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sofar and SOFA#

If you are new to SOFA and/or sofar, this is a good place to start. SOFA is short for Spatially Oriented Format for Acoustics and is an open file format for saving acoustic data, such as head-related impulse responses (HRIRs), loudspeaker directivities, or room impulse responses measured with microphone arrays. Good places to get more information about SOFA after this introduction are

A complete documentation of the sofar package is given here.

Creating SOFA objects#

To cover a variety of data, SOFA offers different conventions. A convention defines the data type, the required and optional meta data and the dimensions (shape) of the data and meta data. The data type can be

  • TF: Transfer Function

  • IR: Impulse Response

  • SOS: Second Order Sections, and

Examples for meta data are - A license - Contact to the author of a SOFA file - Positions of sources and receivers

You should always find the most specific convention for your data. This will help you to identify relevant meta data that you should provide along the actual acoustic data. sofar provides a list of conventions as part of the online documentation and as console output

import sofar as sf
import os

Available SOFA conventions:
GeneralTF (Version 1.0)
SingleRoomSRIR (Version 1.0)
FreeFieldHRIR (Version 1.0)
SimpleHeadphoneIR (Version 1.0)
SimpleFreeFieldHRIR (Version 1.0)
GeneralFIR-E (Version 2.0)
SingleRoomMIMOSRIR (Version 1.0)
FreeFieldHRTF (Version 1.0)
GeneralTF (Version 2.0)
FreeFieldDirectivityTF (Version 1.1)
SimpleFreeFieldHRSOS (Version 1.0)
SimpleFreeFieldHRTF (Version 1.0)
GeneralSOS (Version 1.0)
SimpleFreeFieldSOS (Version 1.0)
GeneralFIR (Version 1.0)
GeneralTF-E (Version 1.0)
SimpleFreeFieldTF (Version 1.0)
FreeFieldDirectivityTF (Version 1.0)
SimpleHeadphoneIR (Version 0.1)
SingleRoomDRIR (Version 0.2)
GeneralFIRE (Version 1.0)
SimpleHeadphoneIR (Version 0.2)
SimpleFreeFieldTF (Version 0.4)
MultiSpeakerBRIR (Version 0.3)
SingleRoomDRIR (Version 0.3)
SimpleFreeFieldHRIR (Version 0.4)

You might have noticed that the data type (TF, IR, SOS) is part of the convention name and that it is appended by -E in some cases. This stands for Emitter and indicates that the convention is used to save data from multiple emitters (loud speakers).

Let us assume, that you want to store head-related impulse responses (HRIRs). In this case the most specific convention is SimpleFreeFieldHRIR. To create a SOFA object use

sofa = sf.Sofa('SimpleFreeFieldHRIR')
sofar.SOFA object: SimpleFreeFieldHRIR 1.0

The return value sofa is a sofar.Sofa object filled with the default values of the SimpleFreeFieldHRIR convention. Note that you can also get a sofa object that has only the mandatory attributes by calling sf.Sofa('SimpleFreeFieldHRIR', mandatory=True). However, it is recommended to start with all attributes and discard empty optional attributes before saving the data.

Getting information about SOFA objects#

A first impression of the data can be obtained with

SimpleFreeFieldHRIR 1.0 (SOFA version 2.1)
GLOBAL_Conventions : SOFA
GLOBAL_Version : 2.1
GLOBAL_SOFAConventions : SimpleFreeFieldHRIR
GLOBAL_SOFAConventionsVersion : 1.0
GLOBAL_APIName : sofar SOFA API for Python (
GLOBAL_APIVersion : sofar v1.1.3 implementing SOFA standard AES69-2022 (SOFA conventions 2.1)
GLOBAL_ApplicationName : Python
GLOBAL_ApplicationVersion : 3.10.13 [CPython - GCC 11.4.0]
GLOBAL_AuthorContact :
GLOBAL_Comment :
GLOBAL_History :
GLOBAL_License : No license provided, ask the author for permission
GLOBAL_Organization :
GLOBAL_References :
GLOBAL_RoomType : free field
GLOBAL_Origin :
GLOBAL_DateCreated : 2024-04-19 14:42:55
GLOBAL_DateModified : 2024-04-19 14:42:55
GLOBAL_Title :
GLOBAL_DatabaseName :
GLOBAL_ListenerShortName :
ListenerPosition : (I=1, C=3)
  [0 0 0]
ListenerPosition_Type : cartesian
ListenerPosition_Units : metre
ReceiverPosition : (R=2, C=3, I=1)
  [[ 0.    0.09  0.  ]
   [ 0.   -0.09  0.  ]]
ReceiverPosition_Type : cartesian
ReceiverPosition_Units : metre
SourcePosition : (I=1, C=3)
  [0 0 1]
SourcePosition_Type : spherical
SourcePosition_Units : degree, degree, metre
EmitterPosition : (E=1, C=3, I=1)
  [0 0 0]
EmitterPosition_Type : cartesian
EmitterPosition_Units : metre
ListenerUp : (I=1, C=3)
  [0 0 1]
ListenerView : (I=1, C=3)
  [1 0 0]
ListenerView_Type : cartesian
ListenerView_Units : metre
SourceUp : (I=1, C=3)
  [0 0 1]
SourceView : (I=1, C=3)
  [1 0 0]
SourceView_Type : cartesian
SourceView_Units : metre
Data_IR : (M=1, R=2, N=1)
  [0 0]
Data_SamplingRate : 48000
Data_SamplingRate_Units : hertz
Data_Delay : (I=1, R=2)
  [0 0]

This lists the names of all entries and a brief summary of the content. Lets take a closer look

  • Global Attributes store meta data that describe the whole data stored in a SOFA file. Global attributes start with the keyword GLOBAL and are as stored as strings.

  • Variables store numeric (double) or string data. Variables are always arrays. Example for Variables are the SourcePosition and Data_IR, which stores the actual acoustic data (impulse responses in this case).

  • Variable Attributes store meta data describing a specific variable and are stored as stings. Variable attributes start with the name of the variable followed by an underscore and the name of the attribute. For example, SourcePosition_Type gives the Type of coordinates that are used to store data for the variable SourcePosition. An exception to this rule are the data variables, e.g, Data_SamplingRate is not an attribute but a double variable.

In the case of variables, the SOFA convention also specifies the dimension (called shape in numpy) that the data must have. For example Data_IR must be of shape \(M \times R \times N\). In this case, \(M=1\) denotes the number of source positions for which HRIRs are available, \(R=2\) is the number of ears - which is two - and \(N=1\) gives the lengths of the HRIRs in samples.

All dimensions can be obtained by

R = 2 receiver (set by ReceiverPosition of dimension RCI, RCM)
E = 1 emitter (set by EmitterPosition of dimension ECI, ECM)
M = 1 measurements (set by Data_IR of dimension MRN)
N = 1 samples (set by Data_IR of dimension MRN)
C = 3 coordinate dimensions, fixed
I = 1 single dimension, fixed
S = 0 maximum string length

For SOFA files, the dimensions are automatically deduced from the data and the output tells us that for example the dimension \(R=2\) was deduced from the shape of the variable ReceiverPosition. It is thus important to be aware of the dimensions and enter the data accordingly. In some cases, variables can have different shapes. An example for this is the ReceiverPosition which can be of shape \(R \times C \times I\) or \(R \times C \times M\).

The dimension S denotes the lengths of the longest string contained in a string variable. S is zero in this case, because the convention does not have any string variables. C denotes the size of coordinate points and is always three, because coordinates are either given by x, y, and z values or by their azimuth, elevation in degree and radius in meter (cf. SOFA coordinate systems).

To get a dimension as a variable, use


A complete description of each convention is given in the sofar documentation. The documentation shows that data can be mandatory, optional, and read only.

Getting and adding data to SOFA objects#

Data can be obtained by simply calling attributes of the Sofa object, e.g.


Note that all variables are stored as numpy arrays, which means that you can use indexing, and slicing just like with any other numpy array. For example you can get only the left ear data with

sofa.Data_IR[:, 0]

Data can be added in the same way. The HRIRs and source position can for example be set with

sofa.Data_IR = [[[1, 0, 0], [0, 0.5, 0]]]
sofa.SourcePosition = [90, 0, 1.5]

Now, the SOFA object contains a single HRIR - which is [1, 0, 0] for the left ear and [0, 0.5, 0] for the right ear - for a source at 90 degree azimuth, 0 degree elevation and a radius of 1.5 meter. Note that we entered lists and that sofar automatically converts the lists to numpy arrays. Sofar handles this in two steps:

  1. When entering data as lists it is converted to a numpy array with at least two dimensions.

  2. Missing dimensions are appended when writing the SOFA object to disk.

Normally, you would continue with filling all mandatory entries of the SOFA object. For this example we’ll stop here for the sake of brevity. Let us, however, delete an optional entry that we do not need at this point


In some cases you might want to add custom data. Although third party applications most likely won’t make use of non-standardized data this can be useful for documentation and research use. Try this to add a temperature value and unit. Note that you have to specify the data type and shape (dimensions) if you are adding SOFA variables

sofa.add_variable('Temperature', 25.1, 'double', 'MI')
sofa.add_attribute('Temperature_Units', 'degree celsius')

After entering the data, the SOFA object should be verified to make sure that your data agrees with the SOFA standard and that if it can be read by other applications.


This will check specific rules determined by the SOFA standard AES69 and general rules such as:

  • Are all mandatory data contained?

  • Are the names of variables and attributes in accordance with the SOFA standard?

  • Are the data types in accordance with the SOFA standard?

  • Are the dimensions of the variables consistent and in accordance to the SOFA standard?

  • Are the values of attributes consistent and in accordance to the SOFA standard?

If any violations are detected, an error is raised.

Reading and writing SOFA objects#

Note that you usually do not need to call sofa.verify() separately because it is by default called if you create, write, or read a SOFA object. To write your SOFA object to disk type

sf.write_sofa(os.path.join('..', '..', 'resources', 'my_first.sofa'), sofa)

It is good to know that SOFA files are essentially netCDF4 files which is based on HDF5. They can thus be viewed with HDF View.

To read your sofa file you can use

sofa_read = sf.read_sofa(os.path.join(
    '..', '..', 'resources', 'my_first.sofa'))
SOFA file contained custom entries
Temperature, Temperature_Units

And to see that the written and read files contain the same data you can check

sf.equals(sofa, sofa_read)

Upgrading SOFA files#

SOFA conventions might get updates to fix bugs in the conventions, in case new conventions are introduced, or in case conventions get deprecated. To find out if SOFA file is up to date, call upgrade_convention to get a list of upgrade choices or the information that the data is up to date.

Convention SimpleFreeFieldHRIR v1.0 is up to date

Next steps#

For detailed information about sofar please refer to the sofar documentation.

License notice#

This notebook © 2024 by the pyfar developers is licensed under CC BY 4.0

CC BY Large


%load_ext watermark
%watermark -v -m -iv
Python implementation: CPython
Python version       : 3.10.13
IPython version      : 8.23.0

Compiler    : GCC 11.4.0
OS          : Linux
Release     : 5.19.0-1028-aws
Machine     : x86_64
Processor   : x86_64
CPU cores   : 2
Architecture: 64bit

sofar: 1.1.3