Acoustic frequency-shelving filter

Abstract

In order to adapt the frequency response of headphones, various filters are used, including shelving or frequency-shelving filters. However, these are usually implemented electronically. In the case of wired headphones without their own voltage supply, however, no electronic signal processing is possible. In order to realise an acoustic shelving filter, an acoustic damping element (DD) having a flat, acoustically effective damping material, which has a defined specific acoustic resistance, is placed in front of the radiating surface of the sound transducer at a small distance (d) in relation to the diameter of the acoustic damping element. It thus forms a flat air gap (AG), which is open at the edge, with respect to the radiating surface of the transducer. Acoustic properties of the shelving filter can be controlled by the size and material of the damping element (DD) and its distance from the sound panel (SW) or from the transducer (W).

Description

[0001] Acoustic cowtail filter

[0002] The present invention relates to an acoustic filter, in particular an acoustic sheaving filter, which is also referred to as a cowtail filter in German.

[0003] background

[0004] One of the main goals in the acoustic development of headphones is to achieve a frequency response that meets specific sonic requirements. For example, the frequency response should be as linear as possible across the widest possible range. Various methods are available for this, especially filters. Electronic filters that modify the electrical signal reproduced by the headphones, such as high-pass filters, low-pass filters, band-pass filters, shearing filters, and equalizers, are available in many variations. Unlike low-pass, high-pass, or band-pass filters, shearing filters are known to have transfer functions that exhibit different, but essentially constant, values ​​at high and low frequencies.

[0005] If the headphones are wired and lack their own power supply, electronic processing is not possible, so the means are limited to purely mechanical constructions such as damping, channels, and volumes. High-pass, low-pass, and band-pass filters can be implemented in a generally known way, allowing the headphone's transfer function to be influenced within certain limits and the frequency response to be adjusted. These limits are determined by the available installation space, material properties, and material tolerances.

[0006] For example, the use of damping in headphones has long been common practice. US4058688A shows a headphone in which the rear space between the transducer or baffle plate and the perforated rear housing is filled with damping material. Within the transducer itself, the volume behind the diaphragm is also separated from the surrounding environment by damping. In front of the transducer is a cover featuring a foam ear cushion with individual openings for sound emission. The diameter and depth of these openings allow for adjustment of the inertia of the air contained within, and thus a specific operating frequency at which the level of the emitted sound is increased. The foam is acoustically open and therefore largely acoustically transparent. The transducer has a diaphragm or...A vibration plate is used, the bead of which is surrounded by an opposing circumferential wave (corrugation) to lower the lower cutoff frequency of the transducer.

[0007] DE21 13963A1 discloses headphones with free-radiating transducers and closable openings behind the transducers. An acoustically transparent foam insert is arranged between the transducer and the ear as a spacer, along with another foam layer with a central recess for damping cavity resonances. Neither has an acoustic effect on the reproduced sound. DE2006249A1 discloses similar headphones, but their closable openings are located in front of the transducers, thus reducing bass response. Damping material placed in front of the diaphragm to dampen cavity resonances should be selected and arranged so that the desired sound does not experience any significant attenuation, particularly of a frequency-dependent nature.

[0008] However, no method is currently known for constructing a shelving filter or sheave filter using mechanical or acoustic (i.e., non-electronic) means. Such a filter can be advantageous for adjusting the frequency response of headphones, especially if it incorporates a low-pass function. In this type of filter, the lower frequencies are attenuated less than the higher ones. Therefore, such a shelving filter could improve the bass response of the headphones. Furthermore, acoustic optimization of the headphones can minimize the need for additional electronic processing of the audio signal, which can, for example, reduce the headphones' power consumption. Similarly, a band-stop filter based on mechanical or acoustic means can be beneficial.

[0009] Summary of the invention

[0010] The present invention therefore aims to provide a passive filter, and in particular a shelving or shelving filter, that operates on a mechanical or acoustic basis and does not require its own electronic signal processing. This objective is achieved by a device according to claim 1. Claim 7 relates to headphones according to the invention.

[0011] According to the invention, an acoustic shelving or shearing filter for a transducer comprises an acoustic damping element with a planar, acoustically effective damping material. This damping material can be positioned at a small distance in front of the transducer's radiating surface relative to its diameter and the diameter of the transducer. It lies parallel to the plane of the diaphragm or radiating surface, thus forming a planar air gap open towards the edge opposite the radiating surface. The damping element can include at least one fastening element. For example, in front of a transducer with a round diaphragm, which is located in the baffle of a headphone, an air gap is formed in the direction of movement of the diaphragm or radiating surface. This air gap is essentially ring-shaped and open in the radial direction. The acoustically effective damping material is planar and usually has pores or similar small openings.It has a defined specific acoustic resistance or acoustic transmittance and, in one embodiment, has a closed surface without openings larger than the pores. Thus, the volume of the air gap forms an acoustic "spring" and the damping material an acoustic resistance, together forming an acoustic filter. In another embodiment, e.g., for ring-shaped transducers or to achieve a bandstop function, the damping material has a ring-shaped surface with a single central opening larger than the pores of the damping material.

[0012] Further advantageous embodiments are described in claims 2-6 and 8-15.

[0013] Brief description of the drawings

[0014] Further details and advantageous embodiments are shown in the drawings. These show

[0015] Fig. 1 Views of a conventional acoustic unit of a headphone with and without ear pads;

[0016] Fig. 2 shows a sectional view of the acoustic unit according to the invention with ear cushions;

[0017] Fig. 3 shows a section through the acoustic unit according to the invention;

[0018] Fig. 4 Frequency responses of a headphone (HD800S) with the acoustic unit according to the invention in various versions;

[0019] Fig. 5 shows a level reduction by changing the distance of a disk with D=54 mm and damping material with 300 Ns / m 3 ;

[0020] Fig. 6 shows a level reduction by changing the distance of a disk with D=42 mm and damping material with 80 Ns / m 3 ;

[0021] Fig. 7 Frequency responses of a headphone (HD560S) with the acoustic unit according to the invention and with a conventional acoustic unit;

[0022] Fig. 8 shows the basic frequency responses of low-pass, high-pass and sheiving filters;

[0023] Fig. 9 shows a ring transducer with mounting points for a sheaving filter; and

[0024] Fig. 10 shows a frequency response of a bandstop filter.

[0025] Detailed description of the invention

[0026] The acoustic unit of a headband-style headphone typically consists of a baffle with a loudspeaker transducer. These may be enclosed in a housing. The baffle has openings around the transducer, which are covered with acoustically transparent material. This creates a defined, open sound-radiating surface, consisting of the transducer and the openings, which can be coupled to the head via an ear cushion. Fig. 1 shows two views of a conventional headphone acoustic unit. The left view shows the ear cushion P, while the right view shows it removed. In front of the transducer is a protective cover B for the diaphragm, which may also have other functions, such as acting as a resonator and / or an acoustic low-pass filter.

[0027] To create an acoustic cowtail filter, an area in front of the transducer is completely covered with an acoustically effective damping material. This area is positioned close to the baffle relative to its diameter and the diameter of the transducer, creating an air gap between the damping material and the baffle or transducer. This air gap is open at its perimeter. Low frequencies can propagate unimpeded and reach the ear without being affected by the damping material. High frequencies, on the other hand, are radiated more directionally due to the increasing focus and can only reach the ear by passing through the air gap and then the damping material. Therefore, to create a cowtail filter, the damping material should not have any openings through which sound could be radiated without damping.In particular, it should not have any openings larger than the material's inherent pores. This achieves a level reduction for high frequencies, which, however, reaches a constant value at higher frequencies instead of continuously decreasing as with a low-pass filter. This behavior corresponds to that of a sheaving or cowtail filter with a low-pass function.

[0028] Fig. 2 shows a sectional view of the acoustic unit according to the invention with ear cushion P, wherein a planar, acoustically effective damping element DD is attached in front of the loudspeaker SW mounted on a baffle such that an air gap AG is formed between the loudspeaker and the damping element, and thus parallel to the diaphragm plane of the loudspeaker, which is open at its edge. The loudspeaker and the damping element, and thus also the air gap AG, can be, for example, round or oval, and they are arranged concentrically. The damping element DD can be attached – for example, by means of a mechanical support element with struts or legs (not shown in Fig. 2) – to the baffle, to the loudspeaker or transducer, to the housing, or to the ear cushion, whereby these mechanical support elements can interrupt the otherwise circumferentially open air gap AG. The damping element DD and the attachment element form an acoustic damping element.The support elements cover only a small portion of the circumferential opening of the air gap AG (e.g., approximately 3%–7%, at most 10–15%), so that the air gap is essentially open at its circumference. In other embodiments, the support elements can cover a larger portion of the circumference, although they then also attenuate the low frequencies more strongly, or they can be located internally, for example in a ring transducer. In the case of a ring transducer, the internal support element can also be designed as a closed cylinder.

[0029] The damping element (DD) together with the air gap (AG) forms a passive shelving filter, as explained above. Various mechanical properties allow the filter parameters of the passive shelving filter, such as the cutoff frequency and the level attenuation, to be set and modified. In particular, the filter parameters of the shelving filter are influenced by the size or diameter of the damping element (DD) relative to the diameter of the transducer, the distance between the damping element (DD) and the transducer diaphragm, and the acoustic impedance or specific acoustic resistance of the damping material. This distance can range from 0.1 mm to 20 mm, depending on the desired effect and the shape and excursion of the diaphragm. In many applications (due to frequency and to minimize the overall height), values ​​between, for example, 1 mm and 5 mm are used.For the specific acoustic resistance, the value can be, for example, in the range of 50-2000 Ns / m. 3The acoustic resistance should be a defined and constant value, as it determines the attenuation of high frequencies. Foams and felt are therefore generally unsuitable as damping materials because their acoustic resistance cannot be clearly defined or can fluctuate significantly. Furthermore, acoustically open foams contain an undefined but relatively large mass of resonating air within their pores. Suitable materials include papers, fabrics (preferably made of natural materials, synthetic materials, or metal), textiles, etched or laser-perforated materials with defined permeability, microperforated films or membranes, fibrous insulating materials such as non-woven fabrics, and other materials with acoustically permeable pores that exhibit a clear, uniform, and constant acoustic resistance and little or practically no resonating (air) mass.The diameter of the damping element should be between 50% and 200% of the diameter of the transducer or the radiating surface.

[0030] Since the sheaving filter according to the invention has a low-pass function and also provides contact protection for the membrane, the conventional contact protection shown in Fig. 1 can be omitted. However, it is possible to combine both filters.

[0031] Fig. 3 shows a section through an acoustic unit (left in detail, right in principle), where the planar damping element DD is mounted parallel to and at a distance d in front of the baffle SW, in which the transducer W is located. The acoustic unit is closed off externally by a rear wall R, which encloses a rear volume RV and can be ventilated by openings VO in a known manner. A planar air gap AG, which is open to the front volume at the edge of the damping element DD and, in this example, also near the edge of the transducer diaphragm W, and is interrupted by (in this example, three) thin struts ST for mounting the damping element DD, allows primarily low frequencies to pass through, since the sound source is small compared to the wavelength for these frequencies. As soon as half the wavelength of the radiated frequencies becomes smaller than the diameter of the sound source diaphragm, beaming occurs.From this frequency and higher, the damping element can influence the level. By dimensioning the damping element DD, the cutoff frequency and level can be adjusted. Since the damping element DD has a defined specific acoustic resistance, this level is attenuated uniformly and not continuously reduced. Therefore, a shearing filter is created, not a low-pass filter.

[0032] Fig. 4 shows exemplary frequency responses of the acoustic module of a specific headphone (Sennheiser HD 800 S) with the acoustic unit according to the invention in two different variants, which differ in the distance of the damping surface from the transducer or the baffle. The material or its specific acoustic resistance, the size of the damping surface, and the size of the transducer are the same in both cases: the damping is a disc with a diameter of 54 mm and a resistance of 300 Ns / m. 3The transducer has a diameter of 56 mm. As already mentioned, a damping material with a defined specific acoustic resistance is suitable for damping, such as paper, woven fabric, cloth, non-woven fabric, microperforated film, or another material microperforated, for example, by etching or laser. In the first embodiment according to the invention, corresponding to curve f2, the damping element DD has a distance of 5 mm from the transducer, while in a second embodiment according to the invention, corresponding to curve fs, it has a distance of 2 mm from the transducer. For comparison, a frequency response f1 without the filter according to the invention is shown. The frequency responses of both embodiments fa and fs according to the invention are consistently below the values ​​f1 of the conventional acoustic module from approximately 200–300 Hz up to the upper end of the audible range at approximately 20 kHz. The parameters of the sheaving filters are selected such that clear differences are visible from approximately 4 kHz.In the first curve, f2, an initial peak, visible at approximately 5.3 kHz (fi), as well as all values ​​above approximately 8 kHz, are significantly attenuated. The second curve, fs, lies predominantly below the first curve, f2, with virtually all values ​​between 4 kHz and 20 kHz being significantly attenuated by approximately 2-5 dB compared to f. The uniform attenuation of the high frequencies leads the user to increase the volume slightly. This shifts the entire f2 or fs curve upwards without altering its shape. As a result, the frequency response of the arrangement according to the invention, at least in this example across the entire range below approximately 4 kHz, is higher than the original frequency response, f. This increases the relative energy content of the low frequencies below 4 kHz, creating the impression of enhanced bass reproduction.

[0033] While Figure 4 shows the absolute frequency response of an HD 800 S acoustic module, Figures 5 and 6 show relative frequency responses, i.e., essentially the differences fa' = f2 - fi and fs' = fs - fi, respectively, compared to the unfiltered transducer. This makes the influence of the distance d (and thus the width of the air gap) and the size and material of the filter disk on the filter's frequency response more apparent. The remaining parameters are as described above. All measurements were performed on the same fiat-plate acoustic coupler. Small deviations, e.g., below 2 kHz, result from measurement inaccuracies.

[0034] Fig. 5 shows the level reduction by a disk with a diameter of 54 mm made of a damping material with 300 Ns / m 3The curves are shown at different distances d, namely 2 mm in curve fa' and 5 mm in curve fs', as in Fig. 4. As can be seen, the two curves are almost identical up to approximately 3 kHz. Above 3 kHz, the wider air gap in curve fs' leads to an even greater reduction in frequency. While a level reduction is already noticeable from 3 kHz onwards in fs' (i.e., with a 5 mm distance), a clear effect only occurs from 8 kHz onwards in f2' (i.e., with a 2 mm distance).

[0035] Fig. 6 shows measurements of a headphone setup according to Fig. 4, where the curves show the relative level reduction compared to an undamped headphone. In this setup, however, the disc with a diameter of 42 mm (75% of the transducer diameter) consists of a damping material with a damping coefficient of 80 Ns / m. 3The distances are again the same as above, namely 2 mm for f4 and 5 mm for fs'. Here, the two curves are practically identical up to approximately 4 kHz and, even at higher frequencies, are generally closer together. Above 4 kHz and throughout the important range up to approximately 15 kHz, the fs' curve of the 5 mm wide air gap lies approximately 0.5–1 dB below the fT curve of the 2 mm wide air gap. Figure 6 shows that with a smaller diameter of the damping element, the distance d to the baffle has less of an effect. In contrast, the small difference between the two curves in the range above 4 kHz is due to the lower specific acoustic resistance of the damping material. With this variant, the frequencies above 4 kHz can be selectively attenuated, which is perceived by the user as an increase in the mid and low frequencies.Thus, by appropriately selecting the damping material as well as the size and distance of the damping element from the transducer, the expert can influence both the cutoff frequency and the value of the level reduction of the sheiving filter.

[0036] Figure 7 shows the frequency response of a headphone (using the Sennheiser HD 560S as an example) with the acoustic unit according to the invention and with a conventional acoustic unit. Curve fi" corresponds to the conventional acoustic unit, while curve fa" corresponds to an acoustic unit according to the invention. For this measurement, a damping disk was selected whose diameter is larger than the diameter of the transducer itself. As can be seen from the difference in the curves, the frequency response of the acoustic unit is attenuated above approximately 100 Hz by the sheaving filter according to the invention in this example. Thus, by appropriate amplification, a perceived boost of the frequencies below approximately 100 Hz can be achieved in the frequency response of the headphones.

[0037] To illustrate the characteristics of the sheaving filter in contrast to high-pass or low-pass filters, the frequency responses of various filter types are idealized in Fig. 8. The frequency response FH of a high-pass filter exhibits a region HPi that rises essentially monotonically at low frequencies and then transitions at a cutoff frequency fe.HP into a region HPa that is essentially constant. In contrast, the frequency response FT of a low-pass filter exhibits a region TPi that is essentially constant at low frequencies and then transitions at a cutoff frequency fe.TP into a region TPz that falls essentially monotonically.Sheaving filters, or shearing filters, on the other hand, exhibit in their frequency response FS a first, essentially constant region Si at low frequencies, a second region S2 at mid-frequencies that falls monotonically (or, alternatively, rises monotonically), and a third, essentially constant region S3 at high frequencies. Thus, sheaving filters allow frequencies of the entire audible range to pass, but the gain or attenuation in the first region S1 differs from that in the third region S3.

[0038] Bandpass filters, bandstop filters, and notch filters are also common (not shown here). These exhibit the lowest or highest attenuation or the highest or lowest gain in a specific mid-frequency range. In contrast, sheaving filters do not have a maximum or minimum in the mid-range S2. Sheaving filters with a decreasing mid-range S2, as in the example in Fig. 8, are called low-pass filters, while those with a rising mid-range S2 are called high-pass filters. Since Fig. 8 only illustrates the principle of different filters, no gain or attenuation values ​​are given on the vertical axis. Furthermore, the curves have been distributed vertically for better visualization. Typically, for example, the constant ranges TPi of the low-pass filter and HPa of the high-pass filter would be positioned at the same height.

[0039] Fig. 9 shows a known ring transducer 90 with an annular diaphragm 91 and a chassis 92 having an inner rim 931 and an outer rim 932. The diaphragm 91 is attached to both rims 931, 932 of the chassis and is driven by a voice coil 94 in the magnetic field of an annular magnet 95. The mounting element ST for the acoustic damping element DD (not shown here) can be attached to the baffle SW or directly to the ring transducer 90. In the latter case, it can, for example, be attached to the outer and / or inner rim 931, 932 of the chassis 92 of the ring transducer. In one embodiment, the acoustic damping element can be annular, like the diaphragm 91. However, such an annular damping element can also be used with conventional circular transducers if the special effect created by the central opening is desired.In the central area of ​​the damping disk, a different damping material with a different, e.g., lower, acoustic impedance can also be placed. In both cases, the effect is that the higher frequencies are again presented with a different, e.g., lower, acoustic impedance. This allows them to pass through the mechanical filter more easily, enabling, for example, the implementation of a bandstop filter. In addition to the parameters mentioned above, the level and the curve of the level increase at higher frequencies can also be adjusted, within certain limits, by the size of the central area. Figure 10 shows an example of a frequency response in which, in particular, the frequencies between 2 kHz and 8 kHz are more strongly attenuated, while frequencies above this range are less so.

[0040] The invention relates, in one embodiment, to an acoustic damping element with a planar, acoustically effective damping material having a defined acoustic resistance and at least one fastening element for attaching the damping material in front of a radiating surface of a sound transducer and parallel to the diaphragm plane of the sound transducer such that a planar gap is created opposite the radiating surface, the width of which is less than the diameter of the damping element.

[0041] In another embodiment, the invention relates to a headphone with at least one ear cup, wherein the ear cup comprises: a sound transducer with a diaphragm, and an acoustic damping element with a planar, acoustically effective damping material having a defined acoustic resistance, and at least one fastening element for attaching the damping material in front of the diaphragm of the sound transducer and substantially parallel thereto, wherein the planar, acoustically effective damping material is arranged in front of the diaphragm of the sound transducer such that a planar gap is located between the diaphragm and the damping material, the width of which is less than the diameter of the diaphragm, and wherein the planar, acoustically effective damping material acts as an acoustic shelving filter with a low-pass function.The invention can be generally used for devices with one or more transducers for acoustic audio reproduction, such as improved headphones. In particular, the invention is advantageous for passive headphones without their own power supply because the filtering is purely mechanical or acoustic and no electronic signal processing is required.

Claims

Patent claims 1. Acoustic damping element with a planar, acoustically effective damping material (DD) with a defined acoustic resistance; and at least one fastening element (ST) for fastening the planar, acoustically effective damping material (DD) in front of a radiating surface of a sound transducer (W) and parallel to the diaphragm plane of the sound transducer (W) such that a planar gap (AG) is created opposite the radiating surface, the width (d) of which is less than the diameter of the damping element.

2. Acoustic damping element according to claim 1, wherein the planar, acoustically effective damping material has pores and no further openings larger than the pores.

3. Acoustic damping element according to one of claims 1-2, wherein the planar, acoustically effective damping material has a specific acoustic resistance of 50-2000 Ns / m 3 exhibits.

4. Acoustic damping element according to claim 3, wherein the damping material is paper, a woven fabric, cloth, nonwoven fabric, a microperforated film or another microperforated material.

5. Acoustic damping element according to one of claims 1-4, wherein the acoustic damping element acts as an acoustic sheaving filter with low-pass function.

6. Acoustic damping element according to claim 1, comprising an annular surface made of an acoustically effective damping material with a first specific acoustic resistance and a central region, wherein the central region either consists of an acoustically effective damping material with a second, different specific acoustic resistance or is open, and wherein the acoustic damping element acts as a mechanical bandstop.

7. Headphones with at least one ear cup, the ear cup comprising: a sound transducer (W) with a diaphragm; and an acoustic damping element comprising a planar, acoustically effective damping material (DD) having a defined acoustic resistance; and o at least one fastening element (ST) for fastening the planar, acoustically effective damping material (DD) in front of the diaphragm of the sound transducer (W) and substantially parallel thereto; wherein the planar, acoustically effective damping material (DD) is arranged in front of the diaphragm of the sound transducer such that a planar gap (AG) is located between the diaphragm and the damping material (DD), the width (d) of which is less than the diameter of the diaphragm, and wherein the planar, acoustically effective damping material (DD) acts as an acoustic shepherd's tail filter with a low-pass function.

8. Headphones according to claim 7, wherein the edge of the gap (AG) is essentially open all around and is only interrupted by legs or struts (ST) of the at least one fastening element for the acoustic damping element.

9. Headphones according to claim 8, wherein the edge of the circumferential gap (AG) is open to at least 90% of its circumference and the legs or struts (ST) make up at most 10% of the circumference.

10. Headphones according to one of claims 7-9, wherein the at least one fastening element is suitable for placing the damping material (DD) at a distance between 0.1 mm and 20 mm in front of the radiating surface of the sound transducer.

11. Headphones according to claim 10, wherein the at least one fastening element is suitable for placing the damping material (DD) at a distance between 1 mm and 5 mm in front of the radiating surface of the sound transducer.

12. Headphones according to one of claims 7-11, wherein the sound transducer has a first diameter and the acoustic damping element has a second diameter, and wherein the second diameter is between 50% and 200% of the first diameter.

13. Headphones according to claim 7, wherein the sound transducer (W) is a ring transducer having a chassis with an inner and an outer rim, and the fastening element for the acoustic damping element is attached to the inner rim of the chassis of the ring transducer.

14. Headphones according to one of claims 7-13, wherein the damping material has pores and no further openings larger than the pores.

15. Headphones according to one of claims 7-14, wherein the damping material is paper, a woven fabric, cloth, nonwoven fabric, a microperforated film or another microperforated material.

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