GB2502282A - A small-volume loudspeaker - Google Patents

Abstract

The loudspeaker is configured so that: · the ratio r between the value of the impedance Z of the loudspeaker at the resonance frequency f0 and the electrical resistance Re of the loudspeaker is greater than 5, and · the Q factor of the loudspeaker is less than 30.

Description

The present invention concerns a loudspeaker.

The current trend in loudspeaker industry is to design compact loudspeakers, much flatter than their previous counterparts. This is done in order to integrate them to systems such as flat panel displays, in/on-wall speakers, cars, laptops, tactile pads and the like.

However, such flat and compact loudspeakers do not render bass frequencies well, if at all.

This is due to the available technology, which suffers from the following drawbacks: reproducing bass frequencies requires a high air volume and consumes a lot of electrical power.

A high power consumption by itself can generate cost issues and more importantly, thermal handling issues. In compact designs, less space is left to cool the device, limiting the performance and speeding up the aging of the loudspeaker.

In the conventional approach, the operating frequency of the loudspeaker is large. In practice, it starts slightly after the resonance frequency and has no real end but the one due to any of the following undesirable effects: mechanical break-up modes of the membrane, electrical inductance effects, directivity drops, etc. Moreover, equalization is easy to implement and leads to a robust output since the system is inherently equalized. Any changes in the environment (e.g. temperature, aging, etc.) will only have unnoticeable effects on the equalized response/output.

However, when using a conventional driver operating in a small volume, the acoustic response levels are significantly decreased. In order to compensate for this effect, the power levels have to be increased, which leads to the aforementioned issues.

From this, one can conclude that the conventional approach is not the most efficient when dealing with small volumes.

Document US 7702114 tries to address the problem of low frequency rendering in small volumes by providing a loudspeaker that operates at or near its resonance frequency.

However, the solution presented in this document focuses on a single resonance frequency, addressing the volume issue on only a small fraction of the bass frequencies.

Furthermore, the proposed loudspeaker does not address the power consumption issue as it has low impedance at resonance frequency.

The present invention has been devised to address one or more of the foregoing concerns.

In order to improve the behaviour of loudspeakers in the low frequency range and in very small volumes, an approach based on a high-impedance resonant system is proposed.

Such a system can be further described as a loudspeaker having a resonance at low frequencies that has a large range, and has high impedance in that range.

Resonant systems in the low-frequency range enable a high volume output, and a high impedance system consumes little power.

Accordingly, the present invention provides a loudspeaker for producing sound over an operating range of frequencies in response to an excitation electrical signal, the loudspeaker being configured to have a resonance frequency (ff3) lying within the range of frequencies, the loudspeaker being further configured so that: The ratio (r) between the value of the impedance (Z) of the loudspeaker at the resonance frequency (ff3) and the electrical resistance (Re) of the loudspeaker is greater than 5, and -the quality factor of the loudspeaker is less than 30.

Such an approach is well-adapted to rendering low frequency sounds despite a low air volume.

The main advantage of a high-impedance resonant system lies in the improvement of the performance in terms of sound pressure levels (SPL).

Furthermore, due to high impedance values, power consumption is low in the operating range.

Despite the operating frequency range being limited compared to prior art loudspeakers, it is not a problem since high frequencies are not so difficult to reproduce as bass frequencies may be. Moreover, crossover techniques can be used to improve the performance.

Conversely, any equalization concerns may be dealt with by using fully numeric robust equalization strategies (e.g. closed-loop adaptable equalization) so that the response remains stable.

The need for a higher voltage is not an issue either as currents are much smaller and in conventional cases, the electric power is much lower.

In order to obtain an efficient magnetic circuit, in an embodiment, the loudspeaker is configured so that the ratio BL2/(Rms Re) is greater than 8, where B is the magnetic field produced by the magnet of the loudspeaker, L is the length of the coil of the loudspeaker and Rms is the mechanical resistance of the moving part(s) of the loudspeaker.

In an embodiment, the loudspeaker encloses a volume of air that is at most 5 litres.

In a particular embodiment, the loudspeaker encloses a volume of that is at most 3 litres, preferably 1 litre.

In an embodiment, the ratio r is greater than 10, preferably taken between 15 and 20.

In an embodiment, the quality factor Q is less than 20, preferably taken between 10 and 15.

In an embodiment the loudspeaker is configured so that the ratio Sd2IV is greater than 0.05, where Sd is the surface of the membrane of the loudspeaker, and V is the volume of air that is enclosed by the loudspeaker.

This represents a good ratio between the volume of radiated air (proportional to Sd2) and the acoustical stiffness of the speaker (proportional to V).

In an embodiment, the loudspeaker comprises amplification means adapted to amplifying an audio signal that is supplied to the loudspeaker.

In other words, the amplification means are able to amplify under high tension and low current conditions, or high impedance conditions that vary with the frequency.

To be more precise the amplification means are able to amplify the signal under the conditions Z> 5 and 0 c 30.

In an embodiment, the loudspeaker further comprises equalization means for equalizing the audio signal that is supplied to the transducer.

In an embodiment, the amplification means are suitable for amplifying the audio signal after it has been equalized by the equalization means.

Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which: -Figure 1 is a graphical representation of the acoustic response of a conventional loudspeaker and of a loudspeaker in the context of embodiments of the present invention in the same enclosure volume; -Figure 2 is a graphical representation of the electrical impedance of a conventional loudspeaker and of a loudspeaker in the context of embodiments of the present invention in the same enclosure volume.

-Figure 3 is a schematic diagram in perspective of a section of a loudspeaker in the context of the embodiments of the present invention -Figure 4 is a schematic diagram in perspective of a section of some of the elements of the loudspeaker shown in Fig. 3 (magnetic circuit and lower part of the enclosure); -Figure 5 is a schematic diagram in perspective of a section of some of the elements of the loudspeaker shown in Fig. 3 (moving part and upper part of the enclosure) -Figure 6 is a schematic diagram in perspective of a section of some of the elements of the loudspeaker shown in Fig. 3 (exploded view of Figure 4); -Figure 7 is a schematic diagram in perspective of a section of an element of the loudspeaker shown in Fig. 3 (membrane and spider former); In Fig. 1 a graphical representation of the acoustic response at constant power of two loudspeakers are schematically depicted. The sound pressure level SPL (vertical axis) produced by the loudspeaker is shown to vary with the frequency f (horizontal axis), the power consumption being constant (1W) and the distance between the top of the membrane and the pressure sensor being constant at 1 m.

The plain line corresponds to the response of a 1.1 L loudspeaker according to the invention while the bold line represents the response of a conventional 1,1L loudspeaker. It should be noted that the ilL figure has been taken merely by way of example and can be changed in other embodiments of the invention.

As can be seen, the SPL of the conventional loudspeaker drastically decreases under a certain frequency fl (around 70 Hz).

On the other hand, the SPL of the loudspeaker according to the present invention reaches a maximum value of 91dB at resonance frequency fo, a value that is substantially maintained (within 5Hz) over a large range of frequencies, spanning 30 Hz, which corresponds to a quality factor Q of 30.

In figure 2, a graphical representation of the electrical impedance of two loudspeakers is schematically depicted.

The electrical impedance IZI (vertical axis) produced by the loudspeaker is shown to vary with the frequency f (horizontal axis).

The plain line corresponds to the response of a 1.1L loudspeaker according to the invention while the bold line represents the response of a conventional ilL loudspeaker. It should be noted that the ilL figure has been taken merely by way of example and can be changed in other embodiments of the invention.

As can be seen, inside the operating range, RI values of the loudspeaker according to the present invention are much higher than the equivalent conventional loudspeaker ones.

The resonance frequency itself is inside this range. The ratio between the impedances inside and outside the range is close to 20.

In order to obtain such a large high resonance peak while being at constant power, i.e. at high impedance Z, the applicant has considered making the following parameters vary: -Mechanical damping Rms; -Force factor BI, that is the product of the density B of the magnetic field in the loudspeaker and the length I of the coil -Electrical resistance of the coil Re; -The weight Mmd of the membrane; and -The diameter CD of the membrane; For instance, in order to obtain resonance in the desired operating range, that is, at low frequencies (lower than 100Hz), Mmd and CD can be chosen accordingly. To be more precise, for a given Mmd, there is only one diameter CD that can yield the desired resonance characteristics (amplitude and width of the frequency range).

In addition, in order to obtain a high SPL value at the resonance peak, the applicant's experimentations have shown that: -the lower the Rms value, the higher the SPL peak value progresses, and -the higher Mmd and CD values, the higher the SPL peak value.

Therefore, Rms has to be relatively low while Mmd and CD remain high in the right proportions as to obtain the desired resonance frequency. By way of example, decreasing the value of Rms can be achieved by modifying the materials in which the suspensions of the loudspeaker are made of.

Moreover, in order to obtain a quality factor Q under the desired value of 30, the applicant has conducted experimentations that have shown that the lower the values of Mmd and CD, the larger the range of the resonance peak becomes.

Finally, low power consumption can be achieved by increasing the speaker's efficiency, i.e. increasing the impedance value Z. At low frequencies, complex electrical impedance Z can be written as: Re Bit'

U

Where v is the complex speed of the membrane and U the complex voltage.

Moreover, the loudspeaker efficiency can be written as: e i.e. the ratio between the generated mechanical power Pm and the consumed electrical power Fe.

Considering that Pm = real (BlvI*) and that Pe = real(UI*) where I is the current in the voice-coil, r can be further written as = 1 + Re111112 real (B 1 i'll) The mechanical equilibrium equation linking the electromagnetic force BIl and the loudspeaker moving part velocity v is F ii BlI=Ijzzr+Rnts+. It' jwCmsi where Mrns is the weight of the moving parts of the loudspeaker, Rms the mechanical resistance of the suspension of the moving parts and Cms the mechanical compliance of the latter.

This can be further written as fill = ABII, A being a constant complex number since Mmms Rms and Cms are held constant.

real (A) Therefore 17 = Re real (A)+ A being constant, maximum efficiency can be achieved by maximizing -. Re

By doing so, high impedance is also achieved.

The loudspeaker shown merely by way of non-limiting example of Fig. 3 is meant to take advantage of the previously-mentioned observations.

It comprises a motor 2 and a membrane 4 encased in an acoustic enclosure 8.

The present invention being centred around compact loudspeakers, loudspeaker 1 is relatively flat compared to prior art loudspeakers. For instance, loudspeaker 1 shown on Fig. 3 is disc-shaped and less than 30mm thick.

The volume contained within the acoustic enclosure 8 is therefore substantially small. In the preferred embodiment shown on Fig. 3, it is around 1L.

Acoustic enclosure 8 is divided into a top enclosure 10 and a bottom enclosure 12.

Top enclosure 10 is itself divided into an annular disc ba and a central disc lOb that rests upon annular shoulder bc.

Annular shoulder bc is regularly pierced on its outer edge by holes lOd in which screws 11 bolt annular disc 10 a and central disc lOb together.

Bottom enclosure 12 comprises a base 12a that is pierced in its centre as to house motor 2. For that purpose, motor 2 rests on a central shoulder 12b.

The outer edge of base 12a arises to form an annular edge 12c upon which top enclosure 10 is resting.

To be more precise, top enclosure 10 rests upon the top face 12d of annular edge 12c. Top face 12d is regularly pierced by holes 12e in which screws 13 bolt top enclosure 10 to bottom enclosure 12.

Now referring to Fig. 4, motor 2 is encased in a casing 14 in the shape of a cylinder that rests upon central shoulder 1 2b.

Inside casing 14 is placed a motor unit 16. Motor unit 16 comprises a

magnet 18, a bottom plate 19 and a field plate 20.

Magnet 18 is being maintained stationary by being stacked between bottom plate 19 and field plate 20. All three are disc-shaped and share the same diameter.

According to a preferred embodiment, magnet 18 and field plate 19 may be of the same thickness, e.g. 5 mm thick, while bottom plate 19 may be slightly thicker, e.g. 6 mm thick.

Referring now to Figs. 3, 5 and 6, the annular space (or magnet gap) 22 that is left between motor unit and casing 14 may house former 24.

In a preferred embodiment, the distance between annular gap 22 and motor unit 16 is 1.35mm.

Former 24, the shape of which can be better seen on Figs. 5 and 6, has a general annular shape with an upper ring that is regularly pierced with holes.

A voice-coil 26 is coiled around former 24 and vertically moveable between an upper and a down position. In a preferred embodiment, the diameter of voice-coil 26 is 56 mm.

As illustrated on Fig. 5, former 24 is mechanically coupled to membrane 4.

As can be seen on Figs. 3 and 7, membrane 4 is disc-shaped and level with top enclosure 10 in the centre of which it is located. It is also of the same thickness.

The diameter 0 of membrane 4 is chosen as to obtain the desired resonance frequency fO, given the mass Mmd of the moving part of the loudspeaker. Alternatively, diameter CD may be given and mass Mmd selected accordingly.

In a preferred embodiment, the diameter CD is 70mm.

Membrane 4 comprises an annular ridge 4a on its lower surface (towards motor 2) that serves as a mechanical link with former 24.

To that purpose, former 24 is inserted within the perimeter defined by annular ridge 4a and is fit so as to touch its inner surface as can be seen on Fig. 4.

Membrane 4 further comprises a spider former 28, an annular ridge placed in the vicinity of the outer edge of membrane 4.

It is longer than annular ridge 4a, as to reach spider 32, a resilient element in the shape of a regularly folded disc, to which it is attached as can be seen on Figs. 3 and 5.

Spider 32 is also attached at its outer end to top enclosure 10.

Therefore it acts a mechanical link between membrane 4 and the outer edge top enclosure 10.

Still looking at Figs. 3 and 5, membrane 4 is also mechanically coupled to surround 30 and to spider 32 through spider former 28.

Surround 30 acts as a mechanical link between membrane 4 and top enclosure 10. It is in the shape of a circular band connecting the top surface of membrane 4 and the inner edge of top enclosure 10.

In order to obtain low mechanical resistance Rms, surround 30 and spider 32 are made in rigid materials so asto obtain less overall dampening.

In addition, the dimensions of the surround and the spider may be chosen so as to further lower the mechanical resistance.

In a preferred embodiment, surround 30 has an external diameter of 115mm and an inner diameter of 100mm while spider 32 has an external diameter of 135mm and an inner diameter of 92mm.

In other embodiments, magnet 18 or casing 14 containing magnet 18 can be arranged so as to be drive membrane 4 instead of former 24. In that case, voice-coil 26 is maintained stationary.

Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention. Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined solely by the appended claims. In particular the different features from different embodiments may be interchanged, where appropriate.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims (11)

1. CLAIMS1. A loudspeaker for producing sound over an operating range of frequencies in response to an excitation electrical signal, the loudspeaker being configured to have a resonance frequency (fO) lying within the range of frequencies, characterized in that the loudspeaker is further configured so that: -the ratio (r) between the value of the impedance (Z) of the loudspeaker at the resonance frequency (f0) and the electrical resistance (Re) of the loudspeaker is greater than 5, and -the Q factor of the loudspeaker is less than 30.
2. 2. The loudspeaker of Claim 1, characterized in that the loudspeaker is configured so that the ratio BL2/(Rms Re) is greater than 8, where B is the magnetic field produced by the magnet of the loudspeaker, L is the length of the coil of the loudspeaker and Rms is the mechanical resistance of the moving part(s) of the loudspeaker.
3. 3. The loudspeaker of Claim 1 or 2, characterized in that it encloses a volume of air that is at most 5 litres.
4. 4. The loudspeaker of any one of Claims 1 to 3, characterized in that it encloses a volume of that is at most 3 litres, preferably 1 litre.
5. 5. The loudspeaker of any one of Claims 1 to 4, characterized in that the ratio r is greater than 10, preferably taken between 15 and 20.
6. 6. The loudspeaker of any one of Claims ito 5, characterized in that, the quality factor Q is less than 20, preferably taken between 10 and 15.
7. 7. The loudspeaker of any one of Claims 1 to 6, characterized in that the loudspeaker is configured so that the ratio Sd2/V is greater than 0.05, where Sd is the surface of the membrane of the loudspeaker and V is the volume of air that is enclosed by the loudspeaker.
8. 8. The loudspeaker of any one of Claims 1 to 7, characterized in that it further comprises amplification means for amplifying an audio signal that is supplied to the loudspeaker.
9. 9. The loudspeaker of any one of Claims 1 to 8, characterized in that it further comprises equalization means for equalizing the audio signal that is supplied to the loudspeaker.
10. 10. The loudspeaker of any one of Claims 1 to 9, characterized in that the amplification means are suitable for amplifying the audio signal after it has been equalized by the equalization means.
11. 11. A loudspeaker substantially as hereinbefore described with reference to, and as shown in Figures 3, 4, 5, 6 or 7.