Electronic – How to drive a speaker/microphone from a digital circuit

There are a lot of things to consider with this, especially if the two speakers are not identical. But let's assume that they are...

Theoretically, if you double the wattage then you'll get +6dB more sound level out. This is true (theoretically) regardless of if you take a single speaker from 200 to 400 watts, or go from one to two 200 watt speakers. In both cases the wattage doubles, so the sound level will rise +6dB.

In theory, theory matches practice. In practice, it doesn't.

Things are rarely so simple, and this is doubly so when it comes to sound. When you go from 1 to 2 speakers, you can get strange effects from the two speakers interfering with each other. Placement and orientation of the speakers can have a huge impact of this. In some cases, certain frequencies can completely cancel out while other frequencies will rise as much as +6dB. This is called "comb filtering". There are other related effects that could be a good thing (like a line array) or a bad thing, creating lots of really bad sound.

There is too much here to go into, but suffice it to say that if you want (effectively) a single speaker at 400 watts then that's what you should go with and not two 200 watt speakers. If you don't have that as an option, then I suggest that you place the speakers in such a way as the speaker coverage patterns don't overlap.

There is one thing that you didn't ask about, but I suspect you need to know, is how to calculate the apparent sound level given the wattage and speaker. Knowing this will greatly improve your chances of getting the right speaker/amp setup. Here are some rules/guidelines to know:

• A 3dB change in sound level is considered "barely perceptible" to the human ear. A 6dB change in sound level is a doubling (or halving) of the power (watts), but is still considered a small change in perceived sound level.

• If you double the distance from the speaker to the listener then the sound level will drop to 1/4th, or -12 dB.

• Typical speech over a PA system should be somewhere in the 65 to 85 dB range. A loud rock concert might be as high as 115 dB. Movies are in the 100-105 dB range.

• Speakers have "sensitivity ratings". A typical rating would be something like 85 dB/Watt/Meter. Meaning that if you put 1 watt into it, and measure it at 1 meter, you'll get 85 dB.

So here's what this means... Let's say that your speaker has a sensitivity of 85 dB/watt/meter, and you are 2 meters away and feeding it 1 watt. The sound you hear will be 73 dB. If you go to 2 watts then you get 79 dB. 4 watts = 85 dB. 8 watts = 91 dB. 16 watts = 97 dB. 32 watts = 103 dB. 64 watts = 109 dB. 128 watts = 115 dB.

Now if you move the speaker to 4 meters away you drop down to 103 dB. To get back up to 115 dB you need a 4x in power, or 512 watts. The point is, very quickly you get into some serious power levels for just a modest increase in sound level.

All of this is irrelevant to the topic of needing a sub woofer. If you need more low frequencies, then get a sub. If you don't, then don't.

Your sample rate must be twice the maximum frequency of interest. This is the Nyquist–Shannon sampling theorem. If you want hi-fidelity audio, your maximum frequency is the limit of human hearing, about 20kHz. Human voice is less demanding: telephone networks work up to about 3400 Hz.

You will find that the AVR's ADC isn't terribly fast, and you don't have much resources to process the data. So probably, your hardware limitations will dictate the sample rate for you.

You probably don't need to worry about codecs. A codec encodes and decodes an audio stream. A common thing to do is compress the audio for more efficient storage or transmission, but it doesn't sound like you are doing either. There are codecs that encode and decode for reasons other than compression, but I can't think of any that would be relevant here. Your computing resources are very limited on the microcontroller anyway, and you won't have much time for encoding or decoding anyway.