Digital sound level meter design

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Dr. Sujbert László
Department of Measurement and Information Systems

This Bachelor Thesis presents the details of the design of a cheap, simple, easily accessible sound level meter, which is appropriate for exploratory measurements.

In the work’s first phase I made market- and literature research, so the specification of the device could be made. It was taken into account that the system will be built around an Atmega128, which calculation capacity is too low for the complex functions.

I made a system design after the specification. It should be emphasized, that all of the system’s components are able to operate from a common voltage source, furthermore both the amplifier and AD converter are programmable. The programmable amplifier allows to change between ranges, while by the programmability of the AD converter various digital filters can be implemented.

I chose the circuits in the design’s next step, which realize the functions of the blocks in the system design. After that, during the wiring diagram’s preparation I used the recommendations of the datasheets and some general design considerations.

When the wiring diagram was ready, I started to design the two sided printed circuit. I took care of the proper arrangement of decoupling capacitors, analog and digital circuits, power lines and ground planes.

The software development started after all components were implanted. If it was possible, I checked the appropriate operation of the circuits meanwhile the implementation. That way soldering and design errors came to light, which could be fixed. It was realized during the software design, that the controller can support only lower sampling frequencies than 48 kHz, but it can be shown, that in the case of noise power measurement it is not a problem. As the result of the process, a device was born, which hardware and software worked properly so the calibration and ranging was the next step.

The developed sound level meter is suitable for linear or A frequency weighted and F or S time weighted sound level measurements in the 40 dB-115 dB range. Measurements were made at a maximum level of 90 dB with really good results: Sound pressure level of a 5 kHz bandwidth noise could be measured with a 0.1 dB accuracy.

To sum up, it could be said, that the initial goal was reached, because the result is such a device, which is really suitable for exploratory measurements.


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