Acoustic shelving filter

Abstract

To adjust the frequency response of headphones, various filters are used, including shelving filters. However, these are usually implemented electronically. With wired headphones that lack their own power supply, electronic signal processing is not possible. To implement an acoustic shelving filter, an acoustic damping element (DD) made of a flat, acoustically effective damping material is placed a short distance (d) relative to the diameter of the transducer (W) in front of the transducer's radiating surface. This creates a circumferential air gap (AG) between the element and the transducer's radiating surface. The acoustic properties of the shelving filter can be controlled by the size and material of the damping element (DD) as well as its distance from the baffle (SW) or the transducer (W).

Description

[0001] The present invention relates to an acoustic filter, in particular an acoustic shelving filter, which is also referred to as a cowtail filter in German. background

[0002] 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, shelving filters, and equalizers, are available in many variations. Unlike low-pass, high-pass, or band-pass filters, shelving filters are known to have transfer functions that exhibit different, but essentially constant, values ​​at high and low frequencies.

[0003] 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.

[0004] However, no method is currently known for constructing a shelving filter or shelving 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. Summary of the invention

[0005] 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 9 relates to headphones according to the invention.

[0006] 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, which can be positioned at a small distance in front of the transducer's radiating surface relative to the transducer's diameter, thereby forming a substantially circumferential air gap with respect to the radiating surface. For example, a substantially annular air gap is formed in front of a transducer with a round diaphragm that is positioned in the baffle of a headphone. The acoustically effective damping material is planar. It has a defined specific acoustic resistance and, in one embodiment, has a closed surface without openings, or, in another embodiment, e.g., for annular transducers or to achieve a bandstop function, it has an annular surface.

[0007] Further advantageous embodiments are described in claims 2-8 and 10-12. Brief description of the drawings

[0008] Further details and advantageous embodiments are shown in the drawings. These show Fig. 1 Views of a conventional acoustic unit of a headphone with and without ear pads; Fig. 2 A sectional view of the acoustic unit according to the invention with ear pads; Fig. 3 A section through the acoustic unit according to the invention; Fig. 4 Frequency responses of a headphone (HD800S) with the acoustic unit according to the invention in various configurations; Fig. 5 A level reduction by changing the distance of a disc with D=54 mm and damping material with 300 Ns / m 3< ; Fig. 6 A level reduction by changing the distance of a disc with D=42 mm and damping material with 80 Ns / m 3< ; Fig. 7 Frequency responses of a headphone (HD560S) with the acoustic unit according to the invention and with a conventional acoustic unit; Fig. 8 Basic frequency responses of low-pass, high-pass and shelving filters; Fig. 9 A ring transducer with mounting points for a shelving filter; and Fig. 10 a frequency response of a bandstop filter. Detailed description of the invention

[0009] 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 Figure 1 shows two views of a conventional headphone acoustic unit. The left view shows the earpad P, while the right view shows it removed. In front of the transducer is a protective cover B for the diaphragm, which can also have other functions, such as acting as a resonator and / or an acoustic low-pass filter.

[0010] 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 the transducer's diameter, creating an annular gap between the damping material and the baffle or transducer. Low frequencies can propagate through this annular gap in the front volume and reach the ear without being obstructed by the damping material. High frequencies, on the other hand, are radiated more directionally due to the increasing directivity and can only reach the ear by passing through the damping material, not the annular gap. Therefore, to effectively create a cowtail filter, the damping material should not have any openings through which sound could be radiated without damping.This achieves a level reduction for high frequencies, but at higher frequencies, the level remains constant instead of continuously decreasing as with a low-pass filter. This behavior is similar to that of a shelving filter with a low-pass function.

[0011] Fig. 2 Figure 1 shows a sectional view of the acoustic unit according to the invention with ear pads P, wherein a planar, acoustically effective damping element DD is attached in front of the loudspeaker mounted on a baffle SW such that an annular gap AG is created. The damping element DD can be – for example, by means of a mechanical support element with struts or legs (not in Fig. 2(as shown) - attached to the baffle, the housing, or the ear cushion, whereby these mechanical support elements can interrupt the otherwise circumferential annular gap AG. The damping element DD and the fastening element form an acoustic damping element. The support elements cover only a small portion of the damping element's circumference (e.g., approximately 3%–7%, at most 10–15%), so that the annular gap remains essentially open. In other embodiments, the support elements can cover a larger portion of the circumference or be located internally, e.g., in a ring transducer. In the case of a ring transducer, the internal support element can also be designed as a closed cylinder.

[0012] The damping element DD, together with the annular 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 of the damping element DD from the transducer diaphragm, and the acoustic impedance or specific acoustic resistance of the damping material. The distance can range from 0.1 mm to 20 mm, depending on the desired effect, with values ​​between, for example, 1 mm and 5 mm being common in many applications (due to frequency and to minimize the overall height). The specific acoustic resistance can range from 50 to 2000 Ns / m³.The diameter of the damping element should be between 50% and 200% of the diameter of the transducer or the radiating surface.

[0013] Since the shelving filter according to the invention has a low-pass function and also provides contact protection for the membrane, conventional contact protection according to Fig. 1 This can be omitted. However, it is possible to combine both filters.

[0014] Fig. 3Figure 1 shows a cross-section through an acoustic unit (left in detail, right in principle), where the planar damping element DD is mounted at a distance d in front of the baffle SW, in which the transducer W is located. A substantially circumferential annular gap AG, interrupted by (in this example, three) thin struts ST for attaching the damping element DD, allows primarily low frequencies to pass through, since the sound source is small compared to the wavelength at these frequencies. As soon as half the wavelength of the radiated frequencies becomes smaller than the diameter of the sound source's diaphragm, beaming occurs. From this frequency onward and for higher frequencies, the damping element can influence the sound level. The cutoff frequency and sound level can thus be adjusted by dimensioning the damping element DD.Since the damping element DD has a defined specific acoustic resistance, this level is attenuated uniformly and not continuously reduced. Therefore, a shelving filter is created, not a low-pass filter.

[0015] Fig. 4Figure 1 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 / m³, and the transducer has a diameter of 56 mm. In the first variant 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 variant according to the invention, corresponding to curve f3, 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 variants according to the invention, f2 and f3, are approximately...From 200-300 Hz up to the upper end of the audible range at approximately 20 kHz, the values ​​are consistently below those of the conventional acoustic module (f1). The shelving filter parameters are chosen so that clear differences are visible from approximately 4 kHz upwards. In the first curve (f2), an initial peak, visible in f1 at approximately 5.3 kHz, as well as all values ​​above approximately 8 kHz, are significantly attenuated. The second curve (f3) 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 f1. The consistent attenuation of the high frequencies will lead the user to increase the volume slightly. This shifts the entire f2 or f3 curve upwards without altering its shape. As a result, the frequency response of the arrangement according to the invention is higher than the original frequency response f 1 at the low frequencies, in this example at least in the entire range below approx. 4 kHz.This increases the relative energetic component of the low frequencies below 4 kHz, creating the impression of enhanced bass reproduction.

[0016] While in Fig. 4 The absolute frequency responses of an HD 800 S acoustic module are shown, which demonstrate the Figures 5 and 6 Relative frequency responses, i.e., essentially the differences f₂' = f₂ - f₁ and f₃' = f₃ - f₁ compared to the unfiltered transducer. This makes the influence of the distance (and thus the width of the annular gap) or 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 flat-plate acoustic coupler. Small deviations, e.g., below 2 kHz, result from measurement inaccuracies.

[0017] In Fig. 5The level reduction caused by a disk with a diameter of 54 mm made of a damping material with a density of 300 Ns / m³ is shown at different distances, namely 2 mm in curve fz' and 5 mm in curve f³', as in Fig. 4 As can be seen, the two curves are almost identical up to approximately 3 kHz. Above 3 kHz, the wider annular gap in curve f 3 ' leads to an even greater reduction in frequencies. While a level reduction is already noticeable from 3 kHz with f 3 ' (i.e., with a 5 mm gap), a clear effect only occurs from 8 kHz onwards with f 2 ' (i.e., with a 2 mm gap).

[0018] Fig. 6 shows measurements of a headphone setup according to Fig. 4The 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) is made of a damping material with a damping coefficient of 80 Ns / m. The spacings are again the same as above: 2 mm at f4' and 5 mm at f5'. Here, the two curves are practically identical up to approximately 4 kHz and, even at higher frequencies, are closer together overall. Above 4 kHz and throughout the important range up to approximately 15 kHz, the f5' curve of the 5 mm wide annular gap is approximately 0.5–1 dB lower than the f4' curve of the 2 mm wide annular gap. Fig. 6It can be seen that with a smaller diameter of the damping element, the distance 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, 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 engineer can influence both the cutoff frequency and the degree of level reduction of the shelving filter.

[0019] In Fig. 7The frequency responses 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 are shown. Curve f1" corresponds to the conventional acoustic unit, while curve f2" 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 shelving 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.

[0020] To explain the characteristics of the shelving filter in contrast to high-pass or low-pass filters, the frequency responses of various filter types are shown in Fig. 8In an idealized representation, the frequency response FH of a high-pass filter exhibits a region HP1 that rises essentially monotonically at low frequencies and then transitions at a cutoff frequency fG,HP into a region HP2 that is essentially constant. In contrast, the frequency response FT of a low-pass filter exhibits a region TP1 that is essentially constant at low frequencies and then transitions at a cutoff frequency fG,TP into a region TP2 that falls essentially monotonically. Shelving filters, or shelving filters, on the other hand, have a frequency response FS that includes a first, essentially constant region S1 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, shelving filters allow frequencies across the entire audible range to pass, but the gain or...The damping in the first area S1 is different from that in the third area S3.

[0021] Furthermore, bandpass filters, bandstop filters, or 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, shelving filters do not have a maximum or minimum in the mid-range S2. For shelving filters whose mid-range S2 slopes downward, as in the example in Fig. 8 In this case, one speaks of a low-pass function, while those with an increasing middle range S2 are referred to as shelving filters with a high-pass function. Since Fig. 8Only the principle of different filters is explained; no gain or attenuation values ​​are given on the vertical axis. Furthermore, the curves have been distributed vertically to make them more visible. Typically, for example, the constant regions TP 1 of the low-pass filter and HP 2 of the high-pass filter would be positioned at the same height.

[0022] Fig. 9Figure 1 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. Fig. 10 shows an example of a frequency response in which frequencies between 2 kHz and 8 kHz are damped more strongly and frequencies above that are damped less strongly.

[0023] The invention can generally be 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 and no electronic signal processing is required.

Claims

1. Acoustic damping element comprising - a planar, acoustically effective damping material (DD); and - a fastening element for attaching the planar, acoustically effective damping material (DD) in front of a radiating surface of a sound transducer (W) such that a substantially circumferential gap (AG) is created opposite the radiating surface, the width (d) of which is less than the diameter of the radiating surface of the sound transducer.

2. Acoustic damping element according to claim 1, wherein the planar, acoustically effective damping material has a closed surface without openings.

3. Acoustic damping element according to claim 1 or 2, wherein the fastening element is suitable for placing the acoustically effective damping material at a distance between 0.1 mm and 20 mm in front of the radiating surface of the sound transducer.

4. Acoustic damping element according to claim 3, wherein the fastening element is suitable for placing the acoustically effective damping material at a distance between 1 mm and 5 mm in front of the radiating surface of the sound transducer.

5. Acoustic damping element according to one of claims 1-4, wherein the planar, acoustically effective damping material has a specific acoustic resistance of 50-2000 Ns / m 3 exhibits.

6. Acoustic damping element according to one of claims 1-5, wherein the acoustic damping element has a diameter that is between 50% and 200% of the diameter of the sound transducer.

7. Acoustic damping element according to one of claims 1-6, wherein the acoustic damping element acts as an acoustic shelving filter.

8. 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.

9. Headphones with at least one ear cup, wherein the ear cup contains: - a sound transducer (W) with a diaphragm; and - an acoustic damping element according to one of claims 1-8; - wherein the planar, acoustically effective damping material (DD) is arranged in front of the diaphragm of the sound transducer such that there is a gap (AG) between the diaphragm and the damping material (DD) whose width is less than the diameter of the diaphragm.

10. Headphones according to claim 9, wherein the gap (AG) is substantially circumferential and is only interrupted by legs or struts (ST) of the fastening element for the acoustic damping element.

11. Headphones according to claim 10, wherein the circumferential gap (AG) comprises at least 90% of the circumference of the fastening element and the legs or struts (ST) comprise at most 10%.

12. Headphones according to claim 9, wherein the gap (AG) is substantially circumferential and wherein the sound transducer (W) is a ring transducer having a chassis with an inner and an outer edge, and the fastening element for the acoustic damping element is attached to the inner edge of the chassis of the ring transducer.

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