This kit gives out between 108 and 110 dB at 1 meter distance when we measured it using a sound meter.  The variation is caused by natural fluctuations of the resultant sound pattern in space caused by construction and destruction interference of the two sound sources. 

We have supplied two piezo sounders preglued into their own resonant cavities with presoldered wires attached.  The PCB in the kit fits into the back of the box and gives you a nice alarm module.  All you have to do is connect between 5V and 12V to the positive (red) and earth (black) wire, and stand back. 

The 556 IC contains two 555 timers.  In this circuit the IC1:B closest to the piezo elements is set to oscillate around their resonant frequency.  The frequency is then varied via input pin 11 from the other 555, IC1:A.  This modulating input is a low frequency oscillation.  However, instead of using the square wave output from the first 555, the step function is converted to a triangular wave by the RC network R3 & C3.  The result of this is a frequency swept output which ensures that the output frequency definitely passes through the resonant frequency of the piezo elements sometime during each modulation cycle.  Thus no trimpot is required in the circuit for frequency calibration.

The output causes Q1 & Q2 to turn on and off.  When the transistors turn on current flows in the 220 turn coil of the autotransformers.  When the transistors turn off the magnetic field collapses and induces a potential in the high turn side of both coils which deflects the piezo element which causes a noise.  A potential of over 200 volts can be induced which can give you quite a shock!

The piezo element is certainly overdriven to give maximum noise output.  We have run this circuit for several hours in test circuits without anything burning out.  It is hard to see how more sound could be generated from such and element.  The sound level generated is very loud.  It can hurt your ears.  It is actually louder than the commercial units which use the same box (they measure 103dB to 106dB).   Even though there is only a 3 to 6bD numeric difference between this unit and the commercial units it is the nature of the logarithmic scale that this corresponds to a big difference in sound energy, and in the current drawn by the circuit.  Three dB is a doubling of the sound energy which draws four times the current.  The commercial units draw around 60 to 100mA.  Our unit draws 400 to 500mA.