The thermoacoustic theory is applied to discussions on cooling power of a regenerator with narrow channels (ωτ
a<<1). Heat flux in the regenerator is shown to be a quadratic equation of the operating frequency ω, and there is a characteristic frequency ω
Q. If ω
Q is independent of ω, the cooling power is also a quadratic equation of ω, and it shows maximum at ω=ω
Q. The cooling power depends on the temperature of the cold end partly through ω
Q, which depends on material constants of the working fluid, the oscillating amplitudes, and the phase difference between pressure and displacement of the fluid. Discussions on ω
Q and the maximum cooling power give a qualitative explanation of recent experimental results on 4K G-M cryocoolers only if ω
Q is identified with experimentally obtained operating frequency corresponding to a maximum cooling power of 4K G-M cryocoolers.
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