An inexpensive sensor for noise

Abstract

Noise is a pervasive workplace hazard that varies spatially and temporally. The cost of direct-reading instruments for noise hampers their use in a network. The objectives for this work were to: (1) develop an inexpensive noise sensor (<$100) that measures A-weighted sound pressure levels within ±2 dBA of a Type 2 sound level meter (SLM; ∼$1,800); and (2) evaluate 50 noise sensors for use in an inexpensive sensor network. The inexpensive noise sensor consists of an electret condenser microphone, an amplifier circuit, and a microcontroller with a small form factor (28 mm by 47 mm by 9 mm) than can be operated as a stand-alone unit. Laboratory tests were conducted to evaluate 50 of the new sensors at 5 sound levels: (1) ambient sound in a quiet office; (2) 3 pink noise test signals from 65-85 dBA in 10 dBA increments; and (3) 94 dBA using a SLM calibrator. Ninety-four percent of the noise sensors (n = 46) were within ±2 dBA of the SLM for sound levels from 65-94 dBA. As sound level increased, bias decreased, ranging from 18.3% in the quiet office to 0.48% at 94 dBA. Overall bias of the sensors was 0.83% across the 75 dBA to 94 dBA range. These sensors are available for a variety of uses and can be customized for many applications, including incorporation into a stationary sensor network for continuous monitoring of noise in manufacturing environments.

Keywords: Hazard; network sensor; noise; sound; sound pressure level.

Figure 1

The inexpensive noise sensor. A simplified circuit diagram (top panel) shows that the electrical signal from the microphone is amplified twice before reaching the sensor microcontroller (Teensy). The actual components of noise sensor incorporated into multi-hazard monitor are shown in the bottom panel. An electret microphone extends from the exterior of the grey enclosure and the sensor microcontroller is shown inside the enclosure. On the red circuit board, a monitor microcontroller communicates with the noise and other hazard sensors and with a database via WiFi.

Figure 2

Setup for laboratory validation of noise sensor. NTI XL2 and noise sensor microphones were located within 30 cm of one another, centered 2.5 cm from the center of the amplifier. The amplifier was connected to the laptop by auxiliary cable.

Figure 3

Mean A-weighted sound levels from noise sensor versus reference SLM at 5 target sound intensities. Points represent mean reading of all noise sensor and reference measurement pairs. Error bars represent one standard deviation of levels, n = 46 sensors.

Figure 4

Box plot of differences in noise sensor and reference SLM at 5 target sound levels. Error bars represent the distribution of A-weighted sound level differences, n = 46 sensors.