Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2002-09-18
2003-09-30
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06626041
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject matter of the present invention is a method for quantitatively determining the airborne portion and solid-borne portion of engine noise such as can be heard, for example, in the interior of motor vehicles or electric vehicles. By flooding the engine compartment with a gas or gas mixture with a different density than air, the acoustic conditions in the engine compartment are changed. Conclusions about the airborne and solid-borne portions of the engine noise in the interior of the vehicle can be drawn from measurements taken before and after this acoustic change. The method can also be applied to other forms of transportation, such as ships and aircraft, as well as power stations, machine shops, etc.
2. Background of the Invention
To achieve a high level of driving comfort in motor vehicles, a pleasant noise level in the passenger compartment of a vehicle is desirable. This noise level or sound level is determined to a considerable degree by the drive unit, in one example, a reciprocating internal combustion engine. The noise emitted by the engine is usually transmitted into the passenger compartment in two ways: noise transmitted directly into the passenger compartment as airborne sound; and sound emitted by the engine transmitted into the passenger compartment through the engine block to the body of the vehicle as solid-borne sound. To selectively reduce the sound level, it is desirable to separate the portion of solid-borne sound from the portion of airborne sound.
The prior art, with respect to the reduction of noise levels, is directed toward reducing airborne sound level of the engine. Thus, for example a test bench for engine sound measurement from BMW AG is described in detail in ATZp. 446 et seq. (1997).
In the same issue of ATZ, p. 414 et seq., measures for reducing the level of airborne sound by sound damping and measures for reducing the level of solid-borne sound by adjustable hydraulic engine bearings are disclosed using the example of the Audi A8.
The solid-borne and airborne portions of the engine noise, as heard in the interior of the vehicle, can currently be determined only at great cost. Two basic methods are described in a paper “Ger ä uschpfadanalyse einmal anders [A Different Approach to Analyzing Noise Paths]” for the 3
rd
Vehicle Acoustics Conference “Ger ä uschminderung in Kraftfahrzeugen [Reducing Noise in Motor Vehicles]” of 10/11.3.1992 at the Haus der Technik e.V., Essen. In the first method, the drive train is partially or even completely decoupled from the vehicle so that the noise in the interior of the vehicle is only the airborne portion of the engine noise. However, for this purpose, costly steel frames must be set up for suspending the entire drive train. In particular, the decoupling of the drive shafts is problematic.
In addition, there is an indirect method in which preliminary measurements of the solid-borne and airborne sound transmission functions, known by the name “Transferpfad-Analyse [Noise Path Analysis]”, are determined. In addition, engine acceleration and noise emitted by the engine are measured. By superposing the signals, which are linear in large regions, it is possible to draw conclusions about the relative contributions of airborne and solid-borne sound. The precision of the conclusions is, however, frequency-dependent as the solid-borne sound transfer functions are imprecise, in particular at low and high frequencies. At low frequencies, it is not possible to carry out excitation sufficiently because the coherence worsens. At high frequencies, the phase can only be measured imprecisely. In addition, the transfer functions also change with temperature (temperature-dependent elasticity of the rubber bearings) and engine load (tensioning of the engine bearings, for example, full-load accelerations at low rotational speeds).
The precise measurement of the airborne sound emissions of the engine is also problematic. Such measurements are carried out in the acoustic close-range field of the engine, as a result of which the sound pressure between two adjacent points can vary greatly. It is, therefore, impossible in practical terms to pick up the emitted engine noise completely and correctly using measuring equipment. Typically, only one microphone is used on each side of the engine to record the engine noise.
Methods in which acoustic effects are studied using gases with properties other than those of as air are known. To influence the acoustics in aircraft, for example, “lightweight” gases, i.e., gases with low density or a high speed of sound are used. In FR 2783498, it is proposed to cause lightweight gases to flow out at the rotor blades of a jet engine to minimize emission of airborne sound. FR2783499 discloses a method in which the shock waves occurring to the engine inlet and to the front edges of the aircraft airfoils when flying above the speed of sound are reduced by causing a lightweight gas to flow out in a selective way. GB 2271387 describes resonators integrated in an aircraft jet engine, which are filled with a lightweight gas, as a result of which the inherent frequencies of the resonator, and thus the effect of the resonator, are influenced in a selective way. In addition to these methods, which utilize the acoustic properties of low-density gases, methods for quantitatively determining gas properties with acoustic means are also known. Thus, DE 19745954 A1 describes a measuring method in which the composition of gases (for example, for anesthetic applications) is determined acoustically. Here, sound pressure oscillations are generated in the gas mixture to be measured and the propagation speed, the sound pressure amplitude, the frequency changes or other acoustic variables of the gas space are determined and the gas composition derived therefrom.
SUMMARY OF INVENTION
An advantage of the present invention is in providing a simple measuring method with which the airborne portion and solid-borne portion of engine noise can be determined easily in a qualitative and quantitative fashion for any operating point and any frequency.
This is achieved by a measuring method for determining the solid-borne portion and the airborne portion of powerplant noise in the interior of a vehicle, by performing a first sound measurement by at least one microphone in an engine compartment and at least one microphone in the interior of the vehicle, the microphones being connected to at least one amplifier and evaluation unit. A gas other than air is introduced into the engine compartment of the vehicle. A second sound measurement is performed when the gas, other than air is in the engine compartment. The first and second sound measurements are compared.
According to the invention, the entire engine compartment or parts thereof is flooded with a gas or gas mixture, which has a density or speed of sound, which is significantly different from that of air. In addition, this gas can also be introduced into the space below the engine cavity and under the floor of the vehicle. The gas used here is preferably helium, hydrogen, xenon or sulfahexafluoride, but other gases or gas mixtures can also be used. Helium, for example, has a higher speed of sound than air by a factor of three, and the speed of sound in xenon and sulfahexafluoride is on the other hand lower than that of air. By flooding the engine compartment with one of these gases, the acoustic conditions change, i.e., the modes shift to higher frequencies in helium and to lower frequencies in xenon and sulfahexafluoride. Here, the airborne sound emitted by the drive train is modified to a very great extent, which can also be picked up by measuring equipment in the interior of the vehicle. In the case of helium, for example, the sound pressure is as a rule lower at low frequencies. However, the sound pressure can also rise at medium and high frequencies.
If it is to be qualitatively decided whether the engine noise in the interior of the vehicle is also influenced by the airborne portion of t
Bellamy Tamiko
Brebob Diane D.
Ford Global Technologies LLC
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