WO2003101147A1

WO2003101147A1 - Improvements in loudspeakers

Info

Publication number: WO2003101147A1
Application number: PCT/GB2003/002243
Authority: WO
Inventors: Mark Alexander Dodd
Original assignee: Kh Technology Corporation
Priority date: 2002-05-24
Filing date: 2003-05-23
Publication date: 2003-12-04
Also published as: GB2403093A; AU2003234023A1; GB0211977D0; GB2403093B; GB0423588D0

Abstract

A loudspeaker cabinet (10) having drive units (20,22) has its internal volume charged with a high proportion of carbon dioxide in substitution for air. The cabinet (10) also contains activated carbon (24), treated to render it hydrophobic, in tubes (26). The carbon dioxide level is maintained by release of the gas from a source (32) by means of a regulator (34), valve (36) and a sensor (38) which may sense for loss of carbon dioxide by monitoring the level of oxygen which increases as air leaks into the cabinet. The combination of carbon dioxide and adsorbent activated carbon can lead to an enhancement of 3- to 6-fold in the acoustic compliance of the speaker.

Description

IMPROVEMENTS IN LOUDSPEAKERS

This invention relates to loudspeakers, and in particular to the achievement of improved bass performance from a minimum size of loudspeaker cabinet.

Numerous attempts have been made to extend the low frequency performance of small loudspeaker systems.

In WO-A-84/03600 (Ward) there is described a technique for reducing pressure variations in an essentially closed volume, such as the interior of a loudspeaker cabinet. The essentially closed volume of the cabinet is effectively reduced in volume when the loudspeaker cones make excursions into the interior of the cabinet. Such excursions tend to increase the pressure in the cabinet and these pressure variations are reduced by locating a mass of material within the cabinet, the material being adsorbent to the gas or vapour in the cabinet. The adsorbing material can be a mass of activated charcoal or carbon in granular form. The granules are described as being held in place by a mesh-like support structure which can be a moulding of plastics material or formed from expanded metal sheet. This can be lined with a porous fabric such as filter paper to prevent small granules from passing through the support structure.

In WO-A-84/03600 it is mentioned that it is desirable to keep the carbon granules free from moisture. Methods of attempting to achieve this described in the aforesaid patent specification include the provision of a moisture impermeable diaphragm located within the box between the granules and the speakers, and also the use of a moisture barrier to prevent moisture from entering the region of the mass of carbon granules .

It has been established that the effect of moisture on the performance of loudspeakers which contain an adsorbing material is not insignificant. Activated carbon adsorbs moisture from the air and this blocks the microporous structure of the granules.

One approach which is described in WO-A-03/013.183 is to use adsorbing material and/or the containment means for it which is at least partially hydrophobic, i.e. water- repellent. In this way one can achieve compliance enhancement of the loudspeaker, i.e. an improvement in the acoustic compliance. In other words, the loudspeaker cabinet can be made to seem acoustically bigger without any physical change to the cabinet. Alternatively, the same acoustic output can be obtained with a cabinet of smaller size.

The aforesaid WO-A-03/013183 describes a loudspeaker comprising a cabinet within which there is provided an adsorbent material which is or which has been treated to make it at least partially hydrophobic. The adsorbent material is preferably activated carbon, suitably treated to provide it with hydrophobic properties.

US-B-6381337 (Greenberg) describes a loudspeaker or microphone which comprises a flexible enclosure and a transducer. The enclosure can be stressed by internal pressure which can be supplied by an inflating gas which can be carbon dioxide. The carbon dioxide is therefore used to provide the enclosure with a degree of rigidity.

US-A-4308431 (Hanbicki) describes a loudspeaker which comprises an air-tight chamber having one major surface formed from a thin gauge plastics sheet and an array of conductive pins extending away from a conductive plate forming the opposite major surface. An electrical signal is applied to the plate and to a conductive coating on the plastics sheet. This creates an electric field within the chamber which the plastics sheet to vibrate in accordance with the electrical signal. The interior of the chamber preferably contains a polar gas which may be carbon dioxide. The carbon dioxide is therefore used as part of the transducer for its polar properties.

US-A-4101736 (Czerwinski) describes a loudspeaker enclosure which contains a bag formed from a soft, pliable membrane and which contains a gas, which may be carbon dioxide, or a combination of a gas and a liquid. The bag is surrounded and separated from the loudspeaker drive unit by an acoustically transparent and porous cocoon which is itself surrounded by acoustical padding.

US-A-2797766 (Sullivan) describes loudspeaker horns and enclosures which contain a relatively heavy gas such as sulphurhexafluoride . It is also mentioned that carbon dioxide has been found to have the effect of increasing or extending the low frequency response of the loudspeaker.

The present invention is concerned with an alternative approach to the solving of the problems associated with the low frequency performance of small loudspeaker systems.

The present invention is based upon the realisation that by providing carbon dioxide within the loudspeaker cabinet in association with an adsorbent material one can achieve up to double the acoustic compliance enhancement of the adsorbent material alone. Moreover, it has been found that the use of carbon dioxide alone leads to only a very small enhancement in the acoustic compliance, typically no more than 10%. The improvement in acoustic compliance achieved with the invention can lead to a lower fundamental resonance frequency and hence improved frequency response. Thus, the combination of activated carbon and carbon dioxide can provide improved bass performance. There may also be a significant reduction in distortion, particularly second harmonic distortion. The adsorbent material is preferably activated carbon which advantageously has been treated to render it at least partially hydrophobic.

It is desirable that the carbon dioxide within the loudspeaker cabinet should be in inherently dry form, i.e. with a low moisture content.

The present invention also provides a loudspeaker assembly comprising wall means defining a substantially airtight rigid enclosure defining a substantially closed interior space, and one or more drive units mounted in respective apertures in the wall means, the interior space containing a gas which has an enhanced proportion of carbon dioxide relative to that of air and a material which is adsorbent to the gas contained in the interior space. Preferably, the or at least one drive unit comprises a moving sound-generating diaphragm which has an outer surface exposed to the ambient atmosphere and an inner surface exposed to the gas contained in the interior space.

Advantageously the assembly also includes a source of gaseous carbon dioxide, means for supplying the interior space with carbon dioxide from the source, sensing means for sensing the gas contained in the interior space and means for controlling the supply means in response to the sensing means to maintain a predetermined content of carbon dioxide in the interior space.

In order that the invention may be more fully understood, an embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing a cross- sectional view through a loudspeaker assembly in accordance with the present invention; and

Figure 2 is a graph showing various characteristics of a speaker in accordance with the invention and a speaker not in accordance with the invention.

Referring to figure 1 of the drawings, there is shown a loudspeaker cabinet 10 having a rear wall 12, a bottom 14, a top 16 and a front wall 18. The rear wall 12, bottom 14, top 16, front wall 18 and side walls (not shown in figure 1) form a substantially rigid enclosure of cuboidal shape. The front wall 18 has in it a smaller circular aperture 18a and a larger circular aperture 18b, behind which are mounted respectively a high frequency drive unit 20 a low frequency drive unit 22.

The drive units consist in conventional manner of conical diaphragms 20a, 22a and drive units 20b, 22b, shown schematically in the figure. The drive units 20, 22 are sealed around their peripheries to the surfaces of the front wall 18 forming the apertures 18a, 18b respectively. The interior of the cabinet 10 is therefore substantially sealed from the ambient atmosphere, front surfaces of the diaphragms 20a, 22a being exposed to the ambient atmosphere whilst their rear surfaces are exposed to the atmosphere within the cabinet 10.

It should be noted that the number, size and frequency responses of the drive units is not materi-al to the present invention: for example, a third drive unit could be provided mounted in a further aperture in the front wall 18 and sealed around is periphery as described above. The third unit could be a further low frequency unit or a unit responsive to some other frequency range. The cabinet may also contain electrical or electronic devices, for example amplifiers and cross-over devices. A mass of adsorbent material 24, for example activated carbon, is located within a plurality of tubes 26 which are suspended within the cabinet from a support arm 28. The tubes 26 can be suspended individually or can be linked together to form curtains extending across the interior of the box. Alternatively or additionally, horizontally extending tubes holding adsorbent material can be provided. The tubes 26 are preferably in the form of tubular bags of material, for example of a multi-filament textile material. The aim is to achieve maximum exposure of the adsorbent material to the surrounding gas in the cabinet 10. The volume of activated carbon relative to the internal volume of the cabinet is shown only schematically in figure 1: in practice up to about two-thirds of the internal volume might be occupied by the activated carbon.

Positioned above the support arm 28 is a shelf 30 on which is positioned a container 32 for carbon dioxide in gaseous or solid form. Connected to the output of the carbon dioxide supply 32 is a pressure regulator 34. The output of the pressure regulator 34 is connected to a control valve 36 which is a slow release valve. The flow through the control valve 36 is controlled by a sensor or sensors 38 connected to the control valve and arranged to sense the carbon dioxide level in the enclosure, either directly or indirectly. Instead of sensing the percentage concentration of carbon dioxide it may be easier to sense the percentage of oxygen in the cabinet enclosure. Assuming a constant ratio of oxygen to nitrogen, this will give a measure, of the volume of air that has leaked into the cabinet and, thus, the volume of carbon dioxide needed to replace it. The aim is always to maintain a constant high percentage of carbon dioxide in the cabinet enclosure. A typical level is about 70% carbon dioxide by volume with the balance air.

In any event, a carbon dioxide level of at least 50% by volume is preferred according to the invention, with minimum levels of 80% or 90% by volume being desirable. Any air entering the cabinet from the ambient atmosphere will be expelled by the carbon dioxide released from the container 32. The drive unit and cabinet enclosure must be airtight apart from a small leakage path, as represented by a vent tube 40 positioned at the bottom of the cabinet enclosure.

The vent tube 40 can include a moisture trap to prevent moisture entering the cabinet from the ambient atmosphere.

As an alternative to the arrangement of the regulator 34, valve 36 and sensor 38, it may be possible to arrange simply for carbon dioxide to be released at a constant low rate from the container 32, in order to compensate for leakage from the cabinet 10 taking place at a substantially similar rate.

Carbon dioxide may be able to flow through some of the materials which constitute the loudspeaker. It is therefore desirable to use materials which will help prevent this or reduce the flow to a minimum. The loudspeaker cone for example might be metallised.

The output of carbon dioxide through the control valve 36 will be dependent upon the particular characteristics of any given loudspeaker. A few cubic centimeters per hour is envisaged as being appropriate for many loudspeakers. The rear wall 12 of the loudspeaker cabinet can be provided with an access door in order to be able to remove and replace the carbon dioxide container 32.

Although in the embodiment which is described above the supply of carbon dioxide and the pressure regulator and control valve are all shown as being located within the loudspeaker cabinet 10, they could alternatively be provided in an add-on box associated with the cabinet.

The adsorbent material 24 within the container bags 26 is chosen to provide the maximum acoustic benefit from its combination with the carbon dioxide. It may be activated charcoal or carbon in granular form. Alternatively, it may be a material such as described in WO-A-03/013183 which is an absorbent material which is or which has been treated to make it at least partially hydrophobic. The key feature of the invention is that the provision of carbon dioxide within the same enclosure as the adsorbent carbon material gives a significant reduction in distortion and improved bass response. The positions of the materials within the cabinet enclosure, the proportion of carbon dioxide in the gas within the enclosure, and the particular adsorbent material used are all matters of choice, selected for the best response for any given loudspeaker.

Figure 2 shows in its upper part the frequency response curves for a loudspeaker according to the invention (curve A) and the same loudspeaker with its cabinet containing air (curve B) . The y-axis shows sound pressure level ("SPL") in units of decibels. It will be noted that curve A shows a significantly higher SPL over the frequency range from 20Hz to about 50Hz and a significant reduction in the resonant hump at around 100Hz, compared with curve B.

The lower part of Figure 2 shows the electrical impedance measured in ohms of the loudspeaker of the invention (curve C) compared with that of the same speaker having a cabinet containing air (curve D) . It will be noted that curve C shows an impedance peak at a significantly lower frequency (about 55Hz) than curve D (about 80Hz) , indicating an increase in acoustic compliance and an improved bass response .

Claims

CLAIMS :

1. A loudspeaker comprising a cabinet whose gaseous content has an enhanced proportion of carbon dioxide and which includes material which is adsorbent to the gas in the cabinet .

2. A loudspeaker according to claim 1, in which the adsorbent material is activated carbon.

3. A loudspeaker according to claim 2, in which the activated carbon has been treated to render it at least partially hydrophobic^'.

4. A loudspeaker according to any preceding claim, in which the cabinet houses a source of carbon dioxide, control valve means, and sensor means to monitor the proportion of carbon dioxide in the cabinet.

5. A loudspeaker according to claim 4, in which the sensor means detects the percentage of oxygen in the cabinet.

6. A loudspeaker according to claim 4 or 5, in which the control valve means can be set to deliver carbon dioxide at a rate of a few cubic centimetres per hour.

7. A loudspeaker assembly comprising wall means defining a substantially airtight rigid enclosure defining a substantially closed interior space, and one or more drive units mounted in respective apertures in the wall means, the interior space containing a gas which has an enhanced proportion of carbon dioxide relative to that of air and a material which is adsorbent to the gas contained in the interior space.

8. A loudspeaker assembly according to claim 7, in which the or at least one drive unit comprises a moving sound-generating diaphragm which has an outer surface exposed to the ambient atmosphere and an inner surface exposed to the gas contained in the interior space.

9. A loudspeaker assembly according to claim 7 or 8, including a source of gaseous carbon dioxide, means for supplying the interior space with carbon dioxide from the source, sensing means for sensing the gas contained in the interior space and means for controlling the supply means in response to the sensing means to maintain a predetermined content of carbon dioxide in the interior space.

10. A loudspeaker assembly according to any of claims 7 to 9, in which the adsorbent material is actuated carbon which has been treated to render it at least partially hydrophobic.

11. A loudspeaker or loudspeaker assembly according to any preceding claim in which the carbon dioxide constitutes at least 50% by volume of the gas within the cabinet or interior space.