米爾布魯克（Millbrook）是英國獨立車輛測試、驗證和工程服務提供商，該公司正在考慮引入聲全息攝影術作為其新型噪聲測量系統(tǒng)的下一步發(fā)展舉措。
“我們的新型測試系統(tǒng)支持最新的汽車技術，包括電動汽車和混合動力汽車，并且能夠引入更先進的測試方法，以減低噪聲、振動和不平順性（NVH），還可以進行測試以檢驗是否滿足法規(guī)要求，并將調查結果和開發(fā)成果應用到車輛改進的過程中。”米爾布魯克的首席工程師Ravi Bal說。
Bal解釋道，新系統(tǒng)對聲源定位和貢獻分析來說非常理想, 而其功能還不止于此。工程師們將能夠測量車輛內部的音響效果和振動，并且將測量結果與外部記錄數(shù)據(jù)同步，在重復測量環(huán)境中實施多種客觀測量。
老系統(tǒng)已經(jīng)運營了20年，Bal說：“盡管一切都已經(jīng)數(shù)字化了，但對于高效而準確的數(shù)據(jù)分析來說，仍存在許多限制。因此，在新設備方面，無論是硬件和軟件，我們都已投入巨資，目的是提升我們的測試能力，以及引入更先進的測試方法。
原來的雙頻道設置已經(jīng)升級為更先進的16頻道：“我們正在考慮引入聲全息攝影術，這是一種必要的聲學攝影機，可以直觀的指引聲源，并且突出顯示潛在問題區(qū)域。這需要讓很多麥克風形成一個陣列，我們的16頻道設備完全能夠實現(xiàn)這一點，并且還能夠通過增加麥克風數(shù)量來提高空間分辨率。”
Bal強調，由于便攜式排放測量系統(tǒng)（PEMS）獲得的投資及其測試部門能力的提升，加上其他升級的實現(xiàn)（包括米爾布魯克測力儀的升級），新的噪聲系統(tǒng)提供了測試、調研的新可能，并有機會使設備水平遠超法規(guī)基本要求。
米爾布魯克的噪聲測試工作在2012年得到提升，當時引入了ISO 10844：2011規(guī)格的噪聲測試路面，提供了對車輛噪聲進行調研和了解的高級設備：“盡管歐洲法規(guī)對車輛型式認證的要求并未如2014年預計的一樣有所進展，但米爾布魯克仍代表了開發(fā)測試和發(fā)展噪聲解決方案的最先進水平。”
全球噪聲測試等級
關于引入全球范圍內監(jiān)管立法等級的工作仍舊在商討之中，但我們現(xiàn)在很難確定下一個法規(guī)變化的時間。
Bal認為歐洲的噪聲旁路限制“相當嚴格”。這些限制應用于所有車輛，包括高能效車型，該車型適用于歐洲共同體整車型式認證（ECWVTA）框架指令。ISO 362設立于1981年，是一項固定測試程序，在今年做了修訂。但目前，歐洲法規(guī)標準仍在使用1998年的版本。
他解釋說：“市場要求高性能車型具備‘正確的’車型尾氣噪聲性能，而我們完全可以做到！”現(xiàn)行的車型認證測試方法不同，根據(jù)車型的輸出功率和車輛的變速箱型號而定。”
“比方說，一輛車的輸出功率為100 kW，裝有手動變速箱（米爾布魯克分類M1），在入口速度為50 km/h時測試第二檔和第三檔。在指定線路AA的某一個特定地點，車輛以“超大”油門加速運行20米，然后加速器停止運轉。第二檔和第三檔的測試結果取算數(shù)平均值，該數(shù)值必須低于適用于此類車型的噪聲限值。應該將大多數(shù)現(xiàn)代車型考慮在內，也就是擁有電子油門的車輛，當駕駛員踩下加速踏板時，他并不是在給車輛提速，而僅僅是讓車輛行駛更快。此種情況下噪聲限值為74分貝（A）。”
但是，對于輸出功率大于140 kW（以及其他規(guī)定）的測試車輛來說，必須只在選定第三檔的前提下進入測試區(qū)域，因為通常這樣會更加安靜（因為發(fā)動機轉速小于二檔的時候），但如果規(guī)定這種情況下的數(shù)值為1分貝，而測試結果的限制定為75分貝（A），通過測試的難度是很高的。
電動汽車和混合動力汽車的發(fā)展，給噪聲測試提供了一個嶄新的契機。Bal解釋說：“基于我們的經(jīng)驗，我們認為分貝數(shù)值從中等到接近70是正常情況，輪胎噪音是主要的噪聲源，輪胎制造商需要申報輪胎產(chǎn)生的聲壓——盡管輪胎噪聲測試方法與整車不同。
電動車的輪胎噪聲降低之后，增設一個人工的人行道預警系統(tǒng)就更為必要。這是噪聲測試的另一個層面。Bal想要看到一種在主要頻率下最低聲壓的限值：“這些主要頻率應該通過使用與個人聲音感知相關的數(shù)據(jù)進行規(guī)定。比如說，行人是否可以聽到典型的背景噪聲？因此，電動車所發(fā)出的聲音不能僅僅設計為持續(xù)的嗡嗡聲，還要有辨識性。”一般來說，比較適宜的噪聲等級約為46分貝，米爾布魯克噪聲場的環(huán)境有利于獲得精確的測量數(shù)據(jù)。
路面測試
盡管米爾布魯克有一個控制路面測試區(qū)（ISO 10844），且測量裝備滿足ISO 362的要求，因此測試結果可以作為參考。輪胎噪聲測試本身并不屬于米爾布魯克的測試內容，但Bal表示提供這種參考的可能性正在商討中。這很有可能需要大規(guī)模投資，而且不同地區(qū)的標準不同，情況就更為復雜。Bal 認為， “歐洲輪胎規(guī)定R 117有四個標準：潮濕路面附著力、滾動阻力、雪地性能和噪聲水平。通過實驗測試，我們可以分析噪聲，但這是一項非常復雜的工作。輪胎必須裝載到最大允許載荷量的特定百分比水平。”
如果米爾布魯克確定涉足輪胎噪聲測試領域，該機構也將會進行另外三個領域的測試研發(fā)工作。Bal補充道：“滾動阻力的測量相對更容易，但濕地附著力的測試需要我們進行更多投資。我們與測試世界（Test World）合作，這是芬蘭的一家冬用車和輪胎測試公司。”這也有助于在全年的時間內方便獲得冰雪路面的測試環(huán)境。
Bal希望米爾布魯克最終可以測試整車噪聲等級、車身結構、接觸面積、空氣動力學特性，以及車輛動力總成所帶來的影響。
但他希望建立一種通過不同程序創(chuàng)建的新型測試規(guī)程：“現(xiàn)階段是由立法者決定需要哪些測試規(guī)程，但如果能夠達成一致，使用更加實際的測試方法，并在早期決策階段就考慮到整車廠和供應商的看法，將能夠可制定出對汽車行業(yè)發(fā)展更合適的標準。”
作者：Stuart Birch
來源：SAE《汽車工程雜志》
翻譯：SAE 上海辦公室
Millbrook’s latest sound advice to the auto industry
UK independent vehicle test, validation, and engineering services provider,Millbrook, is considering introducing acoustic holography as the next step in the further development of its new noise-measurement systems.
“Able to support the latest automotive technologies including electric and hybrid vehicles, our newly commissioned system provides a great opportunity to introduce more advanced test methods to reduce NVH, test to regulatory requirements, and facilitate investigation and development into improvements in vehicle refinement,” said Millbrook’s Principal Engineer, Ravi Bal.
He explains that the new system is ideal for (but not limited to) sound source localization and contribution analysis. Engineers will be able to measure acoustics and vibration inside a vehicle and sync it with data recorded outside, conducting a variety of objective measurements in a repeatable test environment.
The system that it replaces had been in service for 20 years, stated Bal: “Although everything was digital, there were limitations for efficient and thorough data analysis. So we have made a substantial investment in new equipment—both hardware and software—to expand on our test capability and to introduce more advanced test methods.”
The update has seen an advance from a two-channel setup to a 16 channel: “So we are looking at the introduction of acoustic holography, essentially an acoustic camera that can visually indicate sound sources and highlight potential problem areas. This requires multiple microphones built as an array, so our 16-channel capability allows us to do exactly that, with potential scope to improve spatial resolution by increasing the number of microphones used.”
Together with other upgrades, including Millbrook’s dynamometers, investment in its Portable Emissions Measurement System (PEMS), and increased capability within its test department, the new noise system provides the opportunity to test, investigate, and develop “way beyond” basic regulatory requirements, stressed Bal.
Millbrook’s work on noise was enhanced in 2012 with the introduction of an ISO 10844:2011 specification noise test surface, providing an advanced facility for investigating and understanding vehicle noise: “Whilst the anticipated update to European legislative type approval requirements did not go ahead as predicted in 2014, Millbrook remains at the forefront of the latest technology for test and development of noise solutions.”
Global noise test level
Efforts to introduce a global regulated legislative level are still under discussion, but it is difficult to say when the next legislative date will be.
Bal regards European pass-by limits on noise as being “fairly stringent.” They apply to all vehicles, including high-performance cars that are applicable to the European Community Whole Vehicle Type Approval (ECWVTA) framework directive. There is a fixed test procedure—ISO 362—first established in 1981, which was revised this year. Currently, though, the European regulatory standards use a 1998 version.
He explained: “With regard to high-performance cars, the market requires they have the ‘right’ exhaust note for their type—we all do! The current Type Approval test method varies according to the power output of the vehicle category and transmission type of the vehicle.
“As an example, a car with power output of 100 kW and manual transmission (categorized M1 by Millbrook), is tested in second and third gears at an entry speed of 50 km/h. At a specific point, designated Line AA, the car is accelerated on a “wide open” throttle for 20 m and then the accelerator is lifted. In second and third gears, a mathematical average is applied to the results and that must be below the limit applied to that particular class of vehicle. It should be considered for the majority of modern vehicles, that with electronic throttles, when a driver presses the accelerator pedal he is not making the vehicle’s speed increase but merely requesting that it should go faster. The limit is 74 dB(A).”
However, for test vehicles with a power output greater than 140 kW (and other stipulations), it is required to enter the test zone with only third gear selected, which typically tends to be quieter (lower engine speed than in second) but is given a 1 dB provision for its class, consequently raising the limit to 75 dB(A), which is a challenge.
The market emergence of electric and hybrid vehicles has brought a new dimension to noise testing. Bal explained: “We regard dB figures of mid to high 60s as the norm for these vehicle types based on our experience, and we attribute most of this to tire noise, with tire manufacturers required to declare the sound pressure generated by the tire—although this is tested via a different methodology to the whole vehicle.”
Reducing tire noise in electric vehicles increases the need to consider an artificial approach-warning systems for pedestrians. This is another aspect of noise testing. Bal wants to see limits that require minimum sound pressure levels at key frequencies: “These key frequencies should be established using data relating to a person’s perception of sound, i.e., can they hear it over a typical background noise? Therefore, creating a noise that is more than a constant drone, and distinguishable as an electric vehicle.” With a typical ambient noise level of around 46 dB, the situation of Millbrook’s noise site is conducive to achieving accurate measurement data.
Surface test
Although Millbrook has a control surface test area (ISO 10844), coupled with measurement equipment applicable to ISO 362 that can be used as a point of reference, tire test noise per se is not part of Millbrook’s work, but Bal says the possibility is under discussion. Potentially, it would involve a large investment and, again, varying standards across the world would present complications: “The European tire regulation R 117 has four elements: wet grip, rolling resistance, snow performance, and noise. We can assess noise by using a test trailer but it is quite an involved program. The tires have to be loaded to a set percentage of their maximum permissible load carrying capacity.”
If Millbrook did move into tire noise testing, it would also want to enter the other three categories, too. Bal added: “Rolling resistance is relatively straightforward, but wet grip would require further investment. We work in collaboration with Test World, the winter vehicle and tire test facility in Finland.” It facilitates access to snow all year round.
Bal hopes Millbrook will eventually test whole vehicle noise levels, embracing vehicle body structure, contact patch, aerodynamics, and powertrain contributions.
But he wants to see any new test regulations created via a different procedure: “At present, the perception is that legislators decide what is going to be required, but if a more pragmatic approach could be agreed, taking into consideration OEM and suppliers’ views at the earliest stage of decision making, a more appropriate conclusion for the industry could be reached.”
Author: Stuart Birch
Source: SAE Automotive Engineering Magazine
米爾布魯克已獲得一筆“可觀的”投資，以升級該機構的噪聲測量系統(tǒng)。
Ravi Bal表示米爾布魯克正在考慮將聲全息攝影術作為新型噪聲測量系統(tǒng)的一部分。
米爾布魯克的新型噪聲測量系統(tǒng)使用了西門子的技術。
米爾布魯克已經(jīng)增加了對便攜排放測量系統(tǒng)（PEMS）的投資。