Today, the words buzz, squeak, and rattle have a special meaning in the automotive industry. Passenger cars, trucks, and buses now enjoy a major improvement: their interior noise levels are much lower. Drivers and passengers find it much easier to hold a conversation at highway speeds, to hear the radio, or to use their cellular phones.
But there is a downside. Now we hear many little sounds generated by mechanisms inside the passenger compartment. Previously, these sounds were masked by the noise from the engine, drive train, and wind. These little sounds annoy some new car owners. Buzz, squeaks, and rattles (BSRs), perceived as a lack of quality, are factors in some 52% of their complaints.
Terminology
Rather than define the words buzz, squeak, and rattle, let’s recall some familiar sounds. For buzz, try to hear the 120-Hz (and associated harmonics) sound of power distribution transformers. For squeak, try to hear an unlubricated door hinge or a sticky drawer opening. For rattle, mentally shake a coffee can of nails.
BSRs involve relative motion resulting from vehicle and part resonances being excited. Inadequate attachment of parts, typically bolted, riveted, or bonded, can be a factor. So can insufficient clearance.
Buzzes and rattles accompany chatter and impacts. Squeaks occur by sliding between surfaces. Certain material pairs are very likely to squeak.
BSR Problems
As with most noise problems, we can attack:
The source.
The path.
The receiver, the ears of the people involved.
Forget the receiver. Drivers and passengers will not wear earmuffs or ear plugs. Besides, they need to hear outside sounds such as emergency-vehicle sirens. So we are left with the source and the path.
Treating the path is certainly possible. First, we must identify the source and the path(s). The standard pathway treatments of barriers and absorbers can be very effective with BSRs, just as they already have been applied successfully in reducing engine, drive-train, and high-speed airflow sounds.
Treating the source begins by identifying its origin. In a bygone era, the new car owner told his dealer’s service technician about a sound he heard under certain circumstances, such as speed, road surface, or temperature.
Visualize the difficulty in reproducing the driving conditions, hearing the sound, identifying the source, and remedying the problem. It worked better when the owner drove while the service technician attempted to identify the elusive sound.
Preventing BSR problems costs less than hundreds of repeats of this scenario. Prevention is a task for BSR specialists in preliminary design. They avoid stacking resonances by designing instrument panels to resonate at other than major body-bending resonant frequencies. They adequately attach parts to avoid relative motion, or they provide ample clearance between parts. In essence, they avoid pairing certain materials likely to squeak.
Identifying BSR Problems
A few BSRs may appear on pre-production vehicles when specialists drive them on different proving-ground surfaces. They drive at various speeds while listening for BSRs. Sometimes they can identify the sound source. Often, they enlist another BSR specialist who rides as a passenger, not only using his trained ear but also using microphones, a cassette recorder, and a sound-intensity analyzer.
However, the noise only may appear at a particular speed on a particular section of the track and only for a second or two. It takes several minutes of driving around the track to get back to that location at, hopefully, the proper speed for the noise to occur again. Outdoor testing is expensive and time-consuming, and has largely been supplanted by indoor laboratory investigations.
Shaking an Entire Vehicle
Laboratory shaking the pre-production vehicle, as shown in Figure 1, offers many advantages, including independence from outdoor weather. In the quiet of the BSR laboratory, critical road-input conditions or generalized continuous-spectrum random vibration can be applied indefinitely, so that the elusive sound continues while specialists identify the exact source. Once a source is identified, it can be remedied.
At lesser expense, four-poster (vertical only) EH shakers, originally purchased for fatigue testing, are used. Out-of-phase sine forcing that twists the frame sometimes is useful.
Automobile manufacturers and suppliers of large assemblies such as seats and doors generally use electrohydraulic (EH) shakers, as in Figure 2. A large assembly can be shaken on a vibrating platform driven by single-axis or, better, multiaxis EH shakers.
Shaking Smaller Assemblies
Manufacturers of relatively small interior assemblies, such as instrument panels, shake their products on single or multiaxis electrodynamic (ED) shakers. These units are programmed with vibrations that, on the test track, have accompanied BSR.
Thermal Conditioning
Some BSRs only occur at certain part temperatures. For these situations, the ED shaker body usually is outside a specialized chamber with the vibrating table, or a vibrating extension of the table, driving an attachment fixture through the chamber floor or wall that, in turn, holds the DUT.
The chamber has three major functions:
Brings the DUT to the desired test temperature. Then the chamber heating or cooling blowers are turned off briefly (helps attain low background noise levels) for a very few minutes of investigation. Heating also may accelerate the aging of materials involved in BSR.
Blocks outside sounds. The interior is as quiet as possible, so that little noises can be heard, measured, recorded, and analyzed.
Absorbs sounds coming from the DUT over a wide range of frequency, avoiding acoustic resonances and standing waves.
Most BSR testing is single-axis, although multiaxis shaking adds realism and saves time over re-orienting the DUT relative to the shaker axis.
Sine Vibration
Multifrequency (typically 5 Hz to 100 Hz) random vibration inputs are appropriate for many shaker investigations. Alternatively, recordings of road/engine/drive-train vibrations can be used. But for some diagnoses, low-frequency sweeps, such as 5 Hz to 100 Hz in one minute at 1 g, are very useful.
When BSR occurs so does distortion—often quite abruptly—in the response. The distortion often appears in regular bursts. One burst per sine cycle often indicates buzzing or rattling. Two bursts per cycle usually point to squeaking.
Quiet BSR Labs
For elusive BSR sounds to be heard or sensed by microphones, the BSR facility must be quiet. Ideally, BSR investigation teams have their own buildings away from production noises. Briefly turning off building heating and air-conditioning blowers reduces background noise levels.
ED shakers have a significant advantage over EH shakers through which oil must flow. Most BSR investigations are brief and conducted at low force. ED shaker cooling air can safely be turned off for two or three minutes. Sound pressure levels under 20 dB(A) have been achieved.
How Much BSR Is OK?
So far, the BSR limits from most auto manufacturers are overly simple. For example, in most 1/3 octave bands, the BSR cannot exceed 55 dB(A). However, newer and more complicated standards are being influenced by German sound-quality investigations. One goal is >100,000 quiet miles.
Further Study
The Professional Development Division of the Society of Automotive Engineers offers short courses in automotive vibration and BSR testing. For more information, call (412) 776-8531.
About the Author
Wayne Tustin founded Equipment Reliability Institute, an affiliation of reliability educators and consultants. During his career, more than 50 of his articles have appeared in EE-Evaluation Engineering. Equipment Reliability Institute, 1520 Santa Rosa Ave., Santa Barbara, CA 93109, (805) 564-1260, [email protected].
Copyright 1997 Nelson Publishing Inc.
July 1997