Speakers and sound

My son has been curious for a while about the speakers in our CD player and how whatever is behind the grille makes a noise. As we were having a clearout of old broken things from our loft, we came across an old battery-powered CD player I had at university which no longer works. I decided I'd take it apart so he could have a closer look.

If you're thinking of doing this with a 3 year old, I strongly suggest taking it apart when they aren't with you so you don't have little fingers in the way. It took me a good 20 minutes to dismantle everything carefully, discharge the capacitors (with a couple of crocodile clip wires and a light bulb) and figure out what he might find interesting. 

Watching what happens inside when he turns a dial

I took the speakers out (harder than I'd hoped, the glue they were stuck in with was pretty hard to break!) with the wires attached and connected them up to a sound-generating circuit from the snap circuits electronics kit we have got to check they worked. I then put everything loosely back together without the screws or glue so my son would have the excitement of feeling like he'd taken it apart! 

He was keen to find out what was inside, so I explained that I'd unscrewed it and we could carefully take it apart.  I'd thought he would want to find out what was inside the speakers first, but the cogs inside caught his eye - one set were for the volume control and the other the tuning for the radio.  He experimented with turning the dials on the outside and seeing what happened inside.  He was particularly fascinated by the tuning one as it converted the circular motion to a linear motion of a pointer which shows the wavelength it is tuned to.

Speakers connected to an electronics kit circuit

Once he'd finished exploring the cogs, I showed him how the front panel came apart, and he was intrigued to discover the speakers and also what is inside the switches.  He pointed out the LEDs which are like those he uses in electric play dough and snap circuits, and then started to ask how the speakers worked.

I produced the circuit I'd constructed earlier and attached the speaker on with crocodile clip wires.  He turned the switch on and heard a siren sound.  I got him to gently touch the speaker cone so he could feel it vibrating.  He experimented with changing the sound and feeling how the movement of the speaker changed.

Watching salt move as the speaker vibrates

I thought it might help him understand what was happening if we put something on the speaker that he could see moving, so we moved to a table and I asked him to sprinkle a little salt on the speaker cone.  When he turned the circuit on and heard sound from the speakers, he could also see the little salt crystals jumping around which he found quite amusing.

As he changed the sound, he could see the salt moving either faster (higher pitch) or slower (lower pitch).  I explained that the speakers are moving air backwards and forwards, and that our ears hear this movement of air as sound - there's a bit inside our ears (the ear drum) which also moves/vibrates when the air moves.  I'm not sure he was really listening, but he spent a while changing the sound and watching the salt.  He then wanted to go back to playing with the cogs and switches, which he did for a while before losing interest.