deciBel uses your phone mic to measure sound levels in decibels. The results are displayed in numeric and graphical formats.

A Fast Fourier Transform algorithm is used to implement high precision weighting filters and sophisticated digital signal processing techniques to achieve the best possible SPL readings out of your Android phone or tablet.

Please calibrate before the first use (see below)

The Main Display

The Main Display is the upper part of the deciBel screen:

1. The Sound Pressure Level in dB. It updates every second and shows the one second mean sound pressure level.
2. The Weighted dB Unit. Is fixed on dBA in deciBel but may be dBA, dBB, dBC or dBZ in deciBel Pro. To change the weighting filter, touch and hold the main display and then select the new filter.
3. The Minimum dB Value since start or since the last reset. To reset, touch and hold the main display and then select Reset Max and Min.
4. The Dominant Frequency. This is the FFT output frequency component that has the highest amplitude. It is always a multiple of 43.066Hz. If you are measuring a sinusoidal tone, it will be the multiple that is closest to the tone frequency. (the dB value is computed from all the FFT components)
5. The Maximum dB Value since start or since the last reset. See the Minimum dB Value above.

The Panel

The Panel shows one of three chart types or the calibration screen.

To change the chart type, touch and hold the chart and then select a new type. To reset all the charts, touch and hold the Main Display and then select Reset All.

The available charts are:

Fast Chart

The Fast Chart is a dB x Time graph that plots one dB value per second. It is 5 minutes wide and slides to the right so that the last 5 minutes are visible at any time.

Slow Chart

The Slow Chart computes the average sound pressure in each 15 seconds and plots the result along with the maximum and minimum values in this period. It slides to the right and its width may be 15, 30, 60 or 120 minutes. To change the chart width, touch and hold the chart and then select Resize Chart.

Exposition Chart

The Exposition Chart is a sound level histogram. It shows how much time the device was exposed to each level range since start or since the last reset, in 5dB wide columns. The total elapsed time is the sum of the times of all the columns and the time axis grows as needed, so that there is no time limit for the use of the chart. It resembles the functionality of a simple sound level dosimeter.

Screen Capture

To take a screenshot of the displayed chart, touch and hold the chart and then select Save Screenshot.

The chart image goes to the deciBel directory on the SD card and its name has the format yyyy.mm.dd_hh.mm.ss_x.png - the date, the hour and a suffix letter indicating the chart type.

Screen Rotation

Select Menu/Rotate Display to rotate the deciBel screen 180 degrees. This may be very handy if you want to point the mic, which is located at the bottom of the phone, to the sound source.

Note that the screen rotation is complete on devices running Android 2.3 or higher and partial otherwise.

App Termination

It is possible to switch to another app (Home button, then start the other app) and then return to the deciBel (Back button) without loosing SPL data, because the recordings will proceed in the background.

To terminate the deciBel after use, to save battery, hit the Back button instead of the Home button, or the Menu > Exit function.

Calibration

Select Menu/Calibrate to switch to the Calibration Screen.

To be accurate, the deciBel must be calibrated. This is necessary because there are slight sensitivity differences from one microphone to the other, even if they are exactly of the same type. And this sensitivity can change over time, too.

Some possible calibration procedures:

Using a calibrated professional SPL meter

• Simply place your deciBel side by side with the calibrated SPL meter and in the Calibrate Screen (Menu/Calibrate) click on the calibration buttons (+1dB, +0.2dB, -0.2dB or -1dB) until the deciBel readings match the SPL meter readings.
• Select Menu/Save to save the new calibrated reference value. If you select Menu/Cancel instead, the new reference value will be discarded and the old calibration restored.

Note that both the deciBel and the calibrated SPL meter must be set to use A-Weighting and that the calibration environment should be reasonably quiet, without too much transient noise.

Using a calibrated professional SPL meter and a tone generator

This is the best way to calibrate your deciBel, which gives you the best possible accuracy.

In addition to the SPL meter, you need a sinusoidal 1KHz tone generator. If you have a SPL meter, chances are that you also have a SPL calibrator, which is exactly what you need. But if not, any sinusoidal 1Khz tone source may be used: An audio generator from the electronics lab, a computer with a tone generator program (there are free programs both for Windows and Linux) or even another Android phone with a tone generator program installed (there are many of them on the Android Market).

• In a quiet place, where the background noise remains mostly below 40dBA, place the phone, the SPL meter and the tone generator on a flat surface (the floor or a free table) as far as possible from the room walls. The phone and the SPL meter microphones must be as close as possible and both must point to the tone generator.
• Switch the tone generator on and adjust its distance to the phone/SPL meter until you get 50dBA on the SPL meter. The distance between the tone generator and the deciBel/SPL meter should not be less than 50cm (20 inches). If it is, increase the tone generator volume level and repeat the procedure.
• Make sure that both the deciBel and the SPL meter are set to use A-Weighting. (see Main Display above)
• On the deciBel, switch to the Calibrate Screen (Menu/Calibrate) and click on the calibration buttons (+1dB, +0.2dB, -0.2dB or -1dB) until the deciBel readings match the SPL meter readings, in this case, 50dBA.
• Select Menu/Save to save the new calibrated reference value or Menu/Cancel to discard it.

Auto Calibrate (deciBel Pro only)

If you do not have access to a professional SPL meter you still can achieve a reasonable accuracy, within a few dBs of error.

Day Mode

• During day time, in a very quiet room in a residential area (out of the center of the big city) switch off all continuous noise sources such as noisy freezers, air conditioners, aquarium pumps, TV sets, noisy computers, etc. Short duration noises such as a phone call, a passing car, someone walking in the room or a dog barking then and when does not interfere with the calibration process, but all continuous noises must be avoided.

• Switch to the Calibrate Screen (Menu/Calibrate) and then click on the Day (35dB) button, under Auto Calibrate.
• One hour or more later, the deciBel will automatically switch back to normal mode, and this is the sign that the auto calibration process has finished.

Night Mode

Similar to the day mode, but to be done in an extremely quiet environment. To use it, start the Night (25dB) auto calibrate function and place the deciBel phone inside a closet drawer, switch off all continuous noise sources and close all the room doors and windows. Do it late in the night, if possible after 01AM, when most of the city activities have died out.

How?

The deciBel's Auto Calibrate function takes advantage of the fact that the background noise in environments perceived as quiet do not vary very much, and tend to be around 35dB by day and 25dB late in the night.

What it does is filter out any non-continuous noises and record the lowest background noise level in one hour or more, (depending on how stable the noise level is). It then calculates the reference value that makes this noise level evaluate to 35dB (or 25dB) and saves it as the new calibrated reference value.

Accuracy Considerations

The deciBel accuracy is limited by the quality of the phone mic. Fortunately, most (if not all) phones use electret microphones, which tend to have a surprisingly good frequency linearity, at least for the frequency range below 10KHz. Some linearity fluctuations are generally present above this frequency, but fortunately again, they tend to have little impact on the result if you are using the A weighting filter, which strongly attenuates the frequency components above 10KHz.

On the other hand, the amplitude response curve of the electret mic tend to flatten out on the higher sound pressure levels and this is why the deciBel reliability is limited to something between 10-90dB and 10-100dB, depending on the phone model. Above these values the readings are possibly smaller than the actual sound pressure level.

Some phone models may have bandwidth or amplitude limitations. As an example, the HTC Nexus One has an input bandwidth of only 8KHz. The effect is that it deforms the weighting curves and discards all the sound components which are above this frequency.