The Science Behind How Automakers Pump Sound Through Speakers To Actually Make Cars Quieter

The cool thing about being an engineer is that you can measure and visualize unseen phenomena. With the power of technology and the human mind you can bend Mother Nature to your will in ways that the pioneers of science and mathematics like Euler and Fourier couldn’t even imagine. One such way? Making your car less annoying to drive around in by using speakers. It’s called “Active Noise Cancelling” (ANC), and I — a Noise, Vibration, Harshness engineer — am going to tell you how automakers use it to make your car quieter.

ANC goes beyond just trying to create a pleasant drive. Automakers can take advantage of ANC to add lightness, increase your range, even make your car more fun. In fact, the first ANC system in a vehicle was designed by none other than Lotus! And you thought this article would be about a boring old Lexus. 

[Editor’s Note: Everyone please welcome back Steve Balistreri, a fellow enginerd and Detroiter. He’s an expert on NVH, and since I’m fascinated by the topic, you can expect to see lots more from him soon! He already wrote about $1000 hammers used to keep cars quiet and intact. -DT]. 

What Is Active Noise Control?

Let’s talk about what ANC actually is. In a car, passive noise control treatments are things like noise absorbing foam, double paned glass, weather stripping, sound deadening material, and rubber engine mounts. They are just sort of sitting around passively absorbing energy (sound and vibration) because of their physical properties — kind of like my brain before my morning coffee. It’s absorbing things, but it’s hardly an active participant.

Active Noise Control, on the other hand, needs a power source, computers, mind boggling algorithms, sensors and speakers to put the smack down on noise. Let’s say you were being harassed by an awful tuba player. If you wanted to use passive noise control to get back at them, you’d spray them down with acoustic foam or cover him with a rubber blanket. For active noise control you’d place a microphone in the bell of his tuba, plug it into some sort of computing device with an amplifier, and set up a speaker between you and the tuba. By playing his tuba noise through your speaker at the same volume (with a slight alteration) it would actually make him quieter, no add to the cacophony. This is just a theoretical example but it’s pretty wild right?

To understand the mechanics behind this craziness we have to get into what sound actually is. Sound is vibration that propagates through a medium like air or water, and in the form of a wave. Everyone is familiar with sound waves, and graphs like the one above, but they are kind of misleading. Unlike waves in the water, there is no sound “surface” that would make that wave shape you probably visualize in your head. 

Destructive Interference

The sound wave graph is actually plotting the pressure of the medium at a specific point (like on the diaphragm of a microphone). The peak of the wave is the maximum pressure, the trough is the minimum pressure, while the middle is regular boring atmospheric pressure. Where the air particles are squished together, the pressure is high. Where the air particles are spread out, the pressure is low. You’ll notice the air particles aren’t traveling with the wave, they are just oscillating back and forth. This is a good thing, otherwise every sound you hear would be accompanied by a blast of air going about 770 mph.  

The amplitude is the height of the wave and the depth of the trough. The length of the wave is, you guessed it, the wavelength. The wavelength is directly related to frequency. Since sound waves move at a constant speed, having shorter wavelengths will result in more waves crashing into your ear drums ie. a higher frequency. Long wavelengths result in fewer crashes or a lower frequency.  

Another important component is the phase, which is the position of the wave. This is measured in degrees. It’s a little bit confusing but the video below explains it. Just like when you turn 180 degrees you are facing in the opposite direction; two waves 180 degrees out of phase are opposite one another. Think of it as two congruent waves, and now flipped one around the horizontal axis

If you want to reduce this wave without just absorbing it with some foam you can use something called destructive interference — i.e. two waves 180 degrees out of phase. This is adding two waves so the peaks line up with the troughs. So you throw in some high pressure where the wave has low pressure, and low pressure where the wave has high pressure. If the waves have the same frequency and amplitude the energy will cancel out. 

Think of it like standing on a boat bobbing in the waves. When the boat goes up you bend your knees and you extend them when the boat dips down. If you do this dance at the same frequency as the boat’s bobbing, your body will be steady and you won’t spill your drink. The waves are canceled out. When you do the opposite, bending your knees when the boat dips and extending them when the boat pitches up you are adding the energy of the waves and your legs, performing constructive interference which will launch you into the water. If your legs are extending and contracting at a different frequency as the boat you’d probably look ridiculous and fall over. 

Here are a couple cool video examples. The first one shows destructive interference. Two waves of the same amplitude but 180 degrees out of phase collide with each other. At the point of collision they cancel each other out and the cup is spared. 

 

This one shows constructive interference. The waves are in phase, so when they collide their energy is added together and the cup gets smacked in the face. 

Here’s another example using water. The balls bouncing in the water are making waves at the same amplitude and frequency. In some places the waves add together, while there are bands where the water is perfectly smooth even though there are waves moving all around it. That’s constructive and destructive interference. 

How Does ANC Work In Cars?

Scientists have been experimenting with using destructive interference to control noise since the 1930’s. In the late 50’s the first patent was issued for using ANC in the aerospace industry for use in helicopter and aircraft cockpits. The first headphone was developed in 1957 which you can see below. This development probably started in the aerospace industry because aircraft can be pretty loud, and pilots need to be in these loud environments for long periods of time without going crazy. 

Your standard hearing protection earmuffs don’t do a whole lot below 500 Hz, which is true of many passive noise control treatments such as noise absorbing foam. Thankfully this is where ANC really shines. A wavelength at 500 Hz is just over 2 feet long. As you can imagine, when the wavelength is longer it’s much easier to record, process, and play back the noise at the precise moment so it is out of phase with the source sound.  It also helps when the noise is periodic, or repetitive like the thumping of helicopter blades or the firing of an engine.

If you’ve ever worn noise canceling headphones on an airplane, you know they are lifesavers for eliminating that constant droning sound. It’s not surprising that the first consumer noise canceling headphones were designed in 1978 by a guy on a flight who was annoyed he couldn’t hear his music. His name was Dr. Amar Bose. Luckily for us, this Bose guy was a pretty big deal in the headphone world. After 11 years in development, the first model you could buy hit the shelves in 1989. 

You can see how the principle works in cars via the image below. A reference microphone records the ambient noise. This gets processed and filtered with additional feedback from an error or feedback microphone that records what the user can hear. An inverted wave, 180 degrees out of phase to the ambient sound wave is played by the speaker at exactly the right time so it lines up and cancels out the ambient noise. 

To use an imperfect baseball analogy, the pitcher is the noise source, the catcher is the ANC system and you are the umpire. The pitcher throws his noise ball at you which is blocked or canceled by the catcher. In a vehicle things get a lot more difficult than in headphones. For headphones, the microphones and speaker are in line with each other, right next to your ear, directly in the sound path. In a car, the feedback microphones are in the headliner a few feet behind your head while the source microphones are several feet in front of you by the wheel wells capturing road noise. The speakers that play the inverted noise signal are all over the cabin, some in front of you and some behind. Instead of standing behind the plate, the catcher is in the dugout and has to throw their mitt at the exact point above the plate where they predict the ball will be. Not only do they have to get the timing right, they need to put the right amount of spin on the mitt so the ball gets caught and doesn’t just bounce off and hit you in the head (phase matching). 

Obviously this is way more complicated. There are many noise sources that have to be effectively captured. The sources and the speakers that cancel the noise out are in different positions, varying distances from your head. With the razor precise timing needed, the signal response, processing time, and differences in time it takes the sound from the speakers to hit your ears all have to be accounted for. Because of this, automotive ANC will probably never get to the level of headphones, but it can still have a significant impact. 

But enough about stupid math and baseball! This is a car site! Where does Lotus come into this? In 1993 my friend. With that classic icon of British Motorsport, the Nissan Bluebird. Believe it or not, Lotus helped Nissan develop an advanced noise canceling system in the early 90s. The first installed in a production vehicle! When I think of Lotus around this time I think of a bunch of guys about to be sold off by GM, cranking out Elans in a hundred year old horse stable in the bucolic English countryside. Not a bunch of nerdy scientists doing math in an anechoic chamber. 

But really it makes perfect sense. Lotus is known for making consumer cars that handle amazing because they are super light. To make a car that is acceptable to the consumer, the cabin has to be a nice place to sit for extended periods of time. Of course with a sports car you want more engine noise and to be able to hear the tires working. What you don’t want is unbearable engine booming as some body panel acts like a subwoofer on the fritz, the basketball like “pung” of your tire resonating over every tar strip, or howling road noise as you drive over different surfaces. It doesn’t matter how beautifully your car handles if you get a headache after driving it for 20 minutes. You will love a car so much more if you can wring it out on the twisties, then drive 3 hours to the Valley to help Dom boost some VCRs in comfort, without feeling like you’re in a stripped out race car.

And that’s what ANC can do. Vehicles are packed to the brim with potential noise sources. To counteract this, automakers spend tons of time and money designing and testing the optimal noise control package. Unfortunately passive noise control systems add a lot of weight and cost. If you’ve ever picked up a stack of sound deadening material (like Dynamat) you know this. Engine balance shafts reduce vibration but add weight and sap away power. Automakers often lean toward softer suspension and subframe bushings to reduce vibration at the expense of handling. Mass dampers are literally large weights that are mounted to problem areas of the car. There’s a reason why people strip all this stuff out when building a race car. 

With an ANC system you are using a lot of parts already in the car, like the stereo system. Add a few microphones in the cabin and some mics and/or accelerometers by the wheels to measure the noise coming from the road:

Plug all these signals into a small computer along with engine speed from the CAN system:

Use that to send an output to your speakers:

And viola: Your car is quieter, and you can trade about a pound of sensors and computers for dozens of pounds of passive noise control treatments. I’ve read racing forums where people saved over 100 lbs stripping this stuff out of their car. I mean, look how happy that lady in the picture below is, driving around in her Genesis. ANC did that.

In the early 90s Lotus wasn’t just in the business of making lightweight enthusiast cars, they’ve been involved in racing and F1 for decades, which is where all the automotive mad scientists do their magic. ANC was actually borne out of their work on active suspension for their formula one cars. There’s a fascinating two part interview with Colin Peachy, one of the head engineers at Lotus Engineering; it’s worth the read. I’ll quote an important section here:

Having spent years using conventional NVH to knock off the odd dB here and there, suddenly taking 10dB off the dominant engine harmonic right across the speed range is pretty mindblowing.

The Formula 1 approach was really about using active control and active feedback on the suspension systems. The idea is that you have a feed forward signal for a fast system response and a feedback signal error signal to constantly adapt the accuracy of the feedforward performance. Once you actually crack that you then think: “What else can I do with this?” I can use it to minimize the vibrations coming through a suspension system into the car to actually control the ride and handling performance. We can also use this to work on noise or any engine vibration and a whole range of things. So once you’ve delivered the basic system there are a whole lot of things you can do with it. Noise was quite an obvious thing to go for because generally there are loudspeakers already in the car, because you have an audio system, so you’re using hardware that’s already there. If you can then use that system to cancel noise then you get a cost effective benefit.

If you read the full interview you see Lotus was really ahead of its time. The company was thinking of other possibilities for this technology, like engine sound enhancement, or creating acoustic character for electric vehicles. Unfortunately being ahead of your time doesn’t always work out right away. In the ’90s electric vehicles didn’t really exist,  and automakers didn’t care that much about NVH or saving weight. After the Nissan Bluebird, the technology sat on the shelf for a while. 

Now there’s much more concern for saving weight to maximize efficiency, and greater emphasis on a more refined driving experience. Today a surprising number of vehicles use ANC. It isn’t a magic bullet since it has a limited frequency range and can’t block out everything, but it can have a major effect as shown in the frequency graph below. The blue trace is with ANC off, while the yellow is ANC on. You can see it takes out a significant chunk of the noise when it’s on. As the Lotus engineer said, it can be a struggle just to knock bad noise down a couple decibels. ANC is cutting up to 10 dB off certain frequencies which is huge.  

Last year Bose did a study with Ricardo where it bought three cars and benchmarked them for noise, weight, and handling. They then built them multiple ways so see how much cost and weight they could save, and they looked into improving handling by using stiffer bushings. On some of the vehicles Ricardo saved over 30 lbs and up to $130 in cost, with improvements in handling tests. This is pretty big in a time when automakers are trying to save every penny and pound they can. I think right now most companies are using ANC as a cheap way to mask noise problems. But as more start planning for ANC early in the design process, these benefits will become much more substantial. 

There is a lot of work being done in optimizing the number and placement of the microphones and accelerometers that provide feedback to the system, as well as refining the mind boggling algorithms that do all these calculations. I tried reading some academic papers on these studies and they were talking about particle swarm optimization and ant colony algorithms. My brain started leaking out of my head. Since it only takes 0.007 seconds for sound to travel from your tires to your ears, it requires smart people to figure this stuff out.

I used to test these systems when I was doing the NVH benchmarking for one of the major Japanese automakers. We would do the usual instrumented tests at their proving grounds; acceleration and deceleration tests, steady speed cruise tests, idle tests, as well as driving over different calibrated track sections that mimicked specific types of road surfaces. In the cars we couldn’t turn ANC off manually (98% of them) we’d pull any and all fuses related to the audio system and re-run the tests. The effect was definitely noticeable, and some cars had annoying NVH problems that ANC masked perfectly. Fixing these problems with passive noise control would take lots of work adding heavy treatments or expensive tooling changes.

Common complaints about modern cars are that they’ve become too heavy, expensive, and provide a numb, isolated driving experience. I’m hoping someday automakers will start using ANC in the spirit of Lotus. Not just to get rid of annoying noise and save some money, but to make cars lighter and more fun to drive.