Sound Amplitude changing with frequency

[absias]

Hello guys,

I am using NI myRIO-1900. I wrote simple program to generate sine wave from loudspeaker and detect it by two microphones. I tried to input frequencies 50Hz, 100Hz, 150Hz .... until 450Hz and I saw that the amplitude of the sound received by microphones change with the frequency even through it was set to 1. 

for 50Hz the sound detected is small, for 150 is a little larger, then 200Hz is also a little larger, then suddenly drops at 250Hz. So the sound amplitude sometimes increase and sometimes decrease depending on frequency of sinewave. 

Please suggest how I can have constant amplitude received by microphone even after changing frequency of the sound.

Thank you

[GerdW]

Hi absias,

what's the frequency response of your mic? Especially in this low frequency range?

[Henrik_Volkers]

Not only the mike, every part in your signal chain probably has a transfer function.

Say the DA provide always an amplitude of one. But how about the frequency response (transfer function) of:

DA Filter,

Amplifier,

Speaker,

ROOM,

mikrophone,

preamp,

... ?

Thats why you mostly try to change only one (known) part in that chain with a DUT and measure the difference 😉

[absias]

Hi GerdW,

I use GRAS type 46AE microphone. Frequency responce:

±2dB               3.15Hz-20kHz 

±1dB               5Hz-10kHz