Good Vibrations – Phenomenon of Structure-borne Sound
November 15, 2018:
Humans have learned to unconsciously assess acoustic information day and night. Every signal is influenced in the real environment on its way to the ear. The “mixed product” perceived automatically by the ear allows conclusions about the surrounding environment due to the learned experience. Studies show that the perception of sound also takes place through the body surface. In the article, we deal with the phenomenon of structure-borne sound and report how we consider structure-borne sound perception in Mvoid’s auralization process.
The engineers of the car manufacturers and acoustic experts work hand in hand to bring the performance of the sound systems to new technological heights. The OEMs offer vehicles that are equipped with over 30 speakers and groundbreaking technologies. In the development of the “rolling concert halls”, the experts also consider psychoacoustic characteristics. For good reason, since the assessment of sound quality cannot be judged solely on a metric based on numbers. Pure mathematics cannot guarantee an emotional listening experience. The human ear works in much subtler ways. In addition, the perception of sound not only takes place via the ear, but rather several human senses are addressed.
According to modern physiology a human possesses nine senses. Seeing and hearing are the most dominant mediums of receiving information, the visual sense being the prevailing one. In psychoacoustics it also means: that the eye also listens. In addition to our eyes, it is possible for humans to perceive spatiality through their ears and their acoustic experience of life, such as the perceived size of a room or the position in a room. This cognitive performance takes place unconsciously, continuously and automatically.
Humans naturally have a pronounced auditory perception. They have learned to unconsciously assess acoustic information day and night. The auditory perception differs from person to person, it is not universal. Every signal is influenced in the real environment on its way to the ear, contrary to the transmission in an anechoicroom. The “mixed product” perceived automatically by the ear allows conclusions about the surrounding environment due to the learned experience. Studies show that the perception of sound also takes place through the body surface.
Scientific findings on the perception of vibrations
In the past various studies have dealt with the effects of vibrations, generated by audio, on human perception. Exemplary, we are presenting three studies. Two studies relate to the perception of vibration in a vehicle, one study refers to the perception of vibration in a concert hall.
1. William L. Martens et al. investigated the influence of whole-body vibration (delivered via the floor and the seat on which participants were situated) on preferred equalization for binaural recordings of musical sound reproduced via an automotive audio system (binaural sound recording = sound recording of sound signals with microphones, which reproduce a natural sound with exact directional localization via headphone). The research refers to four programs differing in musical genre, selected for their substantial low-frequency content.
The results showed that the preferred level (bass equalization) for low-frequency audio delivered by a headphone was reduced by the study participants, the higher the vibration levels of the four programs were presented. A substantial effect on the listener’s preferred low frequency level was observed when the level of vibration delivered to the observer was changed by 12 dB.
(Source: “Whole-body vibration associates with low-frequency audio reproduction influences preferred equalization”; William L. Martins et. al., AES 36thInternational Conference, Dearborn, Michigan, USA, 2009)
2. Germain Simon et al. also analyzed the presence of whole body vibrations. Listeners’ bass equalization preferences were measured for four different music programs, too, reproduced through a high-quality automotive audio system auditioned in situ (in the car) and through a headphone-based binaural room scanning (BRS) system. The task was repeated while the listener experienced different levels of simulated and real whole-body vibrations associated with the automotive audio system itself.
The results verified that both real and simulated whole-body vibrations produced by an automotive audio system influence listeners’ preferred bass equalizations of music reproduced through it. The evaluations of the virtual sound system prove that adding simulated vibrations to the BRS playback can restore the apparent “missing 3 dB of bass” and bring the experience much closer to what heard and felt when listening in situ.
(Source: “The effect of whole-body vibrations on preferred bass equalization of automotive audio systems”; Germain Simon et al., AES 127thConvention, New York, NY, USA, 2009)
The experiments of both studies were carried out in a parked vehicle with the engine turned off in a research laboratory. The effects of vibration caused by the vehicle engine and road noise and correspondingly preferred bass balance were not investigated in these experiments.
In summary, both studies indicate that binaural spatial measurements may not provide accurate, natural, valid listening results for vehicle audio systems unless full body vibration is considered.
3. Merchel und Altinsoy analyzed the influence of audio-induced vibrations on the perceived quality of the concert experience. Researchers explored the questions of whether whole-body vibration is important for the perception of music, whether the absence of the vibration component can lead to a loss of perceived quality in a concert experience, and whether a concert experience can be enhanced by adding vibration. The study focuses on whole-body vibration of a seated person as perceived in a classic concert hall.
The vibratory intensity and frequency spectra are dependent on various factors, e.g., room modes or construction parameters of the floor. The researchers claim, that in many cases, the concert listener may not recognize the vibrations as a separate feature because the tactile percept is integrated with the other senses (e.g., vision and hearing) into one multimodal percept. Even if the listener is unaware of vibrations, they may have an influence on recognizable features of the concert experience, that are of vital importance in determining the quality of concert halls.
Merchel and Altinsoy summarized that seat vibrations can have a considerable positive effect on the experience of music.
(Source: “Vibration in music perception”, Sebastian Merchel und M. Ercan Altinsoy, AES 134thConvention, Rome, Italy, 2013)
Researchers agree that vibration plays a significant role in the perception of music. Study results show that a change of 10 – 15 dB in the range of low frequencies are perceived by humans via the structure-borne sound.
Consideration of psychoacoustics with the help of auralization
In order to be able to consider the characteristics of psychoacoustics in the development of modern sound systems in vehicles in the early development phase, the process of auralization is applied.
The process of auralization generates acoustic animations of a situation (space) that immediately makes the effects of different situations (spatial modes) audible. The sound of the virtual sound systems can be experienced. The sensory impressions are transmitted. If the stimuli do not match, a cognitive dissonance arises – a contradiction of impressions. Sound shortcomings are perceived as unnatural or even disturbing. The acoustics experts can use the auralization process to examine psychoacoustic characteristics on the digital product and eliminate unwanted noise in the early development phase.
Enhanced listening experience through the perception of structure-borne noise through powerful, natural bass performance
In recent months, we from Mvoid have analyzed the phenomenon of structure-borne sound perception in more detail. We examined how we can implement the scientific results into our auralization process. When researching we discovered the start-up company Lofelt (www.lofelt.com). The company develops haptic technology for different areas.
Lofelt has introduced a product called Basslet at CES 2017. A product that amplifies the perception of structure-borne noise via headphones when playing music. The heart of the Basslet is the “„L5 Actuator“. The actuator creates vibrations that make the low frequencies noticeable. The basses are perceived more intense, although people do not hear them louder than before.
The Basslet begins to vibrate as soon as music is played, and in intensity, frequency, and beat/rhythm to match the bass tones in the music. A bass slowly fading away can also be felt as such. The impact of a bass drum is brief, but all the more noticeable. The wearable works in the deep tones.
The basslet looks like a smartwatch or a fitness gadget. It consists of two parts: a transmitter and a receiver. The wired transmitter is connected to the smartphone, laptop or music player and the headphones. The receiver is worn like a watch on the wrist.
In the meantime, Mvoid has developed simulation processes for the optimization of the “L5 Actuator” for Lofelt and will use the advanced product in our Mvoid auralization process in the next release of VRtool.
Due to the further development of our Mvoid auralization tool (VRtool) using haptic technology from Lofelt, we are able to experience the depth perception of vibrations via headphones in our auralization process. Now, the human brain can combine sound and vibration into a single experience.
If you want to discover more about the Mvoid auralization process, read our article “Music in the ears – How does auralization protect against inappropriate expectations of the sound?“.
©Jrgen Fichle – Fotolia.com #98157119
„L5 Actuator“, Lofelt