“An Immense World”這本書這個段落，講的是「聲音 All Ears, Sound」。
人類耳朵能聽到的聲音的頻率，是20赫茲到20,000赫茲。
(赫茲(Hertz，符號：Hz)，常簡稱赫，是頻率的國際單位制單位，表示每一秒週期性事件發生的次數~維基百科)
低於20赫茲的，叫做超低頻，大海裏的鯨魚和大陸上的大象，都是善用超低頻溝通的動物。
超低頻的聲音，波長較長，傳遞的距離比較遠。
高於20,000赫茲的，叫做超高頻，哺乳動物的鼠類、飛行的鳥類、昆蟲，聽得到超高頻。
超高頻的聲音，波長較短，傳遞的距離比較近。
不過，把聲音如此歸類，將人聽不見的聲音，加上「超高」、「超低」的字眼，就是擬人論(anthropomorphism)的觀點。
這個世界看起來會很奇怪，往往是從人的狹隘觀點去觀看的，才會讓人覺得很奇怪的。其實，如果能從生物自身的角度看(Umwelt)，那麼往往就不足為奇了。
譬如，為什麼鯨魚和大象要用「超低頻」來溝通呢？那也只能從它們的角度，為了在寬廣的大海和大陸中求生存，必須用聲音能傳得遙遠的超低頻，也就是說那是生活的必需，就就可以理解了。
又譬如，蜂鳥唱的歌，已經涵蓋了超高頻的範圍，但是為什麼其中有些部分是連它們自己都聽不到的呢？原來，有些昆蟲是聽得見那些超高頻的，而蜂鳥除了蜂蜜之外也吃昆蟲，所以從Umwelt的角度看，非常有可能那種超高頻的聲音，是為了逼出昆蟲呢！
看完了這一段，就算是夜深人靜之處或者心平氣和之時，也不要忘了，在超高頻及超低頻的世界，那裏存在著永遠的喧囂。
節錄幾段於后，跟大家分享：
“…an elegant experiment,…He presented them with pairs of buzzy sounds. One consisted of repeated chunks in which the pitch rose over a few milliseconds before falling again. In the other, the pitch of the chunks fell over the same range of frequencies, and over the same time period. To a slow ear, both sounds would average out to the same pitch, and seem identical. To a fast ear, they’d be completely different…humans could only distinguish between these sounds if the chunks were longer than 3 to 4 milliseconds. Canaries and budgerigars hit their limit at between 1 and 2 milliseconds. And zebra finches weren’t even slightly duped by the shortest 1-millisecond chunks. This experiment clearly showed that birds can hear complexities that are imperceptibly fast to humans.”
有一個設計得很棒的實驗。
產生一對的兩個響聲，在幾千之分一秒內第一個響聲頻率逐漸上升然後消失，接著第二個響聲的音頻則逐漸下降。
比較鈍的耳朵，這兩個響聲聽起將是一個平均的響聲。比較銳利的耳朵，則可以聽出來那是兩個響聲。
人類要能分出那是兩個響聲，必須響聲是長於千分之3到4秒。金絲雀及虎皮鸚鵡則聽得出千之分2秒的響聲。斑胸草雀則輕輕鬆鬆能聽得出千之1秒的響聲。
這個實驗證明，鳥類能聽到聲音的複雜度，是人類難以想像的。
“…At the time, people thought that hearing was mostly static. It might get duller with age in some species humans, sadly, among them- but it wasn’t thought to change over shorter timescales. But as we’ve repeatedly seen, an animal’s senses are finely tuned to its environment and have evolved to extract whatever information is relevant. When the environment fluctuates from one season to the next, the information that’s relevant also changes. For a North American bird, spring often means sex. The air fills with courtship calls that are absent in other times of year and must now be carefully judged. Fall brings openness: Bare branches make little birds more visible to predators. The ability to localize the sound of approaching danger, which is inextricably linked to fast hearing, becomes paramount. An animal’s Umwelt cannot be static, because an animal’s world isn’t static.”
在以前，人們認為聽力大部分時間是維持一樣的，除非受到老化的影響，但是不管怎樣都不覺得在短時間內會有所變化。
但從科學家反覆的實驗可以知道，動物對環境的變化是很敏感的，而它們也依照從環境中可獲得的各種相關的資訊不斷演化。
當環境隨著季節變化，環境中所透露的相關訊息也跟著不同。
以北美的鳥類為例，春天代表繁殖。空中充滿求愛的召喚，這在其他季節沒有，鳥類必須特別注意。秋天帶來的空曠感：樹葉掉落了露出枝條使得小鳥曝露在掠食者的威脅之下。是否能夠有能力辨識迫近的危險是很重要的，其中重中之重是敏銳的聽力。
動物所面對並感受的世界(Umwelt)不是固定的，因為動物所在的世界不是固定的。
“…To listen for Soviet submarines, the U.S. Navy installed chains of underwater listening posts in the Pacific and Atlantic. This network, known as the Sound Surveillance System, or SOSUS, picked up a deluge of oceanic noises. Some were clearly biological. Others were more mysterious. One especially enigmatic sound was monotonous, repetitive, and low, with a frequency of 20 Hz an octave below the lowest key on a standard piano. This hum was so loud that people doubted it could be coming from an animal…The actual source only became clear when Navy scientists started following the sounds to their sources, and often found a fin whale at the end…”
為了偵測蘇聯的潛艇，美國海軍有一個叫做SOSUS的系統，可以用來聽海底傳來的聲音。
有些明顯是生物所產生的，但有些則顯得頗為神秘。
有一種特別引人注意的聲音，是單調、重覆、20赫茲的低頻，比鋼琴最低的音還要低八度。這種嗡嗡聲很大聲，一開始沒有人相信是動物發出的聲音……當海軍科學家發現這個聲音的聲源總會同時發現長鬚鯨，才慢慢了解原來那些聲音是它們發出來的。
“Human hearing typically bottoms out at around 20 Hz. Below those frequencies, sounds are known as infrasound, and they‘re mostly inaudible to us unless they’re very loud. Infrasounds can travel over incredibly long distances, especially in water. Knowing that fin whales also produce infrasound,…, that their calls could conceivably travel for 13,000 miles…”
人類聽力的下限在頻率20赫茲左右，低於20赫茲叫做超低頻，除非很大聲不然我們是聽不到的。
超低頻的聲音可以傳得很遠，尤其在水這種介質裏。以長鬚鯨發出的超低頻聲音推算，可能傳遞至13,000英哩之外的地方。
“…If you wanted to design a signal that could be used to communicate across oceans, you’d come up with something similar to a blue whale’s song.
Those songs might have other uses, too. Their notes can last for several seconds, with wavelengths as long as a football field….That is, he could map distant underwater landscapes, from submerged mountains to the seafloor itself, by processing the echoes returning from the far-reaching infrasounds. Geophysicists can certainly use fin whale songs to map the density of the ocean crust….”
如果要設計一個可以越洋傳遞的訊號，那能想出來的方式，就應該像長鬚鯨唱出的歌聲那樣。
長鬚鯨的歌聲有其他用途。每個音長達數秒，波長像足球場般大小(按：約75公尺)…可以用傳得很遠的超低頻聲音回音，來量測海底的距離，從海中高山到海床。
地球物理學家，能夠用長鬚鯨的歌聲，繪出海床的密度……
“Krill aren’t evenly distributed across the oceans, so to sustain their large bodies, blue whales must migrate over long distances. The same giant proportions that force them to undergo these long journeys also equip them with the means to do so- the ability to make and hear sounds that are lower, louder, and more far-reaching than those of other animals.”
磷蝦在大海中的分佈並不平均，為了要吃飽，藍鯨必須游得很遠，那就必須配備它們(超低頻的)工具，讓它們可以聽到更低頻，更響亮、傳得更遠的聲音。
“…had been studying African elephants in Kenya’s Amboseli National Park. They’d noticed that elephant families would often move in the same directions for weeks at a time, even though they were separated by several miles. In the early evenings, different groups would also converge on the same waterholes at the same time, but from different directions. Infrasound carries over long distances, even in air, and if elephants use it to communicate, that would explain how they can synchronize their movements across a savannah…”
研究非洲肯亞的大象，發現大象整個家族，雖然它們之間隔數英理，但在幾星期內都會維持朝向同一個方向進行。
在傍晚，不同的象群，從四面八方，聚攏到同一個地方的水池處。
超低頻的聲音，就算透過空氣傳遞，也可以傳遞得很遠，如果大象用超低頻來溝通，就可以解釋在撒哈拉沙漠那麼大的區域，大象們是怎麼同步行動了。
“…The low-frequency parts of the rumbles tange between 14 and 35 Hz- about the same as a large whale’s. Those calls don’t carry as far in the air as underwater, and atmospheric conditions dictate how far they can travel: The colder, clearer, and calmer the air, the greater the range. In the heat of midday, an elephant’s auditory world shrinks. A few hours after sunset, it expands tenfold, theoretically allowing elephants to hear each other over several miles…”
大象低頻的隆隆聲，介於14到35赫茲，大概和大鯨魚發出的聲音一樣。
那些聲音在空氣中傳遞不及在水中，而且會受到天候狀況的影響：天氣較冷、較晴朗、氣流較穩定，聲音傳得較遠。
在氣溫高的日正當中，大象能聽到的範圍會縮小。而在日落幾小時後之後，大象能聽到的範圍會增大10倍，理論上大象縱使相距上幾英浬，都還可以聽到對方的聲音。
“…scientists realized that mice, rats, and many other rodents do indeed make a wide repertoire of “ultrasonic” calls, with frequencies too high to be audible to humans. They make these sounds when playing or mating, when stressed or cold, when aggressive or submissive. Pups that are separated from their nests make ultrasonic “isolation calls” that summon their mothers. Rats that are tickled by humans make ultrasonic chirps that have been compared to laughter. Richardson’s ground squirrels produce ultrasonic alarm calls when they detect a predator (or a tan fedora repeatedly thrown by a scientist to mimic a predator). Male mice that sniff female hormones produce ultrasonic songs that are remarkably similar to those of birds, complete with distinctive syllables and phrases. Females attracted to these serenades join their chosen partners in an ultrasonic duet. Rodents are among the most common and intensively studied mammals in the world and have been fixtures of laboratories since the seventeenth century. All that time, they’ve been spiritedly talking to each other without any human realizing, exchanging messages that slipped beneath the senses of the oblivious researchers and technicians milling around them.”
科學家發現小老鼠、大老鼠及其他各種鼠類會發出各種超高頻的聲音，也就是頻率高到人類的耳朵都聽不到的聲音。
它們在玩耍的時候、面對壓力或寒冷，充滿侵略性或表達順從的時候，都會發出這種聲音。
小幼鼠從巢穴被移開時會發出「隔離叫聲」來召喚母鼠。
大鼠被人搔癢發出的超高頻聲音被視為一種笑聲。
地松鼠發現掠食者也會發出超高頻的叫聲。
公的小鼠聞到母鼠的賀爾蒙時的叫聲好像鳥叫，有清楚的音節和段落。被那些音調吸引的雌鼠會靠近喜歡的雄鼠，而且發出相應的超高頻聲音。
鼠類是研究得最透徹的哺乳動物之一，從十七世紀開始就已經有特定的研究室。在那個時候，鼠類用超高頻興奮地互相溝通，但是一直在它們旁邊的研究人員，當時卻完全沒有意識到呢！
“High-frequency sounds may be easier to locate, but they have an important limitation. They lose energy quickly, and can be easily scattered and reflected by obstacles like leaves, grasses, and branches. This means that ultrasonic calls can only spread over short ranges…”
高頻的聲音或許容易聽到，但是它有一個重大的限制。
高頻聲音的能量會迅速消失，它們容易散射、容易被障礙物(樹葉、草、樹枝)反射。意思是說，超高頻聲音擴散的範圍很小。
“A limited range might be beneficial, however, if animals want to limit their audience. The isolation call of a helpless mouse pup can alert a nearby parent without also alerting more distant predators. In this way, ultrasound really can provide a secret communication channel, not because it lies in an inaccessible frequency range but because it doesn’t travel very far…”
擴散的範圍很小的好處，是只有少數聽眾聽得到。
一隻無助幼鼠的「隔離呼喚」只會引起鄰近父母的注意，而不會驚動到遠處的掠食者。
超高頻的聲音因此可以提供一個秘密的溝通管道，不是因為它位於無法得知的頻率波段，而是因為它無法傳得遠……
“Hummingbirds are even more mysterious….many observers have noticed hummingbirds opening their beaks and fluttering their chests without seeming to sing. The blue-throated hummingbird of North America sings an elaborate song that we can partly hear, but that also extends up to 30 kHz- -well into the ultrasonic range. That was surprising since, …, it can’t hear above 7 kHz. It can still perceive the lower registers of its song, but much of what it sings is inaudible to its own ears…Several other hummingbirds, ……，make calls beyond the hearing of most birds, and the part of these songs that people can perceive sounds like crickets. The Ecuadorian hillstar goes even further, singing entire phrases in an ultrasonic register. Birds tend to have similar hearing ranges that top out before 10kHz. So either these hummingbirds have very unusual ears or they can’t actually hear what they’re saying. And if the latter is true, then why are their songs so high-pitched? Calls demand listeners. If the hummingbirds’ tunes lie beyond their own Umwelten, who’s the audience?
Maybe it’s insects? Even though most insects can’t hear at all, many of those with ears can hear ultrasonic frequencies. More than half of the 160,000 species of moths and butterflies are so equipped. The greater wax moth can even hear frequencies near 300 kHz the highest limit of any animal by some margin. Hummingbirds eat insects as well as nectar, so perhaps they produce ultrasonic calls that they can’t hear to flush out the insects that can.”
蜂鳥更神秘……許多觀察家發現蜂鳥會張開喙嘴鼓動胸腔但是似乎不是在唱歌。
北美藍頸蜂鳥認真唱的歌我們只能聽到一些部份，那些音頻高至3萬赫茲，是到了超高頻。
最令人驚訝的是，蜂鳥聽不到7千赫茲上的聲音，低於那個頻率的它聽得到，但是它唱的大部分它自己是聽不到的……
還有些種類的蜂鳥的歌聲，連大部分的鳥都聽不到，而人聽得到的部分則是像蟋蟀的聲音。
厄瓜多紫金山蜂鳥更絕了，都唱在超高頻的區段。
一般鳥類可以聽到高至1萬赫茲的聲音。所以這些蜂鳥，要嘛自己的耳朶真的那麼好，不然就是自己唱的超高頻其實自己是聽不到的。
如果自己都聽不到，那麼這麼超高頻的歌聲是唱給誰聽的呢？總是要有聽眾嘛！如果以蜂鳥自己的角度來看(Umwelt)，在它的生態棲位裏誰會是聽眾呢？
或許是昆蟲吧！雖然大部分昆蟲都沒有聽力，但其中有很多是聽得到超高頻的。總共16,000種的蛾和蝴蝶之中，有一半以上有這種能力。大蠟蛾甚至能夠聽到頻率為30萬赫茲附近的聲音，這比任何動物能聽到的最高頻率都還要高很多。
蜂鳥吃昆蟲也吃花蜜，所以，或許它們自己都聽不到的超高頻聲音，是用來逼出聽得見那些聲音的昆蟲的呢！
*：ED Yong, “An Immense World,” 2022, Random House/New York
2023/9/9 All Ears, Sound Damakey
