Abstract: The present invention provides an apparatus for enhancing the low frequency response of a loudspeaker. At low frequencies the mechanical and electrical inefficiencies of loudspeakers limit the intensity of the sound output by the loudspeakers. Previous attempts to correct the inefficiencies have been hampered by the non-ideal circuit elements used in conventional feedback circuits. To enhance the low frequency response of a loudspeaker there is provided an apparatus which includes a feedback circuit which is operably connected to an audio amplifier and which is tuned to substantially match the impedance of the loudspeaker within a predetermined frequency range. The apparatus further includes a transformer having a primary winding and a secondary winding. The primary winding is adapted to connect to the drive coil of a loudspeaker, and the secondary winding is connected to the feedback circuit.

Abstract: The invention disclosed comprises a method of and apparatus for inspecting a pipeline in situ. An alternating voltage is impressed between the pipeline and a ground bed. Spaced electrodes are positioned transverse the pipeline at spaced locations along the pipeline. At each location, the current flowing between the electrodes is used to indicate the condition of the coating on the pipeline. In one aspect of the invention, the current flowing between electrodes is obtained by measuring the resistance of the medium in which the pipeline is buried, such as the ground, and the voltage, and then dividing the voltage by the resistance. In another aspect of the invention, the ratio of the resistive and reactive components of the current flowing between the pipeline and the ground bed is used to indicate the quality of the pipeline coating. In yet another aspect of the invention, the third harmonic content of the voltage is measured to obtain an indication of the presence of iron oxide.