roomNoiseCriteria

R2026b

Evaluate room background noise using room noise criteria (RNC) curves

Since R2026b

Syntax

RNC = roomNoiseCriteria(audioIn,fs)

RNC = roomNoiseCriteria(Leq,Lt)

RNC = roomNoiseCriteria(___,Name=Value)

[RNC,rattleRisk] = roomNoiseCriteria(___)

roomNoiseCriteria(___)

Description

RNC = roomNoiseCriteria(audioIn,fs) returns the room noise criteria (RNC) level according to the ANSI/ASA S12.2-2019 standard [1].

RNC = roomNoiseCriteria(Leq,Lt) computes the RNC level using equivalent continuous sound pressure level (SPL) measurements and time-weighted SPL measurements across octave bands.

RNC = roomNoiseCriteria(___,Name=Value) specifies options using one or more name-value arguments.

[RNC,rattleRisk] = roomNoiseCriteria(___) also returns a structure detailing the likelihood of acoustically induced vibrations and rattles.

roomNoiseCriteria(___) without any output arguments plots the octave band SPL measurements against RNC curves and displays the RNC level.

Examples

Room Noise Criteria Evaluation from Room Recording

Load a recording of a room with background noise.

[audioIn, fs] = audioread("office_kitchen_44p1_20s.wav");

If possible, enter the calibration factor for the microphone used to make the recording or calculate it using calibrateMicrophone. The microphone in this example has an associated reference recording at 1 kHz with an SPL meter reading of 64 dB.

[audioRef, fsRef] = audioread("1khz_64db_44p1.wav");
SPLreading = 64;
calibrationFactor = calibrateMicrophone(audioRef,fsRef,SPLreading)

calibrationFactor = 
2.0836

Plot the RNC level for the room recording.

roomNoiseCriteria(audioIn,fs,CalibrationFactor=calibrationFactor)

Room Noise Criteria Evaluation of SPL Meter Readings

Collect equivalent continuous sound level (Leq) measurements and time-weighted sound level (Lt) measurements of a room using a sound pressure level (SPL) meter with octave band filters centered at 16, 31.5, 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz.

Simulate the measurements.

Leq = [66, 58, 52, 49, 44, 46, 44, 38, 31, 19];
Lt = zeros(1500,10) + Leq; % readings collected every 100 ms
fmod = cos(2*pi*(0:1499)/100)';
Lt(:,1) = Lt(:,1) + 7*fmod; % 1 Hz, 14 dB surging in 16 Hz octave band
Lt(:,2) = Lt(:,2) + 6*fmod; % 1 Hz, 16 dB surging in 31.5 Hz octave band
Lt(:,3) = Lt(:,3) + 6*fmod; % 1 Hz, 12 dB surging in 63 Hz octave band
Lt(:,4) = Lt(:,4) + 4*fmod; % 1 Hz, 8 dB surging in 125 Hz octave band

Use roomNoiseCriteria to calculate the room noise criteria (RNC) level using the readings from the SPL meter.

roomNoiseCriteria(Leq,Lt)

Use Room Noise Criteria Instead of Noise Criteria

The ANSI/ASA S12.2-2019 standard requires the Room Noise Criteria (RNC) to be used instead of the Noise Criteria (NC) when surging or large random fluctuations occur in low-frequency octave bands. If the noiseCriteria function detects these conditions, it issues a warning recommending that you use roomNoiseCriteria instead.

Simulate a recording of a washing machine in a shoebox room using acousticRoomResponse.

[audioIn,fs] = audioread('WashingMachine-16-44p1-stereo-10secs.wav');
audioIn = mean(audioIn,2);
roomDimensions = [10,8,5];
tx = [9,7,2]; % near back of room
rx = roomDimensions / 2; % center of room
rng(1);
ir = acousticRoomResponse(roomDimensions,tx,rx,SampleRate=fs,ImageSourceOrder=6,...
    MaxNumRayReflections=50,NumStochasticRays=4000);
x = fftfilt(ir',audioIn);

Calculate the NC from the recording.

NC = noiseCriteria(x,fs)

Warning: Large random fluctuations detected. To follow ANSI/ASA S12.2-2019, use roomNoiseCriteria instead.

NC = 
"NC-57 (1000 Hz)"

The warning states that you should use roomNoiseCriteria due to the presence of large random fluctuations. To evaluate the noise in this room, use the RNC metric instead.

RNC = roomNoiseCriteria(x,fs)

RNC = 
"RNC-56 (1000 Hz)"

Input Arguments

audioIn — Audio signal

column vector

Audio input, specified as a column vector of real finite numbers. If audioIn is not calibrated, you can determine the calibration factor using calibrateMicrophone. When surging or large random fluctuations are present, audioIn must contain enough samples to represent at least 15 seconds of audio at sample rate fs.

Specify either audioIn and fs or Leq and Lt.

For guidance on taking compliant measurements, see Calibration Factor for Audio Recordings.

Data Types: single | double

fs — Sample rate (Hz)

positive scalar

Sample rate in Hz, specified as a scalar equal to or greater than 16 kHz.

Specify either audioIn and fs or Leq and Lt.

Leq — Octave-band equivalent continuous SPL measurements

1-by-10 row vector

Octave-band equivalent continuous sound level (Leq) measurements in decibels (dB), specified as a 10-element row vector. Each element must correspond to a measurement at nominal octave band center 16, 31.5, 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz, respectively.

If the SPL for a given band is unknown, specify it as missing. An RNC level calculated with missing Leq values might not be compliant with ASA S12.2-2019.

Specify either audioIn and fs or Leq and Lt.

For guidance on taking compliant measurements, see Obtaining Standard-Compliant Sound Level Measurements.

Data Types: single | double

Lt — Octave-band time-weighted SPL measurements

N-by-10 matrix

Octave-band time-weighted (Lt) measurements in decibels (dB), specified as an N-by-10-matrix where N is the number of samples from the SPL meter. Each column must correspond to a measurement at nominal octave band center 16, 31.5, 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz, respectively.

For guidance on taking compliant measurements, see Obtaining Standard-Compliant Sound Level Measurements.

Specify either audioIn and fs or Leq and Lt.

Data Types: single | double

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: roomNoiseCriteria(audioIn,fs,IsSurging=false)

CalibrationFactor — Microphone calibration factor

1 (default) | positive scalar

Microphone calibration factor, specified as a positive scalar. audioIn is multiplied by CalibrationFactor before analysis. To compute the calibration factor specific to your system, use the calibrateMicrophone function.

CalibrationFactor is valid only when you specify the first argument as audioIn.

Data Types: single | double

PressureReference — Reference pressure (Pa)

20e-6 (default) | positive scalar

Reference pressure for dB calculation in pascals, specified as a positive scalar. The default value, 20 micropascals, is the common value for air.

PressureReference is valid only when you specify the first argument as audioIn.

Data Types: single | double

IsSurging — Option to specify or screen audio input for surging

"auto-detect" (default) | true | false

Option to specify or screen audio input for surging, specified as "auto-detect", true, or false. If you set IsSurging to "auto-detect", roomNoiseCriteria uses a nonstandard heuristic to detect whether surging is present. If roomNoiseCriteria detects surging or if you set IsSurging to true, roomNoiseCriteria adjusts SPL measurements in low-frequency octave bands according to the standard.

For more information on surging and low-frequency band adjustments, see Screening for Surging and Corrections to Low-Frequency Octave-Band Measurements.

IsSurging is valid only when you specify the first argument as audioIn.

Data Types: string | logical

IsFluctuating — Option to specify or screen audio input for large random fluctuations

"auto-detect" (default) | true | false

Option to specify or screen audio input for large random fluctuations, specified as "auto-detect", true, or false. If you set IsFluctuating to true, roomNoiseCriteria follows screening criteria from ASA S12.2-2019 to determine if large random fluctuations are present. If roomNoiseCriteria detects large random fluctuations or if you set IsFluctuating to true, roomNoiseCriteria adjusts SPL measurements in low-frequency octave bands according to the standard.

For more information on screening for large random fluctuations and low-frequency band adjustments, see Screening for Large Random Fluctuations and Corrections to Low-Frequency Octave-Band Measurements.

IsFluctuating is valid only when you specify the first argument as audioIn.

Data Types: string | logical

Output Arguments

RNC — Room noise criteria rating

string

RNC level, returned as a string. RNC has the form RNC-CurveID (TC), where CurveID is the RNC Curve tangent to the SPL measurements and TC is the tangency frequency in Hz.

For more information on the RNC tangency method, see RNC Tangency Method.

rattleRisk — Risk of acoustically induced vibrations and rattles

structure

Risk of acoustically induced vibrations and rattles, returned as a structure. rattleRisk contains a summary of octave bands that can contain perceptible rattle and the table of values used by the function to determine the risk.

For more information on rattle risk, see Rattle Risk.

More About

Calibration Factor for Audio Recordings

Calibrating the input before calling roomNoiseCriteria using an audio file is recommended for more accurate results. Record a 1 kHz tone using the reference microphone and note the SPL reading from a compliant SPL meter. Use the calibrateMicrophone function to obtain the calibration factor.

The diagram shows a typical setup for microphone calibration. For room noise evaluation, use Z frequency-weighting (some meters use the label linear or flat).

Obtaining Standard-Compliant Sound Level Measurements

The ANSI/ASA S12.2-2019 [1] standard provides the following guidance for collecting an accurate audio recording and SPL measurements collected in indoor spaces:

• Measure near the average standing or seating height for the expected room occupants.

• The microphone or meter must be at least 0.6 m from any acoustically reflective surface, 1.2 m from the intersection of any two surfaces, and 2.4 m from the intersection of any three surfaces.

• Record Leq measurements using a standard-compliant integrating-averaging meter set to fast time weighting and flat (Z-weighted) frequency weighting. For more information, see splMeter.

• Take measurements at a specific point of interest or using a slow sweep around the entire space. When sweeping around a space instead of from a fixed location, measure for at least 20 seconds.

• For the RNC metric, if surging or large random fluctuations are present, the correction procedure requires at least 15 seconds of measurement data.

RNC Tangency Method

RNC curves [1] are used instead of NC curves in indoor spaces where surging or large random fluctuations occur at low frequencies, usually due to the presence of HVAC systems. To determine the RNC level for a given room, the Leq measurements in low-frequency octave bands are adjusted if surging or large fluctuations are present (see Corrections to Low-Frequency Octave-Band Measurements). Then, an RNC curve is selected for each octave band using the equation

$${\text{RNC-X}}_f=\left(L_f-K_f^1\right)\cdot K_f^2\text{,}$$

where f is the center frequency of the octave band, L_f is the Leq measurement in the octave band, and K_f¹ and K_f² are constants given in Table 5 of the standard.

The RNC level of the room is selected as the highest RNC-X_f, and the tangency frequency is the associated frequency f.

Corrections to Low-Frequency Octave-Band Measurements

When surging or large fluctuations are present in low-frequency octave-band measurements, the RNC method corrects SPL measurements at low frequencies to better match psychoacoustic perceptions of loudness [1].

16, 31.5, and 63 Hz octave bands

The time-weighted SPL measurements Lt for octave bands 16, 31.5, and 63 Hz octave bands are combined into a single 3-band sum as

$$L_{\text{LF}}=10{\log }_{10}\left({10}^{\frac{L_{16}-14}{10}}+{10}^{\frac{L_{31.5}}{10}}+{10}^{\frac{L_{63}+14}{10}}\right),$$

where L_X is the SPL measured at the octave band centered at nominal frequency X Hz. The average power level and mean level in dB of L_LF are calculated as

$$\begin{array}{l}{L}_{\text{LFeq}}=10{\log }_{10}\left(\frac{1}{N}{\displaystyle \sum _{i=1}^N{10}^{L_{\text{LF}i}/10}}\right)\\ L_{\text{LFm}}=\frac{1}{N}{\displaystyle \sum _{i=1}^N{L}_{\text{LF}i}}\text{.}\end{array}$$

L_LFm is used to calculate an offset K_LFα that is added to continuous sound level measurement for the 31.5 Hz octave band. When surging is present, the offset is calculated as

$$K_{\text{LF}\alpha }=10{\log }_{10}\left(\frac{1}{N}{\displaystyle \sum _{i=1}^N{10}^{2\cdot \left(L_{\text{LF}i}-L_{\text{LFm}}\right)/10}}\right)\text{.}$$

When only random fluctuations are present, the offset is calculated as

$$K_{\text{LF}\alpha }=0.46{\sigma }_{\text{LF}}^2\text{,}$$

where σ_LF is the standard deviation of L_LF. The corrected Leq measurement for the 31.5 Hz octave band is then calculated as

$${\tilde{L}}_{31.5}=L_{31.5}+K_{\text{LF}\alpha }-\left(L_{\text{LFeq}}-L_{\text{LFm}}\right)\text{.}$$

When selecting an RNC curve using the tangency method, the corrected 31.5 Hz octave band is used, and the uncorrected 16, 31.5, and 63 Hz bands are disregarded.

125 Hz octave band

The average power level and mean level in dB of the 125 Hz octave band are calculated as

$$\begin{array}{l}{L}_{\text{125eq}}=10{\log }_{10}\left(\frac{1}{N}{\displaystyle \sum _{i=1}^N{10}^{L_{125i}/10}}\right)\\ L_{\text{125m}}=\frac{1}{N}{\displaystyle \sum _{i=1}^N{L}_{\text{125}i}\text{.}}\end{array}$$

L_125m is used to calculate an offset K_LFα that is added to continuous sound level measurement for the 125 Hz octave band. When surging is present, the offset is calculated as

$$K_{\text{125}\alpha }=10{\log }_{10}\left(\frac{1}{N}{\displaystyle \sum _{i=1}^N{10}^{1.25\cdot \left(L_{125i}-L_{\text{125m}}\right)/10}}\right)\text{.}$$

When only random fluctuations are present, the offset is calculated as

$$K_{125\alpha }=0.18{\sigma }_{\text{125}}^2\text{,}$$

where σ₁₂₅ is the standard deviation of L₁₂₅. The corrected Leq measurement for the 125 Hz octave band is then calculated as

$${\tilde{L}}_{125}=L_{125}+K_{125\alpha }-\left(L_{\text{125eq}}-L_{\text{125m}}\right)\text{.}$$

When selecting an RNC curve using the tangency method, the corrected 125 Hz octave band is used, and the uncorrected 125 Hz band is disregarded.

Rattle Risk

According to the ANSI/ASA S12.2-2019 [1] standard, if Leq measurements in the low-frequency octave bands exceed the values in the table, acoustically induced vibrations and rattles are likely.

Octave Band	16 Hz	31.5 Hz	63 Hz
Clearly perceptible vibration and rattles likely	75 dB	75 dB	80 dB
Moderately perceptible vibration and rattles likely	65 dB	65 dB	70 dB

Screening for Surging

ANSI/ASA S12.2-2019 [1] recommends aural or visual inspection of SPL measurements to determine if surging is present. Surging is primarily observed as periodic fluctuations in the 16, 31.5, and 63 Hz octave bands. When screening for surging, set the SPL meter to fast time-weighting and flat (Z-weighted) frequency weighting, with measurements reported every 100 ms.

Because ANSI/ASA S12.2-2019 does not define a method to determine if surging is present, the roomNoiseCriteria function implements a heuristic method. SPL measurements in the 16, 31.5, and 63 Hz octave bands are collected using the SPL meter settings. Autocorrelation is calculated on the measurements for each band. If a significant peak is found in the lag domain of any band corresponding to a modulation frequency less than or equal to 4 Hz, surging is detected. For more information, see xcorr.

The standard recommends using the room noise criteria (RNC) instead of NC if surging is present. By default, roomNoiseCriteria screens audio inputs and adjusts low-frequency octave band readings if surging is detected. You can disable screening and manually specify the presence of surging by setting IsSurging to true or false.

Screening for Large Random Fluctuations

Large random fluctuations are detected using the method defined in the ANSI/ASA S12.2-2019 [1] standard. The Leq measurements of the lowest three octave bands are combined as

$$L_c=10{\log }_{10}\left({10}^{\frac{L_{16}}{10}}+{10}^{\frac{L_{31.5}}{10}}+{10}^{\frac{L_{63}}{10}}\right)\text{,}$$

where L_X is the SPL measured at the octave band centered at nominal frequency X Hz.

Fluctuations are detected using the Leq, the maximum observed level (LMAX), and the 90th percentile level (L10) from L_c and L₁₂₅ . If the differences exceed the values in the following table, large random fluctuations are present.

Octave band	Lc	L125
LMAX minus Leq	≥ 7 dB	≥ 6 dB
L10 minus Leq	≥ 3.5 dB	≥ 3 dB

The standard recommends using the room noise criteria (RNC) instead of NC if large random fluctuations are present. By default, roomNoiseCriteria screens audio inputs and adjusts low-frequency octave band readings if large random fluctuations are detected. You can disable screening and manually specify the presence of fluctuations by setting IsFluctuating to true or false.

References

[1] Acoustical Society of America. Criteria for Evaluating Room Noise. ANSI/ASA S12.2-2019 (R2023). Acoustical Society of America, 2019.

Version History

Introduced in R2026b