Noise-Canceling Headphones Without the Headphones

Active noice cancellation in a headrest could one day compete with ANC in a headset

3 min read
Man on a plane
Photo: Getty Images

Active noise cancellation/control (ANC) headsets depend on passive sound attenuation from the padding and cups found in headphones and earphones. This makes ANC headphones effective noise control devices, although wearing these for extended periods of time can be uncomfortable and even cause injury. A group of researchers from the University of Technology Sydney’s Centre for Audio, Acoustics and Vibration have been working on a new kind of virtual noise cancellation system that moves the ANC components away from personal audio equipment and into the headrest of a chair.

Active noise cancellation has an almost century-long history—going back to the 1930s when the first patent was awarded in the US for a device that could cancel out unwanted noise by inverting the polarity of these sounds and playing them back. In 1989 the first working prototype of ANC headphones was released for the aviation industry. In 1991, ANC was used for noise-control in an enclosed space—for the hardtop model of the Nissan Bluebird in Japan.

“However, this ANC headrest system has seen very little progress over the years compared to ANC headphones,” says Tong Xiao, one of the authors of the current study.

There are multiple reasons for this. Existing ANC headrests use microphones set in strategic positions around a user’s head to sample the sounds reaching them. These setups are best for dealing with low-frequency noises up to 1 kHz. However, the passive noise control provided by the cushioning and cupping available in headphones, which reduce high-frequency noises more effectively, are absent. These high frequencies include human speech, which is around 4 to 6 kHz. 

Thus the Sydney team needed to demonstrate an ANC system that worked with both high and low frequencies. So they used a remote acoustic sensing system built around a laser Doppler vibrometer (LDV), which measures non-contact vibrations over a wide range. In their tests, they placed a tiny, jewelry-sized, retro-reflective membrane in the ear as a pick-up for the LDV.

The test was a success, reports Xiao, with noise cancellation active to 6 kHz—and attenuation by 10 to 20 dB

The researchers tested their system on three representative types of  environmental noises—aircraft interior, airplane flyby noise, and speech. Xiao says their system performs closer to a pair of ANC headphones than to any existing ANC headrests, with a wide frequency response but without the need for any bulky headsets or other devices. “We call it ‘virtual ANC headphones,’” he says.

Two secondary loudspeakers were placed behind the HATS for sound control. Multiple primary loudspeakers (three shown) were located arbitrarily to simulate unwanted sound from different directions. The probe laser beam from the LDV was directed toward the membrane in the ear. (b) A membrane was placed close to the ear canal of the left synthetic ear of the HATS. Left: Two secondary loudspeakers were placed behind the head and torso simulator (HATS) for sound control. Multiple primary loudspeakers (three shown) were located arbitrarily to simulate unwanted sound from different directions. The probe laser beam from the laser Doppler vibrometer (LDV) was directed toward the membrane in the ear. Right: A membrane was placed close to the ear canal of the left synthetic ear of the HATS. The LDV remotely determines the surface velocity of the membrane as the error signal for the ANC controller.Images: University of Technology Sydney

The performance of these virtual headphones was demonstrated on a head and torso simulator (HATS) device to record consistent measurements. However, Xiao says, they have performing initial tests with human subjects since the paper was published.  

“The configuration…[is] simple and yet effective,” he says “[One] limitation…so far is the cost, since the system uses LDVs, which are delicate scientific instruments and can be pricey. However, we believe science advancements in the near future can make the system more cost effective.”

There are other aspects of the system to be ironed out still—a more realistic head-tracking system; improved material of the membrane placed in the user’s ears; lasers that are safe and invisible to humans; and, of course, even higher levels still of noise reduction.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.