主题：Output Limitations輸出局限性

輸出局限性

The maximum usable output of and electromagnetic loudspeakers is a function of a number of parameters, including diaphragm displacement , heat transfer, sound quality(maximum acceptable distortion), and/ or wear life due to fatigue of moving parts.

一個電磁揚聲器的最大可利用輸出是許多參數的作用,包括振膜的運動,熱轉換,音質(最大限度內可接受失真),及/或由於振動系統疲勞所致的磨損壽命.

There are two fundamental limitations on a magnetic driver, a displacement limit and a thermal limit. Displacement limits may be caused by either mechanical or electrical factors. Mechanical displacement limiting occurs when a moving part contacts a stationary one or when a suspension element is made unacceptably nonlinear (either temporarily or permanently) by deformation beyond its design range. Electrical displacement limiting occurs when the motor is operated outside its range of linear travel. This is a function of the length of the windings on the voice coil and the thickness of the plates that form the magnet gap. Fig. 17-24 shows three typical voice coil configurations: equal length, overhung, and underhung coils. When any of these coil reach a displacement that causes a reduction in current sensitivity of the motor, excess distortion will result.

一個揚聲器有兩個基本的局限性:位移的限制及熱的限制.位移的限制或許受限於機械因素或電氣因素.機械位移限制發生於當運動部件碰觸到固定部件時或懸吊部件由於變形超出設計範圍產生了無法接受的非線性運動(臨時或永久的).電氣位移限制發生於磁回工作於線性行程範圍以外.這是音圈卷線及華司厚度所形成磁隙的功能.通常有三種典型的音圈結構:平齊式,外露式,內藏式.當其中任何一種位移造成磁回內目前靈敏度降低時,將導致過度的失真.

It has been empirically determined that, due to a magnetic fringe or leakage field at the pole tips, and excursion of 15% further than the gap length is a reasonable distortion limit since it will give approximately 3% harmonic distortion at low frequencies. The equal length voice coil, Fig. 17-24C, has the most potential for magnetic distortion. This design, however, gives the designer the greatest motor strength ( the most conductor in a high density field) and is a common configuration for compression drivers, where excursion is relatively low. The underhung coil, Fig. 17-24B, allow more excursion but requires a larger magnet due to a long gap. For moderate flux density levels(10,000 to 15,000G), this design, as compared to the equal length design, requires approximately twice the magnet weight (twice the area and the same length) for doubling of gap length. This approximately doubles the excursion capability, giving four times the acoustic power output capability (6dB) for a doubling of magnetic weight (3dB). In some cases this is considered a good tradeoff. The overhung coil, Fig. 17-24A, is capable of highest magnetic linearity without a big change in magnet. It is commonly seen on woofers used as direct radiators, where higher excursion is required. The major disadvantage here is that the coil that is not in the gap is not used for transduction. The extra coil length adds both mass and dc resistance, which reduces efficiency. In spite of the foregoing, there are numerous examples of successful commercial woofers using overhung coils. The transducer designer must take into account the often-conflicting demands of high efficiency, high output, and low frequency extension to arrive at an optimum design for a given range of applications.

經驗所示,由於磁場邊緣或极芯頂部漏磁區域,比磁隙多出大約15%漂移,這個合理的失真限度,導致低頻的諧波失真約3%.平齊式音圈,會產生更多潛在的磁場失真.但是,這給了設計者最強的磁場力量(更多的導體集中在高密度的磁場中),而且這也是壓縮式單體的通用結構,其衝程相對較低. 內藏式音圈,允許更多的衝程但是要求較大的磁體以便得到較長的磁隙.能得到適中的磁束密度(10,000到15,000高斯),這種設計,與平齊式音圈設計相比較,需要大約兩倍磁體重量(兩倍的面積及相等的長度)以得到一個成倍的磁隙長度.這大約加倍了衝程能力,以四次的音樂功率輸出能力(6dB)以求加倍的磁體重量(3dB).從某些方面來講,這是被考慮的一個好的折衷方案. 外露式音圈,不需要對磁體作大的改變就可具備最高磁場線性能力.在被用作直接幅射器需要長衝程的低音上最為常見.最主要的缺點是不在磁隙中的線圈未被用作電抗體.超出的線圈增加了重量及直流阻抗,降低了效率.不管前面所講的,已經有大量成功的使用長音圈的低音.揚聲器設計者必須重視高效率,高輸出通常的矛盾要求,以及低頻延伸達到一個為了特定應用範圍的適當設計.

The thermal limit of a magnetic loudspeaker motor is a function of the thermal limit of materials used and the heat transfer from the coil assembly. Most adhesives used in the loudspeaker industry have an upper limit between 120℃ and 177℃ (250℉ and 350℉). Some epoxies go higher but may become difficult to work with due to the exotic curing required. Insulation on wire may tolerate temperatures as high as 218℃(425℉). Anodized aluminum wire has the melting point of aluminum as a limit. Voice coils operated at high temperatures have higher resistance. A 1℃ rise produces approximately a 0.4% rise in dc resistance in both copper and aluminum. Therefore, operating a voice coil 100℃ above ambient (127℃ or 261℉) will raise the voice coil to 40% above its ambient value. The following equation will give voice coil resistance at any temperature in degrees Celsius:

RT=R0+0.004(T-T0)

Where,

RT is the resistance at temperature T in ohms,

R0 is the resistance at ambient temperature T0 in ohms,

T and T0 are in ℃

Heat transfer from the coil determines coil temperature T, and the ability of a loudspeaker mechanism to do this is its thermal resistance, measured in ℃/W. Therefore, as power is doubled, final temperature rise is doubled, which is an important point. Heat transfer in any loudspeaker is primarily a function of the air gap design and voice coil design and the ability of the loudspeaker frame and magnet to dissipate heat to the surrounding or ambient air. Referring to Fig. 17-21, the thermal rise of any stationary voice coil in any air gap △TVC is found by the following:

△TVC=TVC-TS

= QL/ATK

Where,

TVC is the temperature of the voice coil in ℃,

TS is the temperature of structure (magnet) in ℃,

Q is the electrical heating power (I2R) in watts,

L is the effective air gap length or
      L1L2/(L1+L2) in inches,

AT is the total gap area in in^2 exposed to the voice coil,

K is the conductivity of air or 7X10-4W/℃.

揚聲器磁回的熱量的限制是材料使用及音圈熱傳導的熱量限制功能體現.許多應用於揚聲器的膠水上限溫度界於120℃到177℃之間.部分環氧樹脂能達到更高,但是由於需求於國外使得使用起來比較困難.音圈線絕緣層能夠承受高達218℃.陽極處理的鋁線具有鋁的極限熔點.音圈工作在高溫下會使得阻抗變高.銅線及鋁線每升高1℃,直流阻抗上升約0.4%.也就是說,音圈工作溫度超過環境溫度100℃,其直流阻抗將上升超過環境直流阻抗40%.以下方程式可算出音圈在任何攝氏溫度中的直流阻抗:

RT=R0+0.004(T-T0)

其中,

RT:溫度T時的直流阻抗(單位:Ω);

R0:環境溫度T0時的直流阻抗(單位:Ω);

T:直流阻抗上升時的溫度(單位:℃);

T0:標準直流阻抗時的環境溫度(單位:℃).

來自音圈的熱傳導決定音圈的溫度T,揚聲器機械結構決定熱阻抗,用℃/ W測量.因此,如果功率加倍,最終的溫升也加倍,這是非常重要的一點.任何揚聲器的熱傳導是磁隙設計及音圈設計的主要職責,同樣揚聲器盆架及磁回如何將熱量散發到周圍環境的空氣中也是同等重要.在磁隙中定位的音圈溫升△TVC可由下式得出:

△TVC =TVC-TS

= QL/ATK

TVC:音圈溫度(單位:℃);

TS:磁回溫度(單位:℃);

Q:電熱功率(I2R) (單位:W);

L:有效磁隙長度或L1L2/(L1+L2)
     (單位:Inch);

AT:總的磁隙面積
     (單位:Inch);

K:空氣中傳導率或7X10-4W/℃

L1:內氣隙 (單位:Inch);

L2:外氣隙 (單位:Inch);

As the air gap length is decreased and the area increased, heat transfer increases. Making the voice coil former or support out of aluminum will increase the effective heat transfer area as it spreads the heat out; the thicker the aluminum, the more the thermal spreading. Therefore, the effective heat transfer area is higher. A general statement of truth is that voice coils wound on aluminum formers with large diameters in magnets with large gap areas and very tight coil to gap tolerances are capable of handling high electrical power due to good heat transfer in the air gap. In other words, big, expensive, accurately made loudspeakers handle more thermal power. The reverse is also true. As the loudspeaker moves, it may be able to pump or scrub the air in the gap to improve heat transfer many times over. The clever designer may be able to improve this quality. Also, of the three voice coil types shown in Fig. 17-24, the underhung and equal length would have the best heat transfer if the coil were the same height. The overhung coil would only conduct heat well in the gap region, while the coil ends remaining out of the gap would be likely to bake at high power because of relatively poor heat transfer. This is indeed a common failure mode of overhung voice coils. Typical thermal behavior for most coil is on the order of 0.5℃/W to 3℃/W input.

當磁隙長度減少並且面積增加,熱傳導增加.發展鋁音圈骨架或散熱器將增加有效的熱傳導面積使得熱量更容易傳導出去,越厚的鋁骨架,可以傳導出越多的熱量.因此,有效的熱傳導面積非常重要.一個綜合的事實陳述:磁體中的大口徑鋁音圈,加上龐大的磁隙面積以及精準的音圈-磁隙間的空差,使得磁隙中具備良好的熱傳導才能夠輸出強大的功率.換句話說,大的,昂貴的,精確造就的揚聲器才能輸出更多的熱功率.相反也是事實.當揚聲器工作時,將不停帶動磁隙中的空氯以改善熱傳導.聰明的設計師懂得改善此點.同樣,三種類型的音圈設計,假如線圈同樣高. 內藏式及平齊式,具有最好的熱傳導. 外露式僅僅在磁隙區域中有較好的熱傳導,當線圈位於磁隙外時,將由於相當可憐的熱傳導而很有可能接受高功率的烘烤.這是長音圈確實共有的失敗例子.絕大多數音圈典型的熱上升功率輸入規律為0.5℃/W到3.0℃/W.

Lastly, a heat conducting magnetic liquid may be used to improve heat transfer. Known as ferrofluids, these fluids will be retained in a magnetic air gap due to magnetic force. Their thermal conductivity is seven to ten times higher than that of air. Since ferrofluid alters the mechanical damping of the moving assembly, its use must be accompanied by evaluation of the transducer design and possibly by a redesign to account for the changes caused by the introduction of ferrofluid into the magnetic gap. There are also issues related to compatibility of ferrofluid with adhesives and materials used in the construction of a transducer. For these reasons, ferrofluids should generally be designed into a loudspeaker, not added on.

最后,一種熱傳導磁性鐵流體可以用來改善熱傳導.如已知的”Ferrofluids”(磁液),這些流體由於磁場的力量,將停留在磁隙中.它們的熱傳導率高出空氣7到10倍.由於鐵流體改變運動系統的機械阻尼,它的使用必須伴隨分頻器設計的評估以及依照鐵流體注入磁隙中的使用說明作重新設計以解決變化的可能.說明中也介紹了鐵流體與膠水及材料的兼容性.由於這些原因,鐵流體通常設計加入揚聲器內部,而非另外附上.

(譯文僅供參考,不當處請指正.)