應用科學：先民的智慧：古希臘圓形劇場的音效之謎
編輯 Chia-Chen Chang 報導
資料來源 － 科景：http://www.sciscape.org/news_detail.php?news_id=2218
建於紀元前 350 年，現在被聯合國列為世界文化遺產之一的古希臘圓形劇場「Epidaurus」，以其絕佳的音響效果而著名於世。科學家認為箇中原因在於古希臘建築師懂得掌握聲學原理的訣竅。
圖1. The Theater at Epidaurus on the Peloponnese in Greece.
位於希臘伯羅奔尼撒（ Peloponnese）半島的圓形劇場「Epidaurus」是考古學家於西元 1881 年所發現。劇場主體為半圓形結構：舞台位於半圓圓心附近，座位呈階梯狀排列。完工之時座位一共有三十五階；羅馬統治時期曾對劇場進行擴建，另外多增加了二 十一階。自竣工之始「Epidaurus」便以令人讚嘆的音響效果聞名：即便是最後一排的觀眾也可以毫不費力地聽見劇場中央演員的聲音。長久以來建築學家 以及考古學家一直無法理解「Epidaurus」的音響設計，羅馬時期著名的建築師 Vitruvius 曾說過希臘建築師一定是掌握了某種技巧，讓劇場得以增幅人的聲音。
事實上，研究聲學的科學家早已經瞭解波浪狀或者是有皺摺的表面對於聲波來說是個不錯的濾波器：特定頻率的聲波經過多次反射之後會被去除。 基於這個看法，喬治亞理工學院（Georgia Institute of Technology）的兩位科學家 Nico Declercq 與 Cindy Dekeyser 認為「Epidaurus」的階梯型構造是其卓越音響效果的關鍵。透過模擬他們發現：500 Hz 以下的聲波在劇場中傳遞的時候強度會逐漸減弱，而「Epidaurus」劇場四周樹林或者是觀眾低聲交談所產生的噪音絕大部分都是低頻的聲音，過濾掉這些 低頻雜音之後演員的聲音便顯得清晰許多。
或許有讀者會擔心劇場在濾掉低頻雜音的同時也將演員的低頻聲音濾掉了，不過實際上觀眾並不會發現這個差異。這是因為大腦在處理聲音訊號的 時候，會自動補上丟失掉的低頻訊號，這個有趣的現象在神經學上稱為「virtual pitch」。不過 Declercq 強調他們所做的模擬並沒有將劇場的觀眾考慮進去，原因是人體外觀不規則且內部結構不均勻，這兩個因素將會大大地增加模擬的困難度。雖然說現代的劇場設計已 經普遍用音響來加強聲音的效果，Delcercq認為這個發現仍有助於開放空間劇場的音響效果設計。
原始論文：
Nico F. Declercq, Cindy S. A. Dekeyser, J. Acoust. Soc. Am. 121(4), 2011-2022, 2007
參考來源：
News@Nature.com： Why the Greeks could hear plays from the back row
相關連結：
Epidaurus
補充連結：
1. ScienceDaily：Ancient Greek Amphitheater: Why You Can Hear From Back Row
2. Seats Helped Ancient Greeks Hear From Back Row
3. http://en.wikipedia.org/wiki/Epidaurus
英文原文：
Ancient Greek Amphitheater: Why You Can Hear From Back Row
Science Daily — As the ancient Greeks were placing the last few stones on the magnificent theater at Epidaurus in the fourth century B.C., they couldn’t have known that they had unwittingly created a sophisticated acoustic filter. But when audiences in the back row were able to hear music and voices with amazing clarity (well before any theater had the luxury of a sound system), the Greeks must have known that they had done something very right because they made many attempts to duplicate Epidaurus’ design, but never with the same success.
圖2. The Theater at Epidaurus on the Peloponnese in Greece. (Credit: Image courtesy of Georgia Institute of Technology)
Researchers at the Georgia Institute of Technology have pinpointed the elusive factor that makes the ancient amphitheater an acoustic marvel. It’s not the slope, or the wind — it’s the seats. The rows of limestone seats at Epidaurus form an efficient acoustics filter that hushes low-frequency background noises like the murmur of a crowd and reflects the high-frequency noises of the performers on stage off the seats and back toward the seated audience member, carrying an actor’s voice all the way to the back rows of the theater.
The research, done by acoustician and ultrasonics expert Nico Declercq, an assistant professor in the Woodruff School of Mechanical Engineering at Georgia Tech and Georgia Tech Lorraine in France, and Cindy Dekeyser, an engineer who is fascinated by the history of ancient Greece, appears in the April issue of the Journal of the Acoustics Society of America.
While many experts speculated on the possible causes for Epidaurus’ acoustics, few guessed that the seats themselves were the secret of its acoustics success. There were theories that the site’s wind — which blows primarily from the stage to the audience — was the cause, while others credited masks that may have acted as primitive loudspeakers or the rhythm of Greek speech. Other more technical theories took into account the slope of the seat rows.
When Declercq set out to solve the acoustic mystery, he too had the wrong idea about how Epidaurus carries performance sounds so well. He suspected that the corrugated, or ridged, material of the theater’s limestone structure was acting as a filter for sound waves at certain frequencies, but he didn’t anticipate how well it was controlling background noise.
“When I first tackled this problem, I thought that the effect of the splendid acoustics was due to surface waves climbing the theater with almost no damping,” Declercq said. “While the voices of the performers were being carried, I didn’t anticipate that the low frequencies of speech were also filtered out to some extent.”
But as Declercq’s team experimented with ultrasonic waves and numerical simulations of the theater’s acoustics, they discovered that frequencies up to 500 Hz were held back while frequencies above 500 Hz were allowed to ring out. The corrugated surface of the seats was creating an effect similar to the ridged acoustics padding on walls or insulation in a parking garage.
So, how did the audience hear the lower frequencies of an actor’s voice if they were being suppressed with other background low frequencies? There’s a simple answer, said Declercq. The human brain is capable of reconstructing the missing frequencies through a phenomenon called virtual pitch. Virtual pitch helps us appreciate the incomplete sound coming from small loudspeakers (in a laptop or a telephone), even though the low (bass) frequencies aren’t generated by a small speaker.
The Greeks’ misunderstanding about the role the limestone seats played in Epidaurus’ acoustics likely kept them from being able to duplicate the effect. Later theaters included different bench and seat materials, including wood, which may have played a large role in the gradual abandonment of Epidaurus’ design over the years by the Greeks and Romans, Declercq said.
Note: This story has been adapted from a news release issued by Georgia Institute of Technology.