As shown in the Sound level meter schematic diagram of the entire system is in the integrated circuit LM3915, and beyond that there are only a a few passive components. Because the impedance of the speaker in which the measurements were made affecting the outcome, and therefore provided a button to select the load impedance, it may be 4 or 8 ohm.
These Sound level meter devices must be connected, if possible, the terminal itself and baffle not on the amplifier to prevent effect of cable length and voltage drop that is not covered under the measure.
Parts list :
Resistors : 10K (2), 18K, 390K, 2.7K
Diode : Led (10)
IC : LM3915
Battery : 9V