Though the energy in the impulses used is very small, the peak level may be enough to damage loudspeakers. Make sure that the level is set within the working range of the speaker you are measuring.
The circuit for the pulse generator is shown right (Figure 1).
The circuit uses two 555 timers to generate a variable width and variable repetition rate pulse. The circuit has a minimum pulse width of approximately 20 µs and a repetition rate from 50 ms to 5 s. You will need an external d.c. supply of between 9 V and 17 V that need not be regulated.
For this application note we used a small electret microphone. These are inexpensive and provide good frequency response performance. If you need to calibrate your measurements you will need to obtain a calibrated microphone. Microphone calibration is difficult to do unless you have the correct equipment and is only worthwhile if you need to know the absolute level of your acoustic measurements. For relative measurements most good quality capacitor or electret mics will be adequate.
The electret microphone used for these measurements requires a d.c. supply of about 10 V. We used a simple two transistor circuit to supply the mic capsule and give some gain. There are a lot of suitable circuits published for this purpose.
Note that for high-frequency impulse measurements a high pass filter circuit in the pre-amplifier will improve noise immunity.