You can try to escape the outside world inside a pair of headphones, but the outside world -- the rumble of traffic, the screech of a subway, the drone of airplane engines and the racket escaping from other people's headphones -- has a way of getting in.

Instead of forcing users to strap on ever-bigger headphones, electronics manufacturers a few years ago introduced models with "active noise-canceling technology." Such headphones use processors to analyze exterior noise, generate a sound wave opposite of the one intruding from the outside and feed that inverted wave into your ears. The two waves largely cancel each other out, resulting in much less unwanted noise.

The technology was once confined to high-end, audiophile headphones, such as Bose's $299 QuietComfort model. But now it can be had for less than $100 from Sony and Panasonic.

We gave four models a listen: Sony's MDR-NC5, $80; MDR-NC11, $150; MDR-NC20, $180; and Panasonic's RP-HC100, $80. Each runs with a single AAA battery and can be used without audio input just to null out background noises.

The first thing we noticed with noise cancellation enabled was that the buzz of a computer's fan evaporated, replaced by a luxurious silence and a faint hiss in the background. (The Panasonic headphones took a moment to kick in, while the Sony models went to work instantly.) The change wasn't overwhelming, but it was a relief.

In a car or airplane, the difference was more dramatic. We found all but one of the models amazingly effective in blocking the annoying airplane drone, making it easier to nap, listen to music and concentrate on work.

The headphones don't work as effectively against all types of sound. Voices aren't affected much and neither are short, sharp sounds such as ringing telephones. The headphones also seemed less effective against high-pitched noises.

The differences between the Panasonic and Sony headphones were often minor. Panasonic's $80 RP-HC100, for example, was the least expensive model of the four but sounded slightly clearer than the others at low volume and had soft ear enclosures that offered plenty of noise insulation by themselves. (We did not get a chance to review a few other, cheaper, Panasonic models, which sell for as little as $40.)

Sony's MDR-NC5 felt flimsier, and its noise cancellation generated a slightly louder hiss that was noticeable in quiet passages. In its favor, the lighter Sony was more pleasant to wear for long periods -- and, like the other Sony models, included an adapter for airplane audio.

It was impossible to compare the headphones' noise-cancellation techniques during music playback. The Sonys automatically increase music volume with noise cancellation on, while the Panasonic leaves volume alone.

Sony's $149 MDR-NC11 was lighter still, but its earbud-style design produced too much hiss and too little noise cancellation. It stuck so deep in our ear canal that it somehow amplified our own breathing, as if we were underwater.

When price is no object, the $180 Sony MDR-NC20 is the way to go. It has the same soft ear enclosures as the Panasonic but was a little more comfortable. Its silence was blissfully complete, with almost no audible hiss.

All four of the headphones provided richer, more robust sound than the cheapies bundled with most portable CD or MP3 players, but the bulkier models sounded best.

Noice-canceling headphones aren't the cheapest way to hear music. But for people who travel a lot or are routinely subjected to noise, they're much better than earplugs or suffering.

Noise-canceling headphones, from left: Sony's MDR-NC5, MDR-NC11 and MDR-NC20, and Panasonic's RP-HC100.