I'm a student. I'm trying to get this LabVIEW (8.5) program to filter and amplify sound. I have an audio file that plays through speakers into a microphone. The audio is recorded and played in its original form. Then it's run through a low pass and high pass filter and an amplifier (for an amp I used the waveform scale and offset). Then the filtered amplified audio is played. It's doing a good job filtering the unwanted high and low frequencies but the playback over the speakers isn't amplified at all. I have a graph of the sound before and after the amplifier and they show that the waveform is being amplified by about the amount that I scale it but when it plays over the speakers it isn't amplified at all no matter how much I scale it by. Thanks in advance for any help/suggestions that you may be able to offer.

Sounds like a fun project. We do a similar one in an electronics class that I teach, but with op-amps instead of labview for the signal processing. Do you have access to a multimeter or oscilloscope? If you connect either to the signal input and output I bet that you'll find the D/A card can't provide enough current to faithfully reproduce the voltage waveform across the 4ohm or 8ohm speakers (remember, V=IR).

So what you probably need is a power amplifier. You can make one from anything from a 741 op-amp with a gain set to 1 to a couple of transistors. If you contact me directly (PM me), I can get you some designs.

BTW, another way to check if this is your issue is to just turn the volume way (way) down and use the meter/scope on the input/output. I'd recommend putting a sine wave or some other simple signal in for that test. If it works at low volume and not high, make yourself a power amp.

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Kevin Nickels, Assoc. Prof, Trinity University, San Antonio, TX, USA