JP3831263B2 - Duct silencer - Google Patents

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で与えられる。ここで、Ａ，Ｒ_x ，Ｒ_y ，Ｒ_z はそれぞれ任意振幅、ｘ軸方向反射率、ｙ軸方向反射率、ｚ軸方向反射率である。またｋは波数、ｊは虚数単位である。これに対して境界値問題として、図２に示すようにｚ軸方向には無限に続き、ｘ，ｙ方向にはそれぞれａ，ｂの長さを持つ矩形断面ダクトを考える。境界条件として、ｘ軸に平行な面、すなわちｙ＝０、ｂ：０＜ｘ＜ａ上では音響的に剛、ｙ軸に平行な面、すなわちｘ＝０、ａ：０＜ｙ＜ｂ上では音響的にソフトであるとする。
【００１０】
まとめて示すと、
ｖ＝０： ｙ＝０，ｂ， ０＜ｘ＜ａ
ｐ＝ｏ： ｘ＝ｏ，ａ， ０＜ｙ＜ｂ
である。ここでｖは境界面垂直方向粒子速度である。これらの境界条件を前記式（１）に代入して整理すると、一般解はｚ軸の一方向に進行する波動に対して、
【数２】

で与えられる。この場合で対象とする音波の波長がダクト断面寸法よりはるかに大きい場合には、モード次数としてゼロ次となる。すなわちｍ＝ｎ＝０を代入すると、全ての点において音圧はゼロとなり、このような音波はｚ方向にも伝搬しないことになる。
【００１１】
この物理特性を利用して、ダクト内表面を音響的にソフトにすれば非常に騒音低減量の大きなダクトを構成することができる。しかし、音響的にソフトな境界条件を実現するには、受動的な手段では空気よりも軽い材料が必要であり、通常の建築材料で実現することは一般的には困難である。また、能動的には従来の技術であるＡＮＣ（アクティブ・ノイズ・コントロール）システムを利用することができる。本発明では受動的には周波数選択性はあるものの、基本的に音響管を用いることで音響的にソフトな表面を実現し、能動的には従来のＡＮＣ技術をダクトに応用しようとするものである。
【００１２】
前記式（２）ではｘ軸方向に関する固有関数がｓｉｎ（（ｍπ／ａ）ｘ）となっており、この関数の性質によりｍ＝０であればｎ≠０であっても音波が伝搬しなくなる。したがって、ｙ軸方向は境界が剛であっても、別の条件であっても差し支えない。そこで、ｙ軸方向では別の周波数に設計された音響管を配列することができ、１つの区分で２つの周波数に対応できる。これらの区分を繋ぎ合わせることにより、すなわち、複数の異なる深さを持つ音響管を配列することで広い周波数帯域に対応して大きな減衰量を得ようとするものである。また、音響管では低周波数域で管の長さが長くなり、ダクト断面サイズが大きくなり過ぎるような場合には、能動型を利用してダクト全体の断面サイズを小さく抑えようとすることもできる。このように構成されたダクトでは従来のチャンバーのような大きな空間を必要とせず、低い周波数からある程度高い周波数までダクトの断面寸法をあまり大きくすることなく大きな減衰量を得ることが期待できる。
【００１３】
以下、図面等を用いて本発明の受動的な条件での実施例を説明する。図１に実験ダクトモデルを示す。ダクト部１０の断面寸法は１００ｍｍ×１００ｍｍ、長さは２０００ｍｍである。外部騒音の侵入や信号音としての対象音の漏れを防ぐため、ダクト１０は厚さ２０ｍｍのアクリル板１で構成されている。そのダクトの１つの壁面対を単なる剛壁としての厚さ２０ｍｍのアクリル板１、１、および深さがそれぞれ１７０ｍｍ、１３０ｍｍ、８５ｍｍのアルミ管製音響管４、３、２を配列できるようにした。また、それらの組合せも配列できるように音響管１つの単位を５００ｍｍの長さとし、３つの異なる音響管群を配列できるようにした。また、両端には全ての面が剛である部分を設け、受音側にはマイクロフォン７取付用の小孔を穿設し、最後部には長さ１０００ｍｍの吸音楔５を収容した無反射端を取り付けた。音源側では対象信号（１００Ｈｚ〜５ｋＨｚ帯域）を放出できる小型スピーカー６を取り付けた。
【００１４】
各音響管は厚さ２ｍｍ、断面５０ｍｍ×５０ｍｍの角管を必要長さに切断し、１０列２段、計２０個を厚さ２０ｍｍ、幅１００ｍｍ、長さ５００ｍｍのアクリル板に接着した。それを１つの単位としてダクト表面を構成するようにした。図３にその１例として長さ（深さ）１３０ｍｍの音響管群３を示す。このようにして製作したダクトモデルを用いて本発明の消音装置の効果を確認するための実験を行った。図４に示すような剛なアクリル板の壁面をタイプ（ｏ）、１ｋＨｚに設計された長さ（深さ）８５ｍｍの音響管群をタイプ（ａ）、６５０Ｈｚに設計された長さ１３０ｍｍの音響管群をタイプ（ｂ）、５００Ｈｚに設計された長さ１７０ｍｍの音響管群をタイプ（ｃ）とする。今回の実験では、受音側から配列した順で、（ｏｏｏ）（ｏｏａ）（ｏａａ）（ｃａａ）（ｃｂａ）の５つの場合について測定した。
【００１５】
表示配列例（ｏａａ）の場合は、受音側から各５００ｍｍ幅のアクリル板、タイプａ、タイプａの順に総合して１５００ｍｍのダクトに構成したことを示す。測定は音源から受音点までの周波数伝搬特性を測定した。図５に測定された受音点における相対音圧レベルを示す。各測定ではスピーカー６からの音響出力が一定になるように入力電圧を一定に保った。したがって、図５における周波数特性は各種ダクト条件での伝搬特性を表していることになる。また、図６には全ての壁面がアクリル板である場合を基準とした音響管の減衰効果が示されている。図５中にも示されるように、中太実線が配列（ｏｏｏ）の場合で、何も音響管配列がない場合（図６では縦軸が０ｄＢに相当する）である。
【００１６】
中太点線は配列（ｏｏａ）の場合で、１ｋＨｚに設計された長さ（深さ）８５ｍｍの音響管２が５００ｍｍだけ配置され、他は剛なアクリル板だけの場合である。この場合、１ｋＨｚの音が受音点まで殆ど伝搬せず、効果はほぼ４３ｄＢ程度である。細実線は配列（ｏａａ）の場合で、音響管が１０００ｍｍ配置されているが、配列（ｏｏａ）の場合と殆ど変わらない。すなわち、１ｋＨｚの音波の波長３４０ｍｍの約３倍あれば、ソフトな境界条件は大きな減衰を得るに充分であることを示している。経済的に許容されるなら、ダクト全長にわたりソフトな境界条件を採用するのが望ましい。
【００１７】
次に、細点線は配列（ｃａａ）の場合で、５００Ｈｚに対する深さ１７０ｍｍの音響管４が５００ｍｍ加わった場合である。この場合の５００Ｈｚでの減衰は約４３ｄＢであり、１ｋＨｚの場合と同様である。これに６５０Ｈｚに設計された深さ１３０ｍｍの音響管３を５００ｍｍ加えた配列（ｃｂａ）では、太実線が示すように、４５０Ｈｚ〜１．２ｋＨｚにわたって５０ｄＢ程度の非常に大きな減衰が得られていることがわかる。図５中で一点鎖線は背景雑音レベルであり、計測信号レベルはそれよりも充分高く、測定条件は満たされていることがわかる。
【００１８】
また、ダクト境界として最低どの程度の長さ分ソフト境界が必要かを調べたものを図７に示す。ここでは、ソフト境界の長さを１００ｍｍ、２００ｍｍと順次長くしていき、５００ｍｍ（ｏｏａ）までの条件下での音波減衰効果を示したものである。１００ｍｍでは減衰効果もあまり大きくはなく、かつ、その効果の生じる周波数範囲が狭い。しかし、２００ｍｍ程度になると周波数帯域が長さ５００ｍｍのものとほぼ同様となる。この２００ｍｍは対象周波数１ｋＨｚの半波長１７０ｍｍよりも少し長い。以上の結果より、波動が伝搬現象を生じるために必要な最小の距離、すなわち、半波長よりも長くソフト境界を設けることが周波数帯域が広く、かつ大きな減衰を得るに必要な条件であると言える。
【００１９】
同様のダクト断面寸法で、繊維材料（グラスウール板５０ｍｍ厚、密度３２ｋｇ／ｍ³ ）を内張りした条件での予測式
ＴＬ＝（α−０．１）ＰＬ／Ｓ
を用いて計算した場合、吸音率α＝０．９、断面寸法１００ｍｍ×１００ｍｍであれば、周長Ｐ＝０．４、断面積Ｓ＝０．０１、長さＬ＝０．５として、騒音低減量ＴＬ＝１６ｄＢ程度である。たとえ、吸音率αが０．９９になったとしても騒音低減量ＴＬ＝１７．８ｄＢである。これは全周にわたり吸音材料を取り付けた場合であり、前記の実験のように１つの壁面対だけで考えればその半分程度、約８〜９ｄＢ程度であることを考えると、前記の音響的にソフトな境界面による騒音低減量は非常に大きなものであると言える。
【００２０】
しかし、この音響管はダクト内表面に対して開口部が存在し、ダクト内部に存在すべき気流があれば雑音を発生する源となる。その発生は気流の流速にも依存するが、一般的には音響管内部の空気が流体として開口部近傍で激しく出入りし、雑音が発生するものと考えるのが通例である。したがって、このままの開口部をダクト内部に開けておくことは好ましくない。そこで、音響的には抵抗が少なく、流体としての抵抗が非常に大きい膜を開口部に取り付け、音響的な挙動を検討した。図８の実線は配列（ｏｏａ）の状態で、５０μｍの厚みを持つプラスティック膜を音響管開口部に無張力で張った場合の減衰効果を示す。破線は膜を張らない場合の値である。膜がない状態では効果の現れる周波数帯の中心が約１ｋＨｚであったのに対して、膜を張り付けることで３００Ｈｚ下の７００Ｈｚが中心となっている。
【００２１】
図９は配列（ｏｏｂ）の場合の減音性能を示しており、ここでも効果の現れる周波数の中心が７００Ｈｚから５００Ｈｚへと２００Ｈｚも低周波の方へ移動している。図１０は配列（ｏｏｃ）の場合で、この場合も５５０Ｈｚから４５０Ｈｚへと１００Ｈｚも低周波側へ移動している。これらの移動量は元の周波数の２０％から３０％にもなる。これは音響管の長さをその量だけ短くすることができることを意味する。さらに減衰量４０ｄＢのところで見ると、効果の現れる周波数範囲も膜が存在することによって広くなっており、膜の張設が非常に有効であることがわかる。また、高周波数であればあるほど膜の効果が大きく、今回の膜は厚さが５０μｍであることを考えれば、低周波数域ではさらに厚い膜を用いることで高周波数域と同等の効果が得られることが推測される。図１１に配列（ｏａｂ）で膜を張設した場合を示すが、この図で破線で示す膜なしの配列（ｏｂｃ）よりも周波数幅が広い範囲でより大きな効果が得られている。
【００２２】
また、図１２、図１３に音響管開口部に膜が存在することにより気流発生騒音が低減されることを説明する実験結果を示す。図１２は音響管配列（ｏｏａ）で膜を取り付けない状況でのダクト内気流発生騒音レベルを示し、縦軸は音圧レベル、横軸は周波数である。点線（＋）は流速がゼロの場合で、暗騒音に当たるものである。破線（□）から実線（●）まで順次気流速度が１ｍ／ｓ、３ｍ／ｓ、５ｍ／ｓ、７ｍ／ｓである場合の気流発生騒音を示している。図１３はこの音響管開口部に２００μｍのプラスティック膜を取り付けた場合の前記図１２と同様な条件での気流発生騒音である。例えば、流速７ｍ／ｓで比較してみると、膜の有無により低周波数域で最大２０ｄＢ以上も発生騒音が低減している。このように、音響管の開口部に膜を取り付けることは開口部における気流発生騒音を低減し、かつ音響管の長さを短く抑えるという効果が得られることが明らかとなった。
【００２３】
以上本発明の実施の形態について説明してきたが、本発明の趣旨の範囲内で、ダクトおよび音響管の形状（実施の形態の矩形断面の他、筒状であれば特に限定されず、三角形断面も可能で、辺数が４以上の多角形、円形、楕円形等が採用され得る。台形のような平行でない対向二辺を有するものでも、略平行ならばソフトな境界を設定できる）、材質（実施の形態のアルミニウムとアクリル板の他の適宜の素材）、ダクトにおける音響的にソフトに形成される面の部位（矩形断面の少なくとも対向する一対の面の他、対抗する二面のそれぞれの面、さらには辺数が４以上の多角形、円形、楕円形のほぼ全周）、膜の形状、厚さ、材質（実施の形態の合成樹脂の他、金属や他の素材等も採用可能で特に限定されることはない）、音響的にソフトな境界を構成する音響管の深さ（騒音の主成分をなす音波の波長の１／４の長さとする他、騒音の主成分をなす音波の第１次共鳴周波数に一致させてもよい）、音響的にソフトな境界としての構成に組み合わせる能動型制御手法（アクティブ・ノイズ・コントロールシステムの他、適宜のものが採用され得る）等については適宜選択し得る。なお、前述の実施の形態はあらゆる点で例示に過ぎず限定的に解釈してはならない。
【００２４】
【発明の効果】
以上、詳細に説明したように、本発明によれば、矩形断面ダクトの長さ方向において対象音波の半波長程度以上にわたり、対向する一対の壁面境界ほぼ全面が境界面上にて音圧がほぼゼロとなる音響的にソフトな境界として構成されたことにより、ダクト内における少なくとも最小限必要な面のみを所定長さにわたって音響的にソフトな境界として構成するだけで、従来にない大幅な減音性能が得られた（４０ｄＢの減音性能が１／２オクターブ幅にわたって得られた）。
【００２５】
また、矩形断面ダクトの長さ方向において対象音波の半波長程度以上にわたり、対向する二対の壁面境界それぞれのほぼ全面が境界面上にて音圧がほぼゼロとなる音響的にソフトな境界として構成されたことにより、減音性能はほぼ同じであるものの、効果の現れる帯域幅が広がることが確認された。
さらに、辺数が４以上の多角形、円形、楕円形等断面ダクトの長さ方向において対象音波の半波長程度以上にわたり、そのほぼ全周が境界面上にて音圧がほぼゼロとなる音響的にソフトな境界として構成されたことにより、広範な断面形態のダクトに対しても高い減音性能が得られる。
さらにまた、前記音響的にソフトな境界として、ダクト内表面に騒音の主成分をなす音波の波長の１／４の長さを有して終端が閉じた多数の音響管を並設した音響管の集合体で構成された場合は、簡素な構造の終端が閉じた音響管群の設置により、ダクト壁面の境界面における騒音の音圧をほぼゼロにすることができるので、大きな減音効果が得られる。
【００２６】
また、前記音響的にソフトな境界として、ダクト内表面に騒音の主成分をなす複数の音波の波長の１／４の長さを有して終端が閉じた多数の音響管を並設した前記各音波に対応する複数の音響管の集合体で構成された場合は、異なった波長の複数の騒音に対しても幅広く減音機能を発揮させることが可能となる。
さらに、前記音響的にソフトな境界として、ダクト内表面に騒音の主成分をなす音波の第１次共鳴周波数に一致するように調整された共鳴器内蔵の音響管の集合体で構成された場合は、音響管の深さの設計の自由度が向上する。
さらにまた、前記音響的にソフトな境界として、ダクト内表面に騒音の主成分をなす複数の音波の第１次共鳴周波数に一致するように調整された共鳴器内蔵の前記音波に対応する複数の音響管の集合体で構成された場合は、異なった波長の複数の騒音に対しても幅広く減音機能を発揮させることが可能となる。
【００２７】
また、前記音響的にソフトな境界として、騒音の主成分をなす１つまたは複数の音波に対して表面音圧がほぼゼロになるように開口部に膜を張設して終端が閉じた音響管をダクト内表面に多数配列した場合は、音響管内の気流の移動をさらに効果的に抑制して、効果の現れる周波数帯の中心を低周波側へ移動させることができ、結果的に音響管の長さを短くできて装置がコンパクトになる。
さらに、前記音響的にソフトな境界を構成するために、アクティブ・ノイズ・コントロールシステム等の能動型制御手法を組み合わせた場合は、低周波数域で長くなりがちな音響管の深さを短くしてダクト全体の断面サイズを小さく抑えることができる。
かくして本発明によれば、繊維材料等を使用せずして、繊維飛散等がなく、ダクトの断面寸法をあまり大きくすることなく、低周波域でも大きな減衰量が得られるダクト消音装置が提供される。
【図面の簡単な説明】
【図１】 本発明のダクト消音装置の１実施の形態のダクトモデル説明図である。
【図２】 同、ダクト内音波伝搬計算のための座標系を示す図である。
【図３】 同、音響管を２０本配列した、音響的にソフトな境界面を実現したモデル図である。
【図４】 同、音響的に剛な条件をダクト壁面で満たすためのモデル図である。
【図５】受音点における相対音圧レベル周波数特性図である。
【図６】 全てが剛な壁面の場合に対する音響的にソフトな壁面の効果図である。
【図７】 ソフト境界の長さと減衰効果の関係図である。
【図８】 音響配列（ｏｏａ）に対する膜の影響図である。
【図９】 音響配列（ｏｏｂ）に対する膜の影響図である。
【図１０】音響配列（ｏｏｃ）に対する膜の影響図である。
【図１１】音響配列（ｏａｂ）に対する膜の影響図である。
【図１２】膜が存在しない場合の気流騒音レベル図である。
【図１３】膜が存在する場合の気流騒音レベル図である。
【符号の説明】
１ ダクト壁（厚さ２０ｍｍのアクリル板）
１Ａ 壁面境界
１Ｂ 壁面境界
２ 音響管（深さ８５ｍｍのアルミ製管：タイプ（ａ））
３ 音響管（深さ１３０ｍｍのアルミ製管：タイプ（ｂ））
４ 音響管（深さ１７０ｍｍのアルミ製管：タイプ（ｃ））
５ 吸音楔（長さ１０００ｍｍのグラスウール製）
６ スピーカー
７ マイクロフォン
１０ ダクト[0001]
BACKGROUND OF THE INVENTION
The present invention relates to duct noise control for buildings, industrial plants, and the like.
[0002]
[Prior art]
There are many noise sources in buildings and industrial plants. In particular, in a ventilation system for air conditioning and ventilation, the cross-sectional dimension of the duct is smaller than the wavelength and a plane wave propagation phenomenon occurs, so that noise easily propagates through the duct. Therefore, a lot of costs are spent on noise control. Conventionally, in order to prevent these noises, a fiber material processed into a plate shape is pasted inside the duct as a sound absorbing material. In addition, there are some which require covering the entire surface of a relatively large space such as a lining chamber with a sound absorbing material, and fiber materials are often used for noise control in a duct system. The heavy use of this fiber material also has a problem of scattering of the fiber material as the air flow velocity in the duct increases. In addition, in the straight lining pipe, the flow path is bent and deformed to increase the amount of noise reduction, but this causes a pressure loss of the air flow, conversely increases the horsepower of the blower, and consequently increases the power of the noise source Invited.
[0003]
There is also a method in which no fiber material is used for noise control in the duct system. One duct is branched, one is made longer by half a wavelength, and the two are recombined to cause interference between two sound waves and cancel them. There is an interference type. Although this type of noise reduction performance is very excellent, there is also a type in which a 1/4 wavelength acoustic tube called a side branch having a similar function is attached vertically to a duct, which is not frequently used due to its large frequency selectivity. Although it exists (Japanese Patent Publication No. 2000-512369, etc.), it is often used alone for a part of the duct, its noise reduction effect is not so great, and there are few examples of practical use. Although active noise control technology that actively performs such a function instead of passive has been put into practical use, as in the case of the side branch, in order to expect noise disappearance at a certain point, it has a very large effect. Is not obtained. In addition, there is a problem in stability with respect to long-term use peculiar to active control, and it is only used in a form used in combination with a passive type.
[0004]
[Problems to be solved by the invention]
As described above, in the noise control of the duct system, when fiber materials are frequently used, there is a problem of fiber scattering due to air currents and the like, and generally fiber materials are not suitable for noise reduction in a low frequency range. Noise reduction does not increase at low frequencies for inner plane wave propagation. In addition, even a technique that does not use a fiber material is limited to local control, and a large amount of attenuation is not obtained.
[0005]
Therefore, the present invention solves the problems of such conventional noise control, that is, a silencer, and does not use fiber material or the like as a duct system noise control technology, and there is no fiber scattering or the like, and the cross-sectional dimensions of the duct An object of the present invention is to provide a duct silencer capable of obtaining a large amount of attenuation even in a low frequency range without increasing the length of the channel.
[0006]
[Means for Solving the Problems]
The technical solution adopted by the present invention is:
Duct silencer constructed as an acoustically soft boundary where the sound pressure is almost zero on the boundary surface of almost the entire boundary between a pair of opposing wall surfaces over the half-wave length of the target sound wave in the longitudinal direction of the rectangular duct The acoustically soft boundary includes a plurality of acoustic tubes having a length that is ¼ of the wavelength of a plurality of sound waves forming a main component of noise on the inner surface of the duct and closed at the ends. It is a duct silencer characterized by comprising an assembly of installed acoustic tubes .
In addition, as the acoustically soft boundary where the sound pressure is almost zero on the boundary surface over almost the entire surface of each of the two pairs of wall surfaces facing each other over about half a wavelength of the target sound wave in the length direction of the rectangular duct A duct silencer configured as described above, wherein the acoustically soft boundary has a length that is ¼ of a wavelength of a plurality of sound waves forming a main component of noise on the inner surface of the duct, and a plurality of closed ends. It is the duct silencer characterized by comprising with the aggregate | assembly of the acoustic tube which arranged the acoustic tube side by side .
In addition, a sound whose pressure is almost zero on the boundary surface over almost the entire circumference of the target sound wave in the length direction of a polygonal, circular, or elliptical cross-section duct having four or more sides and over the half wavelength of the target sound wave. A duct silencer configured as a soft boundary , wherein the acoustically soft boundary has a length that is ¼ of the wavelength of a plurality of sound waves forming a main component of noise on the inner surface of the duct. A duct silencer characterized by comprising a collection of acoustic tubes in which a large number of acoustic tubes whose ends are closed are arranged side by side .
An assembly of acoustic tubes in which a large number of acoustic tubes are arranged side by side is a duct silencer characterized by being configured as an acoustic tube corresponding to each sound wave .
The duct silencer is characterized in that the acoustic tube assembly has a built-in resonator adjusted to coincide with a primary resonance frequency of a sound wave forming a main component of noise on the inner surface of the duct. It is.
The duct silencer is characterized in that a membrane is stretched at the opening of the acoustic tube.
The duct silencer is characterized by combining active control methods such as an active noise control system in order to form the acoustically soft boundary.
[0007]
Embodiment
Hereinafter, the duct silencer of the present invention will be described in detail with reference to the drawings. FIGS. 1 to 13 are diagrams for explaining a duct silencer of the present invention. FIG. 1 is an explanatory diagram of a duct model of an embodiment of the duct silencer of the present invention. FIG. 2 is a coordinate for calculating sound propagation in the duct. FIG. 3 is a model diagram that realizes an acoustically soft boundary surface with 20 acoustic tubes arranged, FIG. 4 is a model diagram for satisfying an acoustically rigid condition with a duct wall surface, and FIG. Is a relative sound pressure level frequency characteristic diagram at a sound receiving point, FIG. 6 is an effect diagram of an acoustically soft wall surface when all are rigid wall surfaces, FIG. 7 is a relationship diagram between a soft boundary length and a damping effect, FIG. Fig. 9 is a diagram showing the influence of the membrane on the acoustic arrangement (ooa), Fig. 9 is a drawing showing the influence of the membrane on the acoustic arrangement (oob), Fig. 10 is a drawing showing the influence of the membrane on the acoustic arrangement (ooc), and Fig. 11 is a membrane showing the acoustic arrangement (oob). Fig. 12 shows the effects of airflow in the absence of a membrane. Level diagram, FIG. 13 is a stream noise level diagram when film is present.
[0008]
As shown in FIG. 1, the duct silencer of the present invention has a pair of wall surface boundaries 1A facing each other over a half wavelength or more of the target sound wave in the length direction of the duct 10 having a predetermined section (rectangular section in the illustrated example). 1B is characterized in that almost the entire surface is configured as an acoustically soft boundary where the sound pressure is almost zero on the boundary surface (installation of acoustic tubes 2, 4, 3, etc.). This will be described in detail below. In the present invention, it has been considered to realize an acoustically soft boundary surface on the inner surface of the duct. This acoustically soft boundary surface is a boundary surface on which the sound pressure is always zero. The normal inner surface of the duct is generally a rigid member made of a metal plate, and the sound pressure is not zero and the particle velocity is zero. Moreover, even if the boundary surface is covered with a fiber material, neither the sound pressure nor the particle velocity is zero on the surface. Here, if the inner surface of the duct is rigid, the inner surface of the duct is acoustically soft under the condition that the sound wave propagates in a plane wave (that is, the duct cross-sectional dimension is less than half the wavelength of the sound wave). If there is any sound wave propagation, it will be described using mathematical formulas.
[0009]
The solution for the sinusoidal vibration of the wave equation expressed in the Cartesian coordinate system (x, y, z) is as follows: sound pressure is p (x, y, z)
[Expression 1]

Given in. Here, A, R _x , R _y , and R _z are arbitrary amplitude, x-axis direction reflectance, y-axis direction reflectance, and z-axis direction reflectance, respectively. K is the wave number and j is the imaginary unit. On the other hand, as a boundary value problem, a rectangular cross-section duct having an infinite length in the z-axis direction and lengths a and b in the x and y directions as shown in FIG. 2 is considered. As boundary conditions, a plane parallel to the x-axis, ie, y = 0, b: 0 <x <a, is acoustically rigid, and a plane parallel to the y-axis, ie, x = 0, a: 0 <y <b Let's say that it is acoustically soft.
[0010]
In summary,
v = 0: y = 0, b, 0 <x <a
p = o: x = o, a, 0 <y <b
It is. Here, v is the interface vertical particle velocity. Substituting these boundary conditions into the equation (1) and rearranging them, the general solution is for a wave traveling in one direction of the z axis,
[Expression 2]

Given in. In this case, when the wavelength of the target sound wave is much larger than the duct cross-sectional dimension, the mode order is zero order. That is, if m = n = 0 is substituted, the sound pressure becomes zero at all points, and such sound waves do not propagate in the z direction.
[0011]
By utilizing this physical characteristic and making the duct inner surface acoustically soft, it is possible to construct a duct with a very large amount of noise reduction. However, to achieve acoustically soft boundary conditions, passive means require a material that is lighter than air and is generally difficult to achieve with ordinary building materials. In addition, an ANC (active noise control) system, which is a conventional technique, can be actively used. Although the present invention is passively frequency selective, it basically realizes an acoustically soft surface by using an acoustic tube, and actively attempts to apply the conventional ANC technology to a duct. is there.
[0012]
In the equation (2), the eigenfunction in the x-axis direction is sin ((mπ / a) x). Due to the nature of this function, if m = 0, the sound wave does not propagate even if n ≠ 0. . Therefore, the boundary in the y-axis direction may be rigid or under other conditions. Therefore, acoustic tubes designed to have different frequencies can be arranged in the y-axis direction, and two frequencies can be handled in one section. By connecting these sections, that is, by arranging a plurality of acoustic tubes having different depths, a large attenuation is obtained corresponding to a wide frequency band. In addition, in the case of an acoustic tube, when the length of the tube becomes long in the low frequency range and the duct cross-sectional size becomes too large, the cross-sectional size of the entire duct can be suppressed using an active type. . The duct configured as described above does not require a large space like a conventional chamber, and it can be expected to obtain a large attenuation amount from a low frequency to a somewhat high frequency without increasing the cross-sectional dimension of the duct so much.
[0013]
Hereinafter, embodiments of the present invention under passive conditions will be described with reference to the drawings. Fig. 1 shows the experimental duct model. The duct part 10 has a cross-sectional dimension of 100 mm × 100 mm and a length of 2000 mm. In order to prevent intrusion of external noise and leakage of the target sound as a signal sound, the duct 10 is composed of an acrylic plate 1 having a thickness of 20 mm. A pair of wall surfaces of the duct can be arranged as 20 mm-thick acrylic plates 1 and 1 as mere rigid walls, and aluminum tube acoustic tubes 4, 3 and 2 having depths of 170 mm, 130 mm and 85 mm, respectively. . In addition, the unit of one acoustic tube is 500 mm long so that combinations thereof can be arranged, and three different acoustic tube groups can be arranged. Further, both ends are provided with rigid portions, the sound receiving side is provided with a small hole for attaching the microphone 7, and the last portion is a non-reflective end accommodating a sound absorbing wedge 5 having a length of 1000 mm. Attached. On the sound source side, a small speaker 6 capable of emitting a target signal (100 Hz to 5 kHz band) was attached.
[0014]
Each acoustic tube was a square tube having a thickness of 2 mm and a cross-section of 50 mm × 50 mm cut to a required length, and 20 rows and two tiers were bonded to an acrylic plate having a thickness of 20 mm, a width of 100 mm, and a length of 500 mm. The duct surface was constituted as a unit. FIG. 3 shows an acoustic tube group 3 having a length (depth) of 130 mm as an example. An experiment for confirming the effect of the silencer of the present invention was performed using the duct model thus manufactured. The wall of a rigid acrylic plate as shown in FIG. 4 is type (o), the acoustic tube group of 85 mm length (depth) designed to 1 kHz is type (a), and the acoustic length 130 mm is designed to 650 Hz. The tube group is type (b), and the acoustic tube group having a length of 170 mm designed at 500 Hz is type (c). In this experiment, five cases of (ooo) (ooa) (oaa) (caa) (cba) were measured in the order of arrangement from the sound receiving side.
[0015]
In the case of the display arrangement example (oaa), it is shown that a 500 mm-wide acrylic plate, type a, and type a are combined in order from the sound receiving side to form a 1500 mm duct. The frequency propagation characteristics from the sound source to the receiving point were measured. FIG. 5 shows the relative sound pressure level at the sound receiving point measured. In each measurement, the input voltage was kept constant so that the sound output from the speaker 6 was constant. Therefore, the frequency characteristics in FIG. 5 represent the propagation characteristics under various duct conditions. FIG. 6 shows the attenuation effect of the acoustic tube with reference to the case where all wall surfaces are acrylic plates. As shown also in FIG. 5, the middle thick solid line is an arrangement (ooo) and there is no acoustic tube arrangement (in FIG. 6, the vertical axis corresponds to 0 dB).
[0016]
The middle thick dotted line is in the case of the arrangement (ooa), in which the acoustic tube 2 having a length (depth) of 85 mm designed at 1 kHz is arranged by 500 mm, and the others are only rigid acrylic plates. In this case, the 1 kHz sound hardly propagates to the sound receiving point, and the effect is about 43 dB. The thin solid line is in the case of the arrangement (oaa), and the acoustic tubes are arranged 1000 mm, but it is almost the same as in the case of the arrangement (ooa). That is, if the wavelength of the 1 kHz sound wave is about 3 times the wavelength of 340 mm, the soft boundary condition is sufficient to obtain a large attenuation. If economically acceptable, it is desirable to employ soft boundary conditions over the entire length of the duct.
[0017]
Next, a thin dotted line is the case of arrangement | sequence (caa), and is a case where the acoustic tube 4 with a depth of 170 mm with respect to 500 Hz is added 500 mm. In this case, the attenuation at 500 Hz is about 43 dB, which is the same as that at 1 kHz. In addition, in the arrangement (cba) in which the acoustic tube 3 having a depth of 130 mm designed at 650 Hz is added by 500 mm, a very large attenuation of about 50 dB is obtained from 450 Hz to 1.2 kHz as indicated by a thick solid line. I understand. In FIG. 5, the one-dot chain line is the background noise level, the measurement signal level is sufficiently higher than that, and it can be seen that the measurement conditions are satisfied.
[0018]
Further, FIG. 7 shows a result of examining the minimum length of the soft boundary required as the duct boundary. Here, the length of the soft boundary is sequentially increased to 100 mm and 200 mm, and the sound attenuation effect under conditions up to 500 mm (ooa) is shown. At 100 mm, the attenuation effect is not so great, and the frequency range in which the effect occurs is narrow. However, when it is about 200 mm, the frequency band is almost the same as that having a length of 500 mm. This 200 mm is slightly longer than the half wavelength 170 mm of the target frequency 1 kHz. From the above results, it can be said that providing the soft boundary longer than the half wavelength, that is, the minimum distance necessary for the wave to generate a propagation phenomenon, is a necessary condition for wide frequency band and large attenuation. .
[0019]
Prediction formula TL = (α−0.1) PL / S with the same duct cross-sectional dimension and fiber material (glass wool plate 50 mm thick, density 32 kg / m ³ )
When the sound absorption coefficient α is 0.9 and the cross-sectional dimension is 100 mm × 100 mm, the circumferential length P is 0.4, the cross-sectional area S is 0.01, and the length L is 0.5. The reduction amount TL is about 16 dB. Even if the sound absorption coefficient α becomes 0.99, the noise reduction amount TL = 17.8 dB. This is a case where a sound-absorbing material is attached over the entire circumference. Considering only one wall pair as in the above experiment, about half of that, about 8 to 9 dB, the above acoustically soft It can be said that the amount of noise reduction due to a rough boundary surface is very large.
[0020]
However, this acoustic tube has an opening with respect to the inner surface of the duct, and if there is an air flow that should exist inside the duct, it becomes a source of noise. The generation depends on the flow velocity of the airflow, but generally, it is generally considered that the noise inside the acoustic tube enters and exits vigorously near the opening as a fluid, and noise is generated. Therefore, it is not preferable to leave the opening as it is inside the duct. Therefore, the acoustic behavior was examined by attaching a film with low acoustic resistance and very high fluid resistance to the opening. The solid line in FIG. 8 shows the damping effect when a plastic film having a thickness of 50 μm is stretched without tension on the acoustic tube opening in the state of the arrangement (ooa). A broken line is a value when a film is not stretched. In the state without the film, the center of the frequency band where the effect appears is about 1 kHz, but by attaching the film, the center is 700 Hz below 300 Hz.
[0021]
FIG. 9 shows the sound reduction performance in the case of the arrangement (oob), and the center of the frequency at which the effect appears also moves from 700 Hz to 500 Hz and to 200 Hz as a low frequency. FIG. 10 shows the case of the arrangement (ooc). In this case as well, the frequency is shifted from 550 Hz to 450 Hz by 100 Hz toward the low frequency side. These movement amounts are 20% to 30% of the original frequency. This means that the length of the acoustic tube can be reduced by that amount. Further, when viewed at an attenuation of 40 dB, the frequency range in which the effect appears is widened by the presence of the film, and it can be seen that the stretching of the film is very effective. In addition, the higher the frequency, the greater the effect of the film. Considering that the film this time has a thickness of 50 μm, the use of a thicker film in the low frequency range provides the same effect as the high frequency range. It is speculated that FIG. 11 shows a case where the film is stretched by the arrangement (oab), and a greater effect is obtained in a wider frequency range than the arrangement (obc) without a film indicated by a broken line in this figure.
[0022]
In addition, FIGS. 12 and 13 show experimental results for explaining that airflow generation noise is reduced by the presence of a film at the acoustic tube opening. FIG. 12 shows the noise level of airflow generated in the duct in a state where the membrane is not attached in the acoustic tube array (ooa), the vertical axis is the sound pressure level, and the horizontal axis is the frequency. A dotted line (+) corresponds to background noise when the flow velocity is zero. From the broken line (□) to the solid line (●), airflow generation noise is shown when the airflow velocity is 1 m / s, 3 m / s, 5 m / s, and 7 m / s sequentially. FIG. 13 shows airflow generation noise under the same conditions as in FIG. 12 when a 200 μm plastic film is attached to the acoustic tube opening. For example, when compared at a flow rate of 7 m / s, the generated noise is reduced by as much as 20 dB or less in the low frequency range due to the presence or absence of a film. Thus, it has been clarified that attaching a film to the opening of the acoustic tube has the effect of reducing airflow generation noise in the opening and reducing the length of the acoustic tube.
[0023]
The embodiment of the present invention has been described above, but within the scope of the present invention, the shape of the duct and the acoustic tube (in addition to the rectangular cross section of the embodiment, it is not particularly limited as long as it is cylindrical, and the triangular cross section Polygons with 4 or more sides, circles, ellipses, etc. can also be adopted, even if they have two opposite sides that are not parallel, such as trapezoids, soft boundaries can be set if they are approximately parallel) (Other appropriate materials of aluminum and acrylic plate in the embodiment), acoustically softly formed surface portions of the duct (at least a pair of opposing surfaces of the rectangular cross section, and two opposing surfaces) Surfaces, polygons with 4 or more sides, circles, and ellipses almost all around), film shape, thickness, and material (in addition to the synthetic resin of the embodiment, metal and other materials can also be used) Is not particularly limited), acoustically soft The depth of the acoustic tube constituting the boundary (in addition to the length of ¼ of the wavelength of the sound wave forming the main component of the noise, it may coincide with the primary resonance frequency of the sound wave forming the main component of the noise); An active control method combined with a configuration as an acoustically soft boundary (in addition to an active noise control system, an appropriate one can be adopted) or the like can be selected as appropriate. The above-described embodiment is merely an example in all respects and should not be limitedly interpreted.
[0024]
【The invention's effect】
As described above in detail, according to the present invention, almost the entire surface of a pair of wall surfaces facing each other has a sound pressure almost equal to the boundary surface over a half wavelength or more of the target sound wave in the longitudinal direction of the rectangular cross-section duct. Since it is configured as an acoustically soft boundary that becomes zero, it is possible to significantly reduce noise by simply configuring at least the minimum necessary surface in the duct as an acoustically soft boundary over a predetermined length. Performance was obtained (40 dB sound reduction performance was obtained over 1/2 octave width).
[0025]
In addition, as the acoustically soft boundary where the sound pressure is almost zero on the boundary surface over almost the entire surface of each of the two pairs of wall surfaces facing each other over about half a wavelength of the target sound wave in the length direction of the rectangular duct Although the sound reduction performance was almost the same, it was confirmed that the bandwidth in which the effect appears was widened.
Furthermore, a sound whose pressure is almost zero on the boundary surface over almost the entire circumference of the target sound wave in the length direction of a polygonal, circular or elliptical cross section duct with four or more sides in the length direction. By being configured as a soft boundary, high sound reduction performance can be obtained even for ducts having a wide cross-sectional shape.
Furthermore, as the acoustically soft boundary, an acoustic tube in which a large number of acoustic tubes having a length of ¼ of the wavelength of the sound wave forming the main component of noise on the inner surface of the duct and closed at the ends are arranged side by side. In this case, the acoustic pressure of the noise at the boundary surface of the duct wall can be reduced to almost zero by the installation of the acoustic tube group with the closed end of the simple structure. can get.
[0026]
In addition, as the acoustically soft boundary, a plurality of acoustic tubes having a length that is ¼ of a wavelength of a plurality of sound waves forming a main component of noise and having closed ends are arranged in parallel on the inner surface of the duct. In the case of being composed of an assembly of a plurality of acoustic tubes corresponding to each sound wave, it is possible to exert a sound reduction function widely even for a plurality of noises having different wavelengths.
Further, the acoustic soft boundary is constituted by an assembly of acoustic tubes with a built-in resonator adjusted so as to coincide with a primary resonance frequency of a sound wave that forms a main component of noise on the inner surface of the duct. The degree of freedom in designing the acoustic tube depth is improved.
Furthermore, as the acoustically soft boundary, a plurality of sound waves corresponding to the sound waves incorporated in the resonator adjusted to coincide with the primary resonance frequencies of a plurality of sound waves forming the main component of noise on the inner surface of the duct. In the case of an acoustic tube assembly, a wide range of sound reduction functions can be exhibited even for a plurality of noises having different wavelengths.
[0027]
Further, as the acoustically soft boundary, an acoustic film having a closed end with a membrane stretched so that the surface sound pressure is almost zero with respect to one or more sound waves constituting the main component of noise. When a large number of tubes are arranged on the inner surface of the duct, the movement of the airflow in the acoustic tube can be further effectively suppressed, and the center of the frequency band where the effect appears can be moved to the low frequency side. The length of can be shortened and the device becomes compact.
Furthermore, in order to construct the acoustically soft boundary, when the active control method such as the active noise control system is combined, the depth of the acoustic tube, which tends to be long in the low frequency range, is shortened. The cross-sectional size of the entire duct can be kept small.
Thus, according to the present invention, there is provided a duct silencer capable of obtaining a large amount of attenuation even in a low frequency range without using a fiber material or the like, causing no fiber scattering or the like, and without greatly increasing the cross-sectional dimension of the duct. The
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a duct model of an embodiment of a duct silencer of the present invention.
FIG. 2 is a view showing a coordinate system for calculation of sound propagation in a duct.
FIG. 3 is a model diagram realizing an acoustically soft boundary surface in which 20 acoustic tubes are arranged.
FIG. 4 is a model diagram for satisfying an acoustically rigid condition with a duct wall surface.
FIG. 5 is a diagram showing a relative sound pressure level frequency characteristic at a sound receiving point.
FIG. 6 is an effect diagram of an acoustically soft wall surface when all are rigid wall surfaces.
FIG. 7 is a relationship diagram between a soft boundary length and an attenuation effect.
FIG. 8 is an influence diagram of a film on an acoustic arrangement (ooa).
FIG. 9 is an influence diagram of a film on an acoustic arrangement (oob).
FIG. 10 is an influence diagram of a film on an acoustic arrangement (ooc).
FIG. 11 is a diagram of the influence of the membrane on the acoustic arrangement (oab).
FIG. 12 is an air flow noise level diagram when no film is present.
FIG. 13 is an air flow noise level diagram when a film is present.
[Explanation of symbols]
1 Duct wall (20mm thick acrylic plate)
1A Wall boundary 1B Wall boundary 2 Acoustic tube (85 mm deep aluminum tube: type (a))
3. Acoustic tube (130mm depth aluminum tube: type (b))
4 Acoustic tube (170mm deep aluminum tube: type (c))
5 Sound absorbing wedge (made of glass wool with a length of 1000 mm)
6 Speaker 7 Microphone 10 Duct

Claims (7)

Duct silencer constructed as an acoustically soft boundary where the sound pressure is almost zero on the boundary surface of almost the entire boundary between a pair of opposing wall surfaces over the half-wave length of the target sound wave in the longitudinal direction of the rectangular duct The acoustically soft boundary includes a plurality of acoustic tubes having a length that is ¼ of the wavelength of a plurality of sound waves forming a main component of noise on the inner surface of the duct and closed at the ends. A duct silencer comprising a set of acoustic tubes provided. In the length direction of the rectangular cross-section duct, it is configured as an acoustically soft boundary where the sound pressure is almost zero on the boundary surface over the entire surface of each of the two pairs of wall surfaces facing each other over about half a wavelength of the target sound wave. A duct silencer , wherein the acoustically soft boundary has a length that is ¼ of the wavelength of a plurality of sound waves that form a main component of noise on the inner surface of the duct, and a plurality of acoustics whose ends are closed. A duct silencer comprising an assembly of acoustic tubes in which tubes are arranged side by side. Polygonal, circular, elliptical, etc. with 4 or more sides in the longitudinal direction of the cross-section duct over the half-wave length of the target sound wave, and the sound pressure is almost zero on the boundary surface at almost the entire circumference A duct silencer configured as a soft boundary , wherein the acoustically soft boundary has a length that is ¼ of a wavelength of a plurality of sound waves forming a main component of noise on a duct inner surface. A duct silencer comprising a collection of acoustic tubes in which a number of closed acoustic tubes are arranged side by side. The duct silencer according to any one of claims 1 to 3, wherein an assembly of acoustic tubes in which a large number of acoustic tubes are arranged in parallel is configured as an acoustic tube corresponding to each sound wave. The assembly of the acoustic tubes includes a resonator adjusted so as to coincide with a primary resonance frequency of a sound wave forming a main component of noise on an inner surface of the duct. Item 5. A duct silencer according to any one of Items 4 to 4. The duct silencer according to any one of claims 1 to 5, wherein a film is stretched over the acoustic tube opening. The duct silencer according to any one of claims 1 to 6, wherein an active control method such as an active noise control system is combined to form the acoustically soft boundary.

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