Hi,
In my pic, the series cap is 0.4uF, the parallel resistor 22Ohms for an attenuation of 5dB@20KHz and 9KHz x-over. Attention that the quartz reactance might not be the same for a Single section piezo horn.

The treble section of the pro speakers was short on highs that's why I added the piezos. Regards

• My other white DIY disco speaker