Specifically I’m wondering about electronics that play back short noises or songs, like baby toys. Why would the sounds slow down or get distorted when the batteries get low?
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So, in sound you have frequency. If you’ve ever ‘sped up’ a song you know it sounds like chipmunks and the opposite happens when you slow it down, it sounds like it’s in slow motion.
Electronics also operate at a frequency, which is usually set to the ‘speed’ it needs to operate ‘correctly’. Electricity pulsing at the proper frequency makes the electronic work normally.
Basically when a (non-lithium) battery is going dead, the electronic it powers receives less power to be able to operate at the proper frequency, and so it works slower
Slowed down electronic = slowed down frequency being reproduced = slowed down sound, like that slow motion effect
I would bet the toy you are having issues with uses a mechanical tape. Particularly with older style toys it might be cheaper to make them with the old tech then hire an engineer to design a modern, digital, soundbox.
The electronics will be driven by a frequency produced by an oscillator on a chip somewhere. The frequency of this will be dependent on the voltage it gets. The chip will insulate the oscillator from voltage drop somewhat, but can’t do that infinitely: once the voltage drops below the value the chip can protect against, the voltage across the oscillator will drop and so the frequency of the whole of the electronics will slow down directly proportional to this voltage.
This is a trope from the days of mechanical recordings, tapes and phonographs and such, where slowing down the motor slows down the playback.
It doesn’t work that way for modern digital audio, but it still gets used in TV shows and movies because it’s funny.
People have talked about the frequency issues, but there is another, inherently analog reason for distortion.
The sound is produced by sending an analog signal to a speaker. No matter how digital everything is, the last stage is an analog amplifier that generates a powerful enough signal to move the speaker cone.
To keep it simple, lets imagine you are playing a single pure tone. That tone will require a sine-wave signal sent to the speaker. A tiny, low-voltage sine-wave will be sent to the amplifier, which makes a bigger, higher voltage one. The amplifier’s output shape will track the input shape, ideally perfectly. However, if in order to generate a loud enough sound, the output voltage has to go higher than the supplied battery voltage, it won’t. The top of the sine wave will be cut off, because the amplifier cannot generate an output higher than its supply voltage. A cut-off sine wave is not a pure tone, it is a very buzzy, distorted tone.
The same principle applies to more complex sounds. Insufficient voltage to the final amplifier will generate distorted sounds.
High-end gear won’t do this, as the battery voltage is regulated and until the battery is too dead to do much of anything, the amplifier will get the right voltage. For something like a baby toy, the manufacturer will not invest in regulating the output voltage of the battery, and you will get distortion as the battery voltage falls.
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