Another thing I hate about traveling on the freeway, beside the traffic jam, is the cabin noise within my 2003 Honda Accord (about 70 dB at 65 mph). A long afternoon ride from a hard day at work requires a moment of quietness. Maybe I may have to trade in my car for the new 2011 Ford Fiesta. Ford engineers not only designed a fuel efficient car but also achieved low cabin noise as well.
Ford engineers measured the noise level inside the 2011 Ford Fiesta in a wind tunnel at 80 mph on the highway. They recorded the cabin noise inside the 2011 Ford Fiesta at 25.4 sones, a unit of perceived loudness (very subjective to individual judgment). In comparison with the Honda Fit and Toyota Prius, both of these cars received a 27.6 and 26.2, respectively.
You can think of one sone is equivalent to 40 decibels at 1kHz, a unit of sound pressure level. If I had to approximate (very rough estimation), 1 sone = 40 dB at 1kHz. And we add three decibels every time we double the amount of sound, we would get roughly the following:
1 sone = 40 dB
2 sones = 43 dB
3 sones = 44.7 dB
4 sones = 46 dB
A phon is more closely related to a decibel, which a phon is also a unit of subjective sound loudness. You can convert between phon and sone. If you estimate that decibel and phon units are the same, you can convert between decibel and sone. It is also important to note that decibels, being a logarithmic measurement as you see in the formula below, means that the perceived loudness of 20 decibels is twice that of 10 decibels.
Here is a formula if you want to try it yourself:
a sound of x sones = 10 log[sub]2[/sub] x + 40 phons,
a sound of x phons has loudness 2[sup](x – 40)/10[/sup] sones
To give a better picture of noise levels, here are a few examples of noise level scenarios: your quiet living room is about 22 decibels, a jack hammer running is about 110 decibels, the noise on the freeway is about 90 decibel, your high-end computer runs at a full load with all the fans is about 55 decibels, and your car alarm is about 125 decibels.
Usually smaller cars have higher cabin noise than bigger cars due less material used to dampen the noise.
“A smaller car can pose challenges for noise and vibration control. You have smaller spaces to work with, and there are always cost and material concerns to keep the car affordable,” Pintar said. “With Fiesta, we started with an unusually rigid structure that helps prevent rattles and then enhanced the global noise and vibration package to harmonize with a revised chassis, tire tuning and the revolutionary PowerShift automatic transmission – a key factor in allowing Fiesta to deliver up to a projected 40 highway miles per gallon. Small-car buyers don’t need to compromise a quiet cabin to enjoy top fuel economy.”
Here are a few things that Ford engineers used to help reduce the cabin noise in the Ford Fiesta:
• Acoustic-laminated windshield, reducing wind noise
• Wind tunnel-optimized side-view mirrors, reducing wind noise
• Stiffened door modules, reducing the potential for rattles
• Rear-mounted roof antenna for less wind noise and improved aerodynamics
• Wind noise-optimized grille
• Additional sound-absorbent material throughout the cabin for a quiet interior
• Revised door seals for reduced wind noise
To further achieve a quieter car cabin, the Ford Fiesta was built on a more stable B-car platform with high-strength steel. It also features a quiet PowerShift transmission. This transmission is based on hyper-efficient manual transmission technology. Not only it provides the convenience of a traditional automatic, but also improves the interior quietness in Ford Fiesta as well.