# Sound Pressure Sensor

Working with a [Sound Pressure Level](https://www.tinkerforge.com/en/doc/Hardware/Bricklets/Sound_Pressure_Level.html) sensor works as described in the general article on [how to use sensors](https://iot.datalit.de/2-internet-of-things/iot-in-our-apps/program-the-devices/sensors). In this article, we assume you have successfully initialized the devices with the [Tinkerforge Device Manager](https://iot.datalit.de/2-internet-of-things/iot-in-our-apps/connect-to-devices/the-tinkerforge-device-manager), and you stored all connected devices on a global variable `devices`. We also declared a global variable called `sound`. 

Next, we need to know the device identifier, which **290**:

```javascript
// Get the sound pressure level sensor via its device identifer
sound = devices.getDeviceByIdentifier(290);
```

Once we have a reference to the sensor, we can register a callback function:

```javascript
// We want to be informed when a new sensor value arrives
sound.registerListener(soundChanged);
```

Of course, we have to actually define the function:

```javascript
function soundChanged(val) {
   // Do something with the value object
}
```

## Reading the sound pressure level

Instead of just one single value, the sensor delivers four different values:

1. Sound level in decibel
2. Spectrum length
3. Spectrum chunk offset
4. Spectrum chunk data

We can access the array of value via the `val` object:

```javascript
function soundChanged(val) {

    // Get the decibel value
    var decibelValue = val.getValue("decibel_value");
    
    // Check if this actually was decibel event, if not we get -1 as result
    if(decibelValue !== -1 && decibelValue.type === "decibel_value") {
        log("Decibel: " + decibelValue.value)
    }
}
```


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