maybe they just remove the high-pass filter ?
CMC 6 U linear
The CMC 6 U microphone amplifier normally has a gradual rolloff in response below 20 Hz to guard against infrasonic disturbances from various sources such as air movement and vibration. However, when using pressure (omnidirectional) transducers, particularly with digital recording, it can be desirable to pick up frequencies below 20 Hz without attenuation. The special technology of the CMC 6 U linear microphone amplifiers makes this possible; on request we can deliver microphone amplifiers with response that is flat to as low as 10 Hz. Caution must be advised with respect to infrasonics, however. Since pressure transducers can pick up very low frequencies, ventilation systems in large spaces (churches, concert halls) or traffic rumble can create a problem. With pressure gradient transducers the risk is even greater. They are far less sensitive to very low frequency sound, but respond much more strongly to low-frequency mechanical stimuli such as air currents and solid-borne noise. Such signals may be below the audible range of frequencies, but they can overload electronic circuitry and produce severe distortion, particularly in transformer-coupled circuitry.
