Daniel Van Beers
朝起きてから眠るまで、私たちは音に囲まれています。聴覚は、視覚を除いてヒトの最も強い感覚です。はっきりと、あるいは気づかないうちに、音は私たちの生活に影響を与えています。例えば、鳥の声やほかの自然の音を聞くと心地よく穏やかな気分になりますが、パトカーや消防車のサイレンや踏切の音を聞くと、すぐに危険への警戒心を持ちます。
音楽によって私たちは気分が高揚し、リラックスできます。また“表現力豊かな”会話や音楽は、単調でつまらない日常生活とのバランスをとってくれます。総ての音は日々の暮らしを満たし、これまでの経験に意味を与えてくれるだけでなく、人格形成や自己表現に一役買っているともいえます。特に会話は考えだけでなく、微妙な感情も他人に伝えます。でも、音のように複雑なものがどのように脳に届くのかということについては、めったに考えることがありません。
ヒトの目には740nm(赤)から380nm(紫)の光が見え、音は20Hz(超低音)から20000Hz(超高音)まで聞こえますが、年をとるにつれて高い音はだんだん聞こえにくくなります。また、視覚では明るいものは明るく、暗いものは暗く見えるというように、その程度に合った見え方をしますが、音の聞こえ方は違います。これは、1933年アメリカのベル研究所にいたHervey Fletcherと Wilden Munsonによる先駆的な研究です。彼らの実験に基づく“ヒトの聴覚感度”のグラフから、脳に聞こえる音の強さは音の高さによって変化することがわかり、ヒトには2000Hzから5000Hzの音がよく聞こえることが発見されました。大まかに言うと、ヒトの耳の穴（外耳道）の形と長さでは、その範囲より高い音や低い音への感度は対数的に下がります。
このように限界はありますが、私たちはとても微妙な音の違いを聞きとることができます。例えば、小さな音で音楽を聴くと中音(2000Hz～5000Hz)が際立って聴こえ、低音や高音は背後に隠れてしまいます。逆に音量を上げると、低音や高音が強調されて中音は埋もれてしまいます。このように音の強弱で聞こえ方は変わるのに、音色や音質は変化しません。
様々な楽器の音と一緒に聴こえてくる歌声の繊細な抑揚から日々の喧騒まで、私たちの脳は耳なじみのある音のグループに分けることができます。それから、気になる音たちを構築し始めるのです。
Daniel Van Beers
アメリカ在住。音のエキスパート。芸術文化に造詣が深く、鉄道や猫を愛する一面も持つ。
【原文】Familiar Sound Perception
Daniel Van Beers
From morning when we wake until we go to sleep, we are surrounded by sound. Besides sight, sound is one of the most powerful senses we have.
Sound influences our life in many ways, some obvious and some not so obvious.
For instance, when we hear a bird sing, it almost always invokes a feeling of comfort and tranquility, just as many sounds in nature often do.
When we hear a police car or fire truck siren or railway crossing warning, we are immediately on alert and heightened sense of our surroundings and possible danger which causes anxiety.
When we hear music, quite often our spirit is uplifted and our mind is able to relax and enjoy the moment of performance.
Through the “expressive qualities” of speech and music, we are able to enjoy a higher quality of life for which we all aspire. This helps balance the drudgery and struggle of everyday life.
What this illustrates is the idea that through all the sounds we encounter, our life is enrichened and helps give meaning to our overall life experience. It helps shape our character and allows us to express ourselves in a meaningful way.
Speech, in particular, is useful not only in conveying ideas to others, but also subtle emotions.
But we rarely consider how something as complex as sound gets from the source to our brain.
With sight, the visible spectrum of light to which our eyes are sensitive exists between 740nm(red) through 380nm(violet). Our ears are sensitive to frequencies from 20Hz~20Khz. The upper range deteriorates as we age.
Unlike sight which is rather linear in terms of sensitivity, our hearing is very non-linear.
Pioneering work by Harvey Fletcher and Wilden Munson at Bell Laboratories in 1933 demonstrated that human hearing is not at all linear. Their testing produced graphs of “perceived loudness” that humans sense. As actual loudness of an audio source changes, our brain hears this change at a different level which is frequency dependent.
From Fletcher and Munson’s study, it was discovered the human ear is optimized for frequencies in the 2Khz-5Khz range. This is largely due to the shape and size of the ear canal.
Sensitivity to frequencies above and below this range drop off at a nearly logarithmic rate.
Although having this inherent limitation, we are still able to perceive very subtle nuances in sound.
For instance, if one were listening to music at a low listening level, the mid-range frequencies (2Khz~5Khz) would be perceived as prominent with the upper and lower frequencies diminished in the background.
Conversely, if one were listening to music at a high level, the upper and lower frequencies would appear prominent and the mid-range frequencies more subdued.
In spite of this very non-linear perception of sound, the overall tonal quality of sound remains the same.
Either from subtle inflections in a person’s voice to a musical performance with a number of sounds from various instruments to a noisy environment we encounter on a daily basis, our brain has a unique ability to separate the sounds into familiar groups which convey an overall experience of the moment. Our brain then begins the task of forming familiar sounds.