SPEKTRA Volume 5 issue 2 contains 8 papers: 1) The Effect of Cobalt Content on Magnetic Properties of CoFe Alloys; 2) Uncertainty Analysis of Laboratory Measurement of Airborne Sound Insulation; 3) The Stability Studies of Mixed Halide Perovskite CH₃NH₃PbBr_XI_3-X Thin Films in Ambient with Air Humidity 70% using UV-Vis Spectroscopy and X-Ray Diffraction; 4) The Magnetic Properties Measurement of Coil For Gravitational Acceleration Determination; 5) A Preliminary Study of Radiation Quality Correction Factor (k_Q) Determination for Well-Type Ionization Chamber; 6) The Monitoring System of Indoor Air Quality Based on Internet of Things; 7) Measuring Instrument for Refilled Drinking Water using a TDS Sensor; and 8) The Synthesis and Characterization of Zinc Oxide Thin Film Doping Selenium.