diff --git a/lasp/lasp_common.py b/lasp/lasp_common.py
index 60e856b..1933a55 100644
--- a/lasp/lasp_common.py
+++ b/lasp/lasp_common.py
@@ -8,6 +8,7 @@ import appdirs
 import numpy as np
 
 from .wrappers import Window as wWindow
+from collections import namedtuple
 
 """
 Common definitions used throughout the code.
@@ -15,9 +16,77 @@ Common definitions used throughout the code.
 
 __all__ = [
     'P_REF', 'FreqWeighting', 'TimeWeighting', 'getTime', 'getFreq',
-    'lasp_shelve', 'this_lasp_shelve', 'W_REF', 'U_REF', 'I_REF'
+    'lasp_shelve', 'this_lasp_shelve', 'W_REF', 'U_REF', 'I_REF', 'dBFS_REF',
+    'SIUnits'
 ]
 
+# Reference sound pressure level
+P_REF = 2e-5 # 20 micropascal
+
+W_REF = 1e-12  # 1 picoWatt
+I_REF = 1e-12  # 1 picoWatt/ m^2
+
+# Reference velocity for sound velocity level
+U_REF = 5e-8 # 50 nano meter / s
+
+# Wiki: The unit dB FS or dBFS is defined in AES Standard AES17-1998,[13] 
+# IEC 61606,[14] and ITU-T P.38x,[15][16] such that the RMS value of a
+# full-scale sine wave is designated 0 dB FS
+
+# The LASP code converts integer samples to floats, where the mapping is such
+# that the maximum value (POSITIVE) that can be represented by the integer data
+# is mapped to the value of 1.0. Following this convention, a sine wave with
+# amplitude of 1.0 is 0 dB FS, and subsequently, its rms value is 0.5*sqrt(2),
+# hence this is the reference level as specified below.
+dBFS_REF = 0.5*2**0.5 # Which level would be -3.01 dBFS
+
+Qty = namedtuple('Qty', 'name unit_name unit_symb level_unit level_ref_name level_ref_value')
+
+class SIQtys:
+    AP = Qty(name='Acoustic Pressure',
+             unit_name='Pascal',
+             unit_symb=('Pa', 'muPa'),
+             level_unit=('dB SPL','dBPa'),
+             level_ref_name=('2 micropascal', '1 pascal',),
+             level_ref_value=(P_REF, 1)
+            )
+
+    V = Qty(name='Voltage',
+             unit_name='volt',
+             unit_symb='V',
+             level_unit=('dBV',), # dBV
+             level_ref_name=('1V',),
+             level_ref_value=(1.0,),
+           )
+    N = Qty(name='No unit',
+            unit_name='',
+            unit_symb='[-]',
+            level_unit=('dBFS',),
+            level_ref_name=('Full scale sine wave',),
+            level_ref_value=(dBFS_REF,)
+           )
+    types = (AP, V, N)
+    default = AP
+    default_index = 0
+
+    @staticmethod
+    def fillComboBox(cb):
+        """
+        Fill FreqWeightings to a combobox
+
+        Args:
+            cb: QComboBox to fill
+        """
+        cb.clear()
+        for ty in SIQtys.types:
+            cb.addItem(f'{ty.name} [{ty.unit_symb[0]}')
+        cb.setCurrentIndex(SIQtys.default_index)
+
+    @staticmethod
+    def getCurrent(cb):
+        return SIQtys.types[cb.currentIndex()]
+
+
 lasp_appdir = appdirs.user_data_dir('Lasp', 'ASCEE')
 
 if not os.path.exists(lasp_appdir):
@@ -27,6 +96,7 @@ if not os.path.exists(lasp_appdir):
         print('Fatal error: could not create application directory')
         sys.exit(1)
 
+
 class Shelve:
     def load(self, key, default_value):
         """
@@ -69,11 +139,11 @@ class Shelve:
     def __exit__(self, type, value, traceback):
         self.decref()
 
+
 class lasp_shelve(Shelve):
     _refcount = 0
     _shelve = None
 
-
     @property
     def refcount(self):
         return lasp_shelve._refcount
@@ -93,6 +163,7 @@ class lasp_shelve(Shelve):
     def shelve_fn(self):
         return os.path.join(lasp_appdir, 'config.shelve')
 
+
 class this_lasp_shelve(Shelve):
     _refcount = 0
     _shelve = None
@@ -118,16 +189,6 @@ class this_lasp_shelve(Shelve):
         return os.path.join(lasp_appdir, f'{node}_config.shelve')
 
 
-# Reference sound pressure level
-P_REF = 2e-5
-
-W_REF = 1e-12  # 1 picoWatt
-I_REF = 1e-12  # 1 picoWatt/ m^2
-
-# Reference velocity for sound velocity level
-U_REF = 5e-8
-
-
 class Window:
     hann = (wWindow.hann, 'Hann')
     hamming = (wWindow.hamming, 'Hamming')
@@ -154,6 +215,7 @@ class Window:
     def getCurrent(cb):
         return Window.types[cb.currentIndex()]
 
+
 class TimeWeighting:
     none = (-1, 'Raw (no time weighting)')
     uufast = (1e-4, '0.1 ms')
@@ -188,6 +250,7 @@ class TimeWeighting:
     def getCurrent(cb):
         return TimeWeighting.types_all[cb.currentIndex()]
 
+
 class FreqWeighting:
     """
     Frequency weighting types
@@ -216,6 +279,7 @@ class FreqWeighting:
     def getCurrent(cb):
         return FreqWeighting.types[cb.currentIndex()]
 
+
 def getTime(fs, N, start=0):
     """
     Return a time array for given number of points and sampling frequency.
@@ -228,6 +292,7 @@ def getTime(fs, N, start=0):
     assert N > 0 and fs > 0
     return np.linspace(start, start + N/fs, N, endpoint=False)
 
+
 def getFreq(fs, nfft):
     """
     return an array of frequencies for single-sided spectra