I salvaged a miniature speaker from an old cell phone. When the phone was in one piece, obviously the sound worked just fine. In fact, it got pretty loud. Now I'm trying to hook that speaker up to an Arduino-based project. I'm using the EasyVR shield (https://www.sparkfun.com/products/12656) which I'm assuming has some sort of amp built-in. The shield has screw terminals for an 8-ohm speaker, but it also has a 3.5mm output jack.

When I hook up the salvaged speaker to the screw terminals, I can hear sound being played, but it's very quiet. If I solder the speaker to a 3.5mm plug and use the on-board jack, it's nearly inaudible. Is there some sort of circuit that I can put together to amplify the sound? The Arduino is running off an external 12v supply if that makes a difference (but see the caveat below).

PART TWO:
If I use my computer speakers with the 3.5mm jack, it's definitely loud enough, but I get too much feedback (humming/buzzing). If I use my portable, battery-powered speaker, it's also loud enough, but as soon as I plug the speaker in (via USB), there's the feedback again. It seems as though external power is what causes the feedback. What are my options here?

 

is the speaker a magnetic (magnet on back low resistance voice coil) or piezoelectric one? if piezo electric, you might have to use an impedance step up transformer.

 

If it's just the speaker driver in open space you might need an enclosure to stop the cancellation of sound from the back of the driver. Place the speaker in the cup of your hand to see if it gets much louder when playing.

 

not all speakers are 8 ohm. ive seen them @ 32 ohms up to 600 ohms in a set of headphones.

 

is the speaker a magnetic (magnet on back low resistance voice coil) or piezoelectric one? if piezo electric, you might have to use an impedance step up transformer.

I'm not sure about this salvaged one, but the other mini speaker I tried was magnetic and it had the same problem.

If it's just the speaker driver in open space you might need an enclosure to stop the cancellation of sound from the back of the driver. Place the speaker in the cup of your hand to see if it gets much louder when playing.

I thought about that and did jimmy-rig a sort of enclosure for it, but it didn't make a difference.

not all speakers are 8 ohm. ive seen them @ 32 ohms up to 600 ohms in a set of headphones.

I'm aware, but the shield only supports 8-ohm (supposedly).

 

have you used an ohm meter on the speaker?

a 8 ohm speaker will check somewhere around 5 to 6.

 

I'm aware, but the shield only supports 8-ohm (supposedly).

Exactly. So if the salvaged speaker is NOT an 8Ω speaker, what do you think will happen? Did you take the measurement suggested by Kermin2?

 

Something else I have seen from time to time is some speakers seem to lose efficiency when removed from their enclosure. If I haven't heard it myself, with a few devices, with the original device still driving the speaker in question, (yes, a couple were cell phones!) I would never believe it. It's strange, sometimes, how a free-floating speaker can be so quiet, stay quiet when mounted in an enclosure, and then ring your ears (and sound good) in its original mount. Sometimes, it's just a rubber boot the size of a pinkie fingernail (on a Nokia phone I had in 2004) that does the job so well.

 

have you used an ohm meter on the speaker?

a 8 ohm speaker will check somewhere around 5 to 6.

Exactly. So if the salvaged speaker is NOT an 8Ω speaker, what do you think will happen? Did you take the measurement suggested by Kermin2?

Ooh, good call, guys. When I test it, it initially spikes to anywhere from 50-130, then evens out at about 9.2.

 

Okay, I tried a different speaker that read a constant 8.6 and it's only slightly louder.

 

Okay, I tried a different speaker that read a constant 8.6 and it's only slightly louder.

Ok, if you are measuring 8.6Ω on the resistance scale of your meter, this is approximately a speaker with an impedance of 13.23Ω. It could be 16Ω speaker without any additional information. I estimated these values based on informaiton I found online and Kermit2's comments.

In any case, it is too large for the amplifier that is expecting an 8Ω speaker impedance. You want one whose DC resistance reading is around 5 to 6 Ω.

Note that impedance and DC resistance are different values, although they are related. Google "resistance vs Impedance" for a better explanation that I can give.