The present disclosure relates to devices and methods for determining the density of insulation. For example, one aspect of the disclosure is a device that includes a first unit that includes a sound generator and a second unit that includes a sound sensor and a probe. The probe is configured to be inserted into insulation such that the sound sensor is outside of the insulation and is configured to detect sound that is generated by the sound generator outside of the insulation and transmitted through the insulation and the probe to the sound sensor. The device also includes a control system configured to cause the sound generator to generate the sound and to use the sound detected by the sound sensor to generate output that represents the density of the insulation.