RTL-SDR projects
Weak Signal Propagation Reporter is a way of decoding information that is transmitted by radios across the planet. A suitably licensed amateur radio operator can transmit and any user and receive.
An RTL dongle can be used to create a band survey showing radio interference over time. This chart shows time vs frequency vs intensity in two different formats.
To measure the audio response of your transmitter, you can generate an audio file, transmit it, receive it, record it and compare the spectrograms between the original and the recorded audio.