Ya Gotta Listen to your HART, It's Gonna Tell You What to Do!
Traditional 4-20mA HART uses a modulated audio-band communication signal based on the Bell 202 standard. Audio-band means you can hear the HART FSK 1200Hz / 2200Hz communications. The HART Signal Detector allows you to connect to a HART network and listen to the HART communications.  
The human ear is highly perceptive, and after listening for a short time to HART communications, it can distinguish between a healthy network and one that has problems. Listen to the following communications of a host periodically polling a field device.

HART Communication Example 1
The communication is compliant and you can hear both the host and the field device. Sometimes the field device answers very quickly and sometimes you can hear a very brief silence between the request and the response. Also, there is a short silence between the request/response cycles. This silence allows for another host to get on the bus. 
In the next audio you can also hear a host periodically talking to a field device. In addition, several times you can hear a handheld also on the bus. It is has a slightly different tone (it is actually a little louder).

HART Communication Example 2
Using the HART Signal Detector, listen to your "good" loops and the a loop that is having problems.  You will hear the differences. For example, a bad loop may have a loud hum from interference or noise bursts (static) that shouts over the HART communication.  You can even listen to a loop during commissioning for noise and static before attempting communication between the host and field device. 
The HART Signal Detector is a low cost, high value tool that helps you understand your HART loops.  A great addition to your tool bag. 
Fun Fact: The headline for this snippet was inspired by this song by Tom Petty and the Heartbreakers