轉載: 常見超低音喇叭30問

柒喜 · 發表於 2013-6-12 22:42:57

我說我話: 一般客飯廳擺放Subwoofer也不能重現極低頻。簡單來說，低音不可能達至40周(聽音間有三十呎長除外)，調校不適會引至大量駐波。找來一篇有趣文章----台灣劉漢盛先生有以下相關詳細解釋，大家不妨參考

Q什麼是超低音喇叭？
A：超低音喇叭的英文是Subwoofer，按照字面解釋，具有「副」低音或「在低音之下」的意思，也可說成是「輔助低音」或「更低」的低音喇叭。在台灣，長久以來Subwoofer一直被稱為「超」低音喇叭，這是「Sub」的另一種解釋，其實也還合理。家庭電影院盛行之後，音響界的超低音喇叭很快的就被喧染成「超重低音」。老實說，光從字面看，還真不容易理解何來「超重」之義！對於音響迷而言，我們還是以超低音喇叭來稱呼Subwoofer吧！

Q超低音喇叭的用途為何？
A：作為家庭電影院的環繞喇叭系統一份子，超低音喇叭負責再生電影音效中的低頻音效(Low Frequency Effect)。作為二聲道使用（又稱3D式衛星喇叭系統），超低音喇叭用來補足二聲道喇叭不足的低頻量感，或無法達到的低頻、極低頻領域。

Q超低音喇叭可以分為哪幾種？
A：如果以是否內建擴大機來區分，超低音喇叭可以分為內建擴大機的主動式超低音與沒有內建擴大機的被動式超低音喇叭二種。若是以箱體設計來區分，超低音喇叭與一般喇叭相同，大概可分為密閉式設計（Sealed System）、低音反射式設計（Ported System）、帶通式設計（Bandpass System）、傳輸線式設計（Transmission Line System）等。若是以發聲方向來區分，又有朝正面發聲、朝後面發聲、朝地板發聲、朝上面發聲以及朝左右二面發聲等數種。若以單體動作方式來區分，有二個單體組成的推挽式、二個或三個單體組成的同相發聲式、一個單體一個被動輻射器組成的同相發聲式等。

最先進的超低音喇叭則是在內建擴大機之外，還內建一套低音等化線路，可以適度將室內中低頻駐波做某種程度的衰減，讓100Hz或80Hz以下的低頻段達到比較平直的頻率響應曲線。這種超低音喇叭最理想，售價也最昂貴。

Q何謂密閉式超低音喇叭？
A：箱體本身密閉，沒有與外面接觸的孔道或縫隙，低音單體可以安置在面向前方、後方、側面或上方下方。這種箱體在製造上最簡單，也容易達到品質要求，它的好處是暫態反應快速、可以承受較大的功率，箱體體積也可以做得比較小。密閉式超低音是市面上普遍可見的超低音喇叭之一。

Q何謂低音反射式超低音喇叭？
A：箱體本身有一個或一個以上低音反射孔，讓箱體內的低音單體背波可以經由低音反射孔傳出箱體外，與低音單體的正波相混合，藉此增加低頻的量感。它的好處是失真比較低，承受功率比較大。與密閉式箱體比較，低音反射式的最低截止頻率可以更低（假若使用相同的低音單體）。不過其暫態反應就沒有密閉式箱體那麼好。低音反射式超低音也是市面上普遍可見的超低音喇叭之一。

Q何謂帶通式超低音喇叭？
A：帶通式箱體設計通常包括二個獨立箱室，一個隱藏在內，一個以低音反射孔與外面相通。隱藏在裡面的那個箱室是密閉的，低音單體就安裝在這個箱體上，單體振膜面向外面這個箱室，從箱體外觀上是看不到低音單體的。當低音單體運動時，驅動外面箱體的空氣通過低音反射孔，形成聲波濾波器（Acoustic Filter），對某個頻帶產生四階（每八度衰減24dB，也有設計成六階者）濾波作用。它的好處在於承受功率可能是所有不同箱體種類中最佳者，而暫態反應大概只遜於密閉式箱體。帶通式超低音由於箱體製作成本高，市面上比較少見。

Q何謂傳輸線式超低音喇叭？
A：與傳輸線式喇叭一樣，它的內部要有一個由大漸小的密閉管道，大的這頭連接低音單體的背面，承接單體背波，小的那頭就是出口。管道的長度至少要有最低截止頻率波長的四分之一，管道內壁需要安置適當吸音材料。管道的功能是利用低音單體的背波來讓低頻截止頻率往下延伸，達到更低的頻率響應。它的好處是低頻可以向下延伸得更低。傳輸式超低音的箱體製造成本是所有超低音之冠，因此在市面上更加罕見。

Q何謂被動式超低音喇叭？
A：在早期，超低音喇叭內並未內建專門驅動喇叭單體的擴大機，箱體內只有負責分頻的被動式分音網路，這種超低音稱為被動式超低音。所謂「被動式」的名詞由來就是超低音喇叭本身沒有動能，必須由外接的擴大機來驅動。被動式超低音的音樂訊號從後級的喇叭輸出端取得，而它本身還備有一組訊號輸出端子，將經過分音的音樂訊號傳遞給二聲道喇叭。由於二聲道喇叭與超低音喇叭都由同一部後級來驅動，因此後級的變數增加，驅動力也會被分散。目前，這種超低音喇叭已經相當罕見。

Q何謂主動式超低音喇叭？
A：進入家庭電影院時代之後，由於環繞多聲道喇叭系統的制定，超低音喇叭必須內建驅動單體的擴大機，所以主動式超低音喇叭馬上充斥市面。由於內建擴大機，因此可以從AV環繞擴大機的超低音輸出端獲得低電平音樂訊號（以RCA端子或XLR端子接駁）。此外，一般主動式超低音喇叭也會另設高電平輸入輸出端子，也就是利用喇叭線來接駁。

在家庭電影院的使用場合內，超低音喇叭不可能以喇叭線來接駁，因為無論是AV環繞擴大機或AV處理前級，超低音訊號輸出都是經由低電平RCA端子或XLR端子輸出，並未從喇叭線端子輸出。而在二聲道喇叭加超低音喇叭的使用場合中，使用者可以透過前級的第二組輸出端將音樂訊號傳入超低音，或是以後級的喇叭線輸出端來連接超低音喇叭。

Q超低音喇叭內建的擴大機採用什麼放大線路？
A：絕大多數的主動式超低音，其內建的擴大機都採用D類放大線路，現在比較時髦的稱呼也有稱其為數位擴大機者。為何使用D類放大線路呢？因為這種放大線路效率非常高，不發熱，輸出功率輕易可達數百瓦甚至上千瓦，而且成本低廉。再者，超低音喇叭的頻寬大約為20Hz-150Hz之間，D類擴大機失真較高，不利於中頻段與高頻段的缺點在此並無大礙。當然，也有少數較高級超低音喇叭堅持使用AB類擴大機。假若真的以AB類放大工作，超低音喇叭背面的散熱片面積勢必會相當大，因為數百瓦的輸出功率所需散熱片面積就跟一部數百瓦擴大機一樣大。

Q超低音喇叭必備哪些輸出入端子？
A：以主動式超低音喇叭來說，必備的輸出入端子包括低電平輸出端、輸入端（有的還備有Link
Out，用以連接第二個超低音喇叭）、高電平喇叭線輸入端與輸出端。再來必須有分頻點調整、音量調整、相位調整或相位切換等。少數超低音喇叭還設有高通輸出端與低通輸出端。所謂高通就是濾除分頻點以下頻段，讓分頻點以上頻段通過。而所謂低通就是濾除分頻點以上頻段，讓分頻點以下頻段通過。

Q音量調整或分頻點調整容易理解，什麼是相位調整或相位切換呢？
A：所謂相位調整就是從0-270度的連續或分段調整，而相位切換就僅是0度或180度二種相位切換而已。為什麼需要相位調整呢？簡單的說，當超低音喇叭所發出的低頻與其他喇叭所發出的低頻相混合時，如果二者的相位一致或接近，則總低頻量感是二者相加總和。反之，如果二者的相位相反或接近相反時，總低頻量感就會是相減的。理論上當超低音喇叭與前聲道、中聲道或二聲道左右喇叭放在同一條橫線上時，其低頻相位應該跟前聲道中聲道或左右聲道相同或接近。問題是，許多人礙於各種原因，無法將超低音喇叭放置在左右喇叭之間的同一橫線上，而是放在某個角落或側面，此時超低音喇叭所發出的聲音相位與其他喇叭就會不同，因此最好備有相位調整裝置。

一般超低音喇叭上的相位調整分二種，比較簡單的就是0度與180度二檔切換，這種超低音喇叭最好擺放在牆角或前聲道或左右喇叭之間，即使無法這麼擺，也要儘量靠近前聲道或左右喇叭。另一種比較精確的調整就是0-270度無段式或分段式調整，這種相位調整的選擇性較高，超低音喇叭所能擺放的位置也比較靈活。

到底要如何確定所調整的相位是正確的呢？很簡單，您只要用耳朵聽，選擇低頻量感最豐富那檔就對了。當然，您最好備有粉紅色噪音或多種頻率測試片，播出測試片上的20Hz-150Hz各段頻率，或者選擇單一頻率（例如60Hz）聆聽，這樣一定可以聽出低頻量感最豐富那檔。

Q當我們選擇超低音喇叭的擺放位置時，要注意哪些事項？
A：擺放超低音喇叭與擺放左右聲道喇叭一樣，都要注意擺放位置，因為超低音喇叭也會受到鄰近牆面或地板的反射影響，而在某些頻率產生增強（正相）作用，也會在某些頻率產生抵銷(反相)作用。此外，聆聽空間中本來就不可避免的中低頻或低頻駐波也會影響超低音喇叭的表現。所以，當您在選擇超低音喇叭的擺放位置時，第一個要考慮的是：我是否需要藉著牆角的低頻增強作用來增加超低音喇叭的量感。第二個要考慮的是：聆聽空間原本存在的駐波（左右聲道喇叭所引起的）是否會跟超低音喇叭的聲波形成增強（正相）或抵銷（反相）作用。說得更清楚些，那就是當我們在擺放超低音喇叭時，不僅要考慮到超低音喇叭與左右聲道喇叭相互之間聲波的正相反相問題，還要考慮到超低音喇叭本身與相鄰地板、牆面相互之間所引起的正相反相問題。

有關超低音喇叭與左右聲道喇叭之間所引起的正相、反相問題如前所述，可以藉由超低音喇叭附設的相位調整裝置來解決。至於超低音喇叭本身與牆面所引起的正相、反相問題要怎麼處理呢？當然也僅能藉由超低音喇叭的擺位來處理。看到這裡，我想讀者們已經開始意識到，原來看似簡單的超低音喇叭擺放位置還牽涉到這麼複雜的考量。

Q到底超低音喇叭要怎麼擺放，才能解決它與鄰近牆面之間所形成的聲波正相、反相問題呢？
A：要解決聲波正相、反相問題，最好的方法就是，讓鄰近牆面所反射的聲波與低音單體所再生的聲波都保持相同的相位，也就是互為正相。說得更精確些，如果要讓低音單體、地板、側牆、後牆這四股聲波都維持正相，最保險的方式就是讓反射聲波與低音單體聲波維持在90度相位以內。如果把90度相位換算成波長，那就是四分之一波長以內。不過，由於聲波從單體發出之後會先達到牆面再反射回來，其行進路線已經是二倍，所以我們再把四分之一波長除以二，也就是實際計算時，取八分之一波長就可以了。

或許這樣說很多人不了解，讓我舉實例說明：假若超低音喇叭所選擇的分頻點是100Hz，那麼我們就取最高的100Hz波長來計算（其他更低頻率都已涵蓋在中）。100Hz的波長是340公尺（聲波每秒速度大約值）除以100Hz，等於.3.4公尺，3.4公尺的八分之一波長就是42.5公分。所以我們就應該要把超低音喇叭放置在距離側牆、後牆、地板都不超過42.5公分距離以內，這樣一來，從超低音喇叭後牆、側牆與地板上所反射回來的聲波都能夠與低音單體所發出的主聲波維持正相關係。當所有的反射波與主波都成正相關係時，就是低頻量感最豐富的時候。這也是為什麼把超低音喇叭擺在牆角裡可以獲得最豐富低頻量感的原因。

看到這裡，或許您會聯想到一個問題：既然超低音喇叭放在牆角可以獲得最豐富的低頻量感，那麼把左右聲道喇叭放在牆角裡不是一樣可以得到最豐富的低頻量感嗎？沒錯！一樣可以獲得最豐富的低頻量感，不過不要忘了，左右聲道喇叭不僅只有低音單體而已，它還有中音單體與高音單體，這些單體的頻率範圍比低音單體高得多，如果您按照上述公式計算，就會發現每個單體所需要的距離都不同，如此以來很容易產生複雜的聲波相互增強或相互抵銷現象，這樣反而有害。所以，除了超低音喇叭之外，一般喇叭並不適合擺在牆角裡。

將超低音喇叭擺放在牆角裡還有一個好處，那就是由於低頻量感非常充足，因此您可以將音量旋鈕旋低。當音量旋鈕旋低時，意謂著擴大機不必輸出那麼大的功率，低音單體也不必運動得那麼猛烈，這二者都有助於失真的降低。

Q除了以上所言選擇超低音喇叭擺放位置的方式之外，還有什麼方法可以幫助用家找到最適當的擺放位置？
A：還有一個方法是一般人常用的，那就是一邊播放自己熟悉的音樂軟體，一邊嘗試著更換超低音喇叭的位置。這種方法也是有效的方法，不過，您必須對播放的音樂軟體有把握，確定低頻效果應該如何表現才正確。此外，您也必須對樂器的低頻表現有正確認知，知道多少低頻量感、什麼樣的低頻質感才是正確的。假若您具有以上能力，利用聆聽方式，反覆嘗試超低音喇叭的位置，這不失為有效的方法。

Q超低音喇叭可以擺放在聆聽位置前面，當作茶几使用嗎？
A：一般情況並不適合擺在聆聽位置前面，因為這個位置不容易與左右聲道喇叭取得一致的相位。此外，由於太靠近聆聽者，超低音喇叭所發出的低頻不容易與左右聲道喇叭完全混合。假若超低音喇叭的分頻點調得很低（60Hz）以下，在聆聽某些音樂時，可能會感受不到低頻來自超低音喇叭。總之，這不是良好的超低音喇叭擺位。

Q超低音喇叭可以擺在聆聽位置後面嗎？
A：如果是緊貼著聆聽位置後面，同樣的也會產生相位與低頻無法混合的問題，容易讓人聽出低頻段是從後面的超低音喇叭所發出。假若距離聆聽位置有一段距離，那麼還是要考慮到相位問題。

Q超低音喇叭可以擺在聆聽位置側面嗎？
A：如果盡量靠近左右聲道喇叭的側面，問題不是很大，經過相位調整，依然可以取得完美的低頻混合感覺，聽不出低頻是單獨從超低音喇叭發出。假若超低音喇叭擺放在緊鄰聆聽位置的側面，那就不是理想的位置。除非是二個超低音喇叭，而且分頻點夠低，加上妥善的相位調整。

Q有人說因為低頻沒有方向性，所以超低音喇叭可以隨意擺位？
A：要討論低頻沒有方向性這個問題，首先必須確定到底在什麼頻率以下的低頻沒有方向性。確定特定頻率之後，還要在戶外或無響室中實驗到底有沒有方向性。

不過，可以肯定的是，在電影院中，10Hz的極低頻是沒有方向性的，因為電影院中就是以這麼低的頻率加上超大功率，來製造地震的逼真效果。

回到一般聆聽空間中，假設40Hz以下的低頻沒有方向性，但是因為一般空間不夠大，難免都會產生40Hz的二次諧波三次諧波等，所以我們所聽到的聲波並不是單一的40Hz而已，而是混合著80Hz、120H甚至160Hz的頻率，此時我們就聽得出低頻的方向性了。

可以這麼說，因為一般人的聆聽空間都不夠大，低頻一定會受牆面反射影響，真正夠低的低頻很難再生，因此很容易聽出低頻的方向性。所以，超低音喇叭的位置必須反覆嘗試，找出最佳擺放位置。

Q小空間能夠聽到很低的頻率嗎？
A：許多人信誓旦旦的說，他們能夠在四坪大、五坪大的空間中聽到20Hz的管風琴聲音。其實這是「美麗的錯誤」。依照聲學定律，在一個密閉空間中，頻率的再生與空間中最長邊的距離有絕對的關係，若是距離不夠長，能夠再生的低頻就會受限。其定義是：想要再生某個頻率，室內長度最少要等於或大於該頻率的二分之一波長。例如，想要再生20Hz頻率，室內最長的距離至少也要有20Hz波長（約17公尺）的一半，也就是8.5公尺長。

既然如此，為了那麼多人認為自己的小空間中可以聽到那麼低的頻率呢？因為他們聽到的並不是真正的20Hz，而是20Hz的二倍、三倍、四倍、五倍等諧波，而這些諧波被誤解是20Hz。

Q超低音喇叭可以使用二個嗎？
A：可以！當您使用二個超低音喇叭時，一定要做對稱擺放，不可做不對稱擺放。也就是說，您可以左右對稱或前後對稱的位置各擺一個，不能一個擺在右牆角，另一個卻擺在聆聽位置後方中央位置。

Q使用一個超低音喇叭與二個超低音喇叭有什麼不同？
A：除了低音能量增加之外，還要考慮一個問題，就是相互之間的聲波相位問題。如果二個超低音喇叭擺放的位置在同一個牆面的二個牆角，有可能會因為聲波相互抵銷（此時互為反相）作用而消除或降低左右牆面聲波來回累積所形成的駐波，這對低頻率的頻率響應曲線平直有正面幫助。所以，當您想要使用二個超低音喇叭時，最好將它們擺在左右聲道喇叭後面的二個牆角。

Q小空間加超低音喇叭的效果如何？
A：在小空間中，如果低頻量感不足，加一個超低音仍然是有效解決問題的方法。不過，小空間中容易產生嚴重的中低頻駐波，因此在選擇分頻點以及調整音量時，必須反覆嘗試。此外，超低音喇叭的擺放位置也要多方嘗試，以其能夠將中低頻駐波的負面影響降到最低。

Q在二聲道喇叭加超低音喇叭的使用狀態下，我們要怎麼判別超低音喇叭的分頻點選擇與音量大小是正確的？
A：大哉問！這個問題是找到適當的擺位之後最關鍵的二個問題。以我的經驗而言，分頻點選擇太高時，中低頻聽起來容易有膨脹現象，真正的低頻與極低頻聽起來好像沉不下去。反之，分頻點如果選得太低，中低頻的飽滿堅持感就會不夠，低頻會顯得過於虛軟。所以，分頻點的正確與否會影響整體音樂廳感。我的建議是：以70Hz為中點，您可以把分頻點設定在80Hz，也可以把分頻點設定在60Hz，先集中火力反覆嘗試這三個頻率，找到最適合您的二聲道喇叭以及聆聽空間條件者。低於60Hz或高於80Hz的分頻點通常不會有太好的效果。

至於音量大小，那就必須仰賴您對於樂器演奏的正確認識了。例如，爵士Bass演奏時音量如何，樂器質感如何？電Bass演奏時音量如何？樂器質感如何？腳踩大鼓的噗噗聲該有多結實？多軟？多Q？多沉重？這些都必須靠您自己去現場體會，這樣才能調整出最佳音量。可以這麼說，假若腳踩大鼓的噗噗聲太過於軟，那就有可能低頻過量了。如果爵士Bass聲音太軟，那也表示低頻過量。因為這二種樂器在現場聆聽時都不會太軟。

除了以上樂器之外，鋼琴也可以用來檢測低頻量感是否適度。觀察的重點在低音鍵的表現，假若低音鍵聽起來沉重但渾濁，弦振感不夠清楚，整體聲音也不夠活生，那就代表低頻過量。反之，如果低音鍵重量感不夠，整體鋼琴聽起來單薄沒有權威感，那就代表低頻量感不夠。唯有低音鍵鳴起來弦振感豐富、規模感龐大而清晰，整體鋼琴聲音有彈性有重量感，那才是正確的低頻音量。

或許您要問：管風琴適不適合拿來當作測試分頻點與量感的素材呢？我的回答是：若只想測試低頻沉潛的能力，管風琴的低音階演奏可以當作測試素材。不過，由於管風琴的低音階演奏很少牽涉到暫態反應與複雜的音階變化，它多數是長線條的緩慢節奏，所以無法測試超低音喇叭在「動態」方面的諸多表現。結論是：作為測試超低音喇叭的分頻點與音量調整，管風琴音樂的實用性並不如Bass、腳踩大鼓與鋼琴。至於西方大鼓或日本大鼓、中國大鼓，由於我們並不知道錄音時所使用的大鼓尺寸,也不熟悉演奏時大鼓所發出來的能量，所以我認為並不適合作為測試軟體。一般而言中國大鼓或日本大鼓的頻率不算太低，但能量非常驚人。您知道中國大鼓或日本大鼓在室內打起來的能量有多驚人嗎？我曾經親身體驗過，那會讓您耳膜感受到很大的壓力，身體開始產生不舒服的感覺，這麼龐大的能量我還沒有在音響系統上聽過。

Q所謂超低音喇叭與左右聲道喇叭的低頻無法銜接是什麼意思？
A：低頻無法銜接有二個意思，第一個意思是清楚的聽到二種無法混合的低頻，一種來自左右聲道喇叭，另一種來自超低音喇叭。第二個意思室雖然聽到的只有一種低頻，但是這種低頻並沒有讓整體的聲音得到最佳的平衡感。

前者問題大多出在超低音喇叭的擺放位置不良，想要改善，必須重新找尋最適當的擺放位置。而後者的問題大多出在沒有選對超低音喇叭，盡管怎麼調整分頻點與音量，聽起來不是低頻量感不夠，聲音生硬沒有彈性﹔就是低頻量感太多，聽起來上瘦下胖。更有甚者還會覺得暫態反應與速度感不對。在這種情況下，如果更動超低音喇叭擺位仍然無效，那就只好更換超低音喇叭了。

Q一般人在使用小喇叭時會考慮加超低音喇叭，我使用的是落地式喇叭，也適合加裝超低音喇叭嗎？
A：無論是書架型小喇叭或落地式喇叭，只要您覺得低頻量感不足，嘗試過別的方法也無法解決之後，都可以加裝一個超低音喇叭，藉此來達到高、中、低頻的平衡性。在此我要強調，在聆聽音樂時，我們並不需要過量的低頻量感，而是需要能夠與高頻段、中頻段取得平衡的低頻量感。所以，當您加裝一個超低音喇叭時，往往音量並不需要增加太多，就能夠達到高、中、低頻段量感平衡的要求。也因為如此，當我們在選擇聆聽音樂用的超低音喇叭時，首要考慮的並不是它能夠發出多大的音壓，而是它的反應夠不夠靈活，控制力夠不夠好，音質夠不夠優等要求。

Q時下市面上有一些超低音喇叭體積很小，它的效果比得上大體積超低音喇叭嗎？
A：超低音喇叭在設計時，必須考慮到箱體結構、單體選擇、分音器設計以及擴大機推力等等，必須在這些參數都達到一個合理的平衡狀態，才成產生優質超低音喇叭。某些超低音喇叭的箱體特別小，這並不是設計錯誤，而是箱體小可以利用別的方法來彌補其缺點。例如在箱體內佈置大量吸音物質，其作用等於是增大了箱體內的空氣容積，一般而言最多可以等同增加百分之四十的箱體容積。

此外，增加擴大機的功率也是補償箱體小的方式，這也是為何小箱體超低音喇叭往往內建數百瓦功率擴大機的原因。最後，有些小體積超低音喇叭採用二個低音單體做推挽式動作，這種動作方式可以有效降低單體奇次諧波失真，並且增加音壓，這也是補償小箱體的有效方式。

假若超低音喇叭因為設計錯誤導致箱體過小，那麼這個超低音喇叭的向下沉潛能力必然受限，而且較高頻段會有膨脹現象。這樣一來，它就不是一個優質超低音。

小體積超低音喇叭的效果能否比大體積超低音喇叭還差？這個問題的答案是「不一定」。如前所述，小體積設計與大體積設計各有其著眼點，不能單就體積大小來決定高低優劣。不過可以肯定的式，大體積超低音喇叭通常可以得到比較寬鬆的低頻，而小體積超低音喇叭則會傾向比較凝聚的低頻。

Q聆聽音樂所需的超低音喇叭與看電影音效所需的超低音喇叭有所不同嗎？
A：聆聽音樂時，對於低音域樂器的演奏質感要求較高，對細微的音量變化也同樣有著高度要求。因此，針對聆聽音樂所需的超低音喇叭在單體的暫態反應以及失真上有比較高的要求。反之，看電影音效所需的超低音喇叭比較著重在強烈的衝擊性以及量感的要求上，因此對內建擴大機的功率要求更大，喇叭單體的堅固性要求也更高。

通常，針對電影音效所設計的超低音喇叭，其單體振膜會比較厚重，單體懸邊也是厚而堅固。反之，針對聆聽音樂所設計的超低音喇叭，其單體振膜傾向硬而輕，單體懸邊也比較柔軟富彈性。以我的經驗而言，針對電影音效的超低音喇叭體積可以小些無妨，不過針對聆聽音樂所需的超低音喇叭，其體積不妨選擇大者。

Q我要如何選購超低音喇叭？
A：首先要先了解自己的空間大小與需求。空間如果相當大（例如10坪以上），自己使用的也是落地式喇叭，此時應該選用體積較大的超低音喇叭。因為這類喇叭通常都是為聆聽音樂而設計的。此外，體積較大的超低音喇叭低頻也比較寬鬆，這類的低頻特性能夠與落地式喇叭做比較理想的低頻銜接。

假若空間不大，自己使用的也是書架型小喇叭，此時倒不一定要選擇體積大的超低音喇叭，小體積超低音喇叭應該會比較實惠。為什麼？因為空間小，真正的極低頻無法再生，此時左右聲道喇叭需要的並不是很低的極低頻，而是要補足60Hz-100Hz之間的低頻。只要能夠補足這段低頻，音樂的規模感就會變大，整體聲音也會紮實起來。

Q選購超低音時，要注意音質音色、速度反應的搭配嗎？
A：由於超低音喇叭所負責的大多是在20Hz-125Hz之間的頻段，最常出現的頻段還可以縮小到40Hz-80Hz之間，所以對於音質音色的搭配要求並不苛求。何況在這個頻段內，您所聽到的超低音聲音也無法判別音質音色優劣。重要的是，超低音喇叭的暫態反應、速度感要夠快，夠靈活。或者說，暫態反應速度感至少也要與左右聲道喇叭一致，這樣二者的搭配才會成功。

Subwoofer Basics

Subwoofers are often misunderstood, both in name and in application. While they have a reputation for providing hefty bass lift, they need not be proverbial bulls in the china shop of your listening room. Although proper setup and integration require some attention and patience, the results can enhance the sound of a wide range of systems—without being “in-your-face” obvious about it. Put simply, it is possible to greatly improve a system’s performance by adding a subwoofer (or a pair of subs).

First, the basic definition: A subwoofer is a loudspeaker that produces low frequencies that augment and extend the bass output of a full-range loudspeaker system. The term subwoofer is grossly misused to describe any low-frequency driver system enclosed in a separate cabinet. But subwoofer actually means “below the woofer,” and the term should be reserved for those products that extend bass response to below 20Hz. A low-frequency driver in an enclosure with an output extending to 40Hz and used with small satellite speakers is more properly called a woofer.

You’ll also see full-range speakers with a built-in “subwoofer” powered by its own amplifier. Most of these products actually employ woofers that are simply driven by an integral power amplifier. Such a design relieves your main amplifier of the burden of driving the woofer, but the speakers must be plugged into an AC outlet.

Read on to find out about different types of subwoofers—and their pros and cons—as well as their setup, placement, and integration.

Active vs. Passive Subwoofers
Subwoofers come in two varieties: passive and active. A passive subwoofer is just a woofer or woofers in an enclosure that must be driven by an external amplifier. In one variation of the passive subwoofer, the same stereo amplifier driving the main speakers also powers the subwoofer. In this least desirable method of connecting a subwoofer, the full-range output from a power amp is input to the subwoofer, and a crossover in the subwoofer removes low frequencies from the signal and outputs the filtered signal to the main loudspeakers. This technique puts an additional crossover in the signal path, to filter out bass from the speaker-level signals driving the main speakers. Some subwoofers are designed to extend the system’s bass response without filtering bass from the signal driving the main speakers. Although this technique doesn’t add a crossover to the signal path, it doesn’t increase the power handling of the main speakers, either. Keeping low bass out of your main speakers has many advantages, including a much cleaner midrange.

A better way of driving the passive subwoofer is with an electronic crossover and separate power amplifier. This method separates the bass from the signal driving the main loudspeakers at line level, which is much less harmful to the signal than speaker-level filtering. Moreover, adding a separate power amp for the subwoofer greatly increases the system’s dynamic range and frees the main-speaker amplifier from the burden of driving the sub. Adding a line-level crossover and power amp turns the passive subwoofer into an active subwoofer, and also makes the system bi-amplified.

A self-contained active subwoofer combines a subwoofer with a line-level crossover and power amplifier in one cabinet, eliminating the need for separate boxes and amplifiers. Such a subwoofer has line-level inputs (which are fed from the preamplifier), line-level outputs (which drive the power amp), and a volume control for the subwoofer level. The line-level output is filtered, removing low frequencies from the signal sent to the amplifiers driving the main loudspeakers. This crossover frequency is adjustable to allow you to select the frequency that provides the best integration with the main speakers. (I’ll write more about integration methods in subsequent sections.)

Subwoofer Pros and Cons
Adding an actively powered subwoofer can greatly increase your system’s dynamic range, bass extension, midrange clarity, and ability to play louder without strain. The additional amplifier power and low-frequency driver allow the system to reproduce musical peaks at higher levels. Moreover, removing low frequencies from the signal driving the main loudspeakers lets them play louder because they don’t have to reproduce low frequencies. The midrange often becomes clearer because the woofer cone isn’t furiously moving back and forth, trying to reproduce low bass. This improvement in a small speaker’s performance can be dramatic. The small woofer’s excursion is no longer a limiting factor in how loudly the system will play when low frequencies are filtered from the signal driving it. The midrange is much cleaner, and the overall system sounds like a large, full-range speaker.

Now for the bad news: More often than not, subwoofers can degrade a playback system’s musical performance. Either the subwoofer is poorly engineered (many are), set up incorrectly, or, as is increasingly common, it’s designed to reproduce explosions in a home-theater system, not resolve musical subtleties.

Subwoofer Challenges (and Rewards)
Let’s look at the theoretical problems of subwoofers. First, most subwoofers—passive or active—add electronics to the signal path. The active subwoofer’s internal crossover may not be of the highest quality. Even well-executed crossovers can still degrade the purity of very-high-quality source components, preamplifiers, and power amps. This drawback can be avoided by running the main loudspeakers full-range (no roll-off), but you then lose the dynamic advantages and additional midrange clarity conferred by keeping low frequencies out of the main speakers.

Second, the subwoofer’s bass quality may be poor. The subwoofer may move lots of air and provide deep extension, but a poorly designed subwoofer often adds a booming thumpiness to the low end. Rather than increase your ability to hear what’s going on in the bass, a subwoofer often obscures musical information.

Third, a subwoofer can fail to integrate musically with the main loudspeakers. Very low frequencies reproduced by the subwoofer can sound different from the midbass produced by the main speakers. The result is an extremely distracting discontinuity in the musical fabric. This discontinuity is manifested as a change in the sound of, for example, acoustic doublebass in different registers. Ascending and descending bass lines should flow past the crossover point with no perceptible change in timbre or dynamics.

Another factor that can make integrating a subwoofer difficult is matching a slow subwoofer to taut, lean, articulate main speakers. Put another way, the sound from an underdamped subwoofer won’t integrate very well with that from a pair of overdamped speakers.

Fourth, subwoofers often trade tight control, pitch resolution, and lack of overhang for greater sensitivity or deeper extension. This is particularly true of subwoofers designed for home theater. Consequently, many subwoofers sound bloated, “slow,” and lacking in detail.

Finally, a subwoofer can fill the listening room with lots of low-frequency energy, exciting room-resonance modes that may not have been that bothersome without the subwoofer. This problem of room-mode excitation can be ameliorated by using two (or more) subwoofers; each subwoofer will excite different room modes, substantially smoothing out the room’s low-frequency response. Placement is therefore crucial—you can’t put a subwoofer just anywhere and expect musical results.

All of these problems are exacerbated by the common tendency to set subwoofer levels way too high. The reasoning behind this is that if you’ve paid good money for something, you want to hear what it does. But if you’re aware of the subwoofer’s presence in the sound, either its level is set too high, or it isn’t positioned correctly, or it has been poorly designed. The highest compliment one can pay a subwoofer is that its contribution can’t be directly heard. It should blend seamlessly into the musical fabric, not call attention to itself.

Having said all that, my experience suggests that a subwoofer/satellite system can outperform a similarly priced, full-range loudspeaker system.

Here are some compelling reasons why:

The satellite speakers can be positioned for the best soundstage without regard for how that position affects the bass response.
The subwoofers can be positioned for the best bass integration in the room without regard for soundstaging.
It is much more cost-effective to build a subwoofer enclosure than it is to build the large enclosure of a full-range loudspeaker (to accommodate large woofers), which may have expensive cabinet construction and wood or paint finishes. The subwoofer is less likely to be a prominent part of the home decor, and thus doesn’t require lavish finish quality.
The subwoofer/satellite system gives you a large measure of control over the bass performance, including the amount of bass and how that bass integrates with the room. A full-range loudspeaker offers no such control.

However, I must add that I hold these things to be true with two big caveats: The first is that the subwoofer must be of exceptional quality and be designed for musical performance, not booming home-theater effects. Only a tiny handful of the hundreds of subwoofers on the market fall into this category. Second, achieving seamless integration between the subwoofer and satellites and realizing smooth bass response requires skill and patience. The subwoofer/satellite system is a better choice for someone who enjoys the technical side of audio rather than for the music lover who just wants good sound without having to ascend the learning curve.

The counterargument suggests that a loudspeaker system should be designed and engineered with a single vision, not with a piecemeal approach by different designers. (For more on this, see the Subwoofer Integration section below.)

Subwoofer Placement Tips
Subwoofer placement also has a large effect on how much bass you hear and how well the sub integrates with your main speakers. When a subwoofer is correctly positioned, the bass will be clean, tight, quick, and punchy. A well-located subwoofer will also produce a seamless sound between the sub and the front speakers; you won’t hear the subwoofer as a separate source of sound. A poorly positioned subwoofer will sound boomy, excessively heavy and thick, lacking detail, and slow, with little dynamic impact. In addition, you’ll hear exactly where the front speakers leave off and the subwoofer takes over.

Here are some general guidelines for subwoofer placement. As with full-range speakers, avoid putting the sub the same distance from two walls. For example, if you have a 20'-wide room, don’t put the subwoofer 10' from each wall. Similarly, don’t put the subwoofer near a corner and equidistant from the side and front walls. Instead, stagger the distances to the walls. Staggering the subwoofer’s distance from each wall smoothes the bass because the frequencies being reinforced by the wall are randomized rather than coincident.

You can also get more dynamic impact and clarity from your subwoofer by placing it close to the listening position. Sitting near the subwoofer causes you to hear more of the sub’s direct sound and less of the sound that has been reflected around the room. You hear—and feel—more of the low-frequency wavelaunch, which adds to visceral impact.

The simplest, most effective way of positioning a subwoofer is to temporarily put it as close to the listening position as feasible. Raise the subwoofer off the floor, if possible, so that it’s close to where the listeners’ ears will normally be. Play a piece of music with an ascending and descending bass line, such as a “walking” bass in straight-ahead jazz. Crawl around the floor on your hands and knees (make sure the neighbors aren’t watching) until you find the spot where the bass sounds smoothest, and where each bass note has about the same volume and clarity. Avoid positions where some notes last longer, and/or sound slower or thicker, than others. When you’ve determined where the bass sounds best, put the subwoofer there permanently. Now, when you’re back in the listening seat, the bass should sound smooth and natural.

Subwoofer Integration Tricks
It’s relatively easy to put a subwoofer into your system and hear more bass. What’s difficult is making the subwoofer’s bass integrate with the sound of your main speakers. Low bass as reproduced by a subwoofer’s big cone(s) can sound different from the bass reproduced by the smaller cones in the left and right speakers. A well-integrated subwoofer produces a seamless sound, no boomy thump, and natural timbre. A poorly integrated sub-woofer will sound thick, heavy, boomy, and unnatural, calling attention to the fact that you have smaller speakers reproducing the frequency spectrum from the lower midrange up, and a big subwoofer putting out low bass.

Integrating a subwoofer into your system is challenging because the subwoofer is optimized for putting out lots of low bass, not for reproducing detail. The main speakers’ upper bass is quick, clean, and articulate. The subwoofer’s bass is often slow and heavy. The word “slow” in this application is not technically correct, but vividly describes how such a subwoofer sounds. More precisely, a “slow” subwoofer is one that has excessive overhang: the cone keeps moving long after the drive signal has stopped, which conveys the impression that the bass is “slow.”

Achieving good integration between small speakers and a subwoofer is easier if you buy a complete system made by one manufacturer. Such systems are engineered to work together to provide a smooth transition between the sub and the main speakers. Specifically, the crossover network removes bass from the left and right speakers, and removes midrange and treble frequencies from the signal driving the subwoofer. If all these details are handled by the same designer, you’re much more likely to get a smooth transition than if the subwoofer is an add-on component from a different manufacturer.

If you do choose a subwoofer made by a different manufacturer, several controls found on most subwoofers help you integrate the sub into your system. One control lets you adjust the crossover frequency—the frequency at which the transition between the subwoofer and the main speakers takes place. Frequencies below the crossover point are reproduced by the subwoofer; frequencies above the crossover point are reproduced by the main speakers. If you have small speakers that don’t go very low in the bass and you set the crossover frequency too low, you’ll get a “hole” in the frequency response. That is, there will be a narrow band of frequencies that aren’t reproduced by the woofer or the main speakers.

Setting the subwoofer’s crossover frequency too high also results in poor integration, but for a different reason. The big cone of a subwoofer is specially designed to reproduce low bass. When it is asked to also reproduce upper-bass frequencies, those upper-bass frequencies are less clear and distinct than if they were reproduced by the smaller main speakers. Finding just the right crossover frequency is the first step in achieving good integration. Most subwoofer owner’s manuals include instructions for setting the crossover frequency. As a rule of thumb, the lower the subwoofer’s crossover is set, the better.

Some subwoofers also provide a knob or switch marked “phase.” Phase essentially is a time difference between two soundwaves. To understand a subwoofer’s phase control, visualize a soundwave being launched from your subwoofer and from your main speakers at the same time. Unless the main speakers and subwoofer are identical distances from your ears, those two soundwaves will arrive at your ears at different times; that is, they will have a phase shift between them. In addition, the electronics inside a subwoofer can create a phase shift in the signal. The sub’s phase control lets you delay the wave generated by the subwoofer so that the subwoofer’s wave is perfectly in-sync with the wave from the main speaker. When the soundwaves are in phase, you hear a more coherent and better-integrated sound. One way of setting the phase control is to sit in the listening position with music playing through the system. Have a friend rotate the phase control (or flip the phase switch) until the bass sounds the smoothest.

But there’s a much more precise way of setting the phase control that guarantees perfect phase alignment between the subwoofer and main speakers. First, reverse the connections on your main loudspeakers so that the black speaker wire goes to the speaker’s red terminal, and the red speaker wire goes to the speaker’s black terminal. Do this with both speakers. Now, from a test CD or device app that offers pure test tones, select the track whose frequency is the same as the subwoofer’s crossover frequency. Sit in the listening position and have a friend rotate the subwoofer’s phase control until you hear the least amount of bass. The subwoofer’s phase control is now set perfectly. Return your speaker connections to their previous (correct) positions: red to red, black to black.

Here’s what’s happening when you follow this procedure: By reversing the polarity of the main speakers, you’re putting them out of phase with the subwoofer. When you play a test signal whose frequency is the same as the subwoofer’s crossover point, both the sub and the main speakers will be reproducing that frequency. You’ll hear minimum bass when the waves from the main speakers and subwoofers are maximally out of phase. That is, when the main speakers’ cones are moving in, the subwoofer’s cone is moving out. The two out-of-phase waves cancel each other, producing very little bass. Now, when you return your main speakers to their proper connection (putting them back in phase with the subwoofer), they will be maximally in-phase with the sub. This is the most accurate way to set a subwoofer’s phase control, because it’s easier to hear and identify the point of maximum cancellation than the point of maximum reinforcement. Unless you later move the subwoofer or main speakers, you need to perform this exercise only once.

A useful modern subwoofer feature is an automatic equalization circuit that measures the frequency response of your system in your room and tailors the subwoofer’s output to provide flatter response. You connect a supplied calibration microphone to the subwoofer, push a button, and the subwoofer emits a series of tones that are picked up by the microphone and analyzed by a circuit in the subwoofer. The subwoofer then equalizes its output, applying a boost to certain frequencies and a cut to others so that the resultant output is as flat as possible.

It’s worth noting that the best integration results from adding two (or more) subwoofers to your system. Two subwoofers drive the air in the room more uniformly, with fewer standing waves. The result is smoother bass throughout the room, and better integration with the main speakers.