How Acoustic Experts Reproduce Outstanding Sound in the Vehicle
March 14, 2016:
A noise-optimized driving atmosphere and melodious music can greatly contribute to make driving by car much more pleasant. In order to achieve an outstanding sound experience developers and engineers are facing immense hurdles.
Outstanding sound is based on research and development
Anyone who would like to listen to sound similar to a concert hall in the car relies on a customized sound system. Music is to be reproduced as originally recorded: Clear, powerful and full of emotions. In order to meet this requirement, the early integration of the acoustic experts in the early development phase is necessary.
What is sound?
In the physical acoustics and music different definitions of sound are used. What is referred to in the technical acoustic as sound corresponds in music to the term tone. In the technical acoustic sound is called a periodic amplitude response with the period „T“, with individual frequencies (harmonics) in an integer ratio to fundamental frequency „f (1)“ . The following applies: T = 1/f (1). The phase relation of the individual tones to each other is variable. (Dieter Maute). Only in the case where the frequencies of these partial tones to one another is in an integer ratio, the tones relate to each other “harmonious”, the physics is speaking about a “sound”, otherwise from a sound mixture. (Extract Wikipedia)
What is meant by outstanding sound?
In order to enjoy music in highest quality in the car, the sound systems must play music as natural as possible to the passengers. For the acoustic experts of Mvoid “natural” means, to preserve the character of the music, to balance the tonality, to preserve the characteristics of space and dynamics. The intention of the artist is to be maintained by the reproduction through the sound system at its core.
Balance in tonality
In order to reproduce music tonally balanced irregularities and resonances (= acoustic eigenmodes) in the frequency response have to be balanced. A wrong tonality is felt by most listeners, even if it is only unconsciously. Music sounds like a skirl if a sound system emphasized the heights. The systems sounds dull, if the heights remain underemphasized. If the bass is reduced to a minimum, an essential element of the music is lost. If the bass is moved too much to the foreground, other frequency ranges are eliminated. There is a lack in the balance of tonality. The music isn’t felt as in the original. Associations, for example, with a live experience is minimized. In contrast, even details are audible by a well-balanced sound system, such as the pluck of a guitar string or the very first key-stroke of a keyboard.
Spatiality thanks to binaural reproduction
A live recording of a concert should reflect what is happening on stage. Consequently the reproduction must maintain both – width and depth of the stage. The singers are usually found in front of the stage in the middle, next to the bassist, guitarist, saxophonist, the drums are mostly towards the back. It’s a live concert feeling. If the spatiality is not reproduced like the original event on the stage, the music loses its character. Even when recording in studio musician are playing with certain effects. Using binaural reproduction techniques the acoustic experts are able to reproduce spatiality similar to the original recording.
Selecting appropriate speakers for an original dynamics
In music, dynamic means the relationship between loud and quiet passages of a piece of music. High dynamics face a sound system with great challenges, since both the very loud and very quiet sections must be reproduced without irregularities. Here the number and selection of appropriate speakers plays an important role.
Challenges of developers and acoustic engineers in the integration of sound systems in vehicles
A vehicle delivers one of the most difficult environments to reproduce sound. While in the home sound system only two speakers can be sufficient to reproduce outstanding sound, significantly more speakers must be integrated in the vehicle interior. In comparison: Speakers of home sound systems are designed according to different frequency ranges (high, mid, low). The same with vehicles, but in vehicles the installation space is highly limited, so several individual midrange speakers, woofers and tweeters have to be integrated in an excellent manner.
Positioning of loudspeakers
The speakers are arranged asymmetrically to the seats inside the vehicle. Depending on the size of a vehicle – small, mid-range, compact or premium vehicle – the number of speakers vary. In a small vehicle usually 4 – 6 speakers are installed, a premium vehicle can have 10 – 16 speakers. The existing space and acoustic aspects have to be considered when positioning the speakers. For each car model, whether sedan, SUV, Coupé etc. certain guidelines for optimum speaker positioning exist that initially serve as a starting point. In an analysis process the optimal position is to be found. The individual vehicle design or material properties require individual positioning for an outstanding sound.
Below you will find an example of a sedan:
Importance of sound velocity and wavelength
The human acoustic range covers approximately the range of 20Hz to 20,000Hz (at an older age, the ceiling may fall below 10,000 Hz). This corresponds to a wavelength range of about 17 m to 1.7 cm with a sound-propagation velocity in dry air of c = 343.2 m / s (= 1235.5 km/h respectively 1131 feet per second (771 mph). The velocity of sound changes significantly with temperature, something with the humidity, but not with our air pressure.
Wavelength of different audio frequencies in audible range:
|The wavelength affects how sound is radiated by loudspeakers and how it is heard by listeners. This is the reason why we build different sizes of drivers to cover different frequency ranges – tweeter, midrange speakers and woofers. They have to be accurately selected and matched.
Loudness is the perceptual correlate of physical sound level (dB). In addition to the analysis of the individual frequencies (bass, middle, treble) it is necessary to analyze the duration (transient, short or long notes), abrupt or smooth starts and stops, and the bandwidth (how much of the frequency range is covered?) of the loudness of the sound system.
Furthermore it is necessary to take disturbing noise into consideration, primarily caused by vehicle movement. Listening on the highway is very different from listening in the parking lot. Small changes in bass level or extension can have a dramatic effect on the overall impression of music. It explains why low frequency loudspeaker interactions are a dominant factor in sound reproduction.
The subwoofer gives the music its foundation. In order to reproduce low frequencies, a lot of air needs to be moved. As the space in the vehicle is tight, the bass is often difficult to integrate.
Adjust run-time differences to a uniform level
The passengers sit near a speaker and relatively far away and at a different angle to other speakers in vehicles. The result is that the sound is perceived differently. Even with optimum positioning of the speakers. The sound is distorted. The objective of the engineers and developers is to find an ideal frequency response for each of the speakers. This means that each speaker must be analyzed and rectified so that the acoustic energy at the relevant points in the vehicle is added optimally and not subtracted. Every single seat has to be analyzed and disruptions must be eliminated.
Compensate reflection characteristics due to different materials
A mix of different materials in the vehicle cabin, like textiles, leather, plastics, metals, glass, vinyl etc. and their acoustic influences must be analyzed acoustically and need to be rectified respectively eliminated. The surfaces reflect sound in different ways. Glass, for example, reflects sound strongly. In contrast, the headliner and paneling absorb sound due to its damping structure. A vehicle of the same model, but with other features, like leather or textile seats, ideally needs a different sound-tuning.
The exact positioning of the speakers, adjusting run-time differences, compensation of reflection characteristics and background noise are paving the way to the original sound. They ensure the desired tonality, spatiality and rhythm of the music.
Mvoid® – intelligent solution for optimal integration
Mvoid® simulation technology allows to analyze all above mentioned steps in a virtual product development environment, based on the CAD data of the vehicle. The sound system can be made audible on the computer.
In the past, the large number of acoustic influences meant that acoustic experts had to adjust respectively eliminate lots of parameters manually. Today, acoustic experts can achieve reliable results already by using modern simulation methods. Vehicle profile, equipment and material properties of a vehicle typically changes during the development period. The virtual model allows rapid adaptation to different system architectures and system content and analyzes the speaker positions throughout the development cycle, thus allowing a continuous evaluation and improvement. The simulation model can be used from concept development of the next generation of vehicles to the SOP.