Low frequency room adaptation

The low-frequency room problem has proved to be daunting. In a simple sense, the room in which the loudspeaker is placed is a fundamental part of the loudspeaker system. Variations in the room, or in speaker or listener location in the room introduce significant changes in low frequency timbre of the loudspeaker.

Traditional solutions to this have been to equalize loudspeaker performance so that it is suitable for a specific point in the listening room, known as the “sweet spot.” One of the authors (Pedersen) has developed an alternative approach to this problem.[9] As Pedersen states:

"Systems that are based on a measurement of the transfer function from the input of the loudspeaker to the output of an omnidirectional microphone try to measure the complete multidimensional system with a 1 dimensional measurement followed by a 1 dimensional filtering/equalization. At the very position of the microphone these systems do solve the problem when the evaluation is performed using an omnidirectional microphone, i.e. the measured transfer function to the microphone does approximate the target filter response.

"However, if the listening position is changed or the system is evaluated by other means than an omnidirectional microphone then the conclusion could be different. The choice of target function seems to be non-trivial in these systems, and the optimal target function might be a function of the whole system: loudspeaker position, loudspeaker directivity, room size, listening position, etc.

"The Adaptive Bass Control (ABC) system addresses the 1 dimensional problem of equalizing the acoustic power output of the loudspeaker to match the acoustic power output in a reference loudspeaker position in a reference room. This requires a 1 dimensional measurement and a 1 dimensional equalization filter, which is the case in an ABC system."[10]

In the Adaptive Bass Control system, the acoustic power output across the bass spectrum is matched to a reference acoustic power output selected by the designers, an optimum acoustic power output. The acoustic power output is found by determining the radiation resistance of the loudspeaker at its desired position. The radiation resistance is determined by taking a measurement of the force acting on the loudspeaker diaphragm from the sound field and a measurement of the diaphragm velocity. From these two measurements, the mechanical radiation resistance can be calculated.[11]

These two measurements are done sequentially with a single microphone at two different points in space. The microphone is included in the loudspeaker system and is moved approximately 4 cm. (from 6 to 10 cm. from the diaphragm), using a servo motor. Calculation is done in the loudspeaker electronics. The entire process takes about 1 minute per speaker, and is entirely automated.

“The Adaptive Bass Control (ABC) system generates an equalization filter, which yields a much more constant timbre between different listening rooms and loudspeaker positions at low frequencies. An additional advantage is that ABC improves the timbre everywhere in the listening room – it is not limited to a predetermined listening position.”[12] This quality of the technique in turn yields a far more flexible and musical outcome and performance than do more traditional solutions. It is suitable for the general array of musical listeners and corrects for both major anomalies in bass response and for changes that have occurred in the playback space since installation.