But I want it as load as possible and Ideally self contained, I am using a 12v 38mah battery I will using a regulator to run a RF RX module and probably a MCU (Attiny13v).
I have played with a few piezo buzzers but they don't appear to be very loud, suspect this is down to driving them from the AtTiny13v directly at 5vPP and probably not running then at optimum frequency.
Ain't possible. A 38mAH battery is unlikely to have much ability to deliver more than a few mA without serious voltage drop.
The really common alarm transducer is a piezo disk, used in all the personal alarms. The driver runs off a standard 9V battery (like an amp-hr or so) and it maxes out the curent capacity of that battery to do it.
Yeah, when it comes to loud, low-power alarms, I'm loco for piezo. I'm powering a **broken link removed** alarm piezo with a 20mAh Alkalyne with a negligable voltage drop. With such a small battery, it does max out the current capability, so I have to trigger it in short bursts and place a large capacitor across the supply terminals.
The piezo buzzers have the piezo in a tuned acoustical cavity and the piezo's resonance is part of its feedback so it operates at its loudest.
I tried a piezo speaker I got from a phone and its resonance peak is so narrow that it was difficult to adjust the frequency of my oscillator to its peak: