Most of the time, sounds up to 30 dB are pleasant. Above 80 dB, it starts to bother me. And if it gets louder than 100 dB, it could affect your psychomotor performance, making it harder to pay attention and making you feel stressed. Noise pollution can also make it hard for you to hear. Around 47 dB is the level of noise in a home. But hi-fi music systems and TVs turned up loudly add to this noise, which is bad for your health. Here's a simple circuit for a noise meter that can measure and show how loud the noise is in your room. It also sounds an alarm when the noise level goes above 30dB. A sound intensity sensor and a display unit are part of the circuit. The regulator circuit, which is built around regulator IC 7809 (IC1), gives the circuit a stable 9V power supply. The sound intensity sensor is made up of an op-amp IC CA3130 (IC2), a condenser microphone, and other parts. Op- amp IC2 is set up as an inverting amplifier with a high gain. Resistors R3 and R4 split in half the voltage that goes to IC2's non-inverting pin 3, which is also used as the reference voltage. The sensitivity of the condenser microphone is set by resistor R1. Circuit operation

The microphone picks up sound vibrations and turns them into electric pulses, which are then sent to the inverting input of IC2 (pin 2) through capacitor C4 and resistor R2. Capacitor C4 stops any DC from getting into the op-amp, since DC could mess with how the op-amp works. For negative feedback, the output of IC2 is connected to the input of the inverter through resistor R5, which has a value of 10 megaohms. Since IC2 has a very high input impedance, even a small current can turn on the op-amp. Through capacitor C5, the output of IC2 is sent to preset VR1, which is used to control the volume. Capacitor C5 stops DC from going through preset VR1, so only AC can go through. AC signals from VR1's wiper are sent to a diode pump, which is made up of diodes D1 and D2. The diode pump turns the AC around and keeps it at the level of IC2's output. Capacitor C6 acts as a storage area for DC, and resistor R6 is the way for it to flow out. The display circuit is built around the monolithic IC LM3914 (IC3), which reads the analog voltage and drives ten LEDs to make a logarithmic analog display. The current going through the LEDs is controlled by the resistors inside IC3, so there is no need for resistors on the outside. IC3 has a built-in low-bias input buffer that accepts signals all the way down to ground potential and drives ten separate comparators. As the voltage going into IC3 goes up, the outputs go low in a decreasing order from 18 to 10. Each LED connected to the output of IC3 represents 3 dB of sound level. When all ten LEDs light up, this means that the sound is 30 dB loud. To get the dot-mode display, pin 9 of IC3 is hooked up to 9V. In the dot-mode display, some of the segments overlap a little bit. This makes sure that all LEDs are always "on." When output pin 10 of IC3 goes low, pnp transistor

T1 gets base bias (normally cut off by resistor R7), which makes the piezobuzzer (PZ1) connected to its collector sound. Building and testing Any kind of PCB can be used to build the noise meter circuit. To make a condenser microphone more sensitive, it should be connected with a shield wire and put in a tube. For audiovisual indications, use a small DC piezobuzzer and transparent LEDs. Adjust preset VR1 so that the first LED (LED1) is the only one that lights up. Keep the circuit close to the stereo or TV so you can check the sound level.