Add Zabbix Sound Level blog post.

content/System Administration/zabbix-sound-level.md

+Title: Measuring sound level with Zabbix
+Description: Test consumer electronics with enterprise tools!
+Date: March 22, 2019
+Cover: /media/zabbix/graph-real.png
+Featured_Project: true
+
+Here's a way to observe sound level from a microphone using an IT monitoring system.
+
+# Why?
+
+- I wanted to measure how long an old MP3 player could play until the battery died
+- ...without having to listen to the music
+- ...and I wanted to find out as soon as it stopped playing.
+
+![Photo](/media/zabbix/photo.jpg)
+
+# Zabbix
+
+Zabbix is an open-source monitoring tool. I already use it to keep an eye on my servers. Maybe it can help here?
+
+# Measuring volume
+
+We can use the [`rec` tool](https://linux.die.net/man/1/rec) (part of the [SoX](http://sox.sourceforge.net/) audio processor) to grab audio from the microphone and analyze it.
+
+```bash
+rec -n stat trim 0 .5
+```
+- `-n`: record to `null` instead of a file
+- `stat`: calculate statistics
+- `trim 0 .5`: record `.5` seconds from the start (`0`)
+
+We can use the `Maximum amplitude` statistic to infer whether the music is playing.
+
+```bash
+rec -n stat trim 0 .5 2>&1 | awk '/^Maximum amplitude/ { print $3 }'
+```
+
+All that's left is to track this value over time, and alert us when it falls below some threshold.
+
+# zabbix_sender
+
+Zabbix can observe metrics in a few different ways (agent, remote network-based checks, etc).
+Here, I just want the easiest way to watch a numeric value with minimal configuration overhead.
+Seems like a job for [zabbix_sender](https://www.zabbix.com/documentation/current/manpages/zabbix_sender).
+
+This is the syntax:
+
+```bash
+zabbix_sender -z <zabbix-server> -s <monitored-host> -k <metric> -o <value>
+```
+
+First, some housekeeping on the Zabbix server:
+
+1. Create a **Host**. Let's call it `soundmonitor`. ![Host](/media/zabbix/host.png)
+2. Create a new **Item** for the `soundmonitor` host. Let's call it `soundlevel`. ![Item](/media/zabbix/item.png)
+3. Put the Item on a **Graph**. ![Graph](/media/zabbix/graph.png)
+
+Now we can test the `zabbix_sender` command.
+
+```bash
+zabbix_sender -z zabbix.example.net -s soundmonitor -k soundlevel -o "0.5"
+```
+
+Take a look at the graph. It should now have our fake datapoint!
+
+![Graph 0.5](/media/zabbix/graph-0.5.png)
+
+# All together
+
+We can use [command substitution](https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html) to string together a one-liner.
+
+```bash
+zabbix_sender -z zabbix.example.net \
+  -s soundmonitor                   \
+  -k soundlevel                     \
+  -o "$(rec -n stat trim 0 .5 2>&1 | awk '/^Maximum amplitude/ { print $3 }')"
+```
+
+This takes the numeric sound level from the `rec|awk` combination and passes it as the `-o` argument of `zabbix_sender`.
+
+Now, we just need to put this in a loop...
+
+```bash
+while true; do
+  zabbix_sender -z zabbix.example.net \
+    -s soundmonitor                   \
+    -k soundlevel                     \
+    -o "$(rec -n stat trim 0 .5 2>&1 | awk '/^Maximum amplitude/ { print $3 }')";
+done
+```
+
+# Ta-da!
+
+![Graph (for real)](/media/zabbix/graph-real.png)
+
+Pausing the iPod produced that valley in the middle. Based on this test, I set a **Trigger** for `soundlevel` falling below `0.5` for 60 seconds or longer.