Electrical audio signal processing systems and devices – Monitoring/measuring of audio devices – Loudspeaker operation
Reexamination Certificate
2002-01-29
2004-05-04
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Monitoring/measuring of audio devices
Loudspeaker operation
C381S098000, C381S096000
Reexamination Certificate
active
06731760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and apparatus for controlling the performance of a loudspeaker in a room.
2. Description of the Related Art
The actual performance of a loudspeaker is known to be highly dependent on the acoustics of the actual listening room and the actual loudspeaker position within this room. In particular, the performance of a loudspeaker will change very noticeably when it is in proximity to the boundaries of the room. This is caused by the loading of the room on the loudspeaker as a radiator, or, in other words, due to the changing radiation resistance. A change of listener position changes the perceived performance of the loudspeaker, in particular, due to early reflections and standing waves. However, some boundary effects are universal in the room, in particular, in the bass frequency range, and hence, the perception of this range is less influenced by the listener position.
Loudspeaker designers experience this fact by having to make a compromise when optimizing the timbre of the loudspeaker so that the perceived sound will be acceptable under a number of different conditions, i.e., different room acoustics, loudspeaker positions, and listening positions. Even though making this compromise, the designer cannot ensure that the customer will always experience the intended quality. Thus, the listener will experience a performance of the loudspeaker that depends on the acoustic properties of the actual listening room and the position chosen for both loudspeaker and listener. There is a risk that an expensive loudspeaker which performs very well in the shop, will turn out performing badly, or at least disappointingly, when placed in a different environment and/or different position.
In order to compensate for this problem, it is known to fit a switch in the cross-over filter unit in the loudspeaker in order that the bass response may be modified to suit a particular placement of the loudspeaker. At best, this must be considered a poor compromise, and if at all possible, the precise adjustment will be dependent on a measurement of the room characteristics. Some automatic systems are based on measuring the transfer function of the loudspeaker using an omni-directional microphone, placed at the preferred listening position, or a number of representative positions. An equalizing filter is then inserted so that the resulting transfer function approximates a target function, which, e.g., can be flat in the frequency range of interest.
Systems of the above kind are, for instance, disclosed in U.S. Pat. No. 4,109,107 to Boast and in U.S. Pat. No. 5,511,129 to Craven et al.
In particular, U.S. Pat. No. 4,109,107 discloses a method and apparatus for frequency compensation of an electro-acoustical transducer and its environment, in which it is possible to compensate for the acoustics of a specific listening room relative to the acoustics of an anechoic environment, the anechoic environment being regarded as defining ideal performance characteristics of the transducer, specifically, a loudspeaker system. The compensation for room acoustics is based on measurements of sound pressure at a number of different listening positions in the actual listening room and comparison of the result of these measurements with similar measurements performed in an anechoic room. Specifically, microphone locations were chosen at three positions in an actual listening room and the sound pressures were measured by means of a microphone with a cardioid directional characteristic. Three different orientations of the microphone were used in each of said positions thus yielding a total of nine measurements. These measurements were then averaged and compared with similar measurements performed in an anechoic room thereby yielding an acoustic gain factor for the specific room. It is thereafter possible to compensate for this unwanted effect of the actual listening room.
U.S. Pat. No. 5,511,129 discloses compensating filters for use in obtaining a given, desired amplitude and phase response of a loudspeaker used in an actual listening room. According to this patent, the response of the loudspeaker is initially measured by placing the loudspeaker in an anechoic room, passing a test signal through the loudspeaker and picking up the reproduced audio signal via a microphone. The loudspeaker is then placed in the actual listening room and the microphone is placed at a listener position in the room. The electrical test signal is supplied to the loudspeaker and the resulting audio signal received by the microphone is measured and stored. The microphone is then moved to another point and the process is repeated. Once sufficient measurements have been taken, a coefficient calculator determines a room response from a combination of the stored measurements, jointly representative of all the points at which the measurements were taken. This response includes the response due to the loudspeaker itself. The coefficient calculator therefore uses a stored model of the loudspeaker response jointly with the combined measured response to derive the response of the listening room (averaged over the positions in which the measurements were actually made), thereby eliminating the dependence upon the loudspeaker. A compensation response to substantially compensate for the derived room response is then derived, and combined with the loudspeaker compensation response. From the combined compensation responses, the coefficients of a digital correction filter are then derived, and the thus determined correction filter can then be used during subsequent audio reproduction in the actual listening room.
A major problem of such systems is the sensitivity to changes in the position of the sound source as well as the receiver. If the position of the loudspeaker or the listener is changed after calculating the equalizing filter, the effects can be severe coloration, pre-echoes, etc. Another problem of such systems is the choice of a suitable target function, where a flat function may not be found to be optimal.
A substantially different approach to the compensation for unwanted influences of the acoustics of the actual listening room is disclosed in International Patent Application No. WO 84/00274 to Adams, which discloses an environmental-adaptive loudspeaker system. This document discloses the use of pressure sensing means and acceleration sensing means for determining the instantaneous sound pressure at the surface of a loudspeaker diaphragm and the acceleration of this diaphragm, and based this sound pressure and acceleration, determines control signals for controlling the transfer function of a controllable correction filter via which filter input signals are fed to the loudspeaker. The compensation for unwanted influences of the acoustics of a listening room can thus be based on measurements carried out in the near field of the loudspeaker instead of (a number of) measurements carried out at actual listening positions in the room. Specifically, the control signal for the correction filter is the cosine of the argument of the radiation impedance as seen from the loudspeaker diaphragm,
SUMMARY OF THE INVENTION
It has, in the present invention, been realized that since all the involved acoustic phenomena's are considered to be linear, what is actually compensated through the apparently sensible procedures discussed above is the superposition of several phenomena, such as standing waves
atural frequencies of the room, early reflections, reverberation and the reduction of angular space angle due to the boundary effect, and it is considered that this is the reason why the known procedures will only function for one listening position.
It is the purpose of the invention to provide a method and apparatus for controlling the performance of a loudspeaker in a room in order that it becomes independent of the placement of the loudspeaker. This is obtained in a method according to the invention which is particular in that, in a first acoustic environment, the movement, e.g., v
Bang & Olufsen A/S
Mei Xu
Pendleton Brian T.
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