GB2064262A - Arrangement for controlling listening room reflections produced by loudspeaker systems - Google Patents

Abstract

The employment of two or more separate, but closely adjacent speaker/systems, having within their separate frequency responses a common frequency range (or ranges), but with differing radiation patterns (and hence differing rates of attenuation with distance from the source) for the same frequencies within the range(s) considered allows "Far-Field" sound cancellation while at the same time leaving the "Near- Field" virtually unaffected in sound level. As the listener in a normal small domestic listening room lies within the "Near-Field" condition relative to the direct sound from the combined systems, but lies within the "Far-Field" condition relative to the reflected sound, the latter can be considerably reduced in volume level without significantly affecting the former.

Description

SPECIFICATION Improvements in loudspeakers - A means of controlling the (undesirable) listening room reflections produced by loudspeaker systems One difficulty often experienced in sound production or reproduction, is the fact that, in a normal listening environment, a considerable proportion of the total sound received by a listener's ears is reflected from the listening-room walls. This phenomenon is of very great importance, for example, in the reproduction of stereo signals, because these reflections greatly reduce the precision of the stereophonic image created by the speakers. It is rarely realised how much detrimental effect is caused by listening room reflections, and in fact some speaker manufacturers intentionally increase this reflection, either by adding extra drive units in their speaker cabinets (e.g.Bose) or by attempting to use the speaker's own directional properties (e.g. JBL Paragon).

The listening room 'First Reflection', usually from the wall directly behind the listener, is the one which often causes the greatest degradation, not only to the stereo image, but also to the perceived dynamic range, as it 'fills in' any brief silent periods in the programme with delayed reflections of louder parts of the programme occurring a few milliseconds beforehand.

To avoid this degradation, it is necessary to change the ratio of direct sound to reflected sound in a way which ensures that the latter is as quiet as possible relative to the former.

It is possible to use directional drive units, or phased arrays of drive units, to 'beam' the sound at the listener, and this effect is well known.

However, it can (and often does) cause listening fatigue, as the beaming effect is most un-natural.

There is another way in which a similar effect/improvement can be obtained, without the drawbacks of the above methods, and which also allows a wider degree of movement on behalf of the listener without seriously degrading the stereo image.

It is a well known fact that different drive unit tupes and configuration exhibit different polar radiation patterns - for example, a single ciosed unit tends to radiate spherically, particularly at lower frequencies, whereas an 'open-baffle' unit will radiate in a figure of eight pattern, with positive polarity on one lobe, and negative polarity on the other, whilst not radiating at all sideways.

Multiple phased arrays behave in quite a similar fashion, to a greater or less extent, each having its own peculiarities.

What is very seldom considered is that, because of the radiation pattern, the rate of fall-off of signal amplitude with distance from the source is not the, much-quoted, inverse square law in all cases. This law only applies in the case of a spherical or part-spherical radiation pattern. The majority of the others fall off at a much slower rate with distance from the source.

When the above is taken into account, a way can be seen to reduce, or even theroretically remove altogether, the reflected sound from a listening room wall or other reflecting surface, while still leaving a substantial level of direct sound from the speakers. The key iies in the fact that the sound has to travel much further to reach the listener's ears by reflection than it does to reach him directly, (particularly so in a smallish room where sound reflection problems are most acute.) The solution is not difficult to achieve in practice, using available drive units.If two units or systems of widely differing radiation pattens (and hence different rates of amplitude falloff with distance) are combined, out of phase, (That is to say, with reversed polarity), and that with the slowest rate of falloff with distance is considerably attenuated with respect to the other, a degree of attenuation will be found where, at a considerable distance from the combined system, (such as that which the sound has to travel to reach the listener by reflection), the sound from each will mutually cancel that from the other (being of opposite polarity) and no sound at all results. with these settings unaltered, very little canceiation will be found to occur at positions much closer to the source(s) - direct sound, unreflected - because the one unit/system is here very much louder than the other (due to its inverse square law of sound falloff) Hence, direct sound is barely reduced in volume level at all, whilst that reflected, for example, from the wall behind the listener, is effectively removed altogether, with the attendant improvements in dynamic range, stereo imagery and realism ! Of course, the effect can only be made to work well within a given angle of radiation - for to achieve an.

amplitude fall-off which is significantly slower than 'Inverse square' implies a fairly directional system for one of the parts of the final combination. It is of course possible to work the same 'trick' using more than just two units/sets of speakers, but advantages must then be set against cost. It is also easily possible to deliberately alter the amount of cancellation with frequency (by using units or multiple configurations where the directivity varies with frequency). This may be desirable to improve upon the subjective acoustics of the listening room The technique can be used over a wide range of frequencies. It is often convenient to combine 'open baffle' unit/s with enclosed unit/s in the mid range register, to achieve the necessary directivity with a fairly small system, whereas the upper registers can usually be handled with greater precision using enclosed (often horn) tweeters, multiple arrays being used for the low level, highly directional system required to cancel the wall reflections.

Speakers best for use with this technique will have predictable polar, phase and transient response, and in particular, at treble frequencies, the 'new' Piezo-electric tweeters (in either open cone or horn variety) are exceptionally good.

Improvements in realism achieved this way are nothing short of dramatic, when compared with those obtained using other, more conventional methods.

Claims (10)

1. A loudspeaker system in which the speaker drive units (of any suitable type) may be considered to be grouped into two parts, each part handling, for much of its range, the same nominal range of frequencies, but exhibiting entirely different spatial radiation characteristics, (Such as, for example, spherical or plane wave-front propagation), for given frequencies or frequency band/s within that common frequency range.

2. As in claim 1 ,where the systems parts may be temporarily or permanently connected so as to radiate sound with mutually reversed polarity for frequencies in the common range (although not necessarily all such frequencies).

3. As in claim 2, where the relative levels of the two parts can be adjusted so as to produce full or partial sound cancellation at a known distance from the system, for a given frequency or frequencies, or over a given frequency range or ranges.

4. As is claim 3, where separate units within the system are able to be set to produce results as in 3. (But not necessarily having equal effect over all the range of frequencies concerned).

5. As in any claim, where the relative positions of the various drive units can be adjusted in any way.

6. As in claim 1, or 2, where the relative levels of the two parts can be adjusted so as to produce full or partial sound cancellation at a given distance (Not necessarily the same in every case), and at a given angle or angles away from the main system's axis (Not necessarily the same in every case), for a given frequency or frequencies, or over a given frequency range or ranges.

7. Substantially as in any claim, but where the system can be considered to be grouped into more than two parts for the purposes described.

8. A Stereophonic or Multi - Channel system, using the above technique for one or more of its channels.

New claims or amendments to claims filed on 12th January 1981 Superseded claims 1,5,6 New or amended claims: Claim 1. A loudspeaker system in which the speaker drive units (of any suitable type) may be considered to be grouped into two groups or parts whose major axes are substantially parallel (any may even be coincident), each group or part handling, for much of its range, the same nominal range- of frequencies, but exhibiting entirely different spatial wavefront radiation characteristics, (such as, for example, 'spherical' and 'near-planar' or 'cylindrical' wavefront propagation) for given frequencies or frequency bands within that common range.

Claim 5. As in any claim, where the relative position of any of the various drive units is, or can be, temporarily or permanently altered in any way without changing substantially the nature of said claim.

Claim 6. As in any claim where the relative levels of the two parts can be adjusted so as to produce full or partial sound cancellation at a given distance (Not necessarily the same in every case), and at a given angle or angles away from the main system's axis (Not necessarily the same in every case), for a given frequency or frequencies, or over a given frequency range or ranges.

Claim 9. As substantially in any claim, but where the major axes of the groups or parts are NOT substantially parallel.

Claim 10. Any loudspeaker/system which claims to be able to eliminate, control or enchance any characteristics of a listening room, either partially or fully, by means outlined in any other claim.