LOUDSPEAKERS

Abstract: A method of compensating for boundary effects on a primary sound source (28, 30) located in an environment (26) having boundaries (40), comprising the steps of feeding the primary sound source with an electrical signal, positioning an auxiliary sound source (36) in the environment, determining the distance of the primary sound source from at least one boundary, deriving a correction signal from the electrical signal to the first sound source and the determined distance and applying the correction signal to the auxiliary sound source to energise the auxiliary sound source to produce correcting output. A loudspeaker system for use in an environment having boundaries using the method of compensating for boundary effects.

Description: TITLE: LOUDSPEAKERS DESCRIPTION TECHNICAL FIELD The invention relates to loudspeakers and more particularly, but not exclusively, to domestic high fidelity loudspeakers. BACKGROUND ART A room used for listening to loudspeaker reproduction imposes errors on the otherwise ideal intended performance of the loudspeaker. This is because a listener hears sounds radiating from the loudspeaker not only directly but also via paths of different lengths due to reflections from the room boundaries, typically the floor, rear wall and nearest side wall . At higher frequencies, say above 300 to 500Hz, the errors due to the acoustic reflections and standing wave modes progressively become increasingly complex. Termed diffuse, these modes sum to generate an acoustic power envelope which is perceived by the ear as a fairly flat smooth frequency response. Below 300 to 500Hz the modes are progressively less dense and less uniformly distributed. In this region the ear perceives the discontinuities as response errors, as missing sounds, coloration and aurally distressing peaks. Experts agree that the listening room may account for around 30% of the sound quality of a high quality audio reproducing system. Thus variation in the room/speaker interface is a major factor in high fidelity audio reproduction. As an example, consider a single boundary, namely the floor. At a typical speaker position a seated listener hears two sounds, the first being a direct axial path from speaker to listener and the second being an image of the first but reflected from the floor and subject to a time delay due to the longer transit path. The summation of the two signals results in constructive and destructive interference at frequencies where the delay signal either reinforces or cancels the direct signal. This cancellation or reinforcement is due to the phase difference between the signals and is illustrated in Figure 1 which shows the frequency response (10) of a free-field speaker and the frequency response (12) of the same speaker in an environment with a single boundary, namely the floor. At very low frequencies the floor acts as an acoustic mirror. The phase difference between the first sound and the image is small and the sound pressure is doubled i.e. increased by 3dB. At the first frequency at which the relative delay is 180°, the reflection is out of phase and the output falls into a notch. Successive comb-like notches occur at each multiple of 180° until their rising density and general diffusion leads to their general blending and thus their reduced effect and significance. The first few modes are thus the most severe and thus require most attention. In practice the other two local boundaries, i.e. the rear and side walls, will also have their influence on the overall notch distribution, this being dependent upon the relative delays introduced m the secondary paths to the listener. The effects of the three boundaries are illustrated m Figure 2 which shows the frequency responses (14,16,18) for the three boundaries, namely floor, rear and side wall respectively. Prior art proposals to counter the problem have included both active and passive devices to absorb and control the more serious modal resonances m a listening room due to the ratio of the major room dimensions and m some cases boundary effects. Prior art proposals have also concerned the correction of response errors occurring when a loudspeaker is placed in a room and involve the feeding of a compensation signal to the loudspeaker together with the main or primary signal. A limitation m this method of correcting the primary signal is that the dynamic range or maximum loudness of such a reproducing system is reduced by the amount of correction to be applied, and which must be reproduced by the main system in addition to the primary signal and thus reduces its capacity. This limitation is illustrated in Figure 3 which shows the frequency response (20) of a possible single boundary correction according to the prior art together with the maximum loudness or headroom (22) before correction and the maximum loudness (24) after correction. DISCLOSURE OF INVENTION From one aspect the invention is a method of compensating for boundary effects on a primary sound source located in an environment having boundaries, comprising the steps of feeding the primary sound source with an electrical signal, positioning an auxiliary sound source in the environment, determining the distance of the primary sound source from at least one boundary, deriving a correction signal from the electrical signal to the first sound source and the determined distance and applying the correction signal to the auxiliary sound source to energise the auxiliary sound source to produce correcting output. Preferably the method comprises the step of limiting the frequency range of the correction output to below 500Hz. By transferring the responsibility for boundary correction to an auxiliary channel, the main channel is freed to operate unhindered to its full quality and capacity. Furthermore, the main audio channels are allowed to operate to their full capacity to maximise the performance gain to be derived from boundary correction. The auxiliary channel need not be of high power or high quality since it is fed specific signals over a fairly narrow frequency range located away from the aurally critical region, namely the mid range. Only moderate requirements for harmonic distortion need be met due to helpful aural masking introduced by the main system output . If desired the acoustic output of the auxiliary sound source may be modified in the same manner using a second auxiliary sound source. The method may further comprise the step of determining the distance of the primary sound source from two or more local boundaries whereby the correction signal can take account of reflections from more than one local boundary. The method may further comprise the step of positioning the auxiliary sound source at a fixed location, e.g. adjacent to a local boundary or attached to the enclosure containing the primary sound source. From another aspect the invention is a loudspeaker system for use in an environment having boundaries, the system comprising a primary sound source fed with a primary signal to generate a primary acoustic output, an auxiliary sound source, means generating a secondary signal derived from the primary signal and from a determination of the distance between at least one boundary and the primary sound source, and means applying the secondary signal to the auxiliary sound source so that the auxiliary sound source produces a secondary acoustic output which interacts with the primary acoustic output to modify the overall acoustic output of the system. The primary sound source may be housed in a cabinet or enclosure into which the auxiliary sound source may also be integrated. Alternatively, the auxiliary sound source may be a self- contained unit distance from the primary sound source. The means generating a modified signal may sum or otherwise take account of the expected reflections from two or more room boundaries. The means generating the modified signal is preferably limited to a frequency range below the mid range. The frequency range may be below 500Hz. BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which :- Figure 1 is a graph illustrating the effect of a single boundary on the frequency response of a loudspeaker; Figure 2 is a graph illustrating the separate effects of three boundaries on the frequency response of a loudspeaker; Figure 3 is a graph illustrating the effect of a prior art single boundary correction on the maximum loudness of a loudspeaker; Figure 4 is a plan view of a domestic listening room including a typical high fidelity audio system and a loudspeaker/room boundary correction device of the present invention; Figure 5 is a block diagram of the correction device of Figure 4 ; Figure 6 is a front view of the control means shown in the diagram of Figure 5 , and Figure 7 is a perspective view of a loudspeaker incorporating an auxiliary sound source of a boundary correction device of the present invention. BEST MODES FOR CARRYING OUT THE INVENTION Figure 4 shows a loudspeaker system (26) comprising two primary sound sources, namely left and right channel speakers • (28,30) fed with electrical signals from a controller (32) and two auxiliary sound sources or correction speakers (36) fed with electrical signals from a boundary correction processor (34). The speakers (28,30, 36) are located in a listening room (26) having boundaries (40) . The acoustic output from the correction speakers (36) modifies the acoustic output of the system as perceived by a listener (38) . The correction speakers are self-contained units and are self adjusting or pre-programmed for a range of speaker placements. The speakers are positioned adjacent one of the room boundaries . The speakers may be used for every loudspeaker position, or alternatively in reduced numbers to correct several nearby speakers, for example at one end of a room. A microphone (35) is attached to the boundary correction processor (34) to obtain information on the boundary effects on the acoustic output of the system. The information is fed into the processor (34) which automatically computes the required corrections. The processor may be a pre-programmed device using auto calibrating adaptive signal processing to provide more accurate correction. Digital processing may be used in the correction procedure. Figure 5 shows one possible implementation of the invention whereby the channel (54) of one correction speaker (36) is placed in the signal path of one channel (42) in the primary sound source. The signal in channel (42) is generated by a source (44) and passed through an amplifier (46) , a gain control (48) and a power amplifier (50) to the primary loudspeaker (52) . The channel (54) is placed in between the gain control (48) and the power amplifier (50) . A correction signal (56) passes through the channel (54) along a path through a control (58) , a processing unit (60) and a power amplifier (62) to the correction loudspeaker (64) . As an alternative to the automated procedure described in connection with Figure 4, the correction applied may be user programmed via a set of switches or controls to synthesise the required corrections for the boundary induced errors. Figure 6 illustrates a front panel (66) of one embodiment of a boundary correction processor. The front panel (66) has controls for the sensitivity (68) of the main system, the reflectance (76) of the boundaries and the relative distance (70,72,74) of the side wall, floor and rear wall. The controls may be used in two ways, either on the main signal path to the loudspeaker or by feeding the auxiliary correction speaker. Figure 7 illustrates an alternative embodiment of the present invention in which the primary sound source (79) is housed in an enclosure (78) and the correction speaker (82) is built into the enclosure (78) . The primary sound source (79) is fed via leads (80) and the correction speaker (82) fed separately via leads (84) . The correction speaker (82) is suitably isolated from the primary sound source (79) to control any spurious vibration or other related effects. It will be appreciated that the primary loudspeaker (52) in Figure 5 corresponds to the primary sound sources (28,30 and 79) in Figures 4 and 7 respectively. Similarly, the correction loudspeaker (64) in Figure 5 corresponds to the correction loudspeakers (36 and 82) in Figures 4 and 7 respectively. It will also be appreciated that if the auxiliary speaker is • itself placed such that its output will also carry boundary induced errors, these may be programmed for correction, just as the correction is itself programmed. This will require independent settings/ software to cover the independent placement of the auxiliary correction channel and the main channels. Audio electronic components may also be fitted with the correction channel device of the invention, e.g. preamplifiers, integrated amplifier, receivers and power amplifiers .

Claims: CLAIMS 1. A method of compensating for boundary effects on a primary sound source located in an environment having boundaries, comprising the steps of feeding the primary sound source with an electrical signal, positioning an auxiliary sound source in the environment, determining the distance of the primary sound source from at least one boundary, deriving a correction signal from the electrical signal to the first sound source and the determined distance and applying the correction signal to the auxiliary sound source to energise the auxiliary sound source to produce correcting output. 2. A method according to claim 1, further comprising the step of limiting the frequency range of the correction output to below 500Hz. 3. A method according to claim 1 or claim 2, further comprising the step of determining the distance of the primary sound source from two or more local boundaries whereby the- correction signal takes account of reflections from more than one local boundary. 4. A method according to any preceding claim, further comprising the step of positioning the auxiliary sound source at a fixed location apart from the primary sound source . 5. A method according to any one of claims 1 to 3, further comprising the step of attaching the auxiliary sound source to an enclosure housing the primary sound source . 6. A method according to any preceding claim, further comprising the step of providing a second auxiliary sound source for compensating for boundary effects on the auxiliary sound source 7. A method according to any preceding claim, further comprising the steps of providing a microphone m the environment to acquire information on the boundary effects and using said information to calculate desired correction signal . 8. A method according to any preceding claim, further comprising the step of providing a pre-programmed device using digital signal processing to calculate the correction. 9. A method according to any preceding claim, further comprising the step of providing a set of controls for manually applying the required corrections. 10. A loudspeaker system for use m an environment having boundaries, the system comprising a primary sound source fed with a primary signal to generate a primary acoustic output, an auxiliary sound source, means generating a secondary signal derived from the primary signal and from a determination of the distance between at least one boundary and the primary sound source, and means applying the secondary signal to the auxiliary sound source so that the auxiliary sound source produces a secondary acoustic output which interacts with the primary acoustic output to modify the overall acoustic output of the system. 11. A loudspeaker system according to claim 10 wherein the primary sound source is housed in an enclosure and the auxiliary sound source is integrated in the enclosure. 12. A loudspeaker system according to claim 10, wherein the auxiliary sound source is positioned at a fixed 5 location apart from the primary sound source. 13. A loudspeaker system according to any one of claims 10 to 12, wherein the means generating a secondary signal takes account of the expected reflections from two or more room boundaries . 10 14. A loudspeaker system according to any one of claims 10 to 13, wherein the means generating the secondary signal is limited to a frequency range below the mid range. 15. A loudspeaker system according to claim 14, wherein the frequency range is below 500Hz. 15 16. A loudspeaker system according to any one of claims 10 to 15, further comprising a microphone for acquiring information on boundary effects, the microphone output being connected to the means generating the secondary signal to enable adjustment of the secondary signal in 20 accordance with the acquired information on boundary effects.

Inventors: AZIMA HENRY, AZIMA, HENRY
Applicants: AZIMA HENRY, NEW TRANSDUCERS LTD, AZIMA, HENRY, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H04R1
IPC codes: H04R1, H04R3, H04R5, H04R29