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Noise Cancelling Headphones - part IV.

20. listopadu 2007 v 19:23 | Petr, OK1RP |  Technical topics
Noise - Canceling Headphones -> How it work ?
I received many emails regarding the N-C headphones where You asking me to explain how it works...so few words about the basic principles You can find below.
Silence isn't golden anymore. Really, in this world of booming car stereos, telemarketers, and the exact same music playing in every store and office, silence is worth much, much more than gold. If only you could make the din disappear. One small step toward personal tranquility is putting on a pair of headphones. Regrettably, ordinary audio headphones don't really give you the quiet you're looking for; all they can do is cover up outside noise with the music or radio station of your choice. There are also earplugs and noise-reducing earmuffs available, but these can be unsightly or uncomfortable. Fortunately, technology has come to the rescue. There are a number of noise-canceling headphones now on the market. Sony's MDR-NC20 headphones are a $179.99 option (http://www.sonystyle.com), while Aiwa's HP-CN6 headphones aim for more casual users at a price of $59.99 (http://www.us.aiwa.com).
So what are noise-canceling headphones, exactly? We talked to a engineer at Bose about its high-end QuietComfort 2 headphones ($299;http://www.bose.com), shown here in the main diagram. Read on to find out why they have microphones inside their ear cups and how they can produce near-silence with speakers.
Antisound?
It may sound strange, but headphones like these have to make noise in order to silence other noise. It all has to dowith the structure of sound itself. Sound consists of waves inthe air caused by something vibrating, such as a vocal cord ora speaker cone.A simple way to represent a particular sound is a sine wave,which looks like a series of identical peaks and troughs centeredon a baseline representing zero vibration (see DiagramA). As the object vibrates back and forth, it causes waves inthe air like ripples on a pond, which resemble the hills andvalleys of the sine wave. Louder sounds have taller peaks andlower troughs (that is, more amplitude, or volume). Higherpitchednotes have peaks and troughs closer together (meaninghigher frequencies of vibration).Pure notes, such as a single piano key's, produce clean sinewaves, such as the one in Diagram A. More irregular sounds,such as someone's voice or a song, produce more complexsine waves. The noise in an office with air conditioning, backgroundmusic, and conversations might make a sine wave thatlooks like a dense scribble.The key to noise-canceling technology is to produce audiothat matches the sine waves of each sound in the area-butwith a key difference. The new sine waves are exactly 180degrees out of phase with the original sounds, meaning theyhave valleys where the originals have peaks and vice versa.One vibration effectively cancels out the other, resulting in anoise reduction. This results in sound levels that are close tosilence in all but very loud conditions. (See Diagrams B and C.)
Besides a little peace and quiet, there are other benefits to active noise control, says Dan Gauger, Bose Noise Reduction Technology group senior engineer. Noise cancellation improves the fidelity of music played through the headphones and also cleans up distortion and reduces background hiss, he says. If the user simply wants to cancel noise without listening to any other audio source, she can detach the QuietComfort 2's audio cord and just enjoy the silence. Other benefits are more practical. "If you find yourself exposed to low-frequency noise for an extended period of time, like if you're traveling, that can be fatiguing," Gauger says. Many of Bose's noise-canceling headphones customers have contacted the company about using the devices in transit. "About three out of four tell us that they end their trips feeling less fatigued, more able to get out of the plane and go do what they need to do." Bose also makes different models for airline pilots and other professionals who deal with constantly loud environments. Noise-canceling headphones are "like a portable volume control for the world around you," Gauger says. "Maybe I should actually say (they're) more like a portable mute button."
If You are interested in more informations then visit the http://www.smartcomputing.com/articles/2004/s1501/09s01/09Graphic.pdf?guid= ...where You will find details incl. pictures etc.
It is not easy to find detailed informations how it works in case of products available on the market as its mostly "restricted by sales". If You are interested in the N-C effect and You would not like to waste so much money then You can try to build ownself. It's not possible? Oh, yes of course it is. The method maybe will not be as same as the high-end products using but it is good chance to test it easily.
In my case I am planning to try it to reduce noise on 160m band when high noise coming even separate receiving antenna is used. I prefer to use some project which was already tested by somebody so I can recommend to follow instruction in an article below.
The Process of building N-C headphones ownself
Before we started, we considered trying the following experiments:
  • Sound canceling using analog devices
  • Sound canceling using software
  • Noise cancellation specific to erasing ambient noise and keeping directional noise
We decided to start with sound cancelling using an analog circuit, and go on if we had time (which we didn't).
We obtained:
  • Hi-Fi opamps
  • several resistors of varying resistances
  • two stereo audio jacks
  • electret-condenser microphones
  • two pair of headphones
  • perfboard (that we didn't use)
  • bread board, wire, etc
Experimentation
We started by building the circuit. As we went we got a lot of circuit debugging experience! We learned the truly inifinite benefits of the multimeter.
We then moved on to the headphones. We had two pair, one to use for the actual headphones and one to use just for the wiring.
So, for the use of the microphones, we had to sacrifice a pair of (cheapo) headphones. We put them on a stool and said some kind of chant, holding them above our heads. Then we snipped the wires. We found that each of the left and right headphones had two wires going to them, one being ground and one being the signal, of course. We then soldered these wires to the mics.
As you know, we were using low-quality headphones, and when we mounted the mics on the outside of the headphones, we got little beneficial effect. We found that we could get better results using nice headphones, but they still were not very effective. We moved mics inside headphones to decrease sound delay effects. This greatly improved performance, but introduced feedback until we rigidly pointed the mics away from the headphones' speakers and covered the mics with headphone foam (from the pair of headphones that we had to sacrifice).
Soon, we were doing our best to make the whole system work. We found that due to sound propogation delay, non-inverted output (in our case) produced better effect than inverted output. We also observed that headphones were ineffectual for high frequencies, so to reduce noise produced by the circuit, the input was put through a simple RC lowpass filter with Wo=1K.
Results
  • The apparatus heavily attenuates sounds of very low frequencies, and somewhat attenuates all frequencies <= 1Khz
  • The headphones also get rid of the part of your voice that "echoes" in your inner ear-- the extra echo that you typically hear when your ears are covered and you speak.
Inverted Signal
Non-Inverted Signal
Well the article over there tried to explain basic principles of few methods to cancelling the noise. An example of experiment with inverting the phase of the signal etc. If You would like to do it ownself then visit original pages http://www.headwize.com/projects/noise_prj.htm with much more informations and contruction details.
73, Petr OK1RP
...-.-
 

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