Phase (相位)

[999369]

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[Leo Yeh]

999369有書名 or paper嗎...

我覺得這個寫得比較像我要的

[Leo Yeh]

相位(Phase)：用來描述聲波循環中的一個位置點，通常以角度來表示
http://www.indiana.edu/~emusic/acoustics/phase.htm

這個連結的圖中以sine wave為例
Phase angle =0 => 為聲波起始點 (相對振幅0)
Phase angle =90 => 為聲波最高點 (相對振幅1)
Phase angle =90 => 為聲波起始點 (相對振幅0)
Phase angle =270 => 為聲波最低點 (相對振幅-1)
Phase angle =360 => 回到聲波起始點 (相對振幅0)

cosine wave 則是與sine wave一樣，聽起來也會一樣，只不過其從sine wave的90度相位開始(90° out of phase to a sine wave)，這也是sine wave的最高點

[Leo Yeh]

當許多想同或不同相位的聲波結合在一起，就會變成一個組合的聲波(composite wave)。

interference => 指兩個聲波以上的結合作用

相位差(Phase difference)是指當不同相位的相同聲波的角度差異，如cosine wave與sine wave的相位差就90度

(正向作用)constructive interference
若兩個相同頻率與相位的聲波一起，那就是會成為一個新的振幅2倍大的聲波


(反向作用或相位抵消)constructive interference, phase cancellation
若兩個相同頻率但180度相位相反的聲波一起，那新的聲波完全是平的沒有振幅


通常都是有正向與反向的交互作用，最後的強度就會相對高高低低了

[Leo Yeh]

在真實的聲學環境裡，聲音的來源(如音響系統)與空間的反射音為決定最後聲波的關鍵，只要小小的改變就會使得聲音來源與反射音的相位關係改變，立體聲麥克風若擺位不當，也會使得在某些頻率的相位抵消。
兩個頻率差異很小的聲音，如兩個鋼琴的音力道一樣，可能會被認為是同一個聲音，但考慮聲波後，他們可能彼此是in or out out phase，讓聲波振幅產生脈動(pulsation)，這個脈動就叫做beating
脈動率或是beat frequency就是頻率的差異。將旋樂從400Hz調成441Hz就會每秒鐘產生一個脈動



網頁最下方可自己試試
http://www.indiana.edu/~emusic/acoustics/phase.htm

Use the interactive example below to create beats for yourself. Turn on the 440 Hz sine wave, then turn on the 441 Hz tone (see if you can time your click to the 1-second timer). This will produce one beat per second (441 - 440 = 1). Turn off the 441 Hz tone and click on the 442 hz tone. This will produce two beats per second (442 - 400 = 2). What happens when you activate all three tones?

Use the interactive example below to create beats for yourself. Turn on the 440 Hz sine wave, then turn on the 441 Hz tone (see if you can time your click to the 1-second timer). This will produce one beat per second (441 - 440 = 1). Turn off the 441 Hz tone and click on the 442 hz tone. This will produce two beats per second (442 - 400 = 2). What happens when you activate all three tones?

[Leo Yeh]

[chsiang]

Douglas Self 的觀點，全文請參考：
http://www.douglas-self.com/ampins/p...ubjectv.htm#11

11: ABSOLUTE PHASE.
Concern for absolute phase has for a long time hovered ambiguously between real audio concerns like noise and distortion, and the Subjective realm where solid copper is allegedly audible. Absolute phase means the preservation of signal phase all the way from microphone to loudspeaker, so that a drum impact that sends an initial wave of positive pressure towards the live audience is reproduced as a similar positive pressure wave from the loudspeaker. Since it is known that the neural impulses from the ear retain the periodicity of the waveform at low frequencies, and distinguish between compression and rarefaction, there is a prima facie case for the audibility of absolute phase. It is unclear how this applies to instruments less physical than a kickdrum. For the drum the situation is simple- you kick it, the diaphragm moves outwards and the start of the transient must be a wave of compression in the air. (followed almost at once by a wave of rarefaction) But what about an electric guitar? A similar line of reasoning- plucking the string moves it in a given direction, which gives such-and-such a signal polarity, which leads to whatever movement of the cone in the guitar amp speaker cabinet- breaks down at every point in the chain. There is no way to know how the pickups are wound, and indeed the guitar will almost certainly have a switch for reversing the phase of one of them. I also suggest that the preservation of absolute phase is not the prime concern of those who design and build guitar amplifiers. The situation is even less clear if more than one instrument is concerned, which is of course almost all the time. It is very difficult to see how two electric guitars played together could have a "correct" phase in which to listen to them. Recent work on the audibility of absolute phase [21], [22] shows it is sometimes detectable. A single tone flipped back and forth in phase, providing it has a spiky asymmetrical waveform and an associated harsh sound, will show a change in perceived timbre and, according to some experimenters, a perceived change in pitch. A monaural presentation has to be used to yield a clear effect. A complex sound, however, such as that produced by a musical ensemble, does not in general show a detectable difference. Proposed standards for the maintenance of absolute phase have just begun to appear, [23] and the implication for amplifier designers is clear; whether absolute phase really matters or not, it is simple to maintain phase in a power amplifier (compare a complex mixing console, where correct phase is vital, and there are hundreds of input and outputs, all of which must be in phase in every possible configuration of every control) and so it should be done. In fact, it probably already has been done, even if the designer hasn't given absolute phase a thought, because almost all amplifiers use series negative feedback, and this must be non-inverting. Care is however required if there are stages such as balanced line input amplifiers before the power amplifier itself.

簡言之, 有些樂器作用的頻率及其發聲的物理現象, 那相位就有意義,

但是對於其他樂器, 反相就不一定聽得出來,

那一個複雜的聲音組成就更難去聽出相位造成的差異了,

提出這個不是說相位問題無解, 而是希望討論會漸漸深入,

是否某個頻段(樂器)的相位對聽感造成影響會比其他的頻段(樂器)大,

那就該特別注意, 也會是調整的方向

[Leo Yeh]

"絕對相位"又是另一個議題，不過我是音響迷的話，我會care，不管我有沒有聽出來

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Audibility of Phase Distortion
改變聲音的大小振幅是聽的出來的，改變頻率的強度也是聽的出來的，相位不同所造成的聲音大小，聽的出來嗎？
這邊有鋼琴的聲音
這是原來的錄音
http://www.silcom.com/~aludwig/sounds/phasedem1.wav
這是相位改變後的聲音
http://www.silcom.com/~aludwig/sounds/phasedem2.wav
大家可聽看看，差異非常大的，甚至有人可指出相位調整了多少
不過相位的改變在實際聆聽的條件中聽得出來嗎？
大部份的人都任何差異是無法察覺的...

[Leo Yeh]

傅立葉轉換(Fourier Transform)
傅立葉轉換的物理意義是：透過Fourier Transform，把時域和頻域互相轉換，取得想要的資訊



http://netgames123.blogspot.com/2008...transform.html

[Leo Yeh]

近代的研究都認為相位在某些情況下是很容易發現的
http://www.ocf.berkeley.edu/~ashon/a.../phaseaud2.htm

要減少相位失真問題，就要像MY-HIEND的錄音一樣.....

Mainly, phase distortion is difficult to test and to prove. On the implementation side, phase-linear (accurate) source material and reproduction equipment are necessary:
1) minimalist mic'ing required - no multi-mic'ing or studio mic'ing
使用最少的麥克風 => my-hiend使用兩隻配對的
2) no mixing board analog equalization, because analog equalization filters alter phase!
不要使用類比等化混音，會改變相位 => my-hiend不使用
3) preferably no reverb or additional post-processing
不要再事後處理 => my-hiend不處理
4) phase linear amplification required (any good quality amplifier)
好的相位線性擴大機
5) phase linear speakers, e.g. crossover-less single driver speakers like Quad electrostats
好的相位線性喇叭，如分音少、單單體的喇叭，像Quad靜電喇叭
6) or Stax electrostatic headphones
或Stax靜電耳機