Copyright © Ifsttar & CNRS - All rights reserved
Contact - noisecapture[at]noise-planet.org
Web - http://noise-planet.org
Introduction
NoiseCapture App is an application dedicated
to the evaluation of your noise
environment. It allows to perform measurements and, thus, to give
information on your noise exposure. In addition, you can contribute to the
realization of collaborative (or community) noise maps by anonymously
sharing your measurements.
Main steps:
- Start a
measurement
- Give a
description
- Analyze
results
- Explore the map
Additional steps:
Go further:
NoiseCapture App is an application, made by experts,
but do not forget that it will never replace a professional sonometer. If
you need a professional assistance, please contact a specialist.
The quality of the measurement depend of the technical
performance of the smartphone. Depending of the smartphone and the android
version, some smartphone may produce bad estimation of the sound level.
See section concerning the limits of noise
measurement with a smartphone for more information.
Start a measurement
On the Measurement menu item, press the RECORD
button to start a measurement. You can stop the measurement by
pressing the STOP button.
You can pause/resume a measurement using the PAUSE
button. Depending on the corresponding Settings
value, when resuming the measurement, the last seconds of the measurement
can be removed.
What is displaying?
- a horizontal level-meter and a numeric value showing the instantaneous
sound level ('fast' level on 125ms) in dB(A) (A-weighted
decibel). The color depend on the level range (each level range of 5dB
is associated to a given color);
- statistical sound levels in
dB(A): minimum (Min), maximum (Max) and the mean (Mean) values of the
fast sound level;
- a chronometer that gives the
measurement duration in second;
- an information is given concerning the GPS
localization accuracy;
- a noise level spectrum,
showing the instantaneous sound level for each third-octave band between
100Hz and 16kHz. If you tap on the spectrum, it displays a time-frequency
representation of noise.
Give a description
On the Description menu item:
- add a 'Description' , if needed;
- add a 'Picture ', if needed;
- evaluate the Pleasantness of the sound environment, by
moving the cursor from 'Unpleasant' to 'Pleasant';
- select one on more 'Tags' that correspond to your perception of the
sound environment;
- valid the description by pressing the VALID
button.
This step is optional but it could be useful to evaluate
the noise environments, as well as for a research purpose. Your
contribution will be highly appreciated. Only the anonymous elements
'Pleasantness' and 'Tags' are uploads. The 'Description', as well as, the
'Picture', are stored on your smartphone only and, thus, not uploaded.
Analyze results
On the Results menu item, you
can access to several noise indicators.
While during measurements sound level are displayed in a 'fast' mode
(125ms), final results are given in a 'slow' mode (1s), based on the 1s
(one second) equivalent sound level (Leq,1s). Leq,1s is a standard
environmental noise indicator, used for the evaluation of noise annoyance.
What is displaying?
- statistical sound levels in
dB(A): minimum (Min) and maximum (Max);
- percentile noise levels in
dB(A) over the whole measurement duration:
- LA90: A-weighted noise level that is exceeded for 90% of the
measurement period. LA90 is a useful descriptor to quantify the background noise level;
- LA50: A-weighted noise level that is exceeded for 50% of the
measurement period. LA50 is the median
sound level;
- LA10: A-weighted noise level that is exceeded for 10% of the
measurement period. LA10 is a useful descriptor of road traffic
noise, which is correlated with the noise
annoyance;
- equivalent noise level in
dB(A), on the whole measurement duration [at the center of colored
circle];
- repartition of the noise exposure
(RNE): this indicator represents the distribution over the time of your
noise exposure. Each of the five colors correspond to a given noise
level range.
- a noise level spectrum,
showing the sound level for each third-octave band between 100Hz and
16kHz.
Explore the map
On the Map menu item, users can
access to a geo-referenced representation of the measurement that are
stored in the smartphone memory. Users can choose to represent:
- 'Selected record', which corresponds to the last measurement or to a
loaded previous measurement (see History
for information);
- 'All records' stored in the smartphone memory;
- 'Community noise map': this noise map that is built using all data
produced by all participating citizen. This noise map is stored on the
web server and need an internet access to be displayed.
Explore the measurement history
On the History menu item, users
can access to all measurements that are stored in the smartphone memory.
All measurement that are performed with the smartphone are archived in
the history. You can access to the state of the measurement ('Upload' or
'Not upload' to the server) with the 'Cloud' symbol,
By tapping on item from the history, you can:
- 'Upload to the community': if the corresponding measurement has not be
automatically uploaded to the server, you can process to a manual
upload;
- 'Export result': using a given application (message, drive...), you
can export/save a ZIP file that contains all the metadata of the
measurement. See the README document in the ZIP file for the description
of the metadata. This is the same ZIP file that is upload to the web
server;
- 'Edit description': you can modify the description of the
corresponding measurement (same as the Description
menu item). If the item has been already uploaded to the server, you can
only modify the text description and the picture;
- 'Show result': you can access to the results of corresponding
measurement (same as the Results
menu item);
- 'Show on map': you can access to the map for the spatial
representation of corresponding measurement (same as the Map
menu item);
- 'Delete': select this option if you want to remove the measurement
from your smartphone.
If a measurement is removed from the history, but was
previously transferred to the web server, it is not removed from the web
server. Remember that all data that are uploaded to the server are
anonymized; thus, be confident! (see Privacy)
Calibrate your smartphone
On the Calibration menu item,
users have the opportunity to calibrate their smartphone.
- The methodology is based on the knowledge of a Reference
value for the equivalent noise level Leq (in dB, not in dB(A))
for a given duration, from a reference device (an acoustic calibrator, a
calibrated smartphone, a sound level meter).
- Depending on the calibration method, the calibration can be performed
for a given frequency band or for the full range (global value).
- This Reference value is then
compared to the Measured Leq
(in dB) using the smartphone. The difference between these two values
gives a correction factor (i.e.
the signal gain) that will be applied to the input signal.
Several calibration methods are available:
- Manual calibration from a reference
device or a calibrator
- Manual calibration from a calibrator
device
- Automatic calibration from a calibrated
smartphone
The smartphone calibration is a crucial and necessary
step. Without calibration, you can be sure that the measured data will be
wrong!
The correction factor is applied for the whole signal.
Multi-frequency calibration is not possible for now.
The calibration is carried out with dB values, not
dB(A).
The correction factor can also be set manually in the
Settings menu.
When using a 'Global' value for the calibration, make sure
that the Reference value will be
obtained using the same frequency range that your smartphone.
The 'Calibration warm-up time' and the 'Calibration
duration' can be modified in the Settings.
During calibration, the smartphone and the reference device
must be used in a similar way.
Calibration from a reference device
- Performing
the calibration:
- In the 'Calibration mode' list, choose the frequency on which you
want perform the calibration. You can also choose 'Global' from
the list to calibrate over the entire measurement spectrum. In the
latter case, remember to configure the reference device to perform a
measurement in the same frequency band as your smartphone.
- Press the START button.
- Before calibration starts, you have a warm-up time that can
help you to prepare your calibration (prepare to turn on the
reference device, if necessary).
- Then, the calibration measurement starts for a given duration.
For a better accuracy, the measurement duration should be the
same both for the smartphone and for the reference device.
- Once the measurement is done, enter the Reference
value (from the reference device) in the edit box and press
the APPLY button.
- If you do not want to apply the correction factor, you can press
the RESET button to perform
a new calibration or you can leave the Calibration activity.
- Testing
the calibration:
- Once a correction factor is applied, you can test the calibration
by checking the box 'Test gain'.
- Press the START button
for performing a new measurement and turn on (if necessary) the
reference device.
- At the end of the measurement, the Measured
Leq should be equal or quite similar to the reference
device.
- Given the procedure and equipment involved, you may consider
acceptable a difference up to 0.5dB between the two values. If the
difference is not accetable, restart the Calibration procédure.
After several attempts, if you can not calibrate your smartphone
phone with a difference of less than 0.5dB, you can restart with an
error of 1dB.
Calibration with a calibrator
To apply this calibration procedure, you need an acoustic calibrator and an external microphone. The diameter
of the microphone should be compatible with the calibrator.
- Performing
the calibration:
- In the 'Calibration mode' list, choose the frequency on which you
want perform the calibration.
- Press the START button.
- Before calibration starts, you have a warm-up time that can
help you to prepare your calibration.
- Then, the calibration measurement starts for a given duration.
For a better accuracy, the measurement duration should be the
same both for the smartphone and for the reference device.
- Once the measurement is done, enter the Reference
value (from the reference device) in the edit box and press
the APPLY button.
- If you do not want to apply the correction factor, you can press
the RESET button to perform
a new calibration or you can leave the Calibration activity.
- Testing
the calibration:
- Once a correction factor is applied, you can test the calibration
by checking the box 'Test gain'.
- Press the START button
for performing a new measurement and turn on (if necessary) the
calibrator.
- At the end of the measurement, the Measured
Leq should be equal or quite similar to the reference
device.
- Given the procedure and equipment involved, you may consider
acceptable a difference up to 0.5dB between the two values. If the
difference is not accetable, restart the Calibration procédure.
After several attempts, if you can not calibrate your smartphone
phone with a difference of less than 0.5dB, you can restart with an
error of 1dB.
Automatic calibration from a calibrated
smartphone
In this method, a smartphone already calibrated with NoiseCapture, can calibrate one or more smartphones. On the side of the microphones to
be calibrated, the procedure is fully automatic. The entire system is
controlled by the reference smartphone.
- Performing
the calibration:
- The reference smartphone must be in "transmitter" mode.
- The smartphones to be calibrated must be in "receiver" mode.
- On the reference smartphone:
- Select the source signal:
- "Pink noise": in this case, the smartphone generates a reference
signal. It is necessary to use a sound source connected wirelessly
with the reference smartphone, the source being at equal distance
from all smartphones;
- "Ambient noise": Environmental noise is used for calibration. The
sound level must be sufficiently high and the sound field must be
relatively "diffuse".
- Press Start; the rest of the procedure is automatic. You can
cancel the current calibration with the Cancel button.
What are the limits of noise
measurement with a smartphone
Device requirements for Android
compatibility
The Android
Compatibility Definition Document (CDD) "enumerates the
requirements that must be met in order for devices to be compatible with
the latest version of Android. To be considered compatible with Android,
device implementations MUST meet the requirements presented in this
Compatibility Definition" ( text
from Android 5.1 CDD).
According to the CDD:
- The device SHOULD exhibit approximately flat amplitude versus
frequency characteristics: specifically, ±3dB, from 100Hz to 4000Hz.
- Audio input sensitivity SHOULD be set such that a 90dB sound power
level (SPL) source at 1000Hz yields RMS of 2500 for 16-bit samples.
- PCM amplitude levels SHOULD linearly track input SPL changes over at
least a 30dB range from -18dB to +12dB re 90 dB SPL at the microphone.
- Total harmonic distortion SHOULD be less than 1% for 1kHz at 90dB SPL
input level at the microphone.
In addition, it is specified that " While some of the requirements
outlined in this section are stated as SHOULD since Android 4.3, the
Compatibility Definition for a future version is planned to change these
to MUST. Existing and new Android devices are very strongly encouraged
to meet these requirements, or they will not be able to attain Android
compatibility when upgraded to the future version."
What does it theoretically means?
Thus, the quality of the noise measurement:
- depends both of the device and of the Android version.
- should be quite verified between 100Hz and 4000Hz (frequency
response).
- should be quite linear (amplitude response) in a range from -18dB to
+12dB relatively to 90dB (Sound Pressure Level).
In conclusion, in the current situation,
even after a rigorous calibration and without any further correction, it
is not possible to ensure the quality of the measurement for low and
high noise levels, as well as, beyond 4000Hz, whatever the device and
the Android version. One can however assume that next smartphone
generations should ensure a better measurement quality.
Best practice for the best noise
measurement
In order to perform a correct noise
measurement:
- The smartphone must be calibrated
(see Calibration);
- The smartphone must be outside the
clothing pockets;
- The smartphone has to be in the hand;
- The smartphone microphone must not
to be masked;
- If possible, uses an external
omnidirectional microphone.
In addition, if you want to produce noise
measurements that will be very useful for producing collaborative
(outdoor) noise maps, please check these points:
- The measurement must be performed
outside buildings;
- The measurement must be done when the GPS
signal is correct;
- 'Tags' must be specified
when the measurement is complete (see Description).
Resources
- Acoustic
Glossary: a very useful glossary in Acoustics and Vibration,
maintained by Gracey & Associates;
- Acoustics:
the definition of 'Acoustics' in Wikipedia;
- Environmental
noise: many official information about 'Noise' from the European
Environment Agency.