diff --git a/pytta/__init__.py b/pytta/__init__.py
index d8740aa..0d81a8a 100644
--- a/pytta/__init__.py
+++ b/pytta/__init__.py
@@ -108,6 +108,7 @@
 from . import utils
 from . import rooms
 from . import iso3741
+from . import iso717
 
 from .functions import read_wav, write_wav, merge, split, \
     list_devices, print_devices, get_device_from_user,\
@@ -130,6 +131,7 @@
            'generate',
            'utils',
            'iso3741',
+           'iso717',
            'rooms',
 
            # Functions
diff --git a/pytta/iso717.py b/pytta/iso717.py
new file mode 100644
index 0000000..f970c8f
--- /dev/null
+++ b/pytta/iso717.py
@@ -0,0 +1,70 @@
+"""
+PyTTa ISO 717-1 rating:
+----------------------
+Single-number quantity of airborne sound insulation, ISO 717-1:2013.
+
+    >>> Rw, C, Ctr = pytta.iso717.airborne(R)
+
+"""
+import numpy as np
+
+# ISO 717-1:2013, Table 3 (reference values) and Table 4 (spectra No. 1 and
+# No. 2 for C and Ctr), from 100 Hz to 3 150 Hz in thirds and from 125 Hz
+# to 2 000 Hz in octaves.
+_TABLES = {
+    16: dict(reference=[33, 36, 39, 42, 45, 48, 51, 52, 53, 54, 55, 56, 56,
+                        56, 56, 56],
+             spectra=([-29, -26, -23, -21, -19, -17, -15, -13, -12, -11, -10,
+                       -9, -9, -9, -9, -9],
+                      [-20, -20, -18, -16, -15, -14, -13, -12, -11, -9, -8,
+                       -9, -10, -11, -13, -15]),
+             limit=320, at500=7),
+    5: dict(reference=[36, 45, 52, 55, 56],
+            spectra=([-21, -14, -8, -5, -4], [-14, -10, -7, -4, -6]),
+            limit=100, at500=2),
+}
+
+
+def _half_up(x, step):
+    """Round to a multiple of step, halves up (ISO 80000-1), clearing the
+    binary error of the input first."""
+    return np.floor(np.round(np.asarray(x, dtype=float) / step, 6) + 0.5)
+
+
+def airborne(R):
+    """
+    Weighted sound reduction index and spectrum adaptation terms of
+    ISO 717-1:2013, from R, R', Dn or DnT by band.
+
+    Parameters:
+    -----------
+        * R (array-like):
+            16 one-third-octave bands from 100 Hz to 3 150 Hz or 5 octave
+            bands from 125 Hz to 2 000 Hz, in dB.
+
+    Return:
+    -------
+        * (Xw, C, Ctr), integers in dB: Rw (or R'w, Dn,w, DnT,w) and the
+          adaptation terms for pink noise and for urban traffic noise.
+
+    R is rounded to 0.1 dB, halves up (4.4). The reference curve is shifted
+    in steps of 1 dB until the sum of unfavourable deviations is as large as
+    possible but not more than 32.0 dB in thirds or 10.0 dB in octaves, and
+    Xw is its value at 500 Hz. C and Ctr follow Equations (1) and (2).
+    Values are kept in tenths of a decibel as integers, so the limit is
+    compared exactly.
+    """
+    tables = _TABLES.get(len(R))
+    if tables is None:
+        raise ValueError('R must have 16 one-third-octave or 5 octave bands.')
+    tenths = _half_up(R, 0.1)
+    reference = np.array(tables['reference']) * 10
+    shift = int(np.floor((tenths - reference).min() / 10))  # no deviation
+    while np.maximum(reference + (shift + 1) * 10 - tenths, 0).sum() \
+            <= tables['limit']:
+        shift += 1
+    Xw = tables['reference'][tables['at500']] + shift
+    C = [int(_half_up(-10 * np.log10(np.sum(10 ** ((np.array(L) - tenths / 10)
+                                                    / 10))), 1)) - Xw
+         for L in tables['spectra']]
+    return Xw, C[0], C[1]
diff --git a/tests/test_iso717.py b/tests/test_iso717.py
new file mode 100644
index 0000000..0b9ce4c
--- /dev/null
+++ b/tests/test_iso717.py
@@ -0,0 +1,50 @@
+"""
+Tests for pytta.iso717.airborne, the single-number rating of airborne
+sound insulation of ISO 717-1:2013. Each test pins one rule.
+"""
+import os
+import unittest
+
+os.environ.setdefault('MPLBACKEND', 'Agg')
+
+import pytta
+
+# ISO 717-1:2013, Annex C, Table C.1: sound reduction index of a building
+# element, one-third-octave bands from 100 Hz to 3 150 Hz.
+R_ANNEX_C = [20.4, 16.3, 17.7, 22.6, 22.4, 22.7, 24.8, 26.6, 28.0, 30.5, 31.8,
+             32.5, 33.4, 33.0, 31.0, 25.5]
+
+
+class TestISO717(unittest.TestCase):
+
+    def test_example_of_annex_c(self):
+        # Rw(C;Ctr) = 30 (-2; -3) dB, with 31.8 dB of unfavourable deviation
+        self.assertEqual(pytta.iso717.airborne(R_ANNEX_C), (30, -2, -3))
+
+    def test_deviation_limit_is_inclusive(self):
+        # Octave bands: R 5 dB above the reference curve. Shifting it up by
+        # 7 dB gives 2 dB below R in each of the 5 bands, exactly 10.0 dB,
+        # which is allowed; 8 dB would give 15 dB.
+        reference = [36, 45, 52, 55, 56]
+        Rw, C, Ctr = pytta.iso717.airborne([r + 5 for r in reference])
+        self.assertEqual(Rw, 52 + 7)
+
+    def test_input_is_rounded_half_up_to_one_decimal(self):
+        # The standard rounds R to 0.1 dB before rating, halves up. These
+        # five bands end in 5, and round() of Python, which rounds the
+        # binary value, takes each of them 0.1 dB down: the deviations would
+        # sum 32.3 dB and Rw would drop to 29. Rounded half up, the curve is
+        # that of Annex C.
+        R = list(R_ANNEX_C)
+        for band, value in ((5, 22.65), (8, 27.95), (9, 30.45), (14, 30.95),
+                            (15, 25.45)):
+            R[band] = value
+        self.assertEqual(pytta.iso717.airborne(R), (30, -2, -3))
+
+    def test_rejects_other_band_counts(self):
+        with self.assertRaises(ValueError):
+            pytta.iso717.airborne(R_ANNEX_C[:-1])
+
+
+if __name__ == '__main__':
+    unittest.main()