intro-7-noise-source.pdf

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NVH Basics (Intro-7-Noise Sources) Page 1 Author: D. Griffiths Rev. 03/25/2011 1 7. Sources of Radiated Noise 7.0 Introduction As previously discussed, sources for customer perceived noise and vibration can be divided into force sources (structure borne noise) and airborne noise sources. Usually structure- borne noise dominates low frequency NVH and airborne noise dominates high frequency NVH. While the balance for a given vehicle depends on design details the overall situation is roughly as shown in the figure below: Frequency Hz Figure 5.0.1 Approximate % Structure-borne & Airborne NVH vs. Frequency 7.1 Major Sources The major sources are: Powerplant -Engine (top, sides, oilpan), Transmission Tires Axle Tires Induction System Tailpipe 7.2 Powerplant Noise 7.2.1 Measurement Powerplant radiated noise is generally parameterized by the sound measured one meter from the powerplant when the powerplant is run in a hemi-anechoic dynamometer cell. Most quoted test results are of this type and correspond to variants of the standard SAE J1074 test procedure. 7.2.2 Typical Powerplant Radiated Noise Level The total radiated sound power of a powerplant is typically between 85 and 95 dB at 3000 rpm, and increases at between 5 and 10 dB per 1000 rpm. The sound power also increases somewhat with load. There are standard procedures for measuring powerplant sound power; sound power is roughly equal to the average sound pressure level one meter from the powerplant. Structure-borne Airborne 50 125 20 315 800 2000 5000 20 100% 0%

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Page 1: Intro-7-noise-source.pdf

NVH Basics (Intro-7-Noise Sources) Page 1

Author: D. Griffiths Rev. 03/25/2011

1

7. Sources of Radiated Noise

7.0 Introduction As previously discussed, sources for customer perceived noise and vibration can be divided into force sources (structure borne noise) and airborne noise sources. Usually structure-borne noise dominates low frequency NVH and airborne noise dominates high frequency NVH. While the balance for a given vehicle depends on design details the overall situation is roughly as shown in the figure below:

Frequency Hz Figure 5.0.1 Approximate % Structure-borne & Airborne NVH vs. Frequency

7.1 Major Sources The major sources are: • Powerplant -Engine (top, sides, oilpan), Transmission • Tires • Axle • Tires • Induction System • Tailpipe

7.2 Powerplant Noise 7.2.1 Measurement Powerplant radiated noise is generally parameterized by the sound measured one

meter from the powerplant when the powerplant is run in a hemi-anechoic dynamometer cell. Most quoted test results are of this type and correspond to variants of the standard SAE J1074 test procedure.

7.2.2 Typical Powerplant Radiated Noise Level The total radiated sound power of a powerplant is typically between 85 and 95 dB

at 3000 rpm, and increases at between 5 and 10 dB per 1000 rpm. The sound power also increases somewhat with load. There are standard procedures for

measuring powerplant sound power; sound power is roughly equal to the average sound pressure level one meter from the powerplant.

Structure-borne

Airborne

50 125

20

315 800 2000 5000 20

100%

0%

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NVH Basics (Intro-7-Noise Sources) Page 2

Author: D. Griffiths Rev. 03/25/2011

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7.2.3 Typical Powerplant Noise Spectrum The radiated sound spectrum (1/3 octave) typically peaks at around 1000 Hz as

shown in Figure 7.2.3.

Figure 7.2.3: Typical Powerplant Radiated Noise Spectrum at 1 m

7.3 Induction Noise 7.3.1 Measurement

The typical practice for measuring the induction noise is to place a microphone on the axis of the intake system, about 150 mm from the intake, with the hood open.

7.3.2 Spectrum

The ideal induction system would produce a spectrum consisting of multiples of the primary firing frequency fp. For example, 2nd, 4th, 6th, … order engine crank frequency for a four cylinder engine. Also, the orders would drop off rapidly with frequency. In practice, the induction system is far from ideal; extra orders are produced, and the spectrum may be relatively high in the 5th-10th order region.

Figure 7.3.2

1/3rd

Oct.

dB Lin.

100

90

100 1000 Frequency Hz

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NVH Basics (Intro-7-Noise Sources) Page 3

Author: D. Griffiths Rev. 03/25/2011

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7.3.3 Level The measured sound level is highest at wide open throttle (WOT) and is typically in

the 100-120 dBL range. The spectrum shifts up in frequency as rpm is increased; loudness (dBA) increases at about 5 dB per 1000 rpm.

7.4 Tailpipe Noise 7.4.1 Measurement Tailpipe noise is typically measured at about 500 mm from the end of the exhaust on

a line at about 45 degrees to the axis of the exhaust system (not directly in line with the exhaust system which might be bad for the microphone).

7.4.2 Spectrum The spectrum is dominated by low orders of the firing frequency. 7.4.3 Level The sound pressure level is then in the range 90 to110 dBL range at WOT; loudness

(dBA) increases slowly with rpm.

7.5 Axle Noise Generally, axle noise is a pure tone in the 500 - 1000 Hz range. It may be that in this

case mechanically transmitted noise dominates over airborne noise.

7.6 Tire Noise 7.6.1 Measurement and Level Tire noise is generally measured for pass-by noise and hence the measurement is

made at a considerable distance. Muthrukrishnan (SAE 900762) reported measurements at 2 m and various angles.

Typical sound pressures were 80 dBA at 60 mph, with a 6 db increase from 30 to 60 mph (50 kph to 100 kph).

7.6.2 Spectrum The A-weighted 1/3 octave spectrum tends to be peaked at around 1000 Hz (see, for

example, JSAE Review Vol. 13, No. 3, page 75).