Feel the Vibration

FEEL THE VIBRATION

MY FIVE SENSE WORTH ON: WHOLE-BODY VERTICAL VIBRATION

Written by: Julianne Pascual

Source: Forta, N. G., Morioka, M., & Griffin, M. J. (2009). Difference threshold for the perception of whole-body vertical vibration: Dependence on the frequency and magnitude of vibration. Ergonomics, 52(10), 1305-1310.

“I’m picking up good vibrations,” sang the Gym Class Heroes. Of course, in their situation, they can’t help but pick up anything else. No other words in the teenage vocabulary could possibly describe what accompanies the excitation of having the opportunity to interact with one’s dream girl. On the roads of the metropolis however, there are no such things as good vibrations. Sitting on the backseat of one’s car, there’s nothing more uncomfortable and nausea-inducing than feeling a car’s vibrations, coupled with rollercoaster-worthy bumpy motions care of cracked and rock-littered paths.

Car manufacturers have taken note of the discomfort vehicular vibrations have caused to their consumers’ bodies. However, in order to make improvements, they need to know how much vibration reduction is needed in order for the improvement to be noticeable. After all, any change less than the difference threshold will not be noticed. The difference threshold, also known as the difference limen or just noticeable difference, refers to the lowest perceptible change between stimuli detected by an individual (Goldstein, 2010). The size of the just noticeable difference is related to the initial stimulus magnitude. In fact, it is a constant proportion of the original stimulus value. Their relationship is quantitatively expressed as a relative ratio, Weber’s Law:

Source: University of South Dakota Internet Sensation and Perception Laboratory

Forta, Morioka, and Griffin (2009) conducted a study to give these car manufacturers their answer. They designed a difference threshold set-up for whole-body vertical vibration of seated subjects, examining the effect of vibration frequency (number of completed vibration cycles per second) and magnitude (quantitatively represented by the root-mean-square of a sine wave). They hypothesized that difference thresholds would depend on both vibration frequency and vibration magnitude. But they had to contend with the paucity of the research studies on whole-body vibration perception. Personally, I find that this paucity may be attributed to the complexity of the science behind it.

The Science of Vibration

In the skin of the hand alone, there are four psychophysical channels involved in vibrotactile thresholds: Pacinian (P) channel, non-Pacinian channel I (NPI), non-Pacinian channel II (NPII), and non-Pacinian channel III (NPIII). The P channel has the Pacinian corpuscle as the mechanoreceptor. It often has the lowest threshold at high frequencies (e.g., greater than 40 Hertz). The NPI channel, mediated by the Meissner corpuscle, determines thresholds at frequencies lower than 40 Hertz (Hz). The NPII channel has a higher threshold than the P channel, but responds to the same frequency of the P channel. It is directionally sensitive to the stretching of the skin. A lower threshold range of 0.4 to 4 Hz is covered by the NPIII channel. In other areas of the body, there are other sensory channels believed to be responsible for the perception of vibration (Forta, Morioka, and Griffin, 2010). Given this, it can be safely assumed that different frequencies of vertical whole-body vibration are felt in different parts of the body (Whitham and Griffin, 1978).

In the lower body, it has been reported that vertical vibrations are felt at low frequencies (e.g., less than 16 Hz). Intermediate frequencies (16-31.5 Hz) are felt greatest at the head. At higher frequencies, sensations are localized around the input to the body adjacent to the seat’s surface (Forta, Morioka, & Griffin, 2009).

Time to Feel the Vibration

Forta, Morioka, and Griffin (2009) utilized a sample of 12 young and healthy males for their difference threshold study. It was conducted for three one-hour sessions on three different days. In each session, difference thresholds were determined for eight frequencies (i.e., at 2.5, 5, 10, 20, 40, 80, 160, and 315 Hz) at one magnitude of vibration. If the magnitude was low, the vibration acceleration was 0.05 ms-2 root mean square (r.m.s).  For the middle and high magnitudes, the vibration acceleration was 0.2 and 0.8 ms-2 r.m.s., respectively. Subjects, sitting in a comfortable upright posture, felt the vibrations through a contoured rigid wooden seat attached to a vibrator table. Their hands and feet were supported by stationary handles and footrests.

In the sessions, difference thresholds were determined using a two interval forced-choice method. A reference and a test motion were presented in random order, and they were asked to identify the stronger motion. The two intervals were separated by a 1-second pause.

The method used for this difference threshold study was a modified method of limits called the up-down-transformed-response method. The three-down one-up rule was used to track subject responses. When three consecutive correct responses were given, the magnitude of the test stimulus was lowered by one step (0.25 dB) and when an incorrect response was given, the test stimulus’ magnitude was increased by one step.

Auditory masking was done in this experiment. A white noise at 75 decibels (dB) was utilized, and was heard by the subjects using a pair of headphones. However, the white noise was not enough to mask the noise in the session with the highest magnitude of vibration at the highest frequency (315 Hz).

Results showed that median relative difference thresholds were in the range of 9.5% at 2.5 Hz (at the high magnitude) to 20.3% at 315 Hz (at the low magnitude).

Frequency-dependence was revealed only at the lowest vibration magnitude (0.05 ms-2 r.m.s.), such that higher frequencies had higher difference thresholds. In other words, it was more difficult to detect changes in vibrations as one ascends the 2.5-315 Hz range in the experiment’s low magnitude condition. Forta, Morioka, and Griffin (2009) believe that frequency-dependent difference thresholds may be due to the sensitivity of different psychophysical channels. At low frequencies, the NPIII channel is more sensitive than the NPI and the P channels. Thus, at low frequencies, greater NPIII sensitivity leads to easier detection of changes (low difference threshold). This leads to the implication that at higher frequencies, there is a tendency for the difference threshold to increase since NPIII channels become less sensitive.

Magnitude-dependence was only evident at 2.5 Hz and 315 Hz. However, Forta, Morioka, and Griffin (2009), believe that this may have arisen from perception via sensory channels other than tactile channels, specifically vision and hearing. At 2.5 Hz, changes in motion could be seen, especially at the higher magnitude.  The difference threshold then was lower than average because it was easy to visually compare the motion differences produced through vibration, especially in the higher magnitude sessions. The subjects could have referred to the apparent movement between objects in the foreground against objects in the background as a basis for discerning intensity difference. At 315 Hz, it may have been easier to spot vibration differences (lower difference threshold) because the subjects were able to hear the stimulation at this frequency (more apparent at the higher magnitude). They may have detected vibration differences based on auditory input as opposed to basing judgment on tactile perception.

Some Thoughts on Vibration Minimization

Experiment-wise, it is of interest to note that Forta, Morioka, and Griffin (2009) only focused on male subjects. Granted, it may be that society has the popular notion that anything related to automobiles belongs to the male domain. However, this is not justification to exclude females from being a group of interest in such a study. In fact, in the study of vibration and motion, literature has showed that females are more sensitive in discriminating changes than males (Shechter, Hillman, Hochstein, & Shapley, 1991). Perhaps, significant differences would have been observed in the difference thresholds for the two gender groups. Had such a study been conducted, car manufacturers would have been given more information that might have allowed them to have a more informed way of catering to their male and female consumers. They could have developed specific cars wherein apparent vibration felt would have been minimized to a degree appropriate to each gender’s comfort levels based on their respective difference thresholds.

Comfort is not the only issue addressed by minimizing distracting and uncomfortable vibrations. Safety is also an issue that is tackled. According to Griffin (1990), it’s important to limit vibration felt by drivers because too much vibration can impair the acquisition of visual information (e.g., seeing a stable view from the front car window) and the output of information (e.g., coordinated hand or foot movements).

The confounding variables of visual and auditory cues were apparent in the experiment, specifically in terms of magnitude-dependence for 2.5 Hz and 315 Hz, respectively. For the former, the experimenters could have avoided the visual confound by simply using a blindfold in the experiment. For the latter, perhaps, it may have possibly been better to use an auditory mask with white noise higher than 75 dB. They could have increased the decibel level to 85 dB, which is comparable to the level of city traffic sound heard inside a car. However, I do understand the hesitancy of the experimenters to increase the loudness of the white noise. Participants’ sustained exposure to sound levels at the range of 90-95 dB may result in hearing loss (Galen Carol Audio, 2007). Nevertheless, there is still value in this study’s results. Other researchers would be able to have scientific basis for designing a study using stimuli wherein changes in whole-body vertical vibration can only be felt, not seen or heard. They would use stimuli with the current study’s intermediate frequencies: 5, 10, 20, 40, 80, and 160 Hz. In embarking on such a research endeavor, they could be better assured that any significant difference threshold value obtained in their study is based solely on tactile perception.

Based on the results, it appears that higher difference thresholds exist for higher vibration frequencies. The researchers attributed this to the differential mediation brought about by the distribution of vibrotactile perception channels in different body parts. Perhaps, cars may be designed in such a manner that better considers the sensitivity of different body parts to vibrations. Since the literature reviewed by Forta, Morioka, and Griffin (2009) established that the lower body has channels sensitive to low frequencies, car floors may be designed to have greater padding to better attenuate vehicular vibration frequencies. It’s important that such vibrations do not cause too much discomfort at this level of the body since the car floor features two of the most important parts for automobile control: the brake and accelerator. It’s important that there is little perceived disturbance in this region, so as to better control the necessary pedals.

I find that whole-body vertical vibration is an area of study that has yet to be experimentally investigated intensively. In the future, other researchers could focus on its other components and characteristics. For example, modification of duration of exposure to vibration frequency may affect physiological responses of the body (e.g., sweat production, heart rate, etc.). Results in this topic would be of interest not just to manufacturers in the automobile industry, but also to those who are in the amusement park business. In the creation of rollercoasters, there may be an ideal time period for vibration exposure that would make riders feel more excited or physiologically stimulated during a rollercoaster ride.

Back to the study at hand though, I feel that psychophysical research still has a long way to go before providing results that could help create the perfect, comfortable car seat—one which transmits minimal annoying vibrations. Therefore, in the meantime, I think I’ll make do with having my car’s padded and soft seats attenuate any high frequency vibrations felt by my body.  Well actually, it’s either that or I imagine I’m sitting on an uncontrollably uncomfortable massage chair.

References:

Galen Carol Audio. (2007). Decibel (loudness) comparison chart. Retrieved from http://www.gcaudio.com/resources/howtos/loudness.html

Goldstein, E. B. (2010). Sensation and perception (8th ed.). Belmont, CA: Wadsworth.

Griffin, M. J. (1990). Handbook of human vibration. San Diego, CA: Elsevier Academic Press.

Shechter S., Hillman, P., Hochstein, S., & Shapley, R. M. (1991). Gender differences in apparent motion perception. Perception, 20, 307-314.

University of South Dakota Internet Sensation and Perception Laboratory. (2010). Weber’s law of just noticeable differences. Retrieved from http://people.usd.edu/~schieber/coglab/WebersLaw.html

Whitham, E. M., & Griffin, M. J. (1978). The effects of vibration frequency and direction on the location of areas of discomfort caused by whole-body vibration. Applied Ergonomics, 94, 231-239.

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~ by myfivesenseworth on July 26, 2011.

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