JP2023112626A - Method and device for sound quality improvement of speaker by helmholtz sound absorber - Google Patents

Abstract

To provide a method and a device that improve a phenomenon in which sounds from a front face and a back face of a diaphragm interfere with each other to generate a peak dip of frequency characteristics in a rear face open type speaker and a plane baffle type speaker, a phenomenon in which a low frequency range is reduced, and a phenomenon in which a peak of frequency characteristics is generated in the rear face open type speaker owing to resonance of the speaker box and a standing wave.SOLUTION: A whole or a part of a device of a Helmholtz sound absorber suppresses and adjusts a flow of a sound from a diaphragm back face to effectively absorb a sound of a frequency emitted from the diaphragm back face to deteriorate the frequency characteristics. A peak is improved into high sound quality by making large a difference in a sound absorption quantity between the sound emitted from the diaphragm back face and a sound emitted from a diaphragm front face, and a dip and decrease in the flow frequency range due to interference also can be improved.SELECTED DRAWING: Figure 12

Description

The present invention relates to an open-back, flat-baffle speaker.

The present invention relates to an open-back, flat-baffle speaker. Both have a structure in which the rear side of the speaker unit is largely open, and are similar to each other. After explaining the common points and different points, details will be given.

FIG. 1 is a perspective view of the open rear speaker 1, and FIG. 2 shows how the sound emitted from the front and rear of the diaphragm of the open rear speaker 1 reaches the listener 3 through paths L1 and L2. The graph in FIG. 3 is an example of the frequency characteristics of the open-back speaker 1 .

FIG. 4 is a perspective view of the flat baffle-type speaker 2, and FIG. 5 shows how sound emitted from the front and rear of the diaphragm of the flat baffle-type speaker 2 reaches the listener 3 through paths L3 and L4. FIG. 6 shows an example of frequency characteristics of the planar baffle speaker 2. In FIG.

As can be seen by comparing FIGS. 2 and 5, the open-back speaker is a flat baffle speaker with the ends bent. In both cases, the sound emitted from the back of the diaphragm is radiated as it is to the back, and because the back of the diaphragm is open, the movement of the diaphragm is affected by the air pressure on the back of the speaker unit compared to bass reflex and closed speaker systems. It can be said that they are similar in that they are less likely to be affected by noise, and as the depth of the open-back speaker becomes shallower, there is no difference from the flat baffle-type speaker. Fig. 3 shows an example of frequency characteristics when the difference in path length between the sound emitted from the front side of the diaphragm and the sound emitted from the rear side of the diaphragm is 0.34m, for an open-back speaker and Fig. 6 for a flat baffle-type speaker. be. The shapes of FIGS. 3 and 6 are similar because, except for the peak 1002, the disturbance of the frequency characteristics is caused by the interference between the sound wave emitted from the front surface of the diaphragm and the sound wave emitted from the rear surface of the diaphragm. This is because the peak and dip frequencies are determined by the path length difference. The peak 1002, which is the difference, is due to the fact that only the rear-open type speaker has a box-like shape on the back of the speaker unit, and resonance occurs in the box portion. In addition, the standing wave inside the box may deteriorate the frequency characteristics.

In the following, the details of the defects and their causes, and conventional countermeasures will be described, focusing on the open-back type speaker having the defects of the flat baffle type speaker.

である。
図１、図２、図３は、一例として、Ｌ２－Ｌ１＝０．３４ｍ、音速３４０ｍ／ｓｅｃ、としたもので、ｆｃ は５００Ｈｚとなる。なお、平面バッフル型スピーカ２では、図５のＬ４－Ｌ３が、図２のＬ２－Ｌ１に当たり、同じように考えることができる。 FIG. 1 is a perspective view showing the appearance of a rear open-type speaker 1. FIG. FIG. 2 is a diagram showing the positional relationship between the sectional view taken along the line AA' in FIG. 1 and the listener 3. As shown in FIG. FIG. 3 shows an example of frequency characteristics of the open-back speaker 1. FIG.
The sound emitted from the front surface of the diaphragm of the speaker unit 4 attached to the open rear speaker 1 reaches the listener 3 through the path L1, and the sound emitted from the rear surface of the diaphragm reaches the listener 3 through the path L2. Hereafter, the frequency at which the difference between L2 and L1 is 1/2 of the wavelength is called fc. When the speed of sound is c

is.
1, 2, and 3, as an example, L2-L1=0.34 m, the speed of sound is 340 m/sec, and fc is 500 Hz. In the planar baffle type speaker 2, L4-L3 in FIG. 5 corresponds to L2-L1 in FIG. 2 and can be considered in the same way.

The reason why the peak 1001 at 500 Hz corresponding to fc in FIG. This is because the sound coming out from the rear side of the diaphragm of 1 in opposite phase and reaching the listener 3 through the path of L2 has the same phase frequency. At frequencies lower than fc, the distance difference between L2 and L1 becomes shorter than half the wavelength of the sound, so that the peak dip due to interference does not appear, and it is as if a high-pass filter has been passed through the low-frequency level drop 1004 in FIG. As you can see, the low range is lowered. This is a major drawback of open-back speakers and flat baffle speakers. At 2fc corresponding to the dip 1003 in FIG. 3, the difference between L2 and L1 is one wavelength. Since the sound emitted from the rear side of the diaphragm is originally in opposite phase to the sound emitted from the front side of the diaphragm, it becomes completely opposite phase at a distance of one wavelength, canceling each other out and approaching zero at the minimum, but in reality the path of L2 Since the damping is greater in the case of the case, the dip becomes shallower accordingly. Peaks and dips appear alternately after 3fc, but the peaks and dips gradually become smaller.

A disadvantage of only the open-back type is that the sound emitted from the back side of the diaphragm of the speaker unit 4 may cause resonance, standing waves, etc. due to the speaker box, which may deteriorate the frequency characteristics. This is not caused by the interference of sound waves emitted from the front and back surfaces of the diaphragm, but is caused by the relationship between the sound emitted from the back surface of the diaphragm and the speaker box. The peak caused by resonance is steep and high, and it greatly impairs the sound quality, but countermeasures were difficult. Non-Patent Document 1 describes that the resonance is caused by the vibration direction of the cone and that there is no way to prevent it due to the release of the rear surface, so the effect is reduced by minimizing the depth. However, since reducing the depth is directly linked to increasing fc, it tends to be a trade-off with low-frequency reproduction capability. This corresponds to the peak 1002 appearing at 400 Hz in the graph of FIG.

Here are some conventional techniques for improving these drawbacks.

Although the form of the speaker system is not mentioned, Patent Documents 1 and 2 use a Helmholtz sound absorber to improve the frequency characteristics of the speaker system.
However, in both cases, sound absorption holes are opened on the baffle surface, not in the immediate vicinity of the speaker unit diaphragm. When a peak is created in the characteristics, although it has the effect of reducing the peak, it is disadvantageous in terms of effect and sound quality compared to the method of selectively absorbing the sound emitted from the back of the diaphragm that is the cause. . In addition, since the difference in the amount of absorption of sound emitted from the front and back of the diaphragm is small, it is difficult to improve the dip in the frequency characteristics and the drop in the low range caused by wave interference. It is thought that there is no intention to improve the decline in the low frequency range.

Patent Document 1 states, "In a speaker system for a high-quality stereo or the like, a Helmholtz resonator is placed in the speaker cabinet or in the room where the speaker system is installed to attenuate the signal of a specific frequency and improve the characteristics. There was a description that there was something to do.

However, unlike the bass-reflex type, the open-back and flat-baffle-type speakers have a large opening at the back of the speaker unit, so the sound diffuses quickly. If this is not done, improvement measures using Helmholtz sound absorbers are unlikely to be effective. Open-back and flat-baffle loudspeakers that use Helmholtz sound absorbers to improve sound quality and the prior art are unaware.

Japanese Patent Laid-Open No. 2002-200010 discloses a technique for improving the low frequency range of a speaker with an open rear surface. There is a method of dividing the back surface of the diaphragm, making a part of it an open-back type and a part of a phase-reversal type, or outputting it through a passive radiator. However, this method requires special design of the speaker unit and is difficult to apply to general speaker units. In addition, improvement of the low range is the main focus, and unlike the case of using a Helmholtz sound absorber, it is not possible to improve the frequency with a peak-dip with pinpoint accuracy.

In addition, Non-Patent Document 1 describes making the shape of the baffle irregular, devising the mounting position of the speaker unit, preventing the side plates of the speaker box from being parallel to each other, and using glass wool or felt inside the speaker box. , making the depth of the speaker box as shallow as possible, and preventing the dimension ratio of each side of the speaker box from becoming an integral multiple. Non-Patent Document 2 is almost the same, but it also mentions devising the arrangement of the speaker system in the room. However, although these methods are useful in the initial stage of design as standard, many of them are related to the entire housing design, and are sometimes difficult to apply as improvement measures.

As a prior art, the sound emitted from the front of the diaphragm of the speaker unit and the sound emitted from the back of the diaphragm interfere with each other, resulting in a dip in the frequency characteristics or a drop in the level of the low range. I haven't found a way to improve it by absorbing sound and increasing the difference in the amount of sound absorption between the sound emitted from the front of the diaphragm and the sound emitted from the back.

Japanese Utility Model Publication No. 6-44233

Japanese Utility Model Publication No. 56-134889

JP-A-04-023698

Written and edited by Takeo Yamamoto, Speaker System (Part 2), First Edition, Radio Gijutsusha Co., Ltd., July 15, 1977, p. 252-256

Supervised by Tamon Saeki, New Edition Speaker & Enclosure Encyclopedia, Second Edition, Seibundo Shinkosha Co., Ltd., April 10, 2001, p. 114-115

A phenomenon in which the sound emitted from the front of the speaker unit and the sound emitted from the rear interfere with the sound emitted from the front of the speaker unit, causing regular peaks and dips and attenuating the low frequencies in open-back and flat-baffle type speakers.

A phenomenon in open-back speakers where the sound emitted from the back of the speaker unit increases at a specific frequency due to resonance or standing waves caused by the speaker box, creating a peak in the frequency characteristics.

In open-back speakers or flat baffle speakers, the sound emitted from the front of the diaphragm of the speaker unit interferes with the sound emitted from the back of the diaphragm in the opposite phase, causing a dip in the frequency characteristics or a drop in the low-frequency level. By intensively absorbing the frequency sound from the back side of the diaphragm with a Helmholtz sound absorber, the difference in the amount of absorption between the sound emitted from the front side of the diaphragm and the sound emitted from the back side is increased, resulting in a dip in the frequency characteristics or a low-range sound. It is characterized by improving level lowering.

In open-back type speakers and flat baffle-type speakers, the sound emitted from the front side of the diaphragm of the speaker unit interferes in the same phase with the sound emitted from the back side of the diaphragm, causing a peak in the frequency characteristics, and the open rear side. sound emitted from the back of the speaker unit is amplified at a specific frequency due to resonance or standing waves caused by the speaker box, creating a peak in the frequency characteristics. and improving the peak of frequency characteristics by absorbing sound using a Helmholtz sound absorber.

A sound absorbing device for a speaker system, characterized in that it has a sound absorbing opening in a shape such as a funnel shape that can be covered over the rear of the speaker unit, and one or more Helmholtz sound absorbers are connected to the sound absorbing opening. and

The speaker is a rear open type speaker or a flat baffle type speaker, characterized in that a cylindrical sound path is formed on the back surface of the speaker unit, and the sound path is provided with a sound absorbing hole of a Helmholtz sound absorber.

The speaker is an open-back type speaker in which the housing of the Helmholtz sound absorber is arranged so as to form a narrow sound path in the speaker box, and the speaker can be operated by adjusting the shape or placement of the Helmholtz sound absorber or the width inside the speaker box. The degree of opening of the rear surface of the unit can be adjusted, and the sound absorbing hole of the Helmholtz sound absorber is provided at the narrowest part of the sound path or at a position closer to the speaker unit than that.

It is a rear open type speaker, a horn is formed inside a speaker box, a Helmholtz sound absorber is formed inside the horn, and the degree of opening of the back of the speaker unit is controlled by adjusting the width of the narrow part of the horn. can be adjusted, and a sound absorbing hole is provided at the narrowest part of the horn or at a position closer to the speaker unit than it.

It is possible to improve peaks and dips in frequency characteristics and attenuation of low frequencies caused by interference between sound emitted from the front surface of the diaphragm and sound emitted from the rear surface of the diaphragm in a flat-baffle-type speaker with an open rear surface.

It is possible to improve the peak of the frequency characteristics caused by the resonance and standing waves generated inside the speaker box due to the sound emitted from the back side of the diaphragm of the open-back speaker.

1 is a perspective view of an open rear speaker 1 not embodying the present invention; FIG. FIG. 2 is a diagram depicting the positional relationship between the A-A' cross-sectional view of FIG. This is an example of frequency characteristics when L2-L1=0.34 m and the speed of sound is 340 m/s in FIG. 1 is a perspective view of a planar baffle-type speaker 2 not embodying the present invention; FIG. FIG. 5 is a diagram depicting the positional relationship between the plane baffle-type speaker 2 of FIG. 4 and the BB' sectional view of FIG. This is an example of frequency characteristics when L4-L3=0.34 m and the speed of sound is 340 m/s in FIG. 1 is a perspective view of (Embodiment 1) of the present invention in a rear-opened speaker 1. FIG. FIG. 8 is a perspective view showing only the sound absorber 100 of the open rear speaker 1 (Example 1) of FIG. 7; FIG. 8 is a diagram depicting the positional relationship between the C-C′ cross-sectional view of FIG. 7 and the listener 3 of the open-back speaker 1 (Example 1). It is an example of improvement of the frequency characteristic by the open-back type speaker 1 (Example 1) of FIG. The solid line is before improvement, and the dotted line is after improvement. FIG. 11 is a perspective view of (Embodiment 2) in a flat baffle type speaker 2; FIG. 11 is a perspective view of the rear open-type speaker 1 (Example 3). FIG. 11 is a perspective view showing only a cylindrical Helmholtz sound absorber 200 of the open-back speaker 1 (Embodiment 3). FIG. 13 is a cross-sectional view of the open rear speaker 1 (Embodiment 3) taken along the line D-D′ in FIG. 12 ; FIG. 11 is a perspective view of a flat baffle-type speaker 2 (Embodiment 4). It is the perspective view which looked at the plane baffle type|mold speaker 2 (Example 4) from the back side. FIG. 16 is a cross-sectional view taken along E-E' in FIG. 15 of a flat baffle type speaker 2 (Embodiment 4). FIG. 16 is a cross-sectional view taken along line F-F' of FIG. 15 of flat baffle type speaker 2 (Embodiment 4). FIG. 11 is a perspective view of a flat baffle-type speaker 2 (Example 5). FIG. 11 is a perspective view of a flat baffle-type speaker 2 (Example 6). FIG. 11 is a perspective view of a rear open-type speaker 1 (Embodiment 7). Rear open speaker 1 (Embodiment 7) FIG. 21 is a perspective view showing only the Helmholtz sound absorber on the upper left of FIG. FIG. 21 is a cross-sectional view of the open rear speaker 1 (Embodiment 7) taken along the line G-G' of FIG. 21;

Embodiments of the present invention will be described below with reference to the drawings. In addition, in Example 1, the details of the improvement method will be described together.

FIG. 7 is a perspective view showing an embodiment of the rear-opened speaker 1 of the present invention (Embodiment 1). The Helmholtz resonator has the property of absorbing sound at the resonance frequency, so it is used as a sound absorber. The sound absorber 100 installed inside the speaker box consists of four acoustically independent Helmholtz sound absorber container portions 103 and four common sound absorption openings 102 shown in detail in FIG. The four Helmholtz sound absorbers can absorb sound at different frequencies or several at the same frequency. There are no restrictions on the number of Helmholtz sound absorbers, and it is decided based on necessity, cost, size of equipment, and so on.

FIG. 9 is a diagram illustrating the positional relationship between the C-C' sectional view of FIG. 7 and the listener 3. As shown in FIG. The sound absorbing opening 102 is placed on the rear surface of the speaker unit 4 with a gap, and by adjusting the size of the gap, the sound absorption efficiency for the sound emitted from the rear surface of the diaphragm and the rear surface of the speaker related to the operation as a rear open type speaker can be improved. Adjust the degree of opening. The closer the sound absorbing port 102 and the back surface of the diaphragm are, the less the sound emitted from the back surface of the diaphragm is diffused. This is because the sound absorption efficiency for sound is improved. On the other hand, if they are too close to each other, the advantages of open-back speakers, such as the absence of air pressure from the speaker box on the diaphragm and the dipole sound directivity, are lost, so adjustment is important. At the time of adjustment, differences in auditory sensation are checked by listening, and effects on characteristics can be evaluated by measuring changes in frequency characteristics and impedance curve of reproduced sound. It should be noted that the sound absorbing port 102 is not limited to a funnel shape, and may be, for example, a Petri dish shape or a bowl shape, as long as it satisfies the functions listed here.

In FIG. 7, a sponge 12, which is a sound absorbing material with low frequency selectivity, is stuck inside the speaker box to prevent standing waves, but it is finally removed if unnecessary.

Although FIG. 8 shows only the sound absorber 100, the overall shape excluding the sound absorbing opening 102 and the cylindrical sound absorbing hole 104 is a teardrop shape. The purpose is to minimize the blocking of sound and avoid creating a plane parallel to the walls of the speaker box. Therefore, if it is suitable for the purpose, it may be spherical, cylindrical, polygonal, or the like.

An example of improvement method is shown. First, the frequency characteristics of the speaker system are measured before the sound absorber 100 is installed, and the peak frequency due to the interference of sounds emitted from the front and rear of the diaphragm, or the influence of resonance and standing waves inside the speaker box is identified. Next, the frequency dip caused by the interference of sounds emitted from the front and rear of the diaphragm and the range where the low frequency range is lowered due to the interference are specified. If the sound emitted from the front of the diaphragm is the cause, it can be improved by completely covering the back of the speaker box with a plate, for example, so the cause can be isolated. Note that since the sound absorber 100 may change the volume and sound path and change the frequency characteristics, the change is confirmed by measuring even when the sound absorber 100 with the sound absorbing hole 104 closed is inserted.

After specifying the frequency to be improved, it is determined which frequency the four Helmholtz sound absorbers are to be used for, and the length and thickness of the cylindrical sound absorbing hole 104 are adjusted to match the resonance frequency of the Helmholtz resonance. If necessary, the volume of the container portion 103 of the Helmholtz sound absorber is also changed. In Example 1, the sound absorbing frequencies are set to 250 Hz, 400 Hz, 500 Hz and 1000 Hz.

Next, the sound absorber 100 is installed in the speaker box, and the installation position is determined by measurement and listening while changing the distance between the sound absorption opening 102 and the rear surface of the speaker unit 4 . If the sound absorbing port 102 is brought close to the baffle plate so that there is no gap, it will not be a rear open speaker, but the sound emitted from the back side of the diaphragm will not come out, so the sound coming out from the back side will not be. The resulting peak dip and low frequency drop are completely resolved. Conversely, the farther apart the sound absorber 102 is, the closer the operation is to that of a perfect open-back speaker, but the effect of the Helmholtz sound absorber is very small. If there is a good balance between the position where the back of the speaker is completely covered and the position where it is far away, it is complete. While adjusting the position, if both the improvement of the frequency characteristics and the sound quality are at or above the allowable value, it is considered complete. , the number of Helmholtz sound absorbers, the sound quality and performance required for the speaker system, and repeated cut-and-try.

Effect of Example 1.
FIG. 10 shows an example of frequency characteristics before and after incorporating the present invention.
A peak 1001, a peak 1002, a dip 1003, and a low frequency level drop 1004 will be individually described.

FIG. 9 is a diagram illustrating the positional relationship between the C-C' sectional view of FIG. 7 and the listener 3. As shown in FIG. A peak 1001 at 500 Hz is caused by interference between the sound emitted from the front surface of the diaphragm and the sound emitted from the rear surface. Since the peak due to interference is the addition of waves with the same phase, it is doubled or +6 dB at the maximum depending on the conditions. If a sound absorber is used to lower the peak, the peak of the frequency characteristics will be lowered by the amount of sound absorption regardless of whether the sound is emitted from the front or the back of the diaphragm. However, when a large amount of sound emitted from the front surface of the diaphragm is absorbed and the ratio of sound emitted from the rear surface of the diaphragm at the position of the listener 3 increases, only that frequency reaches the path of L2, so sounds of other frequencies You will hear the sound delayed with respect to Considering the sound quality, it is desirable to absorb the sound emitted from the rear side of the diaphragm while not absorbing the sound emitted from the front side of the diaphragm as much as possible. In addition, the sound emitted from the diaphragm diffuses if there is an open space, although the degree varies depending on the frequency and conditions, so the sound absorption efficiency deteriorates after diffusion. Here, as shown in FIG. 9, the sound emitted from the back surface of the diaphragm is absorbed by a funnel-shaped sound absorbing opening before being diffused. The position is also the farthest position from the front surface of the diaphragm. Therefore, the sound emitted from the rear surface of the diaphragm can be absorbed more than the sound emitted from the front surface of the diaphragm. In the example of FIG. 10, the noise is improved by about 2 dB due to countermeasures.

A peak 1002 is a result of the sound emitted from the rear surface of the diaphragm resonating in the speaker box 11 and increasing. Depending on the shape of the speaker box, Helmholtz resonance, air column resonance, standing waves, etc., also generate peaks, but the effects of the present invention can be achieved in any case where the sound emitted from the back surface of the diaphragm creates a peak due to the speaker box. There is This is not the interference of the sound emitted from the front and back of the diaphragm, but the sound emitted from the back of the diaphragm, which is increased. It is more effective to absorb the sound emitted from the back, and it is also desirable in terms of sound quality. Therefore, the countermeasure method is the same as that of the peak 1001 . In the example of FIG. 10, most of the peaks have disappeared due to countermeasures. Note that this peak does not occur in flat baffle speakers that do not have speaker boxes.

Like the peak 1001, the dip 1003 is caused by interference between the sound emitted from the front surface of the diaphragm and the sound emitted from the rear surface, but is different in that it is a dip. In the case of dips, if the sound emitted from the rear side of the diaphragm is not absorbed more than the sound emitted from the front side of the diaphragm, the dip may become deeper, so caution is required. Here is a rough estimate:

スピーカユニット振動板前面から出る音の周波数特性が完全に平坦とすると振動板前面から出て聴取位置に届くＬ１を経由した音の大きさは周波数に関わらず１だから、振動板背面から出て聴取位置にて逆位相で打ち消しを行っているＬ２を経由した音の大きさは
（数３）
１－０．７５＝０．２５
と考えられる。図９のように吸音器を取り付け、その効果により振動板背面から出て聴取位置に届く音を対策前の４０％に減少させたとすると、Ｌ２経由の音は
（数４）
０．２５×０．４＝０．１
に減少する。振動板前面と吸音口は遠いため、Ｌ１経由で聴取位置に届く音はあまり吸音されずに対策前の９５％になったとするとＬ１経由の音は
（数５）
１×０．９５＝０．９５
となる。この２つが逆位相で合成された聴取位置での音の大きさは
（数６）
０．９５－０．１＝０．８５
となり、ｄＢで表すと

となるので、ディップの改善量は
（数８）
２．５－１．４＝１．１ｄＢ
である。図１０の例では対策によりディップ１００３は１ｄＢ程度改善している。 The drop in the sound pressure level at the dip 1003 indicated by the solid line in FIG. 10 is about -2.5 dB. Assuming that 0 dB is 1, -2.5 dB is 0.75. See number 2.

If the frequency characteristics of the sound emitted from the front of the speaker unit diaphragm are completely flat, the volume of the sound emitted from the front of the diaphragm and reaching the listening position via L1 is 1 regardless of the frequency. The loudness of the sound through L2, which is canceled in opposite phase at the position, is (Equation 3)
1-0.75=0.25
it is conceivable that. Assuming that a sound absorber is installed as shown in Fig. 9, and the sound coming out from the back of the diaphragm and reaching the listening position is reduced to 40% before the measure, the sound via L2 is (Equation 4)
0.25 x 0.4 = 0.1
to Since the front surface of the diaphragm and the sound absorbing hole are far away, the sound reaching the listening position via L1 is not absorbed much, and if 95% of the sound before the countermeasure is reached, the sound via L1 is (Equation 5)
1 x 0.95 = 0.95
becomes. The loudness of the sound at the listening position when these two are synthesized in opposite phase is (Equation 6)
0.95-0.1=0.85
and when expressed in dB

Therefore, the amount of dip improvement is (Equation 8)
2.5-1.4=1.1dB
is. In the example of FIG. 10, the dip 1003 is improved by about 1 dB as a countermeasure.

であるから、対策前よりも
（数１２）
２．５－３．７＝－１．２ｄＢ
１．２ｄＢ 悪化することになる。この結果よりスピーカユニット振動板前面から出る音はなるべく吸音せず、背面から出る音は効率的に吸音することがディップを持ち上げる際には非常に重要であることがわかる。 Since it is important as a way of thinking, I will describe the case of deterioration. If the installation conditions of the sound absorber are bad, and the sound coming out from the back of the diaphragm and reaching the listening position is only 60% of the sound before the countermeasure, the sound via L2 is (Equation 8)
0.25 x 0.6 = 0.15
becomes. Also, if the sound absorber absorbs a lot of the sound emitted from the front of the diaphragm and the sound via L1 drops to 80% of what it was before the countermeasure,
(Number 9)
1 x 0.8 = 0.8
becomes. The loudness of the sound at the listening position where these two are synthesized in opposite phase is (Equation 10)
0.8-0.15=0.65
When the sound pressure difference is expressed in dB,

Therefore, (Equation 12) than before the countermeasure
2.5-3.7=-1.2dB
1.2 dB worse. From this result, it can be seen that it is very important to absorb the sound emitted from the front side of the speaker unit diaphragm as little as possible and to efficiently absorb the sound emitted from the back side when lifting the dip.

The low frequency level reduction 1004 will be described. The level of the open-back and flat-baffle type speaker is reduced as if fc is used as a threshold, and frequencies lower than fc are passed through a high-pass filter. 1004 refers to the entire portion of the range below 0 dB. This is the same as the dip 1003 in terms of cause and countermeasure. However, the area where the level is lowered is wide, and it is difficult to raise the whole with a small number of Helmholtz sound absorbers, so it is necessary to use multiple sound absorbers with different frequencies to improve a wide range. In the example of FIG. 10, the noise around 300 Hz is improved by about 0.5 dB by using one Helmholtz sound absorber.

FIG. 11 is a perspective view showing an embodiment of the flat baffle type speaker 2 of the present invention. The structure is obtained by attaching a sound absorber 100 to a flat baffle type speaker 2 . A member for attaching the sound absorber is omitted in the drawing.

FIG. 12 is a perspective view showing an embodiment of the rear open type speaker 1 of the present invention. The sound absorber 200 is formed by stacking petri dish-shaped containers 202 vertically to form a thick cylinder. FIG. 14 is a cross-sectional view of FIG. 12 taken along the line D-D'. The sound absorbing hole 203 is placed closer to the speaker than the narrow portion 205 of the sound path where the inside of the speaker box in FIG. 14 is narrowed. This is because the sound is not diffused, so the sound absorption efficiency is good, and the sound emitted from the front surface of the diaphragm is also difficult to absorb. A major feature of the sound absorber 200 is that its housing itself forms a narrow space with the wall of the speaker box. This is because adjusting the thickness or arrangement of the sound absorber 200 or the inner width of the speaker box changes the width of the narrow portion 205 of the sound path, so that the opening degree of the rear surface of the speaker unit 4 can be adjusted and optimized.

Other advantages of the cylindrical shape of the sound absorber 200 are that petri dish-shaped and cylindrical materials made for other purposes can be easily reused, the inner wall of the speaker box does not have parallel surfaces, and depending on the shape of the speaker box, a cylindrical shape can be used. It is conceivable to put a plurality of sound absorbers in the speaker box. The point is that the sound absorber itself forms a narrow space with the wall of the speaker box to narrow the sound path, and the shape is not limited to a cylindrical shape. For example, if a rectangular parallelepiped with a square bottom is used, the length of the diagonal line is √2 times the length of the sides, so the advantage is that the width of the narrow portion 205 of the sound path can be changed by rotating it within the speaker box. In addition, the width from the narrow part 205 of the sound path to the opening at the rear of the speaker box can be used as an acoustic horn, or the speaker box can be made spherical instead of cylindrical and floated inside the speaker box with a rod or plate. etc. can also be considered.

FIG. 15 is a perspective view showing an embodiment of the flat baffle type speaker 2 of the present invention. A sound absorber frame 301 having approximately the same area as the baffle surface is attached to the baffle plate from the back, and the sound absorber frame 301 has a cylindrical sound path 305 on the back of the speaker unit 4, from which one or more A cylindrical sound absorbing hole 303 is extended, and a container portion 302 of a Helmholtz sound absorber is located at the end thereof. A rear lid 304 is for covering the rear side of the frame 301 of the sound absorber. Due to the presence of the cylindrical sound path 305, the sound emitted from the rear surface of the diaphragm can be effectively absorbed before it diffuses. If the diameter of the cylindrical sound path 305 is narrowed and the length is lengthened, the sound absorption efficiency can be increased. It is possible to give priority to the advantages of the type speaker, namely, that the air pressure caused by the speaker box is not applied to the diaphragm and the directivity of the sound of the dipole type. In addition, since the sound path 305 is positioned farthest from the sound emitted from the front surface of the diaphragm, the amount of absorption of the sound emitted from the front surface of the diaphragm is suppressed. Also, a large flat baffle-type speaker may have ribs to strengthen the plate, and the frame 301 of this sound absorber also functions as a reinforcing rib for the baffle plate. Another advantage is that the external appearance of the flat baffle type speaker does not change much.

FIG. 19 is a perspective view showing an embodiment of the flat baffle type speaker 2 of the present invention. The area of the baffle plate of Example 4 is reduced, and an enclosure 356 for expanding low frequencies is added in the depth direction. The enclosure 356 for expanding the low frequency range compensates for the reduction in the area of the baffle, and prevents an increase in fc by not reducing the value corresponding to L2-L1 in FIG. Although FIG. 19 shows a modified flat baffle type speaker 2, it is more appropriate to call the modified flat baffle type speaker an open-back type speaker.

FIG. 20 is a perspective view showing an embodiment of the flat baffle type speaker 2 of the present invention. This is a further modification of Example 5, in which the sound absorbers are stacked in three stages. It is also easy to increase the number of sound absorbers by stacking them. An enclosure 356 for expanding the low range is added in the depth direction, but when the depth is deepened by stacking many sound absorbers and the fc is sufficiently lowered and the level drop 2004 of the low range is absorbed by a large number of Helmholtz sound absorbers. It is not necessary if Although FIG. 20 shows a modified flat baffle type speaker 2, it is more appropriate to call the modified flat baffle type speaker an open-back type speaker.

FIG. 21 is a perspective view showing an embodiment of the rear open type speaker 1 of the present invention. FIG. 22 is a perspective view of only the upper left sound absorber 500. FIG. A horn shape in which the inside of the open-rear speaker is expanded toward the outside has been practiced for a long time, but the sound absorber 500 is a Helmholtz sound absorber that doubles as a horn, and is installed on the wall surface of the open-rear speaker 1. to form a horn shape. In addition, by providing the sound absorbing hole at the narrowest part 504 of the sound path or at a position closer to the speaker unit than that, the sound emitted from the rear surface of the diaphragm is efficiently absorbed, and the absorption volume of the sound emitted from the front surface of the diaphragm is reduced. By adjusting the width of the narrowest part 504 of the sound path, the opening degree of the rear surface of the speaker unit 4 can be adjusted and optimized.
In FIG. 22, they are installed on the left and right wall surfaces, but they may be installed on the top and bottom or on all of the left, right, top and bottom.

1 Rear open type speaker 2 Flat baffle type speaker 3 Listener 4 Speaker unit 11 Speaker box 12 Sound absorbing material with low frequency selectivity, sponge, glass wool, etc. Sound absorbing port 103 shared by a plurality of Helmholtz sound absorbers Container portion 104 of Helmholtz sound absorber Cylindrical sound absorbing hole 200 Cylindrical Helmholtz sound absorber 202 Petri dish-shaped container 203 Sound absorbing hole 204 Lid 205 of Petri-shaped container Narrow portion of sound path 301 Sound absorber frame 302 Helmholtz sound absorber container portion 303 Cylindrical sound absorbing hole 304 Rear lid 305 Cylindrical sound path 351 Sound absorber frame 355 Sound absorber lid 356 Enclosure 500 for expanding low range Forms a horn shape Helmholtz sound absorber 502 Container portion of Helmholtz sound absorber 503 Sound absorption hole 504 Narrowest part of sound path 1001 fc peak 1002 Peak 1003 due to speaker box resonance 2fc dip 1004 High-pass filter-like low-pass level drop 2001 fc peak 2003 2fc dip 2004 High-pass filter-like low-pass level drop

Claims (6)

In open-back speakers or flat baffle speakers,
The sound emitted from the front side of the diaphragm of the speaker unit interferes with the sound emitted from the back side of the diaphragm in the opposite phase, causing a dip in the frequency characteristics or a decrease in the level of the low range.
By intensively absorbing sound from the back of the diaphragm with a Helmholtz sound absorber, the difference in the amount of absorption between the sound emitted from the front of the diaphragm and the sound emitted from the back is increased.
A method to improve dips in frequency response or low-frequency level drops. In the rear open type speaker and flat baffle type speaker,
A sound with a frequency that causes a peak in the frequency characteristics by interfering with the sound emitted from the front of the diaphragm of the speaker unit from the back of the diaphragm in the same phase, and
In an open-back speaker, the sound emitted from the back of the speaker unit increases at a specific frequency due to resonance or standing waves caused by the speaker box, creating a peak in the frequency characteristics.
By absorbing sound using a Helmholtz sound absorber behind the baffle plate or inside the speaker box,
A method for improving the peak frequency characteristics. A sound absorbing device for a speaker system, comprising:
It has a sound absorption opening that is shaped like a funnel to cover the back of the speaker unit,
A sound absorber in which one or more Helmholtz sound absorbers are connected to the sound absorber. An open rear speaker or flat baffle speaker,
A cylindrical sound path is formed on the back of the speaker unit,
The sound path has Helmholtz sound absorber sound absorption holes,
speaker system. A rear open-type speaker,
The housing of the Helmholtz sound absorber is arranged to form a narrow sound path in the speaker box,
The degree of openness of the back of the speaker unit can be adjusted by adjusting the shape or placement of the Helmholtz sound absorber or the width inside the speaker box.
The sound absorbing hole of the Helmholtz sound absorber is provided at the narrowest part of the sound path or at a position closer to the speaker unit than that.
speaker system. A rear open-type speaker,
A horn is formed inside the speaker box,
A Helmholtz sound absorber is formed inside the horn,
By adjusting the width of the narrow part of the horn, you can adjust the degree of opening at the back of the speaker unit.
A sound absorbing hole is provided at the narrowest part of the horn or at a position closer to the speaker unit,
speaker system.

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