This is a circuit of Audio Decibel Level Meter. This circuit uses the SA604 which not only designed as an RF device but also the Received Signal Strength Indicator (RSSI). In a cellular radio, the radio’s microcomputer uses that function to monitor the received signal strength continuously. A logarithmic response proportional to the input signal level is given by this circuit. The logarithmic response that can be provided by The SA604 is over an 80dB range up to a 15MHz operating frequency. Here is the circuit :

Audio Decibel Level Meter circuit schematic

To build a sensitive audio level indicator circuit, we can uses two integrated circuits: the SA532 and SA604. This circuit requires less than 5mA with a single 6V power supply. So it is ideal for portable equipment that use battery as power supply. This circuit has some advantages such as 10.5mV sensitivity, low-power consumption belie the 80dB dynamic range, and has small size.

There are some equipments that can use this circuit:
1. Microphone tester
2. VU meters
3. Audio dynamic range testers
4. Audio spectrum analyzer
5. Portable acoustic analyzer
6. Audio analyzers (THD, noise, separation, response, etc.)
7. S-meter for direct conversion radio receiver

[Source: NXP Application Note]