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Acoustic Mass and Resistance as Function of Drive Levelfor Straight, Bent, and Flared Loudspeaker Ports

Nonlinear control of bass-reflex loudspeakers requires accurate knowledge of acoustic mass and acoustic resistance of the port. The values of these parameters are hard to measure and previous research indicates that they are both functions of turbulence levels in the port airflow. Well-designed ports are known to accept higher drive levels before flow separation and vortex shedding causes unwanted port noise. This work investigates the relationship between dependence of port variables and port noise as functions of drive level. Measurements show that port acoustic mass is essentially a constant independent of drive level, and that port acoustic resistance is related to drive level much in the same way port compression and port noise are. Absence of proper flaring of ports and introduction of bends in ports can both exacerbate the increase of port acoustic resistance with drive level, along with port compression and port noise. A properly flared port - even if it is bent - performs better than a straight unflared port. Straight and optimally flared ports have values of port acoustic mass and resistance that change less than 2 dB from lowest to highest drive levels.

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