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Lucid Sound: Harmony in Motion

How Lucid is crafting the interior sound of the Lucid Air

Posted by Greg Goetchius, NVH Engineering Manager What is quality, and how does one tell if a product has it? Look behind a high-end piece of furniture, and you will see quality. You will see that the materials, joining techniques, and attention to detail are as well executed there as they are on the surfaces you can see. The extraordinary effort to make every last part of their creation beautiful, whether or not anyone sees it, demonstrates the designers’ passion for their creation. That is quality.

At Lucid, quality, luxury, and refinement are at the core of our mission to create the world’s most extraordinary automobile. In particular, the acoustic signature of our car plays a critical role in creating this sense of quality and luxury. We call this “NVH Engineering” (Noise and Vibration Harmony). We are passionate about achieving harmony in the way the car sounds and feels when in the hands of our customers. This is Lucid Sound.

How are we doing this? Like the craftsmanship that goes into high-end furniture, we labor relentlessly to sculpt even the smallest details into every part that goes into our car, from the shape and durometer of every last piece of rubber, to the thickness of the glass, the stiffness of the body structure, and the composition and thickness of the carpet, and in countless other visible or hidden places. Behind all of the finished surfaces in a Lucid Air, you will find evidence of the labor to instill acoustic harmony and serenity into the car. This is acoustic craftsmanship. It is the passionate and relentless attention to detail where no one will ever see it. It is what creates quality.

Make no mistake, creating quality takes more than just attention to detail. The right technology is just as important. Here, Lucid is forging a new path. The Lucid Air will be one of the first production vehicles to use broadband active-noise-cancellation technology to reduce interior noise created by tire/road interactions but more on that a little later.

First, though, we need to talk about what we consider to be the main source of noise in an electric vehicle: tire/road noise. This emphasis on tire/road noise is borne out of the key difference in NVH behavior between a conventional automobile and an electric vehicle. The lack of engine noise in an electric vehicle acoustically reveals all of the other noises present in a car travelling down the road and normally drowned out by the sound of an engine. This effect is known as “acoustic masking,” and it is a concept that the Lucid NVH team has been intensely researching. To illustrate this, the chart below shows a plot of overall interior noise versus vehicle speed:

<p>Typical Noise Contribution Comparison</p>

Note that the overall noise is composed of three distinct sounds: powertrain noise, road noise, and wind noise. In a typical car with an internal combustion engine, interior noise at around-town speeds (less than 60 MPH) is dominated by engine noise. In an electric vehicle, the interior noise behavior is completely different, with tire/road noise dominating most of what is heard at speeds below 60 MPH, because the electric powertrain is much quieter than the internal combustion engine.

This is especially true for certain kinds of road surfaces. The chart shown above is specific to a common road surface we call “coarse road aggregate,” where the small stones in the road bed are actually exposed from the asphalt. A typical example is shown below:

<p>Structure-Borne Road-Noise Simulation</p>

A normal car rolling over this kind of road surface exhibits a booming/roaring type sound that can be quite loud, with the noise generally concentrated below 500 Hz. Our NVH team is designing the basic body structure and suspension system to minimize this type of noise, converting the road surface you see in the picture to a road profile spectrum that we can use as an input to our mathematical simulation models of the body structure and suspension. The alpha fleet of vehicles we have recently completed is giving valuable validation insight into the success of the decisions we have made using these simulation models.

However, we recognize that much more can be done to reduce tire/road noise in our car. As mentioned above, we are deploying an active-noise-cancellation technology to reduce tire/road noise in the Lucid Air. Active noise cancellation is a technique in which sound is artificially introduced into a noisy environment such that the synthesized sound wave reaching the ear is exactly identical to the sound wave of the unwanted noise but 180 degrees out of phase. When two identical signals out of phase are combined, they completely cancel each other, resulting in zero noise. The diagram below illustrates this, but in the case of a car, the noise source is not a speaker but rather the noise generated by the tires rolling over a coarse road surface.

The anti-noise signal in our car is generated by the audio system and uses multiple speakers to create quiet zones for all the occupants of the vehicle.

The concept is simple, but the application to a car interior is complicated and requires advanced technology and enormous development effort. In a real car, the noise from the tire/road surface is not a simple sine wave like that shown above but is highly random and unstructured. We call this “broadband noise.” The diagram below shows how much more complex a broadband signal is (left) compared to a simple sine wave (right).

An active-noise-cancellation system focused on simple sine waves for reducing engine noise is relatively easy to implement and has now become a feature in several newer cars to reduce engine noise. However, broadband noise cancellation is a whole different matter and is a much more difficult problem to solve.

Because of this level of complexity, the technology behind our active noise-cancellation system is highly advanced. It relies on an unprecedented number of sensors around the vehicle, up to eleven of the audio system’s speakers, multiple powerful digital-signal processors, advanced software developed specifically for broadband noise, and tight integration into the vehicle’s audio system. This ground-breaking technology finally unshackles the car from the compromise between great handling and high levels of noise plaguing motor cars for decades. We can now tune the suspension and tires to give our car incredibly dynamic responses and still achieve levels of tire/road noise unreachable by even the best luxury cars. Interestingly, since the active noise cancellation system only acts on the sound waves, the tactile feel of the tires and suspension interacting with the road surface will still be present, giving the car a unique sporty feel without the unpleasant roar from the road. There will be no other car in the world that can achieve this.

Beyond tire/road noise, Lucid Sound engineers are working closely with the aerodynamics and exterior styling teams to minimize wind noise. As the noise contribution charts above clearly show, wind noise is the dominant noise source at highway speeds and above, and because of the speed potential of the Lucid Air, we are putting tremendous effort to minimize this noise. In particular, we have implemented an advanced simulation tool called computational fluid dynamics (CFD) to calculate the complex airflow over, under, and around our car. We are able to use this data stream to predict the noise level inside of the vehicle and base important design considerations on these simulation models. The following video illustrates the complex flow field around a Lucid Air traveling at highway speed using CFD:

The results of these types of simulations are called noise spectrum plots and often are difficult to interpret just by examining them, even for experienced acoustics engineers.  The NVH team at Lucid has developed a method to listen to the result of these simulation models, months before any physical wind-tunnel test data is available, offering an intuitive approach to making better engineering decisions for the car while still in the design stage. In this example, we used the CFD model to predict the level of interior wind noise for three different kinds of glass in the car. Because the Lucid Air has a large part of its cabin made from glass, it is critical to engineer the glass properly to ensure the lowest possible levels of wind noise. Below is a typical noise spectrum plot that shows three noise spectra: one for standard tempered glass, one for normal safety laminated glass, and one for acoustic laminated glass. The noise plot shows that the acoustic laminated glass is the best overall, but the question is still, how much better is it really, and is it worth the effort to put it in the car? Now we can listen and say for sure:

The fact that the Lucid Air will be produced with much of its glass made from an acoustic laminate can be traced back to this very analysis. This is how we engineer the acoustic signature of the Lucid Air.

This is only a small glimpse of how the Lucid NVH team is working to create acoustic and vibration harmony in the Lucid Air, and we hope that this blog has given you some insight into the team’s ingenuity and passion. This team is an eclectic blend of electrical, mechanical, aerospace, and software engineers, several with musical backgrounds. They are channeling all of their technical knowledge, acoustical and musical intuition, and their deep passion for creating something truly special into the design of the Lucid Air. They are crafting the sounds and vibrations you will experience in this car with the same care as an artist creates art, or a composer creates music. This is Lucid Sound. It is the passionate and relentless attention to detail where no one will ever see it, but everyone will hear it. This is what creates quality. We invite you to reserve one and hear for yourself.