PRODUCT DATA 1/8″ Pressure-field Microphone — Type 4138 Type 4138 is designed for high-level and very highfrequency measurements and measurements in confined spaces. Being externally polarized, Type 4138 must be used with a classical preamplifier. USES • High-level and very high-frequency measurements • Pulse measurements • Applications which require a high degree of spatial resolution or where space is limited, for example, model testing FEATURES • Sensitivity: 1 mV/Pa • Frequency: 6.5 Hz to 140 kHz • Dynamic Range: 52.2 to 168 dB • Temperature: –30 to 100°C (–22 to 212°C) • Polarization: 200 V external Description Use of Pressure-field Microphones Pressure-field microphones should be used for making measurements in small, closed couplers or close to hard, reflective surfaces. Such microphones are optimised to have a flat frequency response in a pressure field. Because of its small size, Type 4138 can also be used for randomincidence measurements at audio frequencies, where its frequency response is less dependent on angle of incidence. Polarization Voltage Type 4138 requires an external polarization voltage and therefore must be used with a classical preamplifier such as Brüel & Kjær Type 2670 together with 1/8″ to 1/4″ Adaptor UA-0160. Calibration The sensitivity of Type 4138 can be calibrated at 250 Hz using Pistonphone Type 4228, or at 1 kHz using Sound Calibrator Type 4231, in both cases using Adaptor DP-0774. The pressurefield response can be measured using Electrostatic Actuator UA-0033 w

Free-field Correction Free-field corrections must be added to the pressure (actuator) response of the microphone in order to obtain the free-field response at a particular angle of incidence. The free-field correction represents the increase of sound pressure caused by diffraction of the sound waves around the microphone and is only significant at very high frequencies where the wavelength is comparable with the external dimensions of the microphone. The free-field corrections for various angles of incidence are given in Fig. 1. Fig. 1 Free-field correction curves for Type 4138 Due to its small...

Fig. 3 Typical frequency response of Type 4138 recorded by means of the electrostatic actuator method dB 5 0 -5 -10 -15 -20 1 Specifications – 1/8″ Pressure-field Microphone Type 4138 IEC 61094–4 Type Designation: N/A Polarization Voltage: 200 V (external) Open-circuit Sensitivity (250 Hz)a: 1 mV/Pa, –60 dB ± 1.5 dB re 1 V/Pa Random-incidence and Pressure-field Responsea: 6.5 Hz to 140 kHz: ± 2 dB Lower Limiting Frequency (–3 dB)a: 0.05 – 5 Hz Pressure Equalization Vent: Side vented Diaphragm Resonance Frequency: 160 kHz (90° phase shift) Cartridge Capacitancea (Polarized): 3.5 pF at 250 Hz Equivalent...

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