Exhaust Noise: How Much is Too Much?

Exhaust Noise: How Much is Too Much?

Exhaust noise sells mufflers and pipes. Exhaust noise can be irritating, as when an original equipment muffler or pipe rots and blows out. Or, it can be pleasing in the case of a high performance vehicle with a throaty growl when it accelerates. Either way, noise generates exhaust work and parts sales.

How people react to noise depends on a number of factors. They say that introverts are less tolerant of noise than people who are extroverts. Unwanted sound from a vehicle’s exhaust system can be very irritating, especially if the sound frequency falls with certain ranges or is extremely loud.

Sound is nothing more than the compression and expansion of air waves. The repetitive nature of the exhaust pulses from the engine create pressure waves that travel through the exhaust system and eventually exit out the tailpipe. The frequency of these sound waves is measured in Hertz (Hz), which is the number of cycles that occur per second. The greater the frequency, the higher the number of cycles per seconds (Hz) and the higher the perceived pitch of the sound.

The average adult with good hearing can usually hear sound frequencies between 20 to 20,000 Hz. A child with good hearing can hear higher frequencies up to 40,000 Hz. According to studies that have been done on how people react to various sound frequencies, those between 2,000 and 8,000 Hz are typically considered to be the most annoying. Those below 500 Hz or above 10,000 Hz are usually considered less annoying. But there are exceptions. The high pitched buzz saw screech from a sport compact with a four cylinder engine and a straight-through muffler with a sewer pipe opening can be very annoying to other motorists.

A V8 engine produces four exhaust pulses per revolution, a V6 or straight six produces three, and a four cylinder produces two. At 2,200 rpm, which is a typical highway cruising speed, a V8 engine is spinning 26 times a second and is producing an frequency of 104 Hz. Such low frequency tones can easily penetrate the passenger compartment and create harmonics in the entire exhaust system, resulting in a droning, moaning or booming noise.

Low frequency tones also carry further distances than higher frequencies, and are harder to dampen inside the exhaust system.

Another factor that bears on how people perceive noise is the intensity of the sound. Louder is the same as cranking up the volume on a stereo. Louder sounds have more of an impact on our ears than softer or muffled sounds.

Sound levels are measured in units called decibels (dB). The decibel scale is an exponential scale rather than a linear scale. This means that the intensity or energy imparted by the sound waves doubles for every 3 dB in the sound measurement.

The decibel level of the exhaust is important because of how we perceive it, and its effect on us psychologically and physically. If you’re driving a hot muscle car, you want it to sound powerful. On the other hand, if you’re driving a $65,000 Lexus, Mercedes or BMW 7-Series, quiet is synonymous with quality. And if you’re stuck behind the wheel of a 20 year-old beater, noise is the least of your worries. Noise levels are important because too many decibels can damage our hearing. Noise induced hearing loss can occur with prolonged exposure to loud noise, or even short term exposure to loud sounds. Any prolonged noise over 85 dB is considered to be potentially dangerous.

The Occupational Safety and Health Act (OSHA) says protective ear equipment is required for continuous exposure to sounds of 85 dB or higher. And the louder the noise, the shorter the acceptable duration of exposure per work day. For sounds over 100 dB, exposure should be limited to less than two hours per day. Normal conversation produces sound in the range of 45 to 60 decibels. City traffic can produce sound in the 70 to 85 dB range, unless a trucker is using his “Jake Brake” (exhaust brake) to slow down his rig, which can push the dB levels considerably higher (one truck, even under normal driving conditions, typically makes as much noise as 10 passenger cars!).

The pain threshold for loud noise is around 130 dB, with permanent hearing loss occurring at levels of 140 dB or higher. If your ears are ringing after being subjected to a loud noise, the noise was too loud! Some studies say that continuous noise levels as low as 65 to 70 dB can interfere with normal thinking in some people!

To reduce unwanted noise from vehicle exhaust systems, many states and municipalities have noise regulations that specify certain decibel limits for motor vehicles. In Ohio, the noise limit for passenger cars traveling at speeds of less than 35 mph is 70 dB. For speeds higher than 35 mph, the noise limit is 79 dB.

In California, aftermarket mufflers must not exceed a 95 dB limit. The measurement must be taken 20 inches from the exhaust pipe with the engine between 3,000 and 5,000 rpm in neutral (no load).

What’s more, police officers can write “fix-it” citations if they feel the exhaust is unusually loud, offensive or “unusual” even if the officer doesn’t have a decibel meter to measure the sound level.

The ideal muffler is one that quiets exhaust noise without increasing power-robbing backpressure. A well-designed muffler can do both, but there is usually some sacrifice in noise control to reduce backpressure.

On the 2006 Corvette, Chevy engineers came up with a novel approach to have it both ways. The “bi-mode” mufflers at the back of the car have vacuum-actuated outlet valves that control noise and backpressure. From idle up to about 3,500 rpm, the bi-mode valves remain closed, forcing all of the exhaust to flow through the baffles and chambers inside the mufflers.

Above 3,500 rpm, the valves open up giving the exhaust essentially a straight shot through the mufflers and out the tailpipes. Backpressure drops, power is not compromised, and the increased roar of the exhaust lets everybody know this car means business. That is just one of the tricks the Corvette Z06 used to make 505 horsepower from its 7-liter (427 cu. in.) LS-7 V8 engine.

Several aftermarket companies have come out with performance replacement mufflers and exhaust systems for the Z06 that provide even less restriction while still retaining the original bi-mode vacuum motors and control valves.

Stock and aftermarket mufflers use a variety of technologies to control noise. Most original equipment stock mufflers are “tuned” for a specific vehicle application and engine. Unlike “universal fit” aftermarket mufflers that have to fit a wide range of makes and models, OEM mufflers can be fine-tuned to eliminate as much noise as possible as well as harmonic sounds that can reverberate throughout the entire exhaust system.

Mufflers can be tuned by varying the size and length of the muffler itself, the size and length of the perforated tubes inside the muffler, the shape and volume of the chambers inside the muffler, by changing the way the exhaust flows through the muffler, and by adding sound-absorbing materials inside the muffler. Many stock mufflers force the exhaust to flow in a serpentine route through a series of tubes and baffles. Perforated tubes and baffles help dissipate the pressure waves in the exhaust, reducing their energy and volume. The typical OEM muffler contains three perforated tubes separated and supported by plates. As the exhaust enters the muffler, some of the pressure pulses spread outward through the perforations and encounter pressure pulses from the other pipes. All of these sound waves bouncing back and forth tends to cancel out a lot of noise. Less expensive “economy” mufflers may only contain two internal tubes, and are not as efficient at suppressing noise.

The ends of the muffler shell also help reflect sound waves back and forth inside the muffler to create “destructive interference.” The collision of the sound waves breaks up the pressure pulses and dampens the noise. The inside of the muffler may also contain long continuous strands of fiberglass called “roving” for additional sound absorption. Some OEM mufflers use a mineral fiber called “basalt wool” instead of fiberglass.

In really quiet mufflers, one end of the muffler may contain a dead-end chamber called a “Helmholtz chamber.” This chamber allows the exhaust to expand and contract, sort of like an air spring. This helps dampen resonant frequencies that can make the exhaust boom or moan.

To further fine-tune the exhaust, many vehicles have a second muffler called a “resonator” that dampens noise frequencies that made it through the main muffler. The honeycomb inside the catalytic converter also helps dissipate the exhaust pulses and reduce overall sound levels, and acts like an additional resonator in the system.

Back in the 1970s, turbo-style mufflers appeared that reduced backpressure by allowing exhaust gases to follow an “S-shaped” path through the muffler. Later designs added “flow straighteners” (curved pieces of metal) to help redirect the exhaust flow from one tube to the next so the exhaust would flow more smoothly as it changed direction. This reduced backpressure without reducing sound control.

“Straight-through” glasspack or bullet-style mufflers have been around for ages, and are are essentially a perforated tube surrounded by sound absorbing material (fiberglass or steel wool).

This type of muffler produces little backpressure, but also allows more pressure waves and noise to pass through unabated. When fiberglass is used as the packing material, the strands must be long enough so they don’t break down and blow out the tailpipe.

Recently, universal fit “sewer pipe” mufflers for sport compact cars have been a popular add-on to reduce backpressure and to give the car a more aggressive appearance. Most of these mufflers provide minimal sound control and are not tuned to the frequencies produced by four-cylinder engines. Many are stainless steel for appearance and added durability, but they are not on the same level as a high-end performance exhaust system and muffler that has been designed and tuned for a specific vehicle application.

Whether a customer has a stock exhaust system or a performance muffler on their vehicle, you should always look, listen and feel for exhaust problems anytime a vehicle is in for service. If you hear exhaust noise from a vehicle that shouldn’t be making any noise, check it out. Inspect the exhaust system from the manifold to the tailpipe.

Mufflers typically rust from the inside out. Sulfur and hydrocarbon byproducts combine with moisture in the exhaust to form corrosive acids that eat away at the inside of the muffler. The further the muffler is from the engine, the cooler it runs and the more internal condensation it accumulates. This increases the rate of corrosion and shortens the life of the muffler. Short trip driving is especially hard on the muffler because it may not get hot enough to evaporate the internal moisture.

Checking the exhaust for leaks is not only good for noise control, but also for the safety of the vehicle’s driver and occupants. Carbon monoxide is a colorless, odorless pollutant in the exhaust that can make people dizzy, pass out or even kill them before they realize what’s happening. Exhaust leaks can be especially dangerous in cold weather when a vehicle is not moving. So advice your customer to have their exhaust leaks repaired ASAP if any leaks are found.

Though most customers will want a stock equivalent replacement muffler if their original muffler has blown out, others may be willing to pay extra for a performance muffler that offers less backpressure for better fuel economy and performance. With gas and diesel fuel selling for more than $3 a gallon in most parts of the country, any improvement in fuel economy helps.

The best prospects for performance mufflers and exhaust systems are people who drive sporty cars, performance cars, and trucks or SUVs that may be used for towing. Turbocharged and supercharged gasoline and diesel engines respond even better to reductions in backpressure than naturally aspirated engines because of the higher volume of exhaust they product under load.

As for noise, it’s a matter of personal taste. As we said at the beginning of this article, some people want dead silence and others want to let the whole world know they are coming. The nice thing about the latter type of customer is that they will replace a perfectly good muffler with an aftermarket muffler to “uncork” their exhaust. Just make sure the muffler meets local noise ordinances so your customer doesn’t attract unwanted attention from the police.

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