2) I have a speaker with 8 ohms, 0.5 W and using P=(V^2)/R, I found that the voltage is 2V's. Is it the output voltage from the speaker or the voltage needed for the speaker to function?
Neither. 0.5W is the
maximum power coming from an amplifier that the speaker can stand without damage and/or excessive distortion. It will produce sound even if the incoming voltage is only 0.1V (0.00125W) or even less.
Yes, I want to implement this to detect baby cries. But I am yet to understand how to get an output from electret microphone that is heard over a simple speaker. I'm a novice, and I am trying to learn bit by bit.
General-purpose opamps like the LM1458 are meant to be used mainly as voltage amplifiers. This means that they are normally expected to have a high impedance load, which in turn means that they are not expected to deliver high currents to the load. A speaker is a low impedance device. Therefore an opamp (except a special high-power opamp) is not suitable for driving a speaker directly. We need a power amplifier, even a simple one, for that.
So, how exactly will I know how much to amplify the voltage from the electret microphone? Can you provide me with a simple, low power and high gain circuit to achieve 5V.
(To simplify matters, let's stick to voltage amplifiers for the time being and ignore current and power requirements at the output)
An amplifier amplifies an input by a given factor. For example, an amplifier with a gain of 100 produces an output which is 100 times the input. Therefore, to calculate how much amplification is needed to get an output of 5V, we have to know the input voltage which is the output voltage of the microphone. If the microphone's output is 10mV, we have to amplify it by a factor of 500 to get an output of 5V. If the input is 100mV, we need a gain of only 50.
One problem that cannot be avoided in a practical design is that the microphone's output is not constant. Different microphones have different sensitivities. Even for the same microphone, the output will vary with the intensity of the sound it picks up. That intensity depends not only on how loud the sound source is, but also on how close it is to the microphone.
When your baby cries, the strength (voltage) of the mic's output depends on how loudly it cries, how close it is to the mic, whether it is facing towards the mic or away from it, whether the sound is muffled by clothes, room acoustics, and so on. Therefore, the mic's output and the amplification needed to get 5V cannot be calculated to a specific number.
The output from the microphone may be anything from less than 1mV to 100mV, depending on the conditions described.
What can be done, and what is done in designing any audio amplifier, is to estimate a ballpark figure for the output voltage coming from the microphone, design an amplifier with enough amplification to get full power from the softest sound expected, and provide a volume control to manually change the sensitivity. Some audio systems have an automatic volume control (AVC) or automatic gain control (AGC) which compresses the wide volume range to a narrower one.
To sum up, it's impossible to design an amplifier which will produce an output of 5V unless the input is known and constant. What can be done is to estimate the output from the microphone under the circumstances in which it is to be used, and then design an amplifier that will produce an output roughly in the range of 5V.