A problem with the TRF receiver built with triode vacuum tubes is the triode's interelectrode capacitance. The interelectrode capacitance allows energy in the output circuit to feedback into the input. That feedback can cause instability and oscillation that frustrate reception and produce squealing or howling noises in the speaker. In 1922, Louis Alan Hazeltine invented the technique of "neutralization" that uses additional circuitry to partially cancel the effect of the interelectrode capacitance. Neutralization was used in the popular Neutrodyne series of TRF receivers. Under certain conditions, "the neutralization is substantially independent of frequency over a wide frequency band." "Perfect neutralization cannot be maintained in practice over a wide band of frequencies because leakage inductances and stray capacities" are not completely canceled. The later development of the tetrode and pentode vacuum tubes minimized the effect of interelectrode capacitances and could make neutralization unnecessary; the additional electrodes in those tubes shield the plate and grid and minimize feedback.