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Bose acoustic noise cancelling headphones The concept of noise cancelling has been around for a long time but now it has been incorporated into a commercial product the Bose Aviation Headset. It uses electronics inside the headset to cancel unwanted noise. Ever flown in a helicopter or military transport? If you have, you will know just how deafening is the noise inside the fuselage. The only way you can talk to anyone else on the aircraft is to use a standard communications headset. These consist of a set of headphones with a good air seal around the ears and a microphone mounted just in front of the wearer's mouth. Now communication headsets are OK as far as they go but they are uncomfortable to wear for long periods and they don't totally eliminate the noise - far from it. The big problem with any headset is that it is just not possible to stop external noise getting to the user's ears. And even if it was possible to have a hermetic seal, some noise would still pass though the headset and cause a problem. In practice, headsets (and ear muffs) produce most of their attentuation at higher frequencies and have little effect at low frequencies. The problem with noise in aircraft, of course, is twofold. Too much noise causes deafness and it also makes communication via radio difficult. Here's where this revolutionary product from Bose comes into the picture - headphones which cancel out ambient noise. The idea of noise cancelling is not new - it's been a dream almost as long as electronics has been in existence. Back in the early 60's one of the American science magazines ran an article on how it might be possible to achieve a quiet bedroom when your next door neighbour is having a noisy party. The idea was to have a microphone to pick up the unwanted noise in the bedroom, amplify the noise in an amplifier and then feed it to speakers in the bedroom to cancel it out. The idea is simple in theory but a lot more complicated in practice. Not surprisingly, not a lot has come of the idea. Noise cancelling headphones though, are a far different proposition. The airspace inside the headphones is a small closed volume which can be tightly controlled and compensated for. The principle of the Bose noise cancelling headphones is similar to that outlined above. Directly in front of each headphone driver is a small microphone which picks up both the unwanted noise and the wanted signal (from the radio or whatever). Bose's technical info represents the circuit with the composite signal being fed to one input of a summing amplifier while the other input of the summing amplifier is fed with a wanted audio signal. The result is the wanted signal with the noise cancelled out (see Fig.1). In effect though, the Bose Acoustic Noise Cancelling (ANC) Headset is a classic negative feedback system as used in all audio amplifiers. The difference here is Radio/lCS Interface Compensating Filter Bose Acoustic Noise Cancellin Limiter Headset Controller (left channel not shown) The scbematic of the Bose ANC headset: a microphone inside each earpiece picks up the noise. 10 SILICON CHIP BoomandCab!e can be attached lo either left or right earcup On/Off Switch1)10Vides failsate operation Fully Articulated Boom Adju&ment Power Input . hteradap!ersupplied) , Noise cancelling has been a dream for decades but now the Bose ANC headset has made it a reality. that the headset driver is included in the feedback loop by virtue of the pickup microphone. Any noise signals picked up by the microphone are automatically cancelled as they appear as an "error".signal in the negative feedback. Of course, there is a little more to it than that, as Fig.1 shows. The headset is equalised, presumably to give it a more intelligible response for radio communications, and the "corrected" signal is fed through a compensating filter and limiter which probably has been included to prevent any possiblity of acoustic "howl" developing. The net result is that the Bose ANC Headset can give an effective total noise reduction at the wearer's ears of around 16dB compared to the standard US Air Force headset and up to 25dB cancellation of noise in the middle and low frequency bands. The Voyager flight Just how effective the Bose headsets are was demonstrated during the 9-day non-stop around the world flight of the Voyager in 1986. Pilots Dick Rutan and Jeana Yeager wore early prototypes of the headset for the whole 9 days. They did so because the noise level in the plane was very high and without the headsets they were expected to have a major hearing loss as well as putting the whole mission in jeopardy because of noiseinduced fatigue. After the flight, the pilots were tested for possible hearing loss. The conclusion was that there was only a slight shift in the hearing threshold for both pilots, which returned to normal after a few days. As well as the noise cancelling feature, the Bose headsets feature more effective and more comfortable ear cushions. The original type, nicknamed " bonecrushers" continued on page 91 FEBRUARY1990 11 The Serviceman Know About Capacitors; Telephone Bell Monitor/ Transmitter; 32-Band Graphic Equaliser, Pt.2; Led Message Board, Pt.2; Fluke's New 80-Series Multimeters. May 1989: Electronic Pools/Lotto Selector; Synthesised Tom-Tom; Biofeedback Monitor For Your PC; Simple Stub Filter For Suppressing TV Interference; LED Message Board, Pt.3; Electronics for Everyone - All About Electrolytic Capacitors. June 1989: Touch-Lamp Dimmer (uses Siemens SLB0586); Passive Loop Antenna For AM Radios; Universal Temperature Controller; Understanding CRO Probes; Led Message Board, Pt.3; Coherent CW - a New Low Power Transmission Technique. July 1989: Exhaust Gas Monitor (uses TGS812 Gas Sensor); Extension For the Touch-Lamp Dimmer; Experimental Mains Hum Sniffers; Compact Ultrasonic Car Alarm; NSW 86 Class Electrics; Facts On the PhaxSwitch - Sharing Your Phone Line With A Fax Machine. August 1989: Build A Baby Tower AT Computer; Studio Series 20-Band Stereo Equaliser, Pt.1; Garbage Reminder - A 7-day Programmable Timer; Introduction to Stepper Motors; GaAsFet Preamplifier For the 2-Metre Band; Modern 3-Phase Electric Locomotives. September 1989: 2-Chip Portable AM Stereo Radio (uses MC13024 and TX7376P) Pt.1; Alarm-Triggered Telephone Dialler; High Or Low Fluid Level Detector (uses LM1830N); Simple DTMF Encoder (uses Texas TMC5089); Studio Series 20-Band Stereo Equaliser, Pt.2; Auto-Zero Module for Audio Amplifiers (uses LMC669); A Guide to Hard Disc Drives. October 1989: Introducing Remote Control; FM Radio Intercom For Motorbikes (uses BA 1404 and TDA7000) Pt.1; GaAsFet Preamplifier for Amateur TV; 1 Mb Printer Buffer; 2-Chip Portable AM Stereo Radio, Pt.2; Installing A Hard Disc in the PC; A Look! at Australian Monorails. November 1989: Radfax Decoder For Your PC (Displays Fax, RTTY and Morse); Super Sensitive FM Bug; Build A Low Cost Analog Multimeter; FM Radio Intercom For Motorbikes, Pt.2; 2-Chip Portable AM Stereo Radio, Pt.3; Floppy Disc Drive Formats & Options; The Pilbara Iron Ore Railways. December 1989: Digital Voice Board (Records Up To Four Separate Messages, Uses Texas TMS3477NL and 256K RAMs); UHF Remote Switch; Balanced Input & Output Stages; National Semiconductor LM831 Low Voltage Amplifier IC Data & Applications; Install a Clock Card In Your PC; Index to Volume 2. January 1990: Service Tips For Your VCR; Speeding Up Your PC; Phone Patch For Radio Amateurs; High Quality Sine/Square Oscillator; Active Antenna Kit; The Latest On High Definition TV; Speed Controller For Ceiling Fans. Note: November 1987, December 1987 & January 1 988 are now sold out. continued from page 49 drives the base more negative, turning the transistor on. In fact, the transistor was leaking internally with normal operating voltages applied, and was turned on permanently. Under these conditions, the collector voltage fell from the correct figure of 4.1 V to something like 0.1 V. I fitted a new transistor and found that the set now worked perfectly; stable sync, full colour and all. Post mortem But why had the set been dead from the tuner on? That 6.3V at the emitter of the 1st video amplifier also supplies a bias to the video IC at pin 6. So the whole IF strip was inactive when the fault was really in the video amplifier. And the tuner was inoperative because it needs an AFC voltage which is developed from a signal provided by the IF chip. Which brings me back to the point made earlier. If I had been a little more careful when replacing the board in the first place, then none of this story would have eventuated. ~ Bose noise cancelling headphones for obvious reasons, maintained a tight seal to the pilot's head. The ear surround seal in the Bose headset uses a combination of silicone gel and soft foam which follows the contours of the head to create a better seal, with only a slight amount of headband pressure. Even without the noise-cancelling electronics, the Bose headsets still boast better passive noise attenuation than many of the ordinary types on the market, especially at low frequencies. Specifications Each earpiece has its own independent noise-cancelling system, allowing for both mono and stereo use. The sound pressure servo system has been built using surface-mount technology, allowing Circuit Notebook - from page 11 it to fit inside the earcup. The impedance of the earpiece is 1500 and the system has a frequency response of 100Hz to 6kHz. The power supply required is 150mA at 11-16V or 22-32V DC. Applications A number of helicopter and aircraft manufacturers are now offering Bose ANG headsets as an optional feature but Bose see the product as having wide application in any situation where people have to work for long periods while subjected to high levels of noise. And who knows, maybe in the future your W alkman will come with noise cancelling headphones. l!t] Acknowledgement: our thanks to Bose Australia Inc, for their assistance in preparing this article. continued from page 26 represent a variable inductance. By varying dual gang pot VR1, the resonant frequency of the inductor is changed and so the circuit can vary the frequency to be boosted or cut. The filter is an LC type based on the .001µF capacitor and the variable inductance. Note that VR1 is wired so that when VR 1 a increases its resistance, VR1b reduces its resistance. As shown, the centre frequency can be tuned from around 32Hz to 17kHz and the available boost and cut is about ± 12dB. The output of the circuit is taken from pin 7 of IC1b via a 1k0 resistor and 1µF capacitor. Darren Yates, French's Forest, NSW. ($30) (Editor's note: while the circuit shown here is run from a single supply, there is no reason why it could not be modified to run with balanced positive and negative supplies). FEBRUARY1990 91