Patent Description:
To be exact, the present invention relates to the preamble portion of claim <NUM>.

In the prior art especially loudspeakers with two or more drivers (multiway loudspeakers) have exhibited problems with sound diffractions created by discontinuities on the front baffle surface (Face) of the loudspeaker. In practice the high frequency driver (tweeter) has been the most critical part in this sense. The applicant of the present application has created solutions where the surroundings of the tweeter have been formed as a continuous waveguide for high and midrange frequency audio signals either merely for a tweeter and/or midrange driver or alternatively for a coaxial midrange-tweeter driver.

In this application, these kinds of sound sources are referred to as waveguide drivers and they include any drivers located in the centre of this three dimensional waveguide structure. By these solutions good sound quality and accurate directing of the sound energy may be achieved. However, the frequency range and effectiveness of the waveguide for controlling the directivity of radiation depends on the size of the waveguide, determined to a great extent by the surface area covered by the waveguide, and therefore the size of the front baffle (Face) of the loudspeaker. Small waveguide area limits directivity control to high frequencies, such as the tweeter range only. A large waveguide area enables extending the frequency range of directivity control towards lower frequencies, such as the midrange driver frequency range.

When a smaller size loudspeaker is designed, all the drivers usually cannot be placed in the center of the waveguide (such as the low frequency radiator, the woofer) the surface area taken by these other drivers and the drivers themselves will either limit the baffle area available for the waveguide or additionally create harmful diffractions of audio energy, causing deterioration of the quality of the audio signal audible to the listener.

In the prior art there have been attempts to create a loudspeaker with one or more waveguides on the front side of the loudspeaker. The applicant of the present application has earlier created various solutions like this, e.g. in <CIT> and application <CIT>. In these applications were presented solutions where non-coaxial drivers were positioned such that they are not disturbing the waveguide form created on the front surface (Face) of the enclosure and if positioned on the same surface (the front side (Face) of the enclosure) they are covered with a material that functions advantageously as a solid surface in selected frequencies and restricts penetration of the frequencies emitted by the sound source(s) for which the waveguide has been designed and on the other hand being permeable to other frequencies, more specifically the frequencies radiated by the non-coaxial driver(s), typically woofer(s), emit.

Covering the low frequency driver may cause some problems with the dynamic performance of the driver because the volume displacement of air by the driver requires sufficient openings to allow flow of air. In addition the sub volume in front of the woofer may cause unwanted resonances.

<CIT> discloses a cabinet having inner space and a sound emission opening in a front section; a corn-type woofer disposed within the cabinet and a baffle plate having a woofer mounting section and vertically disposed in the inner space so as to face the sound emission opening. A resonant chamber is defined in front of the baffle place for communicating with the sound emission opening. The resonant chamber and the sound emission opening are designed such that resonance occurs between an air mass around the periphery of the sound emission opening and an air spring within the resonant chamber.

<CIT> discloses an enclosure of a loudspeaker provided with plurality of tubes of different length via which the sound is emitted in order to achieve frequency spreading of resonance peaks in the sound pressure level. The side walls of the tubes have opening via which the tubes are coupled to common Helmholtz resonators in order to suppress undesirable resonances in the tubes.

In accordance with the invention at least some of the problems described above are solved by acoustically connecting either resistive resonators to the sub volume of the woofer such that the total volume of the loudspeaker stays as small as possible. Advantageously these resonators are located at least partially around the coaxial element. In addition, the aim of the invention is to improve the dynamical performance of the woofer(s).

More specifically, loudspeaker according to the invention is characterized by what is stated in characterizing portion of claim <NUM>.

According to one embodiment of the invention, the loudspeaker includes at least two resistive resonators connected to each sub volume, the resonator being tuned to at least one of unwanted resonances of the sub volume.

Considerable advantages are gained with the aid of the present invention.

With help of one embodiment of the invention the low frequency driver may be covered and yet problems with the resonances caused by the sub volume of the woofer may be suppressed. In some embodiments the suppression may take place in multiple frequencies by multiple resonators tuned to different frequencies.

With help of the invention the entire front surface (Face) of the loudspeaker can be formed as a continuous waveguide for mid- and high frequencies without any disturbing resonances on form the sub volume of the bass driver, yet keeping the total volume of the loudspeaker as small as possible. By this measure the whole audio range from <NUM> - <NUM> may be directed precisely to one "sweet spot" and in addition the rest of the sound energy is divided to the listening room due to the full waveguide form of the loudspeaker such that the loudspeaker enclosure itself does not essentially affect to the frequency response in other directions than the main direction.

In other words, in the traditional loudspeakers where the complete baffle plate is either planar or only partly curved as a waveguide, the signal formed into other directions than the "sweet spot" will be reflected from the walls of the listening room in a non controlled manner. The invention however provides an enclosure where the sound pressure is optimally distributed to all directions, whereby also the wall reflections sound natural to human ear.

In the following, certain preferred embodiments of the invention are described with reference to the accompanying drawings, in which:.

In accordance with <FIG> one embodiment of the invention the loudspeaker <NUM> includes a coaxial waveguide driver <NUM> comprising a tweeter <NUM> and a midrange driver <NUM> around it. The coaxial driver <NUM> is positioned in the centre of the three dimensional waveguide surface <NUM>, also a front surface (Face) of the enclosure <NUM>. The enclosure is typically made of cast metal, advantageously aluminium. Also other castable or moldable materials, such as λtic combination may be used as a material of the enclosure.

The waveguide surface <NUM> radiates the main acoustic power of the driver <NUM>. The waveguide <NUM> has a smooth continuous surface with axially symmetrical features around the centre of the waveguide driver <NUM>. Two woofer drivers <NUM> are positioned symmetrically on both sides of the waveguide driver <NUM> inside the enclosure <NUM> and narrow ports (openings) <NUM>, first ports are formed just behind the waveguide surface for the woofers <NUM> in order to let the acoustic energy out from the enclosure <NUM>. These first ports <NUM> are in this embodiment in the narrow front ends of the enclosure <NUM> and these ports are partially visible from the listening direction. In other words the first port <NUM> is a U-form slot.

With dashed line are presented the woofers <NUM> and outlines of the woofer sub volumes <NUM> and resonators <NUM> connected to the woofer sub volume <NUM>. The function of the resonators <NUM> is to suppress resonations of the woofer sub volume <NUM>. These resonators <NUM> are positioned partially behind the coaxial driver <NUM> and each sub volume <NUM> has two resonators on both sides of the coaxial driver <NUM>. The sub volume <NUM> has width W and height H such that the ratio W/H is around <NUM> and typically in the range of <NUM> - <NUM>. The resonators <NUM> are typically an integral part of the enclosure.

The resonators are dimensioned such that the longest dimension, in this time length is λ/<NUM> or alternatively λ/<NUM> of the wavelength to be suppressed. In other words if the sub volume <NUM> has an unwanted resonance at wavelength λ, the resonator should be λ/<NUM> long. In frequency domain this means that at resonance f<NUM> , λ=v/f<NUM>, where v is the velocity of sound. Advantageously the resonator <NUM> is filled with a suppressive material <NUM> like PES wool, open-cell foam material, fibre glass, mineral wool, felt, or other fiberous or open cell or porous materials, or alternatively of any solid material that is manufactured in the place of the volume such that the material an open cell or fiberous structure where the cell size or the fiber size as in the dimensional area of <NUM> (micrometer) to <NUM> (millimeter).

With reference to <FIG>, the resonators <NUM> may be also are located at least partially behind the coaxial driver <NUM>.

Referring to <FIG> the two woofers <NUM> positioned symmetrically around the coaxial driver form an equivalent large woofer radiating essentially along the same acoustic axis <NUM> through ports <NUM> as the waveguide driver <NUM> even though the woofers have their own acoustic axis <NUM>.

In other words the loudspeaker <NUM> includes a first driver <NUM>, which is configured to produce a first frequency band B1 and a corresponding first acoustic axis <NUM>, and a second driver <NUM>, which is configured to produce a second frequency band B2, which is different from the first frequency band B1 but may overlap in a cross-over region, and which second frequency band B2 has a second acoustic axis <NUM>. The enclosure <NUM> encloses said drivers <NUM>, <NUM> and comprises a three dimensional waveguide <NUM> positioned on a front surface of the enclosure <NUM> and around the first driver <NUM>.

As described above the second acoustic axis <NUM> of individual woofer drivers are non-coaxial with the first acoustic axis <NUM>, however the resultant axis of the multiple symmetrical woofers working together (equivalent woofer driver) has the same acoustic axis as the coaxial driver, waveguide driver <NUM>. This symmetry is however not required in all embodiments of the invention. The axes <NUM> and <NUM> may be parallel or non-parallel.

Referring to <FIG> the woofer <NUM> is positioned inside the enclosure <NUM> such that a sub volume <NUM> is formed in front of the woofer <NUM> and limited by the woofer <NUM> itself and side walls <NUM>. The resonator <NUM> is acoustically connected to the sub volume <NUM>. A suitable suppressing material <NUM> may be used inside the resonator <NUM> in order to further attenuate the unwanted frequencies.

The side walls <NUM> of the sub volume (front space) <NUM> form a spacer between the driver <NUM> and the enclosure <NUM> sealing the sub volume <NUM> from the rest of the inner volume <NUM> of the enclosure <NUM>. In more detail the inner volume <NUM> is limited by the enclosure <NUM> walls, namely front portion <NUM>, side portions <NUM> and back portion <NUM>.

Typically the first ports <NUM> are directed substantially orthogonally in relation to first <NUM> and second <NUM> axes, most preferably in the range of <NUM>-<NUM> degrees in relation to these axes. However when the first ports <NUM> are conducted to the back portion <NUM> of the enclosure <NUM>, e.g. by channels, the difference between the direction of the first ports <NUM> and the axes <NUM> and <NUM> may be even <NUM> degrees.

The total area of the first ports <NUM> is the critical feature, therefore the first ports <NUM> may be only one single first port <NUM> for each woofer <NUM> as presented in the figures or may be formed of multiple first ports <NUM> like a grid with an area corresponding one single port.

The first ports <NUM> should not disturb the three dimensional waveguide surface <NUM>, and therefore they are advantageously positioned on the side portions <NUM> of the enclosure <NUM>. Of course these first ports <NUM> may be conducted to the back portion <NUM> of the enclosure <NUM> by suitable tubes or channels (not shown). In other words the first ports <NUM> form air passages to areas outside the three dimensional waveguide <NUM> of the front portion <NUM> of the enclosure <NUM>.

The graph of <FIG> shows frequency response of the sub volume <NUM> of the woofer <NUM> (solid line) with one resonance at f<NUM> and corresponding frequency response of a resonator <NUM> acoustically connected to the sub volume <NUM> (dashed line), while the resonator <NUM> compensates for the unwanted resonance of the sub volume <NUM>.

<FIG> shows an alternative embodiment with two resistive resonators (<NUM>) with different lengths for two unwanted frequencies of the sub-volume. Also one or two resistive broad band resonator may be used, advantageously filled with suppressive material. In this case the mechanical dimensions (length, width and depth) of the resonator cavity define the tuning frequency or frequencies of the resonator.

<FIG> shows an alternative with one reactive Helmholtz resonator <NUM>, which is not part of the claimed invention. In general reactive resonators have high quality factor and they are very effective narrow band resonators. Also these type of resonators can be installed several in one sub volume <NUM> if there are several sharp unwanted resonances. This type of resonator is also tuned to the unwanted frequency or frequencies f<NUM>. The dimensioning of the Helmholtz resonator is explained in the following:.

The resonance arises from the effect of the acoustic air mass neck of the resonator <NUM> and the series resonance circuit created by the acoustic compliance of the air volume of the chamber of the resonator. Close to the resonance frequency, the Helmholtz resonator attenuates the unwanted resonance of sub volume <NUM>. The neck-cavity system of the resonator <NUM>, can be derived from the air volume of the cavity of the resonator and the diameter of the neck and its length. <MAT> in which f<NUM> is the resonance frequency, c is the speed of sound, A is the cross-sectional area of the neck, L is the length of the neck, and V is the volume of the chamber.

<FIG> shows an alternative with one reactive panel resonator as a resonator, which is not part of the claimed invention. This embodiment is dimensioned in the following way based on the panel <NUM> mass per unit and cavity depth d:.

Panel resonator/membrane absorber resonant frequency f is defined in the following way: <MAT> where m.

<FIG> shows as a top view a woofer <NUM> having a planar cover <NUM> and short tubes <NUM> forming as well a Helmholtz resonator (which is not part of the claimed invention) where the tubes are the necks and the volume between the cover and the woofer cone forms the volume of the resonator. In <FIG> this solution is presented as a A-A cross section. The tuning principle is the same as in <FIG>.

<FIG> shows another alternative solution, where the resonator <NUM> is formed between the frontal baffle portion and <NUM> and the sub volume <NUM> of the woofer. The resonator may be resistive type without any neck portion. The tuning principle is the same as in previous fig-ures.

Typically the loudspeaker in accordance with the invention functions in accordance with well-known bass reflex principle, where the low frequency driver <NUM> is tuned in resonance with help of the compliance of the air volume contained inside the enclosure <NUM> and the air volume contained inside the reflex port <NUM> of <FIG>.

One embodiment of the invention (<FIG>) can be also described in the following way:
The loudspeaker <NUM> comprises an enclosure <NUM> defining an inner volume <NUM> and including a frontal baffle portion <NUM> (front portion), which has a front port <NUM> for providing a fluid passageway between the inner volume <NUM> and the ambient volume <NUM> of the enclosure <NUM> and a side portion <NUM> extending rearward from the periphery of the baffle portion <NUM>. The side portion <NUM> forms side walls or the enclosure <NUM>. The enclosure further includes a back portion <NUM>, which is typically essentially parallel with the frontal baffle portion <NUM> and forming the back side of the enclosure <NUM>. The loudspeaker <NUM> further comprises a driver <NUM> attached to the enclosure <NUM>, such that the driver <NUM> is arranged at a distance from the baffle portion <NUM>, forming a sub volume <NUM> inside the enclosure <NUM> such that a sub volume <NUM> is formed between the driver <NUM> and the baffle portion <NUM> by a spacer <NUM>, wherein said front port <NUM> acts as a front port between the sub volume <NUM> and the ambient volume <NUM> of the enclosure <NUM>. In accordance with this embodiment a first port <NUM> is formed to the enclosure <NUM> either in the side portion <NUM> or back portion <NUM> in order to connect the sub volume <NUM> and the ambient volume <NUM> with each other.

In accordance with <FIG> one embodiment of the invention two woofer drivers <NUM> are positioned on both sides of the waveguide driver <NUM> inside the enclosure <NUM> and suitable ports (openings) <NUM> are formed for the woofers <NUM> in order to let the acoustic energy out from the enclosure <NUM>.

With reference to <FIG>, the openings <NUM> are covered with an acoustically transparent layer <NUM> forming part of the waveguide surface <NUM>. If needed the acoustically transparent layer <NUM> may be supported from below with support bars <NUM>. The woofer driver <NUM> is typically spaced from the acoustically transparent layer <NUM>.

Referring to <FIG> the two woofers <NUM> form an equivalent large woofer radiating essentially along the same acoustic axis <NUM> as the waveguide driver <NUM> even though the woofers have their own acoustic axis <NUM>.

In other words the loudspeaker <NUM> includes a first driver <NUM>, which is configured to produce a first frequency band B1 and a corresponding first acoustic axis <NUM>, and a second driver <NUM>, which is configured to produce a second frequency band B2, which is different from the first frequency band B1 but may overlap in a cross-over region, and which second frequency band B2 has a second acoustic axis <NUM>. The enclosure <NUM> encloses said drivers <NUM>, <NUM> and comprises a three dimensional waveguide <NUM> positioned on a front surface of the enclosure <NUM> and around the first driver <NUM>. The three dimensional waveguide <NUM> comprises an acoustically selectively transparent portion <NUM> which is acoustically essentially reflecting to sound waves of the first frequency band B1 propagating in a direction angled to the first acoustic axis <NUM>, the waveguide portion <NUM> is essentially transparent to sound waves of the second frequency band B2 propagating in the direction of the second acoustic axis through the waveguide portion <NUM>, and the second driver <NUM> is positioned inside the enclosure <NUM> behind the acoustically selectively transparent portion <NUM>.

As described above the second acoustic axis <NUM> of individual woofer drivers are non-coaxial with the first acoustic axis <NUM>, however the resultant axis of the multiple woofers working together (equivalent woofer driver) has the same acoustic axis as the coaxial driver, waveguide driver <NUM>. This symmetry is however not required in all embodiments of the invention. The axes <NUM> and <NUM> may be parallel or non-parallel.

Referring to <FIG> the woofer <NUM> is positioned inside the enclosure <NUM> such that a sub volume <NUM> is formed in front of the woofer <NUM> and limited by the woofer <NUM> itself, side walls <NUM> and the acoustically selectively transparent layer <NUM>. To the sub volume <NUM> is connected a resonator <NUM>, which is tuned to unwanted frequencies created by the sub volume <NUM>. The resonator <NUM> is resistive. With resistive resonator the suppressive characteristics are of broad band type. In other words the notch around the center frequency f<NUM> created by resistive resonator is not so sharp like in the reactive resonators. The side walls <NUM> of the sub volume (front space) <NUM> form a spacer between the driver <NUM> and the enclosure <NUM> sealing the sub volume <NUM> from the rest of the inner volume <NUM> of the enclosure <NUM>. In more detail the inner volume <NUM> is limited by the enclosure <NUM> walls, namely front portion <NUM>, side portions <NUM> and back portion <NUM>.

In some embodiments of the invention the acoustically selectively transparent layer <NUM> may be replaced by a mechanically protective grid, the grid limiting in this case the sub volume, as well as the inner volume <NUM>. Advantageously the first ports <NUM> are formed in the side walls <NUM> of the sub volume <NUM> and to the side portions <NUM> of the enclosure <NUM> in order to optimize the operation of the woofer <NUM>. Without these first ports <NUM> the performance of the woofer <NUM> may be compromised. The first ports <NUM> may be positioned on any of the side portions <NUM>, e.g. on the short side portions <NUM> as shown in the figures or alternatively to the long side portions <NUM>.

The area of these first ports <NUM> is typically <NUM>-<NUM> % of the area of the openings <NUM> for the woofer <NUM>, most advantageously in the range of <NUM>-<NUM>% of the area of the openings <NUM> for the woofer <NUM>. The total area of the first ports <NUM> is the critical feature, therefore the first ports <NUM> may be only one single first port <NUM> for each woofer <NUM> as presented in the figures or may be formed of multiple first ports <NUM> like a grid with an area corresponding one single port.

Typically the second driver <NUM> is positioned inside the enclosure <NUM> behind the acoustically selectively transparent portion <NUM> and spaced from it, such that a sub volume <NUM> is formed inside the enclosure <NUM> and separated from the inner volume <NUM> by the driver <NUM> and side walls <NUM> formed as a spacer between the driver <NUM> and the front portion <NUM> of the enclosure <NUM>.

In connection with the acoustically selectively transparent layer <NUM> essentially reflecting means reflection or absorption of at least <NUM>-<NUM> % of the acoustic energy, preferably in the range of <NUM>-<NUM> %.

In the same way essentially transparent means transparency of at least <NUM>-<NUM>% of the acoustic energy preferably in the range of <NUM>-<NUM> %.

In the following additional advantageous properties of the acoustically selectively transparent layer <NUM> are presented:
The thickness of the layer <NUM> is advantageously:.

The layer <NUM> should attenuate the acoustical radiation of the waveguide driver <NUM>, meaning typically in frequencies above <NUM>.

In other words the layer <NUM> should have an acoustical impedance (or absorption) as a function of frequency therefore functioning as an acoustical filter in the following way:.

Advantageously the layer <NUM> is formed of holes or pores or their combination in the following way:.

The properties for the ideal material for layer <NUM> are the following:.

The layer <NUM> may cover the loudspeaker front (tweeter <NUM> excluded) or only the holes <NUM>.

The layer <NUM> may be also formed as a metal structure, like mesh or grid with on one or several layers in accordance with the above requirements for porosity and frequency properties. This kind of structure could be formed e.g. by a stack of perforated metal sheets or plates of thickness around <NUM>-<NUM>. The properties of this kind of stack could be adjusted by placement (distribution) of the holes or pores, percentage (openness) of the holes or pores, and the spacing of the plates from each other. The hole or aperture diameter may vary typically around <NUM> -<NUM>. The spacing between the sheets or plates is typically around <NUM>-<NUM>.

A metal structure described above is advantageous, because its propertied can be adjusted freely and the external properties like colour can be as well selected without limitations.

The crossover frequency C is typically the following:.

In accordance with the invention in combination with the large waveguide <NUM>:.

Also an embodiment with only one woofer is possible, however directivity for low frequencies will not be obtained beyond what is provided by the size of the air displacing surface of the woofer in combination with the size of the front baffle of the loudspeaker enclosure.

In alternative embodiments of the invention the selectively transparent portion <NUM> may be replaced by a mechanically protective grid not having complete properties of selective transparency.

In accordance with <FIG> the resonator may be divided into multiple independent sub resonators <NUM>', each having its own resonance frequency.

Claim 1:
A loudspeaker (<NUM>) including
- an enclosure (<NUM>) having a front (<NUM>) portion, side portions (<NUM>) and a back portion (<NUM>) defining an inner volume (<NUM>),
- the front portion (<NUM>) is formed as a waveguide surface (<NUM>) and includes at least one driver (<NUM>, <NUM>) in the center of the waveguide surface (<NUM>) and is capable of radiating the main acoustic power of the loudspeaker (<NUM>) in the direction of a first acoustic axis (<NUM>), and
- two additional drivers (<NUM>) attached to the enclosure (<NUM>),
- the additional drivers (<NUM>) are attached inside the enclosure (<NUM>) such that two sub volumes (<NUM>) are formed inside the inner volume (<NUM>), each sub volume (<NUM>) limited by a corresponding additional driver (<NUM>), spacers (<NUM>) between the corresponding additional driver (<NUM>) and the front portion (<NUM>), and the front portion (<NUM>) of the enclosure (<NUM>),
- at least one first port (<NUM>) is adapted to open from each sub volume (<NUM>) to ambient volume (<NUM>) either to the side portion (<NUM>) or the back portion (<NUM>) of the enclosure (<NUM>),
characterized in that it includes
- at least one resistive resonator (<NUM>) acoustically connected to each sub volume (<NUM>), the resonator (<NUM>) being tuned to at least one of unwanted resonances of the sub volume (<NUM>), and wherein
- each sub volume (<NUM>) has two said resistive resonators (<NUM>) on both sides of the at least said one driver (<NUM>, <NUM>), said two resistive resonators (<NUM>) having different lengths for two unwanted frequencies of the sub volume (<NUM>).