Broadband sound power determination in flow ducts

This paper presents a new experimental technique, which enables not only the calculation of in-duct transmitted sound power for dominant tones, but also for broadband noise over a wide frequency range. The new method also distinguishes between sound power transmitted in and against the direction of flow. It can be performed using wall-flush installed pressure sensors as well as radial sensor rakes in the flow. The method is mathematically derived based on the analytical solution of the wave equation describing sound propagation in ducts with superimposed static mean flow. The analysis technique requires cross-correlation measurements, e.g. between azimuthally traversable sensor arrays and at least one fixed reference sensor. It assumes an invariant mean broadband noise sound field. The broadband sound power determination method was experimentally verified in a low speed DLR laboratory fan experiment against the ISO 5136 standard for in-duct sound power determination. The experimental setup consists of a single-stage compressor in a flow duct with a diameter of 500 mm. An azimuthally traversable duct section located 2m downstream of the rotor can be equipped alternatively with a sampling tube for measurements after ISO 5136 or microphone rakes for in-duct mode detection. For mode detection, 8 microphones were installed in the moving duct section, which was traversed over 36 azimuthal positions. A wall-flush mounted microphone was used as a reference sensor. Experimental results are presented for three operating conditions and two different compressor tip clearances