[填空题]
A certain dog whistle operatx7 apgqn11*bfes at 23.5 kHz, wy(wvoyg/b - k-hile another (brand X) operates at an unknown frequency. If neither whistle can be heard by humans when played separately, but a shrill whine of frequency 5000 Hz occurs when they are played simultaneously, estimate the oper-g /bw -(vykyoating frequency of brand X. kHz