Audio wave amplitude decreased after FIR filter I am doing sampling and filtering for a 17KHz sine audio wave signal, the Nyquist frequency is about 20.8889KHz. I found the fact that this audio signal's amplitude decreased dramatically after it went through an FIR bandpass filter( pass band 16.8khz-17.2khz), the amplitude also decreased after an FIR highpass filter(cutoff frequency 17khz). I think that's because the audio signal frequency is kinda close to Nyquist frequency, am I right?
Is there a filter can preserve the original amplitude of the audio signal within the frequency I want?
Shown below is a pic consists of 2 signals before and after filtering(highpass filter)
Thank you in advance
As per Peter K. recommendation, I will reformulate my comment as an answer, instead of starting by asking:

I think the fact that the cutoff frequency of your HPF is 17KHz (your tone frequency) can perfectly explain why your signal is being attenuated. 

I would recommend you to try to move down the cutoff frequency of the High Pass Filter in order to avoid attenuating your tone.

BR