You want to calibrate your station frequency reference to the WWV signal on your receiver. The resulting beat tone must be:

When heterodyning (mixing) two signals together, the difference between them produces a "beat tone." As your station frequency reference gets closer to perfectly matching the WWV standard, the difference in frequency drops closer to zero Hertz. As the difference approaches 0 Hz (a "zero-beat"), the resulting audio tone becomes very low in pitch. Because frequency and period are inversely related, a very low frequency results in a very long, slow pulsing period (often heard as a slow "wah-wah" sound). If the beat tone were at the highest audio frequency possible, it would mean your reference is far off from the WWV signal. Therefore, seeking the lowest possible frequency and longest period is the correct method for perfect alignment.