声波 can add together and subtract from one another cancelling each other out This property of 声 can be 讨厌 but can also be used 创造性
What is Phase?
想像 a 平（坦） pool of water
If you drop a pebble into the middle of the pool ripples small 波 will 流 outwards until they reach the edges of the pool in 纯/完美 concentric circles
But if you drop two pebbles into the pool so that they land at the same 时间 but hit the water different at points then you get two sets of ripples
Instead of the 纯/完美 三角 ripples 重叠 and passing through one another these ripples collide and interfere with one another
This interference causes some 波 to be cancelled out and others to be 扩音（放大）
Exactly the same thing happens when two 声波 波 meet
In or Out – of Phase
声波 is a 波 of back and forth 运动
Where two 波 meet they can either add to one another or subtract from one another
In the 跟随 图像 two identical 波 are in 纯/完美 alignment Therefore the 波 will add together 制作 their 峰 and troughs twice as 高 We 描述 these 波 as being in 相位
But if two identical 波 are alligned in such a way that where one has a 峰 the other has a trough we can 描述 them as being out of 相位 In this situation the 波 will cancel each other out
Activity
We can 经验 this cancellation 效果（效应） by pointing two 扬声器 directly at one another and playing the same 音 through both of them Where the identical 波 meet in the 中心 the 声波 will be cancelled out
When the 扬声器 are set up as in the diagram play this 声波 and place your head between the 扬声器
If you stand beside the 扬声器 you will be able to 听 the 声波
but if you place your head directly in-between the 扬声器 the 声波 will drop in 响度
This is due to the 声波 波 cancelling each other out
Fact
The 频率 of 343Hz has a wavelength of one metre
So one 波 fits exactly in between the 扬声器 as set up If the 频率 of the 音 were different then the 声波 波 would 不 cancel each other out perfectly
相位 Cancellation as a 作曲 Tool
The Phaser Flanger and Chorus all 用 the property of 相位 cancellation to 创作 their 效果
A copy of the 原始/原创 声波 is made and 延迟 by a period of 时间 This is then 混 in with the 原始/原创 信号 ‘out of phase’ 创作 the resultant 声波
These manipulations are a great way to apply 相位 cancellation 效果 to your 声
Activity
Try experimenting with these 工具 in a 作曲 Does 相位 cancellation 作品 for all 种类 of sounds? What new 音色 can you 创作 using 相位 cancellation
相位 Cancellation and 噪 Reduction
相位 can be used to eliminate 讨厌 声 现代 smartphones have a 声筒（麦克风） both on the top and the bottom even though the 用户 only speaks into the bottom These two 声筒（麦克风） are wired in so that they are out of 相位 This 中项 that any 声波 entering both 声筒（麦克风） for 例如 背景 噪 will be cancelled out leaving only the spoken voice
Shotgun 声筒（麦克风） are used to 录 interviews and these 作品 in a 类似 way
相位 Cancellation on the Beach
听 to this 錄/录 and 听 相位 cancellation in action
Beating
We’ve 描述 the 相位 交互 for identical 声 But far more 复杂 happenings 出现 when different 声 are 合并 see also 复杂 声波
Where two 音 are very close to one another less than about 50Hz apart they will begin to 创作 a pulsating or 拍（音） 效果（效应）
You can see the pulsating 拍（音） in the oscilloscope These are 创作 because the 声波 are only slightly out of alignment and one keeps overtaking the other
Think about two cars on a 三角 racetrack one is travelling faster than the other and so this fast car keeps overtaking the second
Exactly the same thing happens with 拍音 声 When the 声 are in alignment in 相位 they add to one another and when the 声 are out of alignment out of 相位 they cancel each other out
The 时间 that it takes for one 拍（音） to 出现 is 相等 to the 时间 it takes for the first 声波 to overtake the second
Extra
The 率 of 拍音 is 相等 to the 距 between 音 for 例如
正弦 音 at 460Hz and 440Hz will 创作 a 拍（音） 音 with a 20Hz 脉冲
460 – 440 = 20
作曲 Tip
Try to 用 拍音 声 within a 作曲 They can be very useful 工具 especially in 周围 or Minimalist 作品 because they allow you to make subtle but very 有效 changes to the 声
Take two copies of the same 声 and place them side by side static 音 作品 best
Transpose one very slightly and they will begin to 拍（音）
The closer the 声 are in 音高 the faster the 拍音
Keywords
振幅 Chorus 镶边 频率 聆 处理 效果（效应） 相位 正弦 波