
+
+def spectrogram(x, overlap=0.5, winType='hann'):
+    S, t, f = _plot._calc_spectrogram(x, FS, overlap, winType, W, 1.0)
+    return S, t, f
+
+
+class TestSpectrogram(unittest.TestCase):
+
+    def test_overlap_is_the_overlapping_fraction(self):
+        # Hop = W (1 - overlap); every window lies inside the signal, so
+        # there are (n - W) // hop + 1 columns, for any overlap in [0, 1).
+        x = np.random.default_rng(0).standard_normal(FS)
+        for overlap in (0, 0.25, 0.5, 0.75, 0.9):
+            hop = W - int(W * overlap)
+            S, t, f = spectrogram(x, overlap)
+            self.assertEqual(S.shape, (W // 2 + 1, (FS - W) // hop + 1))
+            self.assertTrue(np.allclose(np.diff(t), hop / FS))
+
+    def test_columns_sit_at_the_centre_of_their_window(self):
+        # The first column is centred at W / 2, and a click is loudest in
+        # the column whose centre is nearest to it.
+        x = np.zeros(FS)
+        x[5000] = 1
+        S, t, f = spectrogram(x, 0.75)
+        self.assertAlmostEqual(t[0], W / 2 / FS)
+        loudest = t[np.argmax(S[W // 8])]
+        self.assertLessEqual(abs(loudest - 5000 / FS), 128 / FS)
+
+    def test_sine_reads_at_its_peak_amplitude(self):
+        # A sine of amplitude 0.5 centred on a bin reads -6.02 dB re 1, with
+        # the coherent gain of each window taken out.
+        k = 64
+        tt = np.arange(FS) / FS
+        x = 0.5 * np.sin(2 * np.pi * k * FS / W * tt)
+        for winType in ('hann', 'blackman', 'boxcar'):
+            S, t, f = spectrogram(x, 0.5, winType)
+            level = np.median(S[k])
+            print(f'\n  {winType:8s} {level:+.3f} dB (expected -6.021)')
+            self.assertAlmostEqual(level, 20 * np.log10(0.5), delta=0.05)
+
+
+if __name__ == '__main__':
+    unittest.main()