Sound Level Meter Project with Digital SPL Readout and Data Logging

Noise exposure rules — factory floors, traffic junctions, concert venues, generator rooms — are written in decibels, yet real sound level meters cost enough to put them out of reach of most student experiments. An electret microphone capsule, a MAX4466 preamplifier and an Arduino Nano can do the job at a fraction of the cost, provided the signal chain is honest about what it is: the capsule converts pressure to voltage, the MAX4466 amplifies it with an adjustable 25x–125x gain, the Nano samples it at about 9.6 kHz, and the firmware applies an A-weighting digital filter, computes RMS over a 125 ms Fast window, and converts to decibels against a calibration offset. The result appears live on a 16x2 LCD as A-weighted SPL with 0.1 dB resolution, alongside peak-hold and min/max values — and every sample lands in a CSV file on a microSD card with an elapsed-time stamp. Because a student build cannot claim laboratory accuracy, the project ships with a single-point calibration procedure: the buyer sets the offset against a reference meter or a calibrated phone app at one known level (94 dB at 1 kHz is the standard), and the report documents that procedure rather than inventing measured results.

1. The electret microphone capsule on top of the enclosure picks up sound pressure and converts it to a small AC voltage (datasheet typical sensitivity −44 dBV/Pa).
2. The MAX4466 preamplifier module boosts this signal with a trimpot-adjustable gain of 25x–125x to fit the Arduino's 0–5 V ADC range.
3. The Arduino Nano samples the amplified waveform on a 10-bit analog input at approximately 9.6 kHz, fast enough to capture the audio band of interest.
4. Firmware applies an A-weighting digital IIR filter to the sample stream, matching how human hearing perceives loudness across frequencies.
5. RMS level is computed over a 125 ms Fast window; dB SPL is calculated as 20·log10(RMS) plus the buyer-set calibration offset, and shown live on the 16x2 LCD.
6. The HOLD/LOG push-button freezes the current reading on the display and records the session peak; peak, minimum and maximum are tracked continuously.
7. Once per second, the firmware writes a CSV row (elapsed seconds, SPL, peak) to the microSD card via SPI for later plotting and report analysis.
8. The whole unit runs from a 9 V battery, and the calibration procedure in the guide aligns the meter's offset to a reference source before readings are taken.

Tech stack:

• Arduino Nano (ATmega328P, 10-bit ADC, 16 MHz)
• MAX4466 electret microphone preamplifier module (25x–125x adjustable gain)
• Electret microphone capsule (datasheet typical −44 dBV/Pa)
• 16x2 character LCD with I2C backpack
• microSD card module (SPI) with CSV logging
• A-weighting digital IIR filter in firmware
• 9 V battery, snap connector, 3D-printed handheld enclosure
• Arduino IDE (C/C++ firmware)

• [Live A-weighted SPL readout] The 16x2 LCD shows the current A-weighted sound pressure level in dBA with 0.1 dB display resolution, computed from an RMS window with a firmware A-weighting IIR filter — the weighting professional meters use.
• [Peak-hold and min/max capture] A HOLD/LOG button freezes the display reading and separately records the peak, minimum and maximum SPL of the session, so transient events like a machine startup can be captured and noted.
• [microSD CSV data logging] Every sample is written to a microSD card as a CSV row (elapsed time, SPL, session peak) at 1 sample/s, giving the student real data to plot and analyze in the report.
• [Buyer-run single-point calibration] Instead of claimed accuracy, the project documents the calibration procedure: the buyer plays a known reference tone or compares against a reference meter or calibrated app, and sets the offset so the meter reads correctly at the calibration point.
• [Handheld battery-powered build] A 9 V battery, an enclosed handheld case and a top-mounted electret capsule make the meter genuinely portable for field readings around a workshop or traffic junction.
• [Adjustable preamp gain] The MAX4466 module's onboard trimpot lets the gain be set between 25x and 125x, so the same build covers quiet rooms and loud machinery.
• [DS3231 RTC timestamping — optional] A real-time-clock module can be fitted so the CSV log carries wall-clock timestamps instead of elapsed-time stamps.
• [Threshold alarm with relay output — future scope] A future extension can add a buzzer or relay that fires when SPL crosses a set limit, turning the meter into a simple noise-alarm unit.

• Assembled handheld sound level meter (mic capsule, MAX4466 module, Arduino Nano, LCD, SD module, 9 V battery, enclosure)
• Complete Arduino firmware (sampling, A-weighting filter, SPL computation, peak/min/max, CSV logging)
• Wiring diagram and module connection guide
• Buyer-run calibration procedure document
• Project report PDF (theory, signal chain, calibration, sample logged data with plots)
• Abstract PDF (10-section)
• PPT presentation for final review
• Viva Q&A preparation document (dB math, A-weighting, ADC resolution, calibration concepts)