Frequency-weighted filter


The weightingFilter System object™ performs frequency-weighted filtering independently across each input channel.

To perform frequency-weighted filtering:

  1. Create the weightingFilter object and set its properties.

  2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects? (MATLAB).



weightFilt = weightingFilter
weightFilt = weightingFilter(weightType)
weightFilt = weightingFilter(weightType,Fs)
weightFilt = weightingFilter(___,Name,Value)


weightFilt = weightingFilter creates a System object, weightFilt, that performs frequency-weighted filtering independently across each input channel.

weightFilt = weightingFilter(weightType) sets the Method property to weightType.

weightFilt = weightingFilter(weightType,Fs) sets the SampleRate property to Fs.

weightFilt = weightingFilter(___,Name,Value) sets each property Name to the specified Value. Unspecified properties have default values.

Example: weightFilt = weightingFilter('C-weighting','SampleRate',96000) creates a C-weighting filter with a sample rate of 96,000 Hz.


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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects (MATLAB).

Type of weighting, specified as 'A-weighting', 'C-weighting', or 'K-weighting'. See Algorithms for more information.

Tunable: No

Data Types: char | string

Input sample rate in Hz, specified as a positive scalar.

Tunable: Yes

Data Types: single | double



audioOut = weightFilt(audioIn)



audioOut = weightFilt(audioIn) applies frequency-weighted filtering to the input signal, audioIn, and returns the filtered signal, audioOut. The type of filtering is specified by the algorithm and properties of the weightingFilter System object, weightFilt.

Input Arguments

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Audio input to the weighting filter, specified as a matrix. The columns of the matrix are treated as independent audio channels.

Data Types: single | double

Output Arguments

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Audio output from the weighting filter, returned as a matrix the same size as audioIn.

Data Types: single | double

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:


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visualizeVisualize and validate filter response
getFilterReturn biquad filter object with design parameters set
createAudioPluginClassCreate audio plugin class that implements functionality of System object
isStandardCompliantVerify filter design is IEC 61672-1:2002 compliant
cloneCreate duplicate System object
isLockedDetermine if System object is in use
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object
stepRun System object algorithm


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Check the compliance status of filter designs and visualize them.

Create an A-weighting filter with a 22.5 kHz sample rate. Verify that the filter is standard compliant and visualize the filter design.

aWeight = weightingFilter('A-weighting','SampleRate',22500);
complianceStatus = isStandardCompliant(aWeight,'class 1')
visualize(aWeight,'class 1')

Change your A-weighting filter sample rate to 44.1 kHz. Verify that the filter is standard compliant and visualize the filter design.

aWeight.SampleRate = 44100;
complianceStatus = isStandardCompliant(aWeight,'class 1')
visualize(aWeight,'class 1')

Use the weightingFilter System object™ to design an A-weighted filter, and then process an audio signal using your frequency-weighted filter design.

Create a dsp.AudioFileReader System object.

samplesPerFrame = 1024;
reader = dsp.AudioFileReader('Filename', ...
    'RockGuitar-16-44p1-stereo-72secs.wav', ...
    'SamplesPerFrame',samplesPerFrame, ...

Create a weightingFilter System object. Use the sample rate of the reader as the sample rate of the weighting filter.

Fs = reader.SampleRate;
weightFilt = weightingFilter('A-weighting',Fs);

Visualize the filter response and verify that it fits within the class 1 mask of the IEC 61672-1:2002 standard.

visualize(weightFilt,'class 1')

Create a spectrum analyzer to visualize the original audio signal and the audio signal after frequency-weighted filtering.

scope = dsp.SpectrumAnalyzer( ...
    'SampleRate',Fs, ...
    'PlotAsTwoSidedSpectrum',false, ...
    'FrequencyScale','Log', ...
    'FrequencyResolutionMethod','WindowLength', ...
    'WindowLength',samplesPerFrame, ...
    'Title','A-Weighted Filtering', ...
    'ShowLegend',true, ...
    'ChannelNames',{'Original signal','Filtered signal'});

Process the audio signal in an audio stream loop. Visualize the filtered audio and the original audio. As a best practice, release the System objects when complete.

while toc < 20
    x = reader();
    y = weightFilt(x);


Compare the A-weighted, C-weighted, and K-weighted filtering of an engine sound.

Create an A-weighting filter, a C-weighting filter, and a K-weighting filter. Visualize the filters for analysis and comparison.

wF{1} = weightingFilter;

wF{2} = weightingFilter('C-weighting');

wF{3} = weightingFilter('K-weighting');

Create a dsp.AudioFileReader and specify a sound file. Create an audioDeviceWriter with default properties. In an audio stream loop, play the white noise, and then listen to it filtered through the A-weighted, C-weighted, and K-weighted filters, successively.

fileReader = dsp.AudioFileReader('Engine-16-44p1-stereo-20sec.wav');
deviceWriter = audioDeviceWriter('SampleRate',fileReader.SampleRate);

fprintf('No filtering...')
for i = 1:400
    x = fileReader();
    if i==100
        index = 1;
        fprintf('A-weighted filtering...')
    elseif i==200
        index = 2;
        fprintf('C-weighted filtering...')
    elseif i==300
        index = 3;
        fprintf('K-weighted filtering...\n')
    if i>99
        y = wF{index}(x);
        y = x;

No filtering...A-weighted filtering...C-weighted filtering...K-weighted filtering...

The weightingFilter object uses second-order sections (SOS) for filtering. To extract the weighting filter design, use getFilter to return a dsp.BiquadFilter object with the SOSMatrix and ScaleValues properties set.

Use weightingFilter to create C-weighted and A-weighted filter objects. Use getFilter to return corresponding dsp.BiquadFilter objects.

cFilt = weightingFilter('C-weighting');
aFilt = weightingFilter('A-weighting');
cSOSFilter = getFilter(cFilt);
aSOSFilter = getFilter(aFilt);

Create an audio file reader and audio device writer for audio input/output. Use the sample rate of your reader as the sample rate of your writer.

fileReader = dsp.AudioFileReader('JetAirplane-16-11p025-mono-16secs.wav');
deviceWriter = audioDeviceWriter('SampleRate',fileReader.SampleRate);

In an audio stream loop, play the unfiltered signal. Release your file reader so that the next time you call it, it reads from the beginning of the file.

while toc<8
    x = fileReader();

Play the signal processed by the A-weighted filter. Then play the signal processed by the C-weighted filter. Cache the power in each frame of the original and filtered signals for analysis. As a best practice, release your file reader and device writer once complete.

y = [];
count = 1;
while ~isDone(fileReader)
    x = fileReader();
    aFiltered = aSOSFilter(x);
    cFiltered = cSOSFilter(x);
    if toc>8
    xPower(count) = var(x);
    aPower(count) = var(aFiltered);
    cPower(count) = var(cFiltered);
    y = [y;x];
    count = count+1;


Plot the power of the original signal, the A-weighted signal, and the C-weighted signal over time.

    title('Original Signal')
    t = linspace(0,16.3468,count-1);
        legend('Original Signal','A-Weighted','C-Weighted')
        xlabel('Time (s)')


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[1] Acoustical Society of America. Design Response of Weighting Networks for Acoustical Measurements. ANSI S1.42-2001. New York, NY: American National Standards Institute, 2001.

[2] International Electrotechnical Commission. Electroacoustics Sound Level Meters Part 1: Specifications. First Edition. IEC 61672-1. 2002–2005.

[3] International Telecommunication Union. Algorithms to measure audio programme loudness and true-peak audio level. ITU-R BS.1770-4. 2015.

[4] Mansbridge, Stuart, Saoirse Finn, and Joshua D. Reiss. "Implementation and Evaluation of Autonomous Multi-track Fader Control." Paper presented at the 132nd Audio Engineering Society Convention, Budapest, Hungary, 2012.

Extended Capabilities

Introduced in R2016b