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.Ībove 3,500 rpm, the valves open up giving the exhaust essentially a straight shot through the mufflers and out the tailpipes. The “bi-mode” mufflers at the back of the car have vacuum-actuated outlet valves that control noise and backpressure. On the 2006 Corvette, Chevy engineers came up with a novel approach to have it both ways.

A well-designed muffler can do both, but there is usually some sacrifice in noise control to reduce backpressure. The ideal muffler is one that quiets exhaust noise without increasing power-robbing backpressure. 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 measurement must be taken 20 inches from the exhaust pipe with the engine between 3,000 and 5,000 rpm in neutral (no load). In California, aftermarket mufflers must not exceed a 95 dB limit. 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. 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. 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.Ī V8 engine produces four exhaust pulses per revolution, a V6 or straight six produces three, and a four cylinder produces two. Those below 500 Hz or above 10,000 Hz are usually considered less annoying.

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. A child with good hearing can hear higher frequencies up to 40,000 Hz. The average adult with good hearing can usually hear sound frequencies between 20 to 20,000 Hz. The greater the frequency, the higher the number of cycles per seconds (Hz) and the higher the perceived pitch of the sound.

The frequency of these sound waves is measured in Hertz (Hz), which is the number of cycles that occur per second. 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. Sound is nothing more than the compression and expansion of air waves. 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. They say that introverts are less tolerant of noise than people who are extroverts. How people react to noise depends on a number of factors.

Exhaust sound control