Making Wrist Alarm Louder

[Alphabits]

Hi all,

I'm assisting with the design of a wrist-based alarm that must fit into a wrist-watch enclosure. We've sourced a wristwatch case (plastic with rubber-like overmold and integrated rubber strap) that has a screw-on metal (stainless) back. We've also sourced the largest piezo-electric buzzer we can find that fits the recess in the back, but no matter how we attach it to the back, the volume from the buzzer/speaker when we send it a pure tone is still quite low. We've tried cyanoacrylates (crazy glue, loc-tite) and epoxies with no improvements.

I know my Timex watch beeps quite a bit louder, and it's back is a lot smaller. I haven't taken it apart, but I expect the buzzer is attached to the stainless backplate... So what does Timex know that I don't? Anyone?

Thanks in advance,
Rob

 

[IRstuff]

My Timex is built like you describe.

What I don't understand is if you want to solve your problem, why haven't you taken apart your watch? It took less than 1 minute on mine.

TTFN

 

[Alphabits]

The simple answer (IRstuff) is, i've taken apart many watches in the past and already know how a watch back is constructed, at least in as much detail as I could get by taking one apart. I know the buzzer is stuck to the back fairly rigidly.

What I don't know is what timex has done that makes theirs so much louder (qualitatively, i'd say the Timex is twice as loud as our setup, which uses a buzzer that must be 1.5x the diameter). Special buzzer? Special glue? Unfortunately none of that is apparent by looking at an assembled (or even disassembled) watch.

Thanks,
Rob

 

[IRstuff]

Why not do a side-by-side with a Timex watchback in your setup?

If there's still a difference, maybe the next step is to de-bond the Timex buzzer and compare it to yours.

The final test would be to bond the Timex buzzer to your configuration.

TTFN

 

[GregLocock]

(1) I bet they don't use a pure tone

(2) You may need to consider the buzzer + backplate as a resonant system. The mechanical properties of the joint between the two are probably an important tunable parameter

(3) have you measured the actual frequency spectrum and SPL from the two watches?


Cheers

Greg Locock

 

[amorrison]

If you are totally deaf for any frequency >3000Hz like I am the buzzer could be "loud" but you can't hear the frequency at all.

 

[Alphabits]

An update in case any of you are following along (or perhaps in case someone is searching for similar data in the future).

We took apart my Timex watch today and hooked it up to the silly-scope. Turns out, Greg is right, they don't use a pure tone at all. It seems they charge up a coil, and fire that across the piezo... Kinda like smacking a tuning fork on a table, they hit it with a voltage spike and then let it resonate. The voltage spike gets to 50V (!).

It's given us a direction to head now, so thanks for the suggestions everyone.