Combined active deresonator and ambience generator

Abstract: The invention relates to a device to be used for changing the acoustic properties of a room, comprising a pressure transducer (1), a loudspeaker (2) and an electronic control circuit (3) adapted to generate, in response to pressure changes in the room, an electrical signal that serves to cancel the pressure change detected by the pressure transducer (1) when it is reproduced through the loudspeaker. For improving the properties of the room also with respect to reverberation, the device of the invention further includes an ambience generator (4) adapted to receive an electrical signal proportional to the sound present in the room and to generate, in response to said signal, a signal which produces an acoustic field containing early reflections and reverberation in the room when reproduced through the loudspeaker (2).

Description: The present invention relates to a device to be used for changing the acoustic properties of a room, comprising a pressure transducer, a loudspeaker and an electronic control circuit adapted to generate, in response to pressure changes in the room, an electrical signal that serves to cancel the pressure change detected by the pressure transducer when it is reproduced through the loudspeaker. In the connection of this application, a loudspeaker means an entity constituted by one or more loudspeaker elements possibly operating over different frequency ranges. Often when good sound reproduction is the aim attention is paid solely to the properties of the sound reproduction equipment. The listening space also has a great effect on the way how the music sounds. In most cases, the listening room is the greatest factor influencing sound reproduction. The acoustics of the listening room can be improved for instance by using different acoustic panels to line the walls and ceiling. Such passive methods are difficult to realize with frequencies below 200 Hz. To solve problems presented by low frequencies, the equipment described in the introduction has been presented, with which equipment room resonances can be attenuated or eliminated. This is based on the fact that the device can cancel low-frequency acoustic waves. Placed in a corner of a room, the device is able to attenuate standing waves present in said corner. Such a stand-alone active acoustic resonance suppressor has been described in the US patent 4,899,387. This apparatus is mounted in an upright, free-standing housing for eliminating unwanted reflected waves in a room. The housing is intended to be placed in a location where undesired wave patterns, such as standing waves, are formed in order to offset or cancel such conditions. These conditions are offset by generating an acoustic signal which is the inverse of pressure waves at a particular location. The pressure waves are sensed by a module, including a microphone which generates a corresponding electrical signal. This signal is sent to an electrical circuit where an inverse signal is created which is then transmitted to a loudspeaker. The loudspeaker output is directed toward the location where the standing waves would be formed. The loudspeaker output nulls local acoustic waves so that no standing waves are formed. A combination of an adaptive noise cancellation device with a car entertainment apparatus is described in WO 90/09655. The noise cancellation device described in this document is designed to attenuate noise present in the vehicle cabin which is generated by the vehicle engine. The car entertainment apparatus may contain a tuner, a compact disc player, a cassette player and the like. In addition to room resonances described above early reflections and reverberation found at higher frequencies have a substantial effect on the character of the room as a listening space. A device of the kind described in the introduction has no effect on the properties of the space in this respect. Typically, a normal room has too little early reflection and the reverberation is too sparse and has too short duration. Solutions to add reverberation or to modify reverberation in acoustic space have been described e.g. in the US patent 5,025,472 or in an article by Geoffrey Berry and Gordon L. Crouse "Assisted Resonance" published in the Journal of the AES, April 1976. The importance of early reflections besides reverberation for a realistic acoustical room reproduction has been described in GB 22 55 884. This document discloses a signal processor creating the illusion of a sound source having an apparant distance from the listener. The device of the invention as it is defined in the appended claims influences the properties of the room also with frequencies exceeding 200 Hz. The device of the invention further includes an ambience generator adapted to receive an electrical signal proportional to the sound present in the room and to generate, in response to said signal, a signal which produces an accoustic field containing early reflections and reverberation in the room when reproduced through the loudspeaker. The ambience generator receives the electrical signal proportional to the sound present in the room either directly from the sound reproduction equipment or through a microphone included in the equipment, said microphone most preferably being the same component that serves as a pressure transducer in connection with the control circuit. Thus the pressure transducer is most preferably a pressure sensing microphone. In the following, the device of the invention will be described in more detail with reference to the accompanying drawing, wherein Figure 1 shows a device of the prior art for the attenuation of particularly low-frequency room resonances, Figure 2 shows a device according to the invention for changing the acoustic properties of a room, Figure 3 shows a room with acoustic waves present therein, and Figure 4 shows a signal generated by the ambience generator of the device of the invention on a time-power scale. Figure 1 shows a circuit of the prior art for the active cancellation of low-frequency room resonances. This circuit comprises a pressure sensing microphone 1 adapted to feed a control circuit 3 the output of which has been connected to a loudspeaker 2. The pressure sensing microphone 1 senses the pressure present in the room at the location in which the device is placed. The control circuit 3 strives to maintain the pressure sensed by the pressure sensing microphone 1 at zero by generating a signal which, reproduced through the loudspeaker 2, cancels the pressure at the pressure sensing microphone 1. Thus a so called acoustic throat is created. If a positive pressure peak (acoustic wave) arrives at the pressure sensing microphone 1, the beam of the loudspeaker 2 will move backwards, whereupon the pressure at the microphone 1 decreases. Thus the device "absorbs" the acoustic wave. When the device is placed in a corner of a room, the result is the elemination or attenuation of room resonances. In practice, several such devices are needed in the room, and the effective frequency range is about 20 - 200 Hz. Figure 2 shows a device of the invention for changing the acoustic properties of a room. This device according to Figure 2 comprises as a basis the device of Figure 1 in its entirety. In addition to this, the device comprises an ambience generator 4 which also feeds its output to the loudspeaker 2. Therefore, the outputs of the control circuit 3 and ambience generator 4 are summed at point 6. It may be stated that in the known solution of Figure 1, the loudspeaker 2 can be a loudspeaker capable of reproducing only low frequencies. In the device of the invention, however, the loudspeaker 2 must be able to reproduce the entire frequency band of the audio range, that is, about 20 to 20 000 Hz. In the device of the invention, the ambience generator 4 has been adapted to receive its control signal, which must be a signal proportional to the sound present in the space where the device is located, either directly from the sound reproduction equipment 5 or from the pressure sensing microphone 1. Figure 3 shows the acoustic waves present in a room when the listener 7 and the sound source 8 are stationed centrally in the room opposite one another. In that situation, the listener 7 receives from the sound source 8 first a direct acoustic wave indicated with a large arrow and additionally early reflections indicated in solid line and reverberations indicated in broken line. There must be a sufficient quantity of early reflections, and also the reverberation field must be dense enough and have sufficient duration. Then music reproduced in a room furnished with the equipment of the invention can sound the same as for instance in a concert hall. Since there are too few early reflections and the reverberation is too sparse and short in a typical room, the ambience generator 4 is adapted to produce for instance signals of the kind shown in Figure 4. In this Figure, direct sound received by the ambience generator either directly in an electrical form from the sound reproduction equipment 5 or by "listening" in the room by means of a microphone, for instance a pressure sensing microphone 1, is illustrated furthest left on the time axis t. In response to the signal received, the ambience generator 4 generates signals which have in Figure 4 been denoted as early reflections and reverberations. These are signals corresponding to direct sound, but they are appropriately delayed and attenuated. The reverberation produced by the ambience generator should preferably be adapted to the inherent reverberation of the room in order for the final result to be the optimum. For this reason, the ambience generator is preferably able to generate several signal patterns of different types corresponding to Figure 4, and in these patterns the number and power level of the early reflections vary to some extent, as do the level and number of the reverberation signals. By adjusting the level and number of the early reflections and also the level and number of the reverberation signals, one can materially influence the way how the music sounds in the room in question. In the foregoing, the device of the invention has been described mainly schematically by means of one exemplary embodiment, and it will be appreciated that the device described can be realized by means of electronic solutions of many different kinds without, however, departing from the essential idea of the invention as defined in the appended claims.

Claims: A device to be used for changing the acoustic properties of a room, comprising a pressure transducer (1), a loudspeaker (2) and an electronic control circuit (3) adapted to generate, in response to pressure changes in the room, an electrical signal that serves to cancel the pressure change detected by the pressure transducer (1) when it is reproduced through the loudspeaker, characterized in that the device further includes an ambience generator (4) adapted to receive an electrical signal proportional to the sound present in the room and to generate, in response to said signal, a second electrical signal which produces an acoustic field containing early reflections and reverberation in the room when reproduced through the loudspeaker (2). A device as claimed in claim 1, characterized in that the ambience generator (4) receives the electrical signal proportional to the sound present in the room directly from sound reproduction equipment (5). A device as claimed in claim 1, characterized in that the ambience generator (4) receives the electrical signal proportional to the sound present in the room from said pressure transducer (1). A device as claimed in any one of claims 1 to 3, characterized in that the pressure transducer is a pressure sensing microphone (1). A device as claimed in any one of claims 1 to 4, characterized in that the loudspeaker is a loudspeaker (2) for the entire audio range.

Inventors: KUUSAMA JUHA, MAEKIVIRTA AKI, KUUSAMA, JUHA, MAEKIVIRTA, AKI, Mäkivirta, Aki
Applicants: NOKIA TECHNOLOGY GMBH
Application year: 1993
Main classification: G10K15
IPC codes: G10K15, G10K11