A Brief Tour of Automotive Sound Sources

Guest Contribution by Michael Hermes

Modern cars are boring. They’re safe, practical, and quiet to the point of being uninteresting. A passing sedan glides by with a gentle whoosh of air passing over the car and the tires on the pavement. It makes for a pleasant ride to work but a short article on sound.

Cars didn’t start out quiet, though. Decades of engineering and research have identified the potential noise sources on a vehicle and reduced them to almost nothing. The internal combustion engine-powered car has a vast array of potential noise sources, all of which are quantified and treated by Noise, Vibration, and Harshness (NVH) engineers.

The goal of this article to step through the various parts of the car and identify potential sources of noise. Being a gearhead is not required, but some familiarity with acoustics will be helpful. For the sake of brevity, each topic will be covered with broad strokes. All of these noise sources have been studied in-depth and each could warrant their own book.

For now, let’s set aside the ominously silent electric or hybrid vehicle. Their noise, or lack thereof, is an interesting topic for another day. This article deals with the tried and true internal combustion engine and the cars it powers. It could apply to the largest truck and the smallest motorcycle. Imagine a new, untreated prototype that has not yet been softened by committee or international noise regulations. It exists as a cacophony of mechanical noises to be identified and eliminated–or amplified.

It all begins with air. Engines, like people, need clean air to function. Air is drawn into the engine via the intake, a commonly overlooked source of noise. Depending on the engine and the intake design, it can be one of the loudest sources on an engine. It has two features any good noise source would possess: a narrow passage and airflow. Add a few valves and we’re halfway to a trumpet.

The sound of an intake depends greatly on its size as well as the engine it is feeding. It is not unlike the sound of an exhaust, though in most cases it will be a higher pitch than the exhaust tone. It has a low- to mid-range honk (or growl, if you prefer) that is usually masked by other sources but can be very distinct.

This tone can be capitalized on. If passenger cars are meant to be quiet, sports cars are designed to be just a touch louder. Not too loud, but loud enough to give the perception of performance and validate the owner’s choice of opting for the Sport Package. Some vehicles have, in the past, routed intake noise into the cabin to get that aforementioned sporty tone to the driver. Intake noise is closely coupled to throttle position, so in normal driving conditions it is unoffensive and not noticed. When a minivan needs to be passed, however, the intake can growl away as the sports car accelerates.

The air passes through the intake and is mixed with gasoline inside one of the engine’s cylinders. The gasoline has been riding along in the fuel tank until needed, at which pointed it is sent on its way via the fuel pump. The fuel pump can be the source of ear-piercing, obnoxious noise, especially on a motorcycle where it exists between the rider’s knees in the gas tank. A pump can be small and compact but in turn has to spin very, very fast. This results in piercing, tonal noise source and can be in the 6 to 1o kHz range. On most vehicles it’s been buried away and will be difficult to hear.

Prior to the intake there may be a method of forced induction, such as a turbocharger or a supercharger. These systems work on the principle that some air is good, but more air is better. Both use a compressor to cram more air into the cylinder, allowing for more power. Both usually add a certain amount of tonal whine. A turbocharger waste gate, or blow-off valve, can also add the interesting noise of a raccoon sneezing.

Both the gasoline and the air are allowed into the cylinder by an intake valve that opens briefly and then slams shut again. This happens very quickly, and even if one valve operating once isn’t a loud noise, many valves operating very quickly will create a lot more noise. There will be a minimum of one intake valve and one exhaust valve per cylinder, though often times there are more.

This is a good example of where tonal noise comes from. The actual event of a valve closing (or any impact) is a brief event of broad bandwidth. If the valve closes twice in one second, it’s operating at a frequency of 2 Hz. Three times a second, 3 Hz, etc. Gradually increase the frequency, and the sounds of the events get closer and closer to the point where they are indistinguishable from one another and instead sound like a constant hum.

Back to the air, though. The piston moves up in the cylinder and compresses the air-fuel mixture. The spark plug sparks and ignites the mixture. This sudden increase in pressure forces the piston back down. The exhaust valve opens up and the resulting gas mixture is pushed out by the the piston.

The four-stroke cycle of intake, compression, combustion, and exhaust (easily remembered as “suck, squish, bang, blow”), then concludes with exhaust gases leaving the combustion chamber and trundling out of the engine via the exhaust. The exhaust is one of the most prominent noise sources on a vehicle and one could spend their entire career on exhaust design and noise. Let’s put that on hold until the end though, and continue on by quantifying some of the less obvious sources of noise.

The piston is connected to the crankshaft by a rod; which is appropriately named the “connecting rod”. The Whole Point of the entire process up until now has been to turn the crankshaft. The up and down motion of the piston is converted into a rotating motion by the connecting rod. The connecting rod turns the crankshaft, and the crankshaft is hooked to… anything we want, really. Up until this point the engine is just an engine, not necessarily a car. The crankshaft can turn a big-ass alternator and the result is a generator that provides electrical power. It could turn a belt that drives a pump or a propellor on a boat. In one of my earlier jobs I worked in the parts department at a Chevrolet dealership that sold crate engines directly to the consumer. Many of the big-block V8s we sold did not go into cars, but rather into boats. Really, really fast boats.

On a car, however, the crankshaft is connected to the transmission. The transmission transmits power from the engine to the driveshaft and eventually the wheels. A transmission is a case full of gears. Ask a person to imagine a gear in their head and he or she will most likely imagine the spur gear, also known as the straight cut gear. It consists of a disk with teeth around its diameter that engage teeth of other gears to continue mechanical motion.

Gears are very agreeable, predictable sources of noise. Multiply the number of teeth on a gear by the speed at which it is rotating and the result is the “gear mesh frequency”. This mostly tonal noise will manifest itself as a whine or hum. Like the previous example, gear noise is the result of one small event happening very quickly and is therefore perceived as a constant tone.

A transmission is not full of spur gears, however, but rather helical gears. Helical gears use curved splines which allow for smoother engagement and thus quieter operation. Most transmissions use helical gears and are therefore very quiet. The exception to this is the reverse gear, which often uses a spur gear. This explains the familiar whine that accompanies some vehicles as they back up.

The business end of the crankshaft connects to the transmission, but before moving on it should be noted that the front end gets put to work, too. The front of the crankshaft is connected to a pulley that drives a very important belt called serpentine belt. This belt powers the car’s accessories, such as the alternator that charges the battery and the air conditioning. Belts are generally quiet except for when they are starting to wear and lose their ability to grip, and then they are exceptionally loud. Especially when they’re wet. If you’ve ever pulled up to a car at a stop light and been greeted with a horrific screeching noise, it’s a worn or slipping serpentine belt. It’s a very distinct noise that only belts and cats in heat can make.

The vehicle in question is a rear wheel drive car, so the transmission turns the drive shaft, a long, metal rod that runs to the back of the car and is the reason there is a hump between the rear passenger seats. The drive shaft is connected to the differential (more gears!) which takes the mechanical movement of the drive shaft and sends it to the individual wheels.

Attached to the wheel hubs are brakes. Brakes can create all manner of horrific, terrible noises. Woe to the NVH engineer who has to test and diagnose brake noise. The only way to diagnose an unwanted noise is to find a way to repeat it and then listen to it over and over and over and over again. (But I’m not bitter.) The noises can range from a low groan to the most delightfully ear-piercing tones in the high range of 10 to 15 kHz.

The final destination of all this work is the tires. Tire noise is a function of the tread pattern on the tire and vehicle speed. The quietest tire would be the racing slick, but they have a nasty habit of hydroplaning on water. The tread pattern molded into the tire has many facets that individually come into contact with the pavement. This is yet another tonal source of noise that manifests itself as a hum.

Now that the vehicle has been covered from air to mechanical motion, it’s time to revisit the exhaust. The combustion event can be pretty loud, and a lot of combustion events happen in a small amount of time.

Engine speed refers to how fast the crankshaft is rotating. It’s often referred to as revolutions per minute, or rpm. A normal passenger car will idle at about 1000 rpm and be restricted to probably 5000 or 6000 rpm. A 2013 Formula One engine turned at a blistering 18,000 rpm.

The fundamental of exhaust noise can be approximated by converting rpm to Hertz and then multiplying by the number of cylinders. Divide 18,000 rpm by 60 seconds and the result is 300 Hz. Combustion occurs every other crankshaft rotation (150 Hz), but also once for each of the eight cylinders: 1200 Hz. That’s the starting point for the exhaust note of a Formula One car. There will be many other tones and harmonics present, but 1200 Hz will be prominent. 2014 saw a lower engine speed and two less cylinders, resulting in some unhappy Formula One fans. The signature screaming exhaust tone was slightly neutered due to the reduced engine speed and cylinder count. (15,000 rpm and 6, respectively.)

Now consider the loping, rumbling Harley-Davidson exhaust tone. The pistons are much larger and therefore are limited on how quickly they can move back and forth. The 96 cubic inch H-D engine has almost the same displacement as the F1 engine but using only two cylinders. Assume a cruising engine speed of about 3000 rpm, or 50 Hz. Dividing by two for combustion and multiplying by two cylinders gets us back to 50 Hz, and a good indicator of where the low-frequency rumble of those vehicles comes from.

A car exhaust can be a simple tube, otherwise known as “straight pipes”. This setup is very, very loud. Deafeningly loud. Obnoxiously loud. The other end of the spectrum is the fully treated passenger car exhaust. A combination of helmholtz resonators, expansion cans, tortuous muffler paths, and fiberglass treatments can knock almost all of the exhaust noise down to nothing. The sports car exists somewhere in between. Some exhaust systems use variable butterfly valves to allow the vehicle to (ostensibly) perform better and sound a little tougher when desired while still allowing for quiet cruising under normal driving conditions.

Most countries have regulations that specify how loud vehicles can be under specific driving conditions. Europe leads the way with the most strict regulation, followed by Japan. The U.S. is a bit more lax. In the U.S. these regulations only apply to Original Equipment Manufacturers, however, and the sky’s the limit on aftermarket exhausts and how loud they can be.

Each of the noise sources listed above is one small piece of an orchestra that creates a unique signature sound for any vehicle. Again, the sources listed above are just the tip of the iceberg, but hopefully this helps the acoustically-minded individual understand the myriad of challenges and noise sources that modern cars can present.

Michael Hermes is a NVH Test Engineer with nine years of experience in automotive and engine applications. His work includes noise and vibration testing and analysis, sound quality, and materials testing for architectural acoustics. He also enjoys editing audio for podcasts, mixing and mastering music, and recording ambient spaces.