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# project2doc - Mechanical Engineering at Michigan Tech

By Lauren Jones,2014-12-23 20:48
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project2doc - Mechanical Engineering at Michigan Tech

Data Analysis:

Background test

90

80

70

60

50

dB40

30

20

10

0

20

401/3 octave band

80

160

Sound Pressure Level Results

Location(64” to the center of 315

the law mow) Weighting no muffler stock original new

630exhause side A*(dBA) 98.6 90.5 90.7 91.5

Lin*(dB) 102.6 96.5 98.6 99.8 1.25 kright side A*(dBA) 92.6 90.5 90.7 91.5

Lin*(dB) 99.1 96.5 98.6 99.8 2.5 kfront A*(dBA) 95.9 89.4 89.2 89.7

Lin*(dB) 100.6 95.3 96.1 97.4 5 k

back A*(dBA) 91.4 86.6 86.9 87.6

10 k Lin*(dB) 97.4 91.9 94.9 96.6 above A*(dBA) 95.3 88.2 88.6 88.5

Lin*(dB) 99.3 94.4 95.5 96.4 average SPL @64" A*(dBA) 95.5 89.3 89.4 90.0

Lin*(dB) 100.2 95.2 97.0 98.3

Background noise concern:

The background noise A weighting overall level is 61.6 dBA, linear weighting overall

level is 83.4 dB. And the sound pressure level results for each location are over 10dB

larger than the background noise. So the noise effects can be ignored here. And for dBA

measurement, we can not really see the different between these three mufflers. They are

really close to each other.

We made the 1/3 octave band analysis for each muffler at 5 locations. Calculate the average overall level . we make the average for each frequency at five locations.

N10.1*LpiL10log(*10)10 pavgN1i

N = Number of locations here it is 5 locations.

Lpi = SPL at location i

Then calculate the A weighting and linear weighting over all level .

From figure , we can see that when there is no muffler, the curve for sound pressure level is above the other three curves. The new muffler works better in low frequency, unfortunately, man’s ears are not sensitive to the low frequency noises. The stock muffler works better around 125 Hz. At around 200 Hz, the sound pressure level for new muffler is larger than that without muffler .The possible reason is that the new muffler might work as an amplifier for that frequency. There are some air resonance happening in that frequency range.

Average 1/3 Octave @ 64"

100

95

90

85No Muffler

Stock80OriginalSPL (dB)New75

70

65

60

20

31.5

Frequency (Hz)50

80 125

200Average Insertion loss calculation:

315

500Insertion loss=Lp(without muffler)- Lp(with muffler) avgavg

800

1.25 k

2 k

3.15 k

5 k

8 k

Further, we can see that stock muffler works best among the other mufflers. The original muffler is the second and the new muffler is the third. In low frequencies, the new muffler works better, then the original , and the stock muffler is the third. We can see that around 200 Hz for new muffer and around 150 Hz for original muffler, the insertion loss is negative. The reason is the same as it was talked about in sound pressure level around the same frequency range.

Insertion Loss Average @ 64"

20

15

10

Stock

Original5

New

Insertion loss (dB)0

20

31.5-5

50

80-10

125Frequency (Hz)

200

315

Sound power calculation: 500

800

For a Free Field: 1.25 k2A = Area = 4πR for a point source 2 k;；LL10logAie. A = Area over which the average SPL is taken wp103.15 k

Here we will be using a hemi-spherical surface with a radius, R = 1.626 m (64”). 5 k

8 kSince, R = 1.626 m 22 A = 2πR = m

10log(A) = 12.2dB so, 10

LL12.2dB wp

From the sound power level plots, we can see the same trend as the SPL. The sound power level without muffler is above the one with mufflers. And for frequency range between 800 Hz and 5K, we can not see the difference for the three mufflers. For new muffler around 200 Hz, the new muffler has some problems.

Sound Power Levels

110

105

100

95No muffler

Stock90Original

New85

Sound Power (dB)80

75

70

20

31.5

Frequency (Hz)50

80 125Loudness Calculation: 200

315Loudness Results:

500 No muffler Stock Original New

Sones 213.27 107.05 118.92 125.31 800equal to 3150 1.25 kHz (dB) 101.5 92.3 94 94.8 2 kApplying Mark IIV, we can get the loudness equivalent to 3150 Hz shown in table. From

3.15 kthe table we can see that all the muffler did work well to reduce the noise. It shows that

5 kthe stock muffler works better than the original muffler and new muffler. The difference

8 kbetween original muffler and new muffler is only about 1 dB.

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