GB2516936A - Loudspeaker driver - Google Patents

Abstract

The driver has an electro-mechanical transducer 205 for converting at least part of an electrical input signal into an output comprising mechanical oscillations. Coupling means 206 couple these oscillations to a displacement member, such as a rigid diaphragm, for converting the oscillations into sound waves. The coupling means 106, 206 acts as a mechanical low-pass filter to absorb energy from frequency components of oscillations of the transducer above a normal operating band for the displacement member so as to prevent said energy from being transmitted to the displacement member. The mechanical filter may replace or augment an electrical crossover for the loudspeaker, and may be incorporated into a build ring which also acts as the coupling means.

Description

Title: Loudspeaker Driver

Field of the Invention

This invention relates to a driver for a loudspeaker, a loudspeaker fitted with such a driver and to a build ring for such a driver.

Background to the Invention

a A loudspeaker driver is, in essence, an electro-acoustie transducer that converts an electrical audio input signal into an audio output. Typically, the driver will have an electro-mechanical transducer, such as a voice coil and associated permanent magnet, which generates linear output oscillations in response to the electric current fed to the voice coil. Those oscillations are coupled to a diaphragm which is vibrated by the is transducer so as to generate the sound waves that constitute the audio output. A range of different materials, and constructions, have been used for the diaphragms.

Traditionally, a diaphragm would be formed from paper, but more recently more rigid forms of construction, of metal or a sandwich construction which may include a metal layer, have become popular among speaker manufacturers. These types of diaphragm exhibit low internal loss.

A given speaker diaphragm will not be capable of operating effectively over the frill range of audible frequencies, so that, for example, a diaphra which is required to reproduce lower frequency sounds, say from 2OHz-3kHz, may not be able adequately to operate at higher frequencies. This is in part due to the inherent inertia in the diaphragm, and can manifest itself in modal behaviour of the diaphragm. In addition, some of the more rigid constructions of diaphragm can suffer from breakup as a result of the diaphragm flexing so as to set up standing transverse waves across its surface.

Accordingly, the electrical signal that reaches the transducer is normally filtered by a crossover which splits the audio input signal into separate frequency bands that can be separately routed to drivers specifically designed for reproducing audio in the relevant frequency bands.

I

A crossover can be an expensive component, especially if it has to be able to attenuate the input signal to the transducer sufficiently to prevent a breakup mode (Lc. undcrdampcd or uncontrolled modal bchaviour) forming in a rclativcly rigid, metal diaphragm.

Summary of the Invention

According to a first aspect of the invention, there is provided a driver for a loudspeaker, the driver being operable to generate, from an electrical input signal, an io audio output in one of a number possible bands of audible frequencies, the driver comprising an electrical-mechanical transducer for convcrting at least part of thc electrical input signal into an output comprising mechanical oscillations, a displacement member for converting said oscillations into sound waves constituting the audio output, and coupling means for coupling the transducer to the displacement is member, whcrein the drivcr includes a mechanical filter for absorbing cncrgy from frequency components of oscillations from the transducer outside said band so as to prcvcnt said energy from being transmitted to the displacement member.

Preferably the mechanical filter is a low pass filter for absorbing energy from the frequency components of the oscillations of the transducer above said band.

The filter can thus prevent higher frequency oscillations from being transmitted to the displacement mcmbcr or can at least attcnuatc thosc higher frequency vibrations that are transmitted. The mechanical filter thus can replace or augment the electrical crossover circuitry used in a loudspeaker which employs a driver as one of its band limited drive units. In the latter case, the mechanical filter can enable the electrical crossover to be of a simpler and cheaper construction than would otherwise be the case. Alternatively, the performance of a loudspeaker having a crossover of a given cost can be improved by the mechanical filter.

Preferably, the displacement member comprises a diaphragm, for example a speaker cone or dome.

Preferably, the diaphragm is substantially rigid, in the sense that it exhibits little or no internal flcxing damping. Such a diaphragm may comprisc a metal cone or domc, and for example may be constituted by an Aramid sibstrate sandwiched between ceramic coated metal (for example aluminium) skins. Additionally or alternatively, such a diaphragm may comprise a woven fibre (for example carbon fibre or a paranaramid synthetic fibre such as Kevlar) cone or dome.

Rigid diaphragms are, in speaker design generally preferred to diaphragms of a traditional cone material, such as paper or polypropylene, but can be more prone to larger breakup modes, corresponding to standing wave modes occurring across the diaphragm. The invention is therefore especially beneficial for drivers that have rigid diaphragms as the mechanical filter helps to prevent oscillations that cause such breakup modes being transmitted to the diaphragm.

Preferably, the mechanical low pass filter is incorporated into the coupling means so that thc transduccr acts on thc diaphragm through thc filter.

This enables the mechanical filter to be provided without the need for the driver to have additional components, compared with a conventional driver.

The diaphragm may extend over the front, i.e. output portion, of the transducer, and the mcchanical filter may be attached to the rear of the diaphragm.

The mechanical filter preferably comprises an upstream attachment portion attached to an output portion of the transducer and a downstream attachment portion attached to the displacement member, the filter further comprising a plurality of spaced apart fingers which connect the upstream portion to the downstream portion.

The fingers can have the necessary resilience and damping qualities to provide the desired filtering effect, in which case the characteristics of the filter can, at the design stage, be relatively easily selected by suitable selection of for example, the length, angle, thickness and/or cross-sectional shape of the fingers.

The spaced fingers also allow the filter to have openings that permit air to pass between the rear of the displacement member and the transducer, to reduce air compression behind the displacement member and/or improve cooling of the s transducer. Where the transducer comprises a magnet and voice coil assembly, the arrangement may be such that the openings allow air to pass between the rear of the diaphragm and the space immediately in front of the magnetic pole of the assembly.

Where the oscillations generated by the transducer are generally linear, each finger preferably comprises a respective pillar which has an axis in the same plane as the axis of said oscillations.

This limits the number of parameters affecting the per%rmance of the mechanical filter, and thus facilitates the designing of a filter with the desired characteristics.

Preferably, the pillars are narrower than the spaces between neighbouring pillars.

This facilitates the passage of air as discussed above.

Preferably, the upstream and/or downstream attachment portions are annular.

The mechanical filter may conveniently be of a plastics material lbr example Acrylonitrile Butadiene Styrene (ABS) or nylon.

Preferably, the mechanical ifiter is an injection-moulded component The mechanical filter may to advantage constitute a build ring for the driver.

The mechanical filter may to an advantage be such as to magnify oscillations, fixm the transducer, at frequencies just below a cut off frequency of the mechanical filter.

The cut off frequency may be in the region of 3-8kHz, or one-two octaves below a breakup frequency of the diaphragm. Alternatively, where the driver is fbr use as a tweeter, the cut off frequency may be in the region of 30kHz, or 1-2 octaves below the breakup frequency.

The frequencies at which said magnification occurs may comprise the 1-4kHz band.

Additionally or alternatively, said frequencies may be 2-3 octaves below a breakup frequency. This magnification can increase the usable range of the transducer.

Preferably, the mechanical filter is formed from a material having a Young's Modulus a in the range 0.25-2GPa, ifthe driver is to be used for low or mid-range frequencies or in the range 5-1OGPa if the driver is to be used as a tweeter.

According to a second aspect of the invention, there is provided a driver for a loudspeaker, the driver comprising an electrical-mechanical transducer for converting is at least part of an electrical input signal into mechanical output oscillations, a displacement member for converting said oscillations into sound waves constituting audio output and a mechanical filter through which the transduccr output is attached to the displacement member, wherein the mechanical filter comprises an upstream mounting portion attached to the transducer output, a downstream mounting portion attached to the displacement member and damping means connecting said portions, said damping means being such as to absorb energy from frequency components, above a cut off frequency of the oscillations produced by the transducer.

Preferably, the damping means comprises a plurality of spaced apart fingers or pillars.

Preferably, the mounting portions are annular.

Preferably, the mechanical filter constitutes a build ring for a loudspeaker driver.

The invention also lies in a loudspeaker having a driver as aforesaid, and in a build ring for such a driver, the build ring incorporating said mechanical filter.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a sectional side view of a prior art loudspeaker driver in which a metal diaphragm is attached to a voice coil former by a build ring; Figures 2A-2C show the prior art build ring respectively from above, in side elevation a and in side section (Figures 2B and 2C showing only part of the build ring); Figure 3 is a graph showing the frequency response of the driver of Figure 1; Figure 4 is a sectional side view of part of a first embodiment of loudspeaker driver in Is accordance with the invention; Figure 5 is perspective view of the build ring used in the embodiment ofFigure 4; Figure 6 is a sectional side view, corresponding to Figure 1 of a second embodiment of driver in accordance with the invention; Figures 7A-7C respectively correspond to Figures 2A-2C, and show the build ring for the second embodiment; Figure 8 is perspective view of the build ring of the second embodiment; Figure 9 is a graph of transmissibility against frequency for a vibration isolator; Figure 10 is a lumped parameter model of idealised loudspeaker drivers, relating the various characteristics of the different loudspeaker components to a model of a forced vibration system; Figure 11 illustrates a way of modelling the mechanical pmperties of the build ring, of either embodiment of driver in accordance with the invention, lbr the purpose of designing a build ring with the desired ifitering characteristics; s Figure 12 is a graph showing a simulated comparison of voice coil and loudspeaker cone accelerations fbr a loudspeaker driver fitted with a build ring that incorporates a low pass ifiter; Figure 13 is a graph showing the measured responses of a driver when fitted with various different types of nylon build rings, each acting as a low-pass filter, and an aluminium build ring which has substantially no filtering effect; Figure 14 is a graph showing the impedance responses of the drivers using the nylon build rings used lbr Figure 13, compared with a driver using an aluminium, non-is filtering build ring; Figure 15 corresponds to Figure 13, but in this case plots the difference in the response of each nylon build ring against frequency; and Figure 16 is a graph corresponding to Figure 13 showing the responses of various different types ofABS build rings and of an alumithumbuild ring.

Detailed Descrivtion The speaker driver shown in Figure 1 is of the kind having a one piece rigid diaphragm I which is coupled to the former and voice coil 2 of an electrical-mechanical transducer 4. Traditional dynamic drivers for loudspeakers use paper cones which have inner openings/necks which would be glued directly on to the former of a transducer, with the joint then being hidden with a central dust cap which glues on to the face of the cone. The one piece diaphragm I however, has no such neck, and this type of driver therefore uses a build ring 6 to couple the voice coil and %rmer2tothecone 1.

The components of the driver are mounted on a chassis 8 of, for example, stamped steel, the inboard end of which is attached a magnet assembly 10 which forms part of the transducer 4. The magnet assembly 10 has a annular top plate 12 of ferromagnetic steel which is attached by suitable means to the chassis 8 and, in use, helps to focus the magnetic field from the assembly 10 to the voice coil and the former assembly 2.

The chassis 8 is either riveted or screwed to the top plate 12. The contact surface between the chassis 8 and the top plate 12 is normally bonded, this ensures that there is not movement between the parts. The steel parts are normally bonded to the a magnet, due to the large surface area and the strength of the adhesive bond there arc no issues with detachment The source of the magnetic field produced by the assembly 10 is an annular permanent magnet of ferrite or Neodymium sandwiched between the upper plate 12 is and an annular pole piece 16. The pole piece 16 has a lower annular plate 18 from which a generally cylindrical pole 20 extends into the central passage defined by the former and voice coil assembly 2. It will be appreciated that both the former and voice coil of this assembly are cylindrical, and that the voice coil is denoted by the reference numeral 3 and is Tapped around the exterior of the former, denoted by reference numeral 5.

The diameters of the former 5, voice coil 3, pole 20 and aperture within the top plate 12 arc such that the pole 20 and aperture in the plate 12 define a small gap into which the assembly 2 extends with a small amount of radial clearance from the pole 20 and from the aperture in the plate 12. The pole piece 16 is made from the same type of material as the top plate 12 and focusses the magnetic field on to the voice coil 3.

The former 5 is held in position in the chassis 8 by means of an annular spider 22.

This is the main suspension component within the driver and is the source of most of the driver's stiffness. The spider is normally made out of doped cotton, although there are many grades and specifications that are used in speaker design. Flying leads 24 connect the coil 3 to an input terminal 26 for the electrical input signal.

With reference to Figures 2A-2C, the build ring 6 has an outer annular platform portion 28 which slopes down towards its radial inner edge 30 (Figure 2C) where the platform 28 meets a wall 32 which is cylindrical and (with the ring oricntated in thc way shown in Figure 2B and 2C) vertical. At the end of the wall 32 opposite the platform 28 is an annular inturned lip 4.

The former 5 fits over the wall 32 and is adhered thereto, whilst an adhesive applied to the top of the platform 28 attaches the build ring 6 to the underside of the diaphragm 1. *Io

Thc diaphragm 1 in this particular case is of a suitable mctal, such as aluminium, and although generally dome shaped is normally referred to as a colle. The diaphragm 1 is connected around its circular outer periphery to the upper edge of the chassis 8 by means of an annular, resilient surround (for example of rubber) 36. The suround separatcs thc air bchind the diaphragm 1 from air in front of thc diaphragm 1 and also acts as part of the suspension for the drive unit, helping to ensure it stays centrally locatcd. Thc surround 36 also adds to thc sound-radiating surface area. Thc drivcr will be scaled onto a suitable cabinet (not shown) at an annular gasket 38 extending around the outer periphery of the chassis 8.

As can be seen from Figure 1 the spider 22 is substantially coaxial with the diaphragm I and former 5, and is mounted on an upwardly facing (with the driver orientated as shown in thc figurc) annular shoulder formcd on thc chassis 8.

In use, the leads 24 will be connected to a source of the audio signal through an electrical crossover which will act as a low pass filter for preventing higher frequency components of the audio signal reaching the coil 3 and hence influencing the diaphragm 1. Figure 3 however shows the response of the diaphragm I to input signals over thc fiiH range of audible frcqucncies. Thc driver has a vcry distinctive frequency response, showing an extended flat region 40 in the lOOHz-lkHz band (below which the bass region rolls of at 6dB per octave). This is followed by a transition region 42 in the lkRz-3kHz band, in which the movement of the diaphragm is still controlled, but is starting to show some signs of modal behaviour. This is followed by a messy region, 44 where breakup occurs. The usable region of the driver is normally, but not limited to, the pistonic region.

After the pistonic region, the diaphragm goes into a region of low order bending modes. At 4kHz, the diaphragm is bending out of phase significantly as it builds up to the first main mode (of transverse standing waves across the diaphragm surface) causing a suck-out in sound pressure levels. The graph then shows a series of spikes above 7kHz each one of which relates to a specific bending mode on the diaphragm surface. These modes can cause poor audio quality, and have to be avoided by means a of the electrical crossover. In general, a two-way loud speaker with an electrical crossover exhibits breakup modes which arc anywhere from 15dB to 30dB below the average output of the speaker.

Ideally, no electrical signal should go to the loud speaker at the frequencies in which is the breakup modes occur, but this is not possible without high order expensive crossovers or an active crossover.

The driver shown in Figure 4 is similar in many respects to the driver shown in Figure 1, and corresponding features are therefore denoted by the reference numerals of Figure 1 raised by one hundred. This driver differs from the driver shown in Figure 1 only in the construction of the build ring 106, which is shown in more detail in Figure with reference to Figure 5, the build ring 106 is an injection moulded plastics component having an upstream annular attachment portion 150 which is connected to a downstream annular attachment portion 152 by means of eight pillars, such as the pillars 153-156. As can be seen from Figure 5, the attachment portions 150 and 152 are coaxial, and each of the pillars extends parallel to the axis of those portions.

The outer surface of the attachment portion 150 has a lower, reduced diameter connector part 158 which is, in use, a close fit within the former 105 to which the ring 106 is adhered at the surface 158. An annular radial flange 160 at the top of the connector 158 acts as a stop limiting the extent to which the build ring 106 can be inserted into the former 105. The attachment portion 152 has a downstream surface 162 which corresponds to the rear surface of the diaphragm 101, to which the ring 106 is attached at the surface 162 by means of a suitable adhesive.

The ideal type of adhesives will depend on the material of the build ring and cone.

One example of an adhesive that may be used is a Z component (Z -C) Methacrylate Adhesive (MMA) such as Hard Loc G-55-03 A and G-55-03 B. The pillars that connect the attachment portions are spaced from each other and thus allow air to flow between the space immediately above the pole 120 (as viewed in io Figure 4) and the space behind the diaphragm 101 surrounding the assembly 102.

This facility for cxchange of air hclps to avoid compression of air immediately behind the central portion of the diaphragm 101, whilst also improving the cooling of the coil 103.

is In addition, the pillars have inherent compliance which gives them damping and resilience properties that cause the ring 106 also to act as a mechanical low pass filter.

The filtering properties of the ring are discussed below.

The embodiment shown in Figures 6 and 8 is very similar in most respects to the embodiment shown in Figures 4 and 5, and corresponding components are therefore denoted by the same reference numerals as are used in Figures 4 and 5, raised by 100.

Figure 6 does show various features which have not been included in Figure 4, but which correspond to features of the prior art driver shown in Figure 1, and these are denoted by the reference numerals used in Figure 1, raised by 200. Indeed, the only difference between these two embodiments lies in the configuration of the build ring 206.

As can be seen from Figure 8, the build ring 205 has slightly different proportions from the build ring 106, having a wider upstream attachment portion 250 (i.e. a portion having a larger axial dimension than the portion 160) and a thinner series of pillars, for example pillars 253-256 connecting the upstream attachment portion 250 to the downstream attachment portion 252. The downstream attachment portion 252 has a slightly larger internal diameter than the diameter of the downstream attachment portion 250, and the pillars therefore diverge slightly from the upstream portion 250 to the downstream portion 252. Although the pillars are no longer parallel with the axis of the attachment portions 250 and 252, each pillar nevertheless has its axis within the same plane as the axis of the attachment portions.

The filtering effect achieved by the rings shown in Figures 5 and 8 will now be discussed in more detail.

Vibration isolation filters can be found in many mechanical systems. For example, in a a car these filters arc used to stop engine vibrations from reaching the driver, in heavy industry thcy isolate large machincs from buildings or structures to reduce or prcvcnt transmission of vibrations to sensitive areas. Vibration isolators work as mechanical first order low pass filters. At low frequencies the energy fed to the isolator is transmitted without any change. As the frequency rises there is often a region where is the filter gives some magnification (in amplitude of transmitted vibrations) before starting to cut the amount of energy being transmitted. Figure 9 shows a general curve tbr a vibration isolator, plotting the transmittability of the isolator against frequcncy. It will be appreciated that, for the magnification in vibrations to occur, the vibrated mass on the output side of the filter would have to be such that less Drk is done on that mass than is being supplied to the filter. In a loudspeaker system, this would equate to thc mass of the diaphragm being less than the mass of the voice coil and former.

After the peak, shown at 270, the transmittability decreases with increasing frequency and at the cut off frequency 272 drops below I, indicating that the filter attenuates higher frequencies. Figure 9 sets out the equation that gives the approximate cut off frequency (where mis the driven mass and f is the frequency).

Referring to Figure 10, in a MDOF (multiple degree of freedom) system such as a loudspeaker driver, the transfer fImction between the input, in this case force F, and the output, displacement x is atlected by all terms in between. The model is correct at low frequencies and is provided so that a transducer engineer can gain a working idea of the fundamental physics.

The diagrams A and B shown in Figure 10 can be thought of as lumped parameter models of idealized loudspeaker drivers.

Diagram A is a simplified Lumped Parameters Model of a known driver (for example as shown in Figure 1), in which: * F is the force supplied by the voice coil to the mass mi.

* mj is the total moving mass of the drive unit. This is the sum of the Voice Coil, Former, Build Ring, Cone, 1/3"' of the surround and 1/3"' of the spider.

* k1 is the stiffness of both the surround and spider. These could be drawn separately as in B -(Ic;, k4/).

* R1 is the total damping, this is a sum of the mechanical damping.

* x is the displacement output when combined with the surface area of the cone gives a volume velocity equivalent to the sound pressure level output of the is driver unit.

Diagram B is a simplified Lumped Parameter Model of a Loudspeaker with Mechanical Breakup Attenuation, in which: * Pis the force supplied by the voice coil to the mass in. This time in1 is just the mass of the voice coil and the former as the bufld ring now separates these from the cone and surround, nt2.

* m1 is the mass of the voice coil and the former it might also contain a percentage of the new build ring but not the whole.

* Ic1 is the stiffness of the spider which attaches below the new build ring.

* R1 is the damping of the spider and the electrical damping generated by the voice coil.

* k2 and R2 are parameters that affect the new build ring (106, 206). The aim here is to tune these so that the filter created attenuates the high frequency breakup region without affecting the low frequency pass band.

* mi is the mass of the cone and surround, it will also contain a percentage of the build ring.

* k is the stiffness of the surround which attaches to the outside of the cone.

It3 is the damping of the surround.

By selecting a favourable material and geometry it is possible to tune the parameters of the build ring 1i2 and 112. This essentially will change the tuning frequency allowing s it to be tuned to complement the driver's peribrmance.

In order to build a real build ring with these properties we need to know the matcrial and geometry required fbr the build ring. The easiest way of doing this is to use a fully coupled multi physics Finite Element Analysis (FEA) modeL This method can avoid some of the limitations of the simplified mathematical model above and allow us to create a much more accurate modeL An acoustic engineer with a background in both transducer design and FEA modelling can produce a simulation of sufficient accuracy to design a build ring which achieves the desired filtering function.

3 Adjusting Tuning Frequency The tuning frequency can be adjusted by changing a number of factors on the build ring. This includes both the geometry of the build ring and the material.

Geometry There are three major parts to the geometry; 25. A top surface (e.g. 162) to adhere the diaphragm onto.

* A number of pillars which together form the tuned mechanical ifiter.

* A bottom small surface (e.g. 158) to adhere the voice coil former onto.

The top and bottom surfaces can be adjusted to provide the strength and contact area required without drastically affecting the tuning frequency while the central section made up of the pillars will be focused on below.

The tuning frequency is affected by: * The total cross-sectional area of all of the pillars * The length of the pillars * The angle between the pillars and the vertical axis (i.e. the axis of the attachment positions of the build ring).

Increasing the total cross-sectional area of the pillars will push the tuning frequency up as the system becomes stronger. A sufficient number of pillars serves to provide a good distribution of forcc around thc build ring and to allow pillars which arc thick io enough to be injection moulded.

Increasing the length of all of the pillars or the angle of the pillars will decrease the tuning frequency as the pillars will be weaker.

is It should also be noted that the size of the pillars affects the air flow through the build ring. If the open area between the pillars is made larger there will be improvements in air flow and cooling.

Material Thc choicc of material for thc ring will also affcct its filtering pcrformancc.

The preferred material will: * have a high loss factor or damping * have a Young's modulus that is within a specific range If the strength of the material is too high then the pillars will become too small to manufacture.

If the strength of the material is to low then the pillars will become too large and the whole build ring structure could become uncontrollable.

After analysing the requirements and running some initial models the applicant believes that the target material would have a Young's Modulus of between 1 and 2 GPa. This will allow the build ring to achieve a cut off frequency in the desiredrange for most drive units.

Material Range of Young's moduli available _________________________________ (GPa) Acrylonitrile Butadiene Styrene (ABS) 1.4-2.8 Polypropylene 1.1-1.6 Polyamide Nylon 66 1.5-3.8 Table 1 -Table of materials with suitable stiffness Table I shows a (by no means exhaustive) list of materials that have properties that make them potential candidates for this design.

Simulation Using multi-physics FEA techniques it is possible to predict the response of a drive unit before prototyping it. In the last few years these methods have become the main way of developing a transducer. A starting point of the design is to model the build is ring on its own to determine the required pillar dimensions for a given cut-off frequency. A simple way to model the mechanical properties of the build ring is to lay it out as a flat sheet as in Figure 11 and define the domains with appropriate masses, thicknesses and materials.

The voice coil part of the sheet is then given a defined acceleration and this is plotted against the acceleration modelled in the cone area of the sheet. The effect of the build ring on the cone acceleration can be seen in Figure 12.

As the cone breakup is expected to occur around 5kHz, the interaction between cone and build ring will not be as straightforward as in Figure 12. There will be interaction between the frequency response of the build ring and the cone. A fully coupled FEA model, which models the magnetic, mechanical and acoustic domains of the drive unit, can be used to determine this interaction.

6 Measurements This section will explore the results of the first prototypes, future improvements in both the geometry and the material would improve on these results so they should be seen as proof of concept. One material will be shown in three different geometries which adjust the tuning frequency (NY1 would have the highest tuning and NY3 the lowest).

a Figure 13 shows the measurements made using prototype drivers having build rings of the general type shown in Figure 5 with differing pillar widths. The referencing trace is from a driver with an aluminium, non-filtering build ring. NY3 denotes a driver with an injection moulded build ring of Nylon 66, with a pillar width of 3mm, NY2 a build ring with 2 mm wide pillars and NYI a build ring which uses a 1 mm pillar is width.

The frequency response of the prototypes shows the build ring working as a low pass filter. As the theory sows the filter gives some gain around the tuning (or cut-off) frequency before attenuating above this frequency. The results also show a shift down in frequency in the main breakup at 5kHz. This is an undesired effect of the build ring, but it is small and outweighed by the reduction in level.

Figure 14 shows that the measured impedance response of the drive unit is not affected by the change in build ring.

The difference graphs in Figure 15 show the attenuation in the clearest manner. The graphs show a small change in the tuning frequency however this change is much smaller than expected. The main difference between the graphs appears to be the amount of gain at 4kHz and then the amount of attenuation at 6kHz. The results show that simple geometry changes can result in large changes to the frequency response.

Above 6kHz the mechanical interaction is dominated by the cone breakup. This explains why the attenuation does not continue to increase at higher frequencies.

The features of build rings (of the embodiments of the invention) can be summarised thus: * A plastic part which mechanically connects a single piece metal cone to the voice-coil lbrmer in a loudspeaker drive unit.

s * The part consists of a number of narrow pillars between two gluing surfaces.

* Thc pillars arc designed to act as a mechanical low-pass ifiter and arc tuned to attenuate the cone breakup frequencies.

* The gaps between the pillars are large compared with the pillars to increase airflow, reduce compression and aid in the cooling of the voice coil.

* The application of the ring is not limited to single piece or domed cone but could cover the whole range of possible cone geometries.

ABS Build Ring Figure 16 shows the perlbrmance of driver prototypes with ABS build rings. The reference design using an aluminium build ring with no breakup reduction. Again, the number at the end of the reference label lbr each trace denotes the pillar width in millimetres. The peak at 3kHz is a design issue which is not related to the build ring and as such should be ignored where possible.

The graph shows that all of the designs benefit from a slight lift below 4kHz and a cut above 5kHz. The first two designs ABSI and ABS2 show little imprevement at the breakup and the measurements would support a hypothesis that the tuning frequency of the build ring is a little high in frequency and therefore not providing a cut.

The third build ring ABS3 shows more of an effect with a significant difference in breakup. The breakup has now dropped in level from 92dB to 88dB. The build ring has also added significant lift between 1kHz and 3kHz which could be useful.