There are three reason might cause the problems,

1. Lower sensitivity of microphone

• The sensitivity of the condenser microphone is from -36 dB to -48 dB (0dB= 1V/Pa). If we use -44 dB microphones, the sound will be lower.

2. Defective design of amplifier

• The impedance for the amplifier will affect the sensitivity and the frequency response.
• Normally, the higher the impedance is, the better the sensitivity will be. However if the impedance is too high to get enough voltage/current, the high frequency response may get worse.
• The lower the impedance is, the lower the SPL will be. The too much current will cause the insufficient voltage.
• Input higher voltage will increase the sensitivity. Depend on the design of the amplifier and the frequency, the sensitivity might increases around 0.3 dB to 0.6 dB by 1 V.

3. Defective design of mechanism

• The over-length or curved sound receiving holes leads to lower or unnatural sound.