AP146A

AP146A - Acoustic speaker device.

Info

Publication number: AP146A
Application number: APAP/P/1990/000184A
Authority: AP
Inventors: Krister Amneus
Original assignee: Amneus Eng
Priority date: 1989-06-20
Filing date: 1990-06-15
Publication date: 1991-10-02
Also published as: DE3920041C1; AU5748390A; AP9000184A0; WO1990016135A1

Abstract

A speaker device has at least on speaker provided in an opening (15)in a casing. An additional opening (16)in the casing is a resonator port against the environment and has a size giving resonance with the casing volume (vb)at a frequency which is substantially higher than the natural frequency of the speaker. The resonator port contains a flow limiting material (24, 25). The resonator port (16)is followed by an acoustic tunnel (17)forming a tunning unit (17, 24, 25)which shifts the resonance downwards approximately to the natural frequency (fs)of the speaker or below that due to the fact that its length is approximately equal to the largest extension of the casing volume (vb)or larger and that it is bent at least once along its length. The flow limiting material forms a dampening plug in the tunnel mouth.

Description

Amneus Engineering S-162 30 Vallingby

FIELD OF THE INVENTION

The invention relates to a speaker device consisting of at least one speaker being provided in an opening of a casing ^£which has at least one casing opening forming a resonator port of a size giving resonance with the csing volume at a -frequency which is substantially higher than the natural frequency of the speaker and which contains a flow limiting material, whereby an acoustic tunnel follows the resonator port forming a tuning unit which shifts the resonance downwar approximately to the natural frequency of the speaker or belt that, whereby the flow limiting material forms a dampening plug in the tunnel mouth.

STATE OF THE ART

From the DE 27 08 872 Al there is known a speaker device of the above mentioned kind, having at least one speaker mounted in the opening of a casing, with at least one casing opening forming a resonator port (see Claim 1) having such a size that resonance occurs with the casing volume at a frequency which is essentially greater (see Fig. 4 and associated description) than the natural frequency of the speaker (page 13, 10th line from the bottom), and contains a flow limiting material (Fig.

and 3, reference number 59).

In the case of this known speaker device, an acoustic tunnel— follows the resonator port (Claim 5 and Fig. 2 and 3, reference number 57) and forms a tuning unit which shifts the resonance downward approximately to the natural frequency of the speaker or below that (page 17,

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last paragraph to page 18, first paragraph), with the flow limiting material forming a dampening plug in the tunnel mouth (Fig. 3).

The DE-GM 78 21 830 discloses a closed speaker box, the front wall of which has, apart from the opening for the speaker system, still at least one additional opening for the emission of acoustic reflexion radiation from the back wall, said opening(s) having a circular (alternatively a square) crosssection and a cyclindrical (alternatively a square), inwardly protruding, open tube. The diameter and the length of a tube are adapted to a certain frequency range. In particular, the length may be greater than the diameter, which in turn should be smaller than the diameter of the speaker. The tubes are intended to amplify the sound by reflex action, i.e. it is a modification of the bass reflex system.

The DE 29 11 849 B2 discloses a speaker box having a tunnel provided at the wall carrying the speaker, the walls of said tunnel being made from a porous material or being lined therewith in order to dampen stationary waves in the tunnel providing a connection between the interior and the exterior of the box, wherein obviously only frequencies above 500 Hz are of interest.

The DE 31 13 281 C2 gives a schematical description of a speaker box formed as a bass reflex casing wherein the bass reflex opening area in the front wall carrying the speaker is divided into two openings, and each opening is allocated a direction-dependent flow resistance formed by a tubular hollow profile of a certain length, which tube is claimed to have, due to funnel means, a valve effect. The bass reflex system, the opening or the funnel of which is of such a size that the box casing forming a

cavity resonator is tuned to a predefined resonance frequency was improved in such a way that heat supply is facilitated, because air circulates through the two openings.

In the printed matter Burr, Hans Martin, Vergleichstest: Sechs Standboxen um 600 Mark, published in Audio 9/1988, pages 8284, 88-89, a box named MB Quart 350 is described which is claimed to have an outlet dampened with a foam material, which is, according to the manufacturer, no bass reflex opening. -. This is much rather a compensator opening receiving a plastic insert filled with a dampening foam material. However, according to the author of the printed matter this is a bass reflex opening, however, with a resonance frequency which is far too low. A photograph of the front side of this box shows the insert demounted, so that on the one hand the wall opening having a certain wall thickness is visible, on the other hand the rear end of the tubular neck of the plastic insert, which is to be pushed into said opening, said neck obviously being slightly longer than the wall thickness of the box. As regards the measuring data it is said that on this box phase rotations (up to 50 degrees) were found in the low bass range.

APO00146

A device for carrying out a pressure control of speaker devices of the pressure type is known from the DE-C-17 62 237. This device has an opening resealed by a heavy fibre glass layer. The DE-C-17 62 237 states as the characterizing feature of such a device that its effect is essentially such that apart from a certain and significant pressure reaction which is gradually effective against lower frequencies, also a reduction of the pressure maximum of up to 6 dB (i.e. approx. 50%), which occurs in the case of the system resonance

frequency f^, is achieved, where the maximum of the developed electrical impendance curve is statically dampened to such an extent that also a substantial widening of the acoustic quality and Q-factor is achieved in the case of the said system resoncance frequency.

The system tuning ratio prescribed for the known device in the DE-C-17 62 237 causes a deviation from the frequency linearity of the sound level curve against lower frequencies - with other words: If a bass speaker is used, a frequency characteristic is obtained which rather rapidly decreases at frequencies below approx. 200 Hz.

Another characteristic feature of the acoustic system disclosed in the DE-C-17 62 237 is dependent on the tuning ratio intended therein between the size of the casing opening and the natural frquency of the used speaker, i.e. the * resonance frquency of the opening with the casing volume, which needs to be high enough to be capable of providing the intended pressure regulation by the system resonance f^.

Apart from the above mentioned loss in sound pressure in the low frequency range the said dimensioning ratio also bears the risk that the obtained reduction in pressure may become too strong for a really low frequency. This might cause the oppositely directed pressure effect, which is essential for the linear cone decay, to become too weak, and a strong, nonlinear, acoustic distorsion (triangular wave development) as well as bubble sound distorsion may develop, which originates, if it occurs, from the resonator opening which is completely covered by the porous material.

Summarising, it can be said that for achieving an essential dampening of the frequency range itself at fp as intended according to the DE-C-17 62 237, an opening frequency fp is to be chosen as the necessary tuning frequency according to the formula for the Helmholtz resonance (see equation 10 on page 26), which lies considerably above the normal frequency according to the equation fft <«f_s (^see equations 5 and 6 on page 26 as well as the fpj definition on page 24). The frequency fp is assumed to be >>f_s, and Z_s will decrease at the extent in which fp exceeds the natural frequency.

This as well as the other systems constitute attempts to improve the frequency characteristics of the emitted sound level, which, however, succeeds only partially and under a considerable reduction of the whole acoustic efficiency.

DESCRIPTION OF THE INVENTION <0

It is the object of the invention to provide, by means of a particular arrangement of the resonator opening of a speaker device of the kind mentioned at the beginning, an improved, dynamically regulated dampening effect which is essentially effective at the basic frequency of the speaker.

According to the present invention this object is achieved by means of a speaker device characterized in that the length of the tunnel is at least approximately equal to the largest extension of the casing volume (Vp) and that the tunnel is bent at least once along its length.

The invention provides an acoustic system, wherein the dynamic regulating effect achieved according to the invention can , control the system up to the basic

APOO

frequency fj and is effective up to DC (direct current at a frequency of almost 0 Hz). The construction according to the invention can stand high signal levels even at very low frequencies, if a low-frequency speaker is used, and it has a high acoustic efficiency as well as a low distorsion. It is very suitable for the series production of units having essentially identical effects, which is important e.g. for stereophony. The device has the properties of a pressure chamber.

Further features characterizing the system according to the invention are given in the subclaims.

BRIEF DESCRIPTION OF THE INVENTION

The tuning arrangement of the device according to the invention is described below with reference to the attached drawings, wherein

Figure 1 shows a front view of a speaker casing for a speaker device according to a first embodiment example of the invention having a flow resistant insert (not shown) covering the exterior mouth of the tunnel;

Figure 2 shows a section along the line II-II of Fig. 1, where the flow resistant insert has not yet been pushed into the tunnel mouth;

Figures 3-8 show an end view and an axial section of a first, a second or third embodiment of a further tuning unit, which are e.g. tuned to the border frequency occurring in the speaker device;

Figure 9 shows a further tuning unit which is filled with a flow limiting material;

Figures 10-18 show curves obtained with a speaker device according to Fig. 1 and 2 with the use of an FFT

as of an XY writer, type 2308 Briiel & Kjaer, wherein Fig. 10 illustrates the compliance behaviour of a speaker device according to the invention;

Fig. 11 illustrates the compliance behaviour of the same speaker device with an unfilled opening (Helmholtz character);

Fig. 12 illustrates the compliance behaviour in the case of an opening sealed with an adhesive film (near pressure chamber characteristics);

Fig. 13 illustrates the curve according to Fig 10 (as curve 1) as compared with a sealed f^ tuning unit;

Fig. 14 illustrates the relative movement velocity in the case of the speaker device according to the invention as per Figure 10;

Fig. 15 illustrates in the case the relative movement velocity of a device according to Fig.

ii;

Fig. 16 illustrates the sound pressure level differential between the external (P_e) and the internal (Pj-,) sound pressure of the system according to the invention as shown in Fig. 10;

Fig. 17 illustrates an axially measured sound pressure level curve of the speaker device arranged according to the invention;

Fig. 18 illustrates the impedance curve of the device according to the invention;

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Fig. 19 to 22 show tone-burst” measuring curves.

PREFERRED EMBODIMENTS OF THE INVENTION

Figures 1 and 2 show a box-shaped speaker casing having a bottom portion 10, a front wall 11, a back wall 12, side walls 13, and an upper cover wall 14. The front wall 11 has an opening 15 for a speaker (not shown) and a second, relatively large, gap-like port 16 extending over the whole width of the front wall. The port 16 forms the external mouth of a long tunnel 17 extending, as shown, along the bottom portion 10 of the back wall 12 and along part of the upper side 14. The tunnel 17 is defined by the bottom portion 10, the walls 12 and 14, the side walls 13, and wall elements extending between the side walls 13. The tunnel 17 has an essentially constant cross-section and a volume V_t starting at the internal mouth 21 of the tunnel following the actual speaker chamber or the volume of the speaker casing. The actual speaker chamber Vj-, encompassed by the tunnel 17, the side walls 13 and the front wall 11, is internally lined at all sides with disks 22 of a dampening material e.g. mineral fibre mats. Reference number denotes a relatively thick strip of a material having acoustic flow resistance, e.g. a mineral fibre material, and denotes two nets corresponding each other and being made e.g. of an expanded metal. An insert for effecting a flow resistance in the port 16 is realized by compressing the strip 24 between the rigid nets 25 to achieve the density 26 as well as by an airtight insertion of the unit thus obtained in the port 16. If necessary, the tunnel 17 may completely or partially be filled or lined with a fibre material e.g. acrylic fibres, however, with a very low density and a low flow resistance. The reference number 23 of Figure 2 denotes a tubular tuning unit which is explained in more detail with reference to Fig. 7 and 8.

The tuning unit 27 shown in Fig. 3 and 4 has a relatively thick tube 28 made e.g. of aluminium. On one end of the tube there is provided a plug 29 of a flow limiting material, advantageously of mineral fibres or an acoustic foam material, which is in airtight connection with the internal side of the tube 28 and forms a pressure differential zone in the tube, the length of the plug causing an essential differential time dt, which is much greater than this would be the case if there were no such plug. The differential time is the time necessary for a noise condition (e.g. compression) to pass through the plug.

The differential time dt is due to the presence of the flow limiting plug in the mouth area of the acoustic tunnel against the environment and due to the fact that it is a well defined object with only limited extension relative to the overall < physical length of the acoustic tunnel, the size dt being proportional to the extension of the plug and to the effective flow resistance thereof. Due to the fact that the plug is inserted in the tunnel, it obtains, apart from its intrinsic longitudinal dimension, also an acoustically complex function effective in the physical longitudinal extension of the tunnel. The dynamic limiting component thus obtained has a complex dimension with different properties than the purely resistive flow limitation, which the plug on its own has. The value of this dimension is dependent on the frequency and on the flow that can pass through the plugged tunnel per time unit.

and 31 are perforated layers with fixed positions, e.g. of an expanded metal or perforated sheet metal, and have a high proportion of perforated area. The plugged end of the tube 28 may be received by a speaker casing

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opening and has a shoulder area 32 for airtight glueing against the internal side of the speaker casing. In practice, the plug 29 should have an axial length of 15 to 30 mm, advantageously a length in the order of magnitude of 1.0 (or more) times the tube diameter. The tube 28 advantageously terminates at a distance from that side of the speaker casing which is opposite the plug 29 of at least 1.6 times the internal diameter of the tube.

Fig. 5 and 6 show a further tuning unit 33 in the form of a tube 34 of e.g. aluminium, with an air permeable plug 35 having a far lower flow limiting capability than the plug 29. For example, the plug 35 consists of a foam material with open pores and a density of 30 to 80 ppi, advantageously in the order of 45 ppi. The plug 35 as well as the plug 29 may be provided at their one end or on both ends with thin layers of a densely structured material such as staple fibre layers or a fine-meshed metal wire net, which layers rest against their border surfaces in a mechanically fixed manner. The plug 35 is in an airtight manner fixed to the internal side of the tube 34 and should have a length sufficient to cause a differential time and to prevent any oscillation or shifting about its balancing position. The length may e.g. correspond to the internal diameter of the tube. Alternatively, the plug may be stiffened by means of an expanded metal net or the like. The unit 33 is arranged such that it can be glued into the opening of a speaker casing, with an offset surface 36 on the tube 34 being intended to rest against the internal side of the speaker casing.

The tuning unit 33 can be tuned to a frequency lying considerably below the natural frequency of the speaker, advantageously to a frequency approximating the lower

border frequency of the speaker (according to equation 8 of page 26) in the speaker device comprising the speaker casing and the associated speaker, or to a frequency falling below this border frequency. In practice, the tuning frequency may not fall below the frequency f_1# and it is advantageously approximately 0.5 times or less.

L.

The tube 34 may, by the way, be formed or arranged as described above with reference to tube 28. £

Fig. 7 and 8 show an alternative embodiment of the tuning unit according to Fig. 5 and 6. The mouth plug of the tube 37 is, however, replaced by a very thin (e.g. 0.4 to 0.015 mm), tensioned, fine-meshed net 38 of e.g. metal, e.g. with a mesh size of 30-400 mesh, which has the effect of an acoustic resistance. In this net, a differential time with a small dt value compared to the unit according to Fig 5 and 6, which has the flow limiting plug 35, develops. The tube 37 has a shouldered surface 39.

The tuning units 33 and 23 may have another than a circular cross-section and may be arranged in the tunnel 17 or 28, advantageously parallel and particularly coaxially to the tunnel 17 or 28, although this is a less advantageous embodiment as compared to arranging the units 33 or 23 outside the tunnel 17 or 28. For example, in particular in the case of small speaker casings the tuning unit 33 or 23 may be a slotlike channel which is arranged diametrically above the crosssection of the channel 16 or the tube 28 - as indicated at 23' in Fig. 1. The tube 28 too may have another than a round cross-section.

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In an embodiment of the invention which is preferred for acoustic reasons, a parallelepipedal speaker unit is used, the width (face) of which is equal to the cubic root of the casing volume, whereas the height is 1.25 times the face width and the depth 0.8 times the face width. The bass speaker is arranged with its centre at a distance from the bottom, which is a third of the height, advantageously slightly excentric to the vertical centre line of the face side. The dampening material 22 is preferably at least twice as thick on the rear wall of the casing than at the bottom, at the top and the sides of the casing, and the preferred dampening material is fibre glass wool having a density of approx. 24 kg^-3. The partitioning walls of the casing are advantageously provided with reinforcing slats. For example, a reinforcing slat (not shown), which may be glued to the said walls, may extend between the walls 10 and 18 in the longitudinal direction of channel 17, or the wall 10 may be provided at the internal tunnel end on the top side thereof with a transversely extending reinforcing slat increasing the acoustic length of the tunnel.

Advantageously at least 50% of the casing volume Vj-, are filled with an acoustically absorbent material. The possible further tuning unit 2 3 or 33 is arranged e.g. near the speaker next to a corner between the bottom the the side wall of the volume Vjj (Fig. 1). The speaker can, in the case of an optimally tuned tuning unit 33 or 23, momentarily be supplied with air, and it can therefore even faster and better follow a dynamically varied signal programme such as reciprocating string passages on a double bass, a big drum and similar sound patterns. In constructions for the medium sound range, the speaker receives, due to the dynamical dampening of the resonance frequency, a reduced, negative, acoustic shading, and

the switch-off time is advantageously shortened.

Fig 9 shows another unit 41 designed to be inserted into the opening of a speaker casing, said unit having a relatively short tunnel 42 filled with a plug of a flow limiting material 43 with flow limiting properties sufficient to ensure a pressure chamber character of a speaker casing. The reference numbers 44 and 45 denote air permeable layers fixing the position, e.g. of expanded metal or perforated sheet meta^.

The port or the tunnel 42 has an offset surface 46 to provide an airtight seal against the interior of the speaker casing. The tunnel 42 is dimensioned such that it gives resonance with the casing volume at a frequency at least as high as the natural frequency or higher than the resonance frequency achieved by one of the tuning units 17, 24, 25; 27; 33; 23; the tunnel 42 is advantageously tuned near or above the * frequency f2 developed in the tunnel 16, 17 in the Helmholtz resonance circuit.

There is a good reason for specifying in greater detail herein the preferred way to optimize the tuning of the frequency according to the invention. It may be favourable to tune the tuning units according to one of Fig. 1 to 9 to a frequency which is considerably lower than the ones that can be calculated on the basis of the formulae (page 26) - e.g. by moving calculated f₂ downwards towards 0.5 X f_lr as well as moving f_s towards f'_s, or even upwards towards 0.7 X f'_s. The effective flow limitation active in the respective tuning . device should be chosen such that it is sooner larger than too small. This is associated with the fact that it is not practical to reduce the dynamic pressure factor in the system according to the invention to such an extent that the speaker unit can overshoot in an acoustically

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uncontrolled way. With other words, according to the invention the pressure force dynamically opposed to the deflection of the speaker must approximate the value which would be prevalent in the case of a construction designed as pressure chamber equivalent.

If it is intended to modify system characteristics in the case of transient sound passages with low frequencies, then this can be achieved, according to a specific embodiment of the invention, by hyperventilating the acoustic system. The only way to effect such a hyperventilation is by opening the acoustic system, so that the enclosed air can theoretically freely flow into the environment. In order to ensure that this opening action can be done without any subsequent essential disadvantages with regard to the dynamically effective control of the speaker system, a further tuning unit 23 is used in the acoustic system according to the invention, which is arranged as a tube or tunnel having a very high length as compared to its cross-sectional area, and according to equation 10 of page 26 is theoretically tuned from 0 Hz to a value near f^ according to equation 8 of page 26.

If such a unit is incorporated into the acoustic system, the existing dampening of the mobile part of the acoustic system, i.e. of the speaker unit, occurs as follows: The column of air enclosed in the channel can be regarded as otherwise separate from the acoustic system. The mass representing the column of air is dynamically reciprocated depending on the acceleration level in the speaker unit along the channel, and the level of reciprocation can be mathematically calculated.

Theoretically applies that if the pressure level p developed by the speaker unit in the acoustic system is maintained constant (e.g. at 1 Nm^-2), also the acceleration level (a ms“²) at theoretically 0 Hz and above is constant. Consequently, the speed level ( v ms¹) of the speaker unit is doubled for each division of the frequency in half, and the reciprocation level (d m) of said speaker level is quadratically increased with the acceleration level.

At a higher frequency than the tuning frequency, the columh of air in the channel will always act as an acoustic barrier, while the tuning frequency forms a threshold value, starting from which the channel increasingly opens acoustically, whereby more and more dynamic kinetic energy is allowed to pass through the channel per time unit.

The effect of these facts on the speaker is such that in the regulating area defined by the channel the dampening of the amplitude of motion acting on the speaker in the acoustic system develops in such a way that it may adopt a course which is negative progressive relative to 0 Hz, i.e. declining. With other words, the hyperventilation according to the invention, which is made possible in the preferred embodiment, means that a column of air with a variable reciprocation speed partially imposes a dynamic load on the decay properties of the speaker unit and partially enhances the decay properties of the speaker unit at short courses, i.e. that a uniformly dynamic control of the starting and stopping times of the speaker ds achieved. Thus, the speaker can momentarily supplied with air, i.e. it can breathe. In this way, the additional provision of hyperventilation serves to provide a higher quality quickresponse speaker system.

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The hyperventilation function may be initiated for various intervention levels, with one or several units acting as hyperventilators being selectable. If at least two such devices are used, then one of them should be arranged as disclosed in Claim 3 or 4, they should have circular crosssectional areas, a specific length, be provided with a plug 35 of an adapted foam material with open pores and tuned to a lower frequency, e.g. to f_x (equation 8 in Annex II).

The additional device 47 (Fig 1) can according to Claim 6 or 7 be formed like a slot will then have a hyper-hyperventilating effect compared to the ventilation of the former device. The latter device should, even if it is the only hyperventilator in the system, have an extraordinarily small slot height, e.g. in the order of 0.1 to 2 mm, the width can be approx. 10 times the height (e.g. 20 mm), and it will tune itself automatically to a frequency near 0 Hz, in any case, however, below fj.. In order to ensure a negligible sound radiation, the opening is chosen to be narrow and long. The application of hyperventilation is effective also with regard to the fact that otherwise in the case of densely repetitive, strongly transient sound passages the speaker unit might build up a stationary air pressure developed in the volume of the casing in the medium range, and this air pressure could cause a shifting of the symmetrical operating zero of the swing pole, with other words, the mid-position of the speaker cone in the casing volume may be shifted into one or the other direction, which is unfavourable in functional respect.

Even though a system with hyperventilation according to the invention may be obtained by simple measures by inserting a tubular unit not including any form of mechanical flow limitation, i.e. by means of a net 38 according to Fig. 7 and 8 or by means of a plug 35 according to Fig 5 and 6, the use of a net 38 in front of the otherwise open ventilation unit is to be preferred. This is, because a completely open channel may cause a whistling or a flow noise, the frequency of which might become audible at the natural tuning frequency of th< tube.

In order to modify the dampening relationships that will be prevalent at the resonance frequency f^, a further tuning unit according to Fig. 4 or 9 may be used, which is tuned to a much higher frequency than the tuning frequency of the former unit 23 or 33, whereby a thus synergistically effective tuning ratio can be achieved by varying the intervention frequency of the tuning unit 27 or 41 or the flow limiting portion 29 or 43 thereof. The use of this other and in a synergistically way pressure controlling device requires that the flow resistance in the further units according to Fig. 4 or 9 is maintained on a high level. The easiest way to control the effect of this device on the acoustic system is by studying the electric impedance characteristics at the resonance frequency f(j of the system. Such a pressure regulating unit effects a possibly desirable dampening of f^ and in the direct vicinity of f^ with respect to both impedance and frequency response.

This makes it possible to influence and modify in the manner described above the frequency response of the speaker and to obtain, starting from approx. 100 Hz, a certain flattening against low frequencies, which may be

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desirable in certain applications. With other words, in contrast to the acoustic function of the actual tuning units 23, 27, 33, the device 41 is an acoustic hole or leak. The modification of the frequency response is really a function of the fact that the acoustic quality Q is being controlled. The general formula for the quality Q is found as equation 12 in Annex II.

Another way of developing the function of the acoustic system further is given through the use of the synergistic tuning facility stated in Claim 9, wherein the bent, long tunnel 17 is tuned to f^ or even to a lower frequency and cooperates in a synergistic way with at least one further, shorter tunnel 23, 27, 33 or 41, which will then be tuned to an essentially separate, higher frequency - e.g. near f_s or to a higher frequency.

In a construction of this kind, an optimisation of the flow limiting component for the respective tuning units used can be achieved by trying out, however, the flow limiting component of the device tuned to the lowest freqency should be arranged such that it has a small or negligable resistance.

In order to illustrate the diagrammatic characteristics of a 10 speaker arranged as a model1 system with high volume parameters, Fig. 10-22 are enclosed, wherein Fig. 19-22 relate to tone-burst analyses. Annex II shows equations that can be used according to the invention by somebody skilled in the art for dynamic system constructions.'

Fig. 18 shows the usually mostly used diagrammatic illustration of speaker modells. The curve shown relates to the electrical impedance measured with the 10 high

compliance speaker unit (f_s = 24 Hz) of the modell, the speaker inserted in a speaker casing having a total (V^ + V^) volume of approx 70 dm³ and being filled with an acoustically dampening mineral fibre material. As is directly evident from the characteristic of the impedance curve, this is a normal (one-phased) impedance function essentially corresponding to the characteristic that would be present if the modell had been arranged as a conventional pressure system.

During the measurement procedure, a long tunnel 17 according to Claims 1 to 3 was used with a straight, tubular device, which device was arranged as the tuning unit 23 shown in Fig.

and in Fig 7 and 8 (the grid 38 was a 50 mesh brass grid).

In the mouth 16 of the main tuning unit 17, 24, 25, two expanded metal grids 25 as well as a mineral fibre plug 24 of a free thickness of 40 mm and a density of 24 kg^-3 was inserted. As a result of inserting said plug arrangements, the length 26 of the portion 24 was reduced to 20 mm.

As a typical example for the transient characteristic that can be found in a dynamically acoustic system according to the invention, Fig 19 and 21 illustrate the acoustic signal response, which was measured at the acoustic mouth of the speaker by means of a standard microphone 4165 of Fa. Briiel & Kjaer (B & K), to the electrical signal delivered to the speaker unit, which are shown in Fig 20 and 22, respectively. During the measurement process, 10 sinusoidal periods at a frequency fj = 42 Hz (Fig 19 and 20) or at the characteristic frequency f_s = 24 Hz (Fig 21 and 22) were delivered to the speaker unit (8 Ohm impedence). The signal voltage level was kept constant and was the same in both measuring processes.

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It is evident that the transient response is extraordinarily good, which means that both the transient and the decay functions are dampened extremely short and strong - in a way that is to be regarded as aperiodic -, which means that the system may only have one sinusoidal period as transient resultant after the electrically supplied burst” period, i.e. at f<j and lower frequencies. This fact in connection with a compulsory sinusoidal oscillation added to the signal voltage is due to the fact that the acoustic system as such has a resonant period having the system resonance frequency f<j.

Advanced measuring technology using an FFT analysing system B & K 2033 made it possible to illustrate also measurements of complex impedance values occurring at the construction modell. These are shown in Fig 10 to 17, and the curves shown in Fig. 10 and 13 were obtained by means of a standard microphone arranged in the interior (Vj-, + V^) of the system and an accelerometer (4375 B & K). The acceleration signal was integrated into a velocity or displacement level signal by means of a pre-amplifier, the microphone signal (4165) was passed through a microphone amplifier (2619 B & K) and the curves of Fig 10 to 13 were obtained by means of Fourier analysis (FFT differential analysis).

The curves n Fig 10 to 13 are calibrated in the measured specific system compliance M_c(m³ N”¹), which is in this case

1.6 X 10² m³ N^-1 for the 0 dB level. The displacement signal was used.

Fig 10 shows the system according to the invention (with two effective tuning devices), and it becomes evident that this is a modified pressure chamber version: The curve shows that the mobility of the speaker unit, having

been maintained nearly constant, receives a modified compliance ratio by the dynamic control function, said compliance increasing at lower frequencies and following a defined system characteristic of 17 Hz up to DC (i.e. near 0 Hz) .

Fig 11 shows the course of the compliance function, if both tuning elements are freed from their flow limiting part 24 and from the exterior part of grid 25 or the net 38 (Helmholtj. approximation). This curve shows an impedance maximum at fp =

Hz for the unit 23.

Fig 12 shows an impedance curve corresponding to the function shown in Fig. 10. However, the mouths of the two tuning units are entirely sealed against the environment by an adhesive film (characteristics of a pressure chamber). The inflexion characteristic shown in the curve between 32 Hz and 80 Hz or the low but sharply defined lower cut-off frequency at minimum impedance shows an acoustic system which is neither dynamical nor has a defined pressure characteristic - with other words, a random system is generated. Maximum compliance was measured at -15 dB.

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Fig. 13 shows in curve 2 what happens if only the one tuning unit 23 is sealed, while the curve one is identical with the curve according to Fig. 10. This allows to evaluate for the construction the compliance enhancement by the hyperventilation unit, in order to avoid decompression tendencies at higher frequencies than f_s. The maximum compliance is here (Fig. 10, curve 1 of Fig 13), increased to approx. -10 dB, the result of the invention. This corresponds to a 3.16 times higher compliance compared to the one in Fig.

12. The voltage remained constant in each case in order to develop 1 Watt at 8 Ohm.

Fig 14 and 15, which are to be compared with the compliance functions shown in Fig. 10 and 11, show the velocity level (V_s) obtained with the system according to the invention (Fig. 14) or at Helmholtz characteristic (Fig. 15) if the supplied speaker voltage (U_s) was maintained constant. The dampening level, η = 20 log v_s/u_s, which was present in the acoustic system according to Fig. 10, is shown in Fig 14, while Fig 15 shows the dampening level present in the acoustic system according to Fig. 11. The fact that the modell construction according to the invention is in fact a pressure system modified in the manner described, is confirmed by the fact that the dampening minimum (velocity maximum) of the curve according to Fig. 14 is at fj = 42 Hz. The acceleration signal was converted into a velocity signal.

Fig. 16 and 17 show sound pressure levels. Fig. 16 shows the differential level which was measurable between the exterior pressure level (p_e) and the internal sound pressure (p^,). Fig 17 shows the sound pressure slevel at a constant, supplied electrical signal (p_e Nm^-2W_e ^_1), which could be measures axially to the cone of the speaker at a point located on the same level as the mouth area thereof relative to the environment. As is evident, the frequency response is the same as in the case of a well designed pressure system and is reduced relative to the cut-off frequency in a well controlled manner by 4 per 5 Hz.

COMMERCIAL EXPLOITABILITY

Speaker devices of the kind described above are commercially exploitable in acoustic industry, e.g. in recorder rooms.

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- 23 DEFINITION OF ABBREVIATIONS

Ap: Port area in Helmholtz resonator systems ffj: Helmholtz resonance frequency in a Helmholtz resonator system; usually equals f's < f_s fj: lower border frequency in Helmholtz resonator systems;

f2^ upper border frequency in Helmholtz resonator systems;

f(fb): system resonance frequency in a pressure chamber system;

(O f_s: natural frequency in an electro-acoustic speaker unit;

fp: port resonance frequency in a Helmholtz resonator system; usually at f'_s < f_s;

f'_s: the natural frequency f_s displaced by acoustic load of the speaker unit towards lower frequencies;

f<j: new and frequency displaced (+ or -) fp occurring in the dynamically, acoustically regulated system;

V_AS: air volume, which under the load of an electroacoustic speaker unit having the natural frequency f_s will give a compliance ratio s = 1.0, on the basis of which also fp and f^ can be calculated;

AP 0 0 0 1 4

* ♦ * 4 ^vb^: ^vt^:

It:

s:

^sb^: ^sd^:

Q:

^vb^: ^vt^:

It:

s:

^sb = ^sd^:

Q:

volume of a pressure chamber system;

a physical volume for the cross-section area of the tunnel, which is determined by the length 1^ of the tunnel;

the physical length of an acoustic tunnel;

compliance ratio, when the electro-acoustic speaker unit is loaded by a pressure chamber volume to such an extent that f^ applies (infinite face wall);

compliance ratio applicable to an infinite V^+V^ baffle construction, in the compression volume of which is subtracted;

is for systems according to the invention normally sj < 0.9 sjj and represents the enhanced compliance usually obtained by dynamic, acoustic control (see equation 4, Annex II);

Q = fd/(^f2 ” ^fl) i-^s the magnification factor of an acoustic circuit, from which the dampening factor of the same system can be calculated as an inverted value. The measurement of and f2 is on a level which is at 3 dB lower than the level at resonance frequency f_o.

dampening factor.

Claims

1. Speaker device, consisting of at least one speaker being provided in an opening (15) of a casing which has at least one casing opening forming a resonator port (16) of a size giving resonance with the casing volume (V^) at a frequency which is substantially higher than the eigenfrequency (f_s) of the speaker and which contains a flow limiting material (24, 25), whereby an acoustic tunnel (17) follows the resonator port (16) forming a tuning unit (17,

24, 25) which shifts the resonance downward approximately to the eigen-frequency (f_s) Of the speaker or below that, whereby the flow limiting material forms a dampening plug in the tunnel mouth, characterized in that the length of the tunnel is at least approximately equal to the largest extensions of the casing volume (V&) and that the tunnel is bent at least once along its length.

2. Speaker device according to claim 1, characterized in that the tunnel (17) is screening the casing volume (V^) proper containing the speaker against the bottom (10) and the back side (12) of the speaker casing, as well as possibly against the top side (14) thereof.

3. Speaker device according to claims l or 2, characterized in that the tunnel (17) has its exit in the casing volume proper, containing the speaker, at a place positioned near the front of the casing (11).

4. Speaker device according to one of the claims 1 to 3, characterized in that it has at least a second tuning unit (33, fig. 4; 23, fig. 6) comprising a port with acoustic tunnel (34; 37) and possibly flow limiting or acoustic resistive material (35; 38), being tuned to a frequency being substantially different to the resonance frequency effected by the first mentioned tuning unit and being substantially below the eigenfrequency of the speaker and approximating the boundary frequency (f^) of the speaker in the speaker device or falling short of such boundary frequency.

5. Speaker device according to one of the claims 1 to 4, characterized in that the flow limiting material is foam stuff, preferably acoustic foam stuff.

6. Speaker device according to claim 4, characterized in that a second and a third additional tuning device (23 and 47 in fig. 9, respectively) is included, of which the second tuning unit is tuned and adapted to the flowing air amount per time unit in such a way that it effects an increase of the compliance enhancement in the direction of lower frequencies to very low frequencies, while the third tuning unit (47) is tuned and adapted with respect to the flowing air amount per time unit in such a way that it effects with respect to the second unit (23) a likewise limited and, in the direction of still lower frequency additional progressive increase of the total compliance enhancement provided by the first mentioned as well as the second tuning unit.

7. Speaker device according to claim 6, characterized in that the third tuning unit (47) comprises a narrow slit extending through the casing wall carrying the speaker, and that the slit mouth is positioned in basically the same plane as the mouth part of the speaker and preferably in the vicinity of the speaker.

8. Speaker device according ta claim 7, characterized in that the slit (47) is provided at its mouth against the environment or against the casing volume with a flow limiting component, e.g. of a fine meshed metal net, fine structured web, a thin layer of staple fibres or consisting of preferably foam stuff (43).

AP 0 0 0 1 4 6

9. Speaker device according to one of the claims 1 to 8, characterized in that it comprises at least one additional port or tunnel (42) which has a size giving resonance at a frequency being substantially higher than the tuning frequency of the first mentioned tuning unit (17, 24, 25), and which contains flow limiting material.

10. Speaker device according to one of the claims 1 to 9, characterized in that the plug consisting of flow limiting material is provided on one or both front sides with one or more thin, disk-like sheets of material (staple fibre layer or fine meshed metal wire net) having acoustic resistance.

11. Speaker device according to one of the claims 4 to 10, characterised in that a tunnel of the second tuning unit (23, 33) is provided inside the tunnel of the first tuning unit (17, 28), preferably parallel and especially coaxially with the latter.

12. Speaker device according to one of the claims 4 to 11, characterized in that a tunnel of the second tuning unit (23, 33) has a slit like cross-section.

13. Speaker unit according to claims 1 to 12, characterised in that the casing volume (Vj-,) proper containing the speaker is covered inside with acoustically absorbing material (22), as mineral fibre mat, preferably filling at least 50% of the casing volume (V^).