Laser Vibrometry - See What Your Structure Sounds Like

Transcription

1 Laser Vibrometry - See What Your Structure Sounds Like Some Fundamentals: Roger Traynor EIS Sound Quality & Perception Workshop, Brunel University, June 8 th 2014 What is Sound? Sound & Noise Vibrations transmitted through an elastic solid or a liquid or gas, with frequencies in the approximate range of 20 to 20,000 hertz, capable of being detected by human organs of hearing. What is Noise? Sound that is loud, unpleasant, unexpected, or undesired. are both received by the ear as pressure waves originating from the vibrating source. The brain processes signals from the ear to decide if the received vibrations are pleasant/neutral Sound or unpleasant Noise! 1

2 Laser Vibrometry How does it work? Traditional pickups (microphones) sense air motion and provide sound pressure level values and frequency information for the sound/noise. Microphones arrays can give an idea of distribution of sound pressure variation over an area, but only offer low spatial resolution. Sensor Head HeNe Laser Reference Beam Beam Subject To be able to accurately locate the source of a sound or noise problem requires a method that can provide high spatial resolution over a structure. Photo-Detector Laser Vibrometry can provide that capability Controller Time History Impulse Response Frequency Response Laser Vibrometry How does it work? By adding mirrors to steer the laser spot, an HD camera to view and capture an image of the test structure, plus data acquisition and control hardware/software, an area scanning vibrometer can be realised. Laser Vibrometry How does it work? By measuring with three laser vibrometer heads, both the shape geometry and the tri-axis (X-Y-Z) vibrational response can be measured, to establish the sound field propagation from the structure s surface, in all vectors. 2

3 Laser Vibrometry Key Advantages The vibrometer s laser spot size is very small (~100µm diam. per 1m standoff), so multiple data points can be defined on the test structure, to obtain high spatial density information. The laser spot doesn t weight very much (!), so there are no mass-loading effects on the test surface, such as can be caused by using accelerometers and other contact transducers. Laser vibrometers have a flat response (~1%), from near DC out to MHz frequencies - their response to vibration is accurate and flat. No correction factors need to be applied to correct for sensor non-linearity. s Laser Vibrometry Performance What can it do? Frequency response: well beyond audible (DC to 24 MHz) Displacement resolution: 2pm (to > ± 41 mm) Velocity range: sub µm/s to ± 30 m/s Key advantages for acoustic applications Non-contact, zero mass loading - no effect on the structure High spatial resolution ~100µm diam. Spot /m standoff Flat frequency & phase response Measures from rough/smooth, light/dark, hot/cold surfaces Easy to use low power, visible class2 laser safety beam Single point or area scan options s Practical Applications: Laser vibrometry is extensively used for sound & vibration studies in all areas: NVH - Body Panels - Engines & Drivetrains - Brake Squeal - Acoustic Tuning Practical Application Examples: Gas Turbine Vibration - Control Surfaces - Cabin Noise -Undercarriages Product Development - Noise Isolation -Transformer Hum - General Testing Hearing Research - Medical Device Development - Insect Behaviour HDD Dynamics - Mobile Phone Microphones /Speakers - GPS Sensors s s 3

4 Automotive: A vehicle s sound Autograph can be a major factor in its success or failure in the market. A great deal of time, effort and money is invested in obtaining the right NVH signature. Modern customers no longer accept squeaks and rattles, cabin booming or brake and suspension harshness, neither in lowend economy cars and certainly not in the more expensive luxury vehicles. Engine Sound & Vibration Characterisation Alternator vibration mapping with the PSV During pre-production prototype testing of a new engine configuration, there was evidence of whine from a newly sourced alternator Using the PSV scanning vibrometer, the noise hotspots were identified and the problem designed out Deflection shapes from the PSV With most vehicles now being assembled from 3 rd party suppliers rather than manufactured on a single site, the crosscompatibility of components, and their testing, has taken on major importance for vehicle makers. Courtesy: JaguarLandRover Floor Pan Vibration Mapping Panel boom on a car s floor pan can seriously affect the cabin quality perception of the vehicle. Mapping the vibration distribution and identifying the unwanted vibration frequencies allows NV engineers to optimise the amount and location of sound deadening material, without excessively increasing the vehicle s weight. Motorcycle Noise Comparison Testing Polytec were asked to measure and characterise the sound of motorcycles from two different manufacturers. versus Frequency and amplitude responses of a vehicle floor pan Undamped (left) & Damped (right) One was the market leader in the Sports Road Bike category, outselling its competitor by significant numbers, despite both having similar engine sizes and configurations, road handling, price and other factors. From market research, the target customers felt that the competitor s bike had a more sporty image, due in part to its sound signature. 4

5 Comparative Product Testing The two bikes were mounted on a rolling road and scanned with a PSV series laser vibrometer system. At ~6000 rpm (their power-loaded speed) the following was observed: Note that the left hand motorcycle has a lower overall noise (dba) level The competitor s machine has a more dominant low frequency noise, plus a higher frequency resonance spike Comparative Product Testing A definite acoustic difference was observed in the two bikes. (The overall sound signature of the right hand machine was found to have a mix of more low frequency (bass) volume, plus notes in the higher frequency region. This was what customers were perceiving as the Sportier sound of the competitor s bike. During the measurements, it was also noticed that vibrating surfaces on the front of the engine appeared to be working with the air intakes to project sound forward from the bike. This effectively announces the imminent arrival of the bike both a safety warning and a desirable feature for a Lairy Café Racer! Vehicle Body-in-White Vibration 3D Vehicle Body Vibration Full BIW vehicle vibration mapping can be quickly carried out, using an industrial robot-mounted PSV-3D scanning vibrometer Whole vehicle body operational deflection shape Full geometry and vibrational data (frequency, amplitude and excitation reference values) can be exported for FE model correlation and design optimisation, where appropriate 5

6 Excessive Noise Leading to Damage Polytec s PSV Scanning Vibrometer was used to assess vibration levels on the rear panels of the Thrust SSC Mach 1 Record Holding Supersonic Car Noise levels on the panels were ~170 dba Panel displacements were ~15mm at 28 Hz Vibration levels on titanium composite panels adjacent to the exhausts were sufficient to cause stress-cracks as well as tear rivets from the panels (The accelerometers used to try to measure the vibration levels were thrown off the rear of the structure soon after the engine was started. Laser vibrometry proved to be the only viable method to map and provide frequency and energy distribution information from the violently-excited hot panels) Other Acoustic Work: Although automotive applications are an important area for us, laser vibrometry is extensively used for sound and vibration studies in a multitude of other areas Domestic white goods Medical equipment Internal building panel noise transmission Other Acoustic Work: Summary & Conclusion Some acoustic tasks are obvious, such as loudspeaker optimisation.. Wavefront distortion in a small loudspeaker 1 khz frequency..but LDV is also able to image sound field propagation by measuring subtle optical density changes in the airspace near to speakers, hand tools and other sound generators: Non-contact Laser Doppler Vibrometry has proven itself to be a versatile tool in the examination of vibrating surfaces, i.e. those that emit Sound and Noise. LDV provides flat response analysis of the frequency mix, relative amplitudes and the source of that vibration, with high spatial resolution, enabling the researchers and test engineers to work on eliminating bad sound (noise) or enhancing good sound characteristics of a structure. Loudspeaker time response to click excitation..allowing the designer to optimise the sound for best performance and quality 6