Active noise cancellation or Passive noise isolation?

為了不要聽到耳機播放音樂以外的噪音，目前有兩種解決方案：一種是主動式消音(active noise cancellation)，另一種是被動式隔音(passive noise isolation)。 第一種主動式消音的方法是耳機本身會附上一個麥克風，但是這個麥克風不是用來給你說話用的，而是取外界的噪音反相處理後，混合到音樂中，達到噪音互相抵銷的功能。然而這種作法有三個缺點，第一：需要額外的電池供應主動式消音功能。 第二：因為多了一個電池，耳機本身就會變笨重。第三：(對某些"欣賞"音樂的人來說)對音訊動手腳本身就是一種失真，這種技術頂多能讓你"聽"音樂，而不是"欣賞"音樂。至於被動式隔音，如果隔音做得好的話，外界的聲音至少可以被減到隨身聽只要放一點音量就可以完全覆蓋剩下的噪音的程度，對於耳朵的保護、音質的堅持都不失為一種很好的解決方案。

以下的文章取自ZDnet ( FW from ZDnet/News&Blogs/David Morgenstern's article)

A vote for passive noise isolation, not active reduction

flight to NYC I took this week, many travelers are using active noise-reduction technology to shut out the din of the airplane. However, I prefer the passive approach of noise isolation, which is a greener solution, and provides more accurate sound.
The earbuds in question were a pair of Etymotic Research ER•4 MicroPro earphones that I purchased at the Macworld Expo in January. The company offers a substantial discount during the show and I splurged on the ER•4s, which have a list price of $299. While no bargain, I’ve found these earphones to be the real deal.
Instead of countering noise with counter-noise as is the case with the active noise-cancelling headphones, the passive earphones fit inside the ear canal to block out the noise. The better they fit and stop the noise, the better they work.
Whenever I’ve tried the active headphones, I’ve found them big and bulky, and a product that I didn’t want to travel with. Worse, the sound wasn’t as good to my ears as my passive earphones, especially in less-noisy environments (remember that not everywhere you go will be noisy).
Over the past 5 years or so, I’ve purchased many different brands and styles of the passive earphones, including entry-level, prosumer and professional flavors from Etymotic, Sennheiser, Shure and Ultimate Ears. The ER•4 are the best so far.

I found the earbuds very comfortable and quiet — I used the 3-flange soft-plastic eartips, which provide 35dB reduction, according to the company. ER says that you can get about 7 decibels more with the foam eartips that come with the package, but I take the buds in and out of my ears a lot and I’ve found the foam eartips don’t take as well to frequent removal.
The sound produced by Etymotic Research earphones are always a bit “dryer” (or less-warm depending on how you hear it) than comparable Shure models. This is a matter of personal taste.
But what I know for sure is that the earphone isn’t adding or subtracting sound from the recording — something that must be happening with active noise canceling technology. In addition, there’s no battery to worry about during the trip, a concern with the active systems.
Of course, the isolation earphones are smaller and more robust than the active headphones. The ER•4s use a braided wire for part of the cable. Perhaps this prevents some of the referred noise that sometimes happens when clothing rubs against the wires? No idea, but I didn’t notice those acoustic artifacts.
As with most technologies, you get what you pay for. The ER•4s list price is less than most of the active headphones on the market. I admit they’re not cheap. (And it’s an odd feeling listening to music on ear buds that cost more than my iPod.)
However, please don’t judge the isolation category by the results you may have found with entry-level products. I’ve been less-than-satisfied with low-cost lines of earphones for comfort and isolation. The ER•4 is a much different piece of equipment.

NOV 16 MON 2009 22:16 - migrate from pixnet