Article VARI BASS Vicoustic - Research & Development

Transcription

1 Article VARI BASS Vicoustic - Research & Development 1

2 Acoustics is the science that covers sound, ultrasound, and infrasound. It s important to know how materials can change the acoustic environment of a certain space, how sound waves are actually generated, how they spread out in a room, how they behave when eventually hit a surface, how mechanical vibrations are transferred through a medium, and the reasons why they have these behaviors. Applying these acoustical principles in technology gives way to acoustical engineering. This branch involves working with acoustic equipment to take audio measurements, carry out signal processing tasks, have a specific knowledge regarding architectural acoustics and environmental acoustics, understand the concepts of acoustic transduction and ultrasonic acoustics, and the acoustic properties of a room. Understanding the acoustic properties of a room will help us to know how sound will behave in a closed space. When we consider closed spaces like Home Cinemas, Sound Control Rooms, and Laboratories, we already know each one serves a different purpose, however, the sound quality we actually want to obtain in each of them is quite similar. In other words, we want to give each of these rooms the correct treatment, so we can have good speech intelligibility. This article will therefore talk about a few of the main acoustic principles, as well as explaining the solutions we should opt in order to reach acoustic standards. In closed rooms we can have inconvenient sound reflections, reverberation and flutter echoes. Each of these processes depend on the size of the room, the construction materials used on the house, the type of windows and the nature of surfaces. To solve these problems we need to use absorption and diffusion materials. Absorption materials are porous and fibrous, and function as an energy converter, capturing sound energy and transforming it into heat. Diffusion materials have a different behavior. They will act upon the reflected sound waves scattering them and transforming them in smaller waves with less energy. With lower frequencies of sound present in a closed space, we are most likely to encounter resonance frequencies (also known as resonant modes). This happens when the sound waves generated by a source have the exact wavelength as the room. If this happens, then sound quality will be low. If we consider that a room has 6 surfaces (4 walls, the ceiling and the floor), the sound waves will travel inside the room along several axes. If the properties and characteristics of the these sound waves are not controlled, then the room will end up having bad audio performance, regardless the sound technology present. At the moment, the audio market offers 2 solutions. The first solution consists in using Bass Traps. They can be found in a flat shape, or a cylindrical form, and are placed in the corners of the room with the intention to soften the intensity of resonance frequencies. This can be achieved through a process of absorption. This solution is relatively standardized, however, the results can be quite limited. At the moment, none of the Bass Trap solutions available are enough to solve the problem. Scientific research has shown a very simple reason: the dimensions of a room and it s physical properties are a critical variable when it comes to picking the right solution. In other words, each room or space has a unique resonance frequency. Therefore, we have custom made solutions, specifically designed for each room, which include resonance mechanisms such as Helmholtz, and acoustic membranes. These are built after a careful analysis of the dimensions, characteristics and the materials that compose the room. Yet these solutions can be expensive, and lack some flexibility, because during a long period of time, the layout of the room can change, which will bring changes to the acoustic environment. Throughout these years, there has been a lot of analysis and research around this subject. We highlight in particular the work done by Everest, F. Alton (1994), Beranek (1996) ou Kuttruff (1997). Vicoustic created a new solution to deal with low frequencies - a new tunable device with the capability to absorb low frequencies, allowing the user to control resonance frequencies in his/her room. It s called Vari Bass. 2

3 A lot of theoretical research and development has been done, including prototyping and tests. The universities of Huddersfield and Salford in England, and in particular the work done by Mr. Bruno Fazenda have made an important contribution to this project. A tunable Bass Trap is definitely the revolutionary product the acoustic market needed. This new product follows the do it yourself idea that nearly all other Vicoustic s products have. However, the Vari Bass will add a new concept in the market called tune it yourself. During all the process of investigation and work that myself an Mr. Bruno Fazenda were doing, we eventually came across a whole lot of potential solutions that could fulfill the requirements. At a certain point in time, it became clear that the Helmholtz resonator method would be the best way to make the tunable mechanism of the Vari Bass. Taking into account the requirements of the construction industry, this would definitely be the best and most efficient option. The first prototype model was basically a simple Helmholtz resonator, which has a syringe shape, with an air cavity system in the interior, sealed with an insulating material, and a central tube connected to the air chamber inside. Image 1 - Vari Bass with an air chamber Image 2 - Measurements taken at Salford University, impedance tube When it came to the testing phases, we actually had some trouble finding the correct spots to take low frequency audio measurements, because we needed a room with a stable sound field in the low range of the sound spectrum. The reverberation chambers can only ensure a stable sound field above 100Hz. We therefore added an impedance tube to our device alongside the support from Salford University. We then concluded the first tests. The initial aim was to understand if the device was able to resonate, and if it was effective enough. The preliminary tests turned out to give positive results in the tuning chapter, however, the efficiency was quite low. Throughout the investigation process, a few alternative solutions were adopted. The main concern at this time was to build a device capable to be efficient and tunable at the same time, and we accomplished a model that looked promising. 3

4 How the system works As we know, the losses of energy happen by viscosity in the tube that is in contact in the air chamber. We eventually had the idea of increasing the number of tubes in the device. However, this would not be a good idea because it would modify the tuning range, due to changes in the mass-spring ratio of the overall system. The only way to get around this problem, would be to make a readjustment on the air chamber taking into account the number of tubes present. Keeping in mind these variables, we made the final model which turned out to have great tuning characteristics and excellent efficiency. The losses on the Vari Bass do not only happen through the Helmholtz resonator, but also through the interaction between the isolation (the rubber) and the air cavity, creating a sort of piston system that allows you to increase the efficiency of the device. Figura 3 - Vari Bass with 5 chamber Audio measurements in the room We carried out a few tests with one Vary Bass unit, in a room with 10m3 without any absorbent material. We tested out 4 different frequency modes - 63Hz, 66Hz, 91Hz. 124Hz. The Vari Bass was tuned to the same frequency as room modes. It s extremely important to know where to place the Vary Bass units in the room. As we know, Helmholtz resonators work best when placed in the exact location where the pressure is the highest. These locations are normally in the surfaces or corners of the room. The following images show the results of the audio measurements taken in this room. The first images show a comparison in the decay time for the different acoustic modes. We carried out one measurement using the sine sweep measurement to identify the acoustic modes. After identifying the modes, we generated sinusoidal sound waves with the same frequency as the modes. Then we interrupted the sinusoidal sounds to analyze the decay time and the outcome results with, and without the Vari Bass. As we can observe in the images below, the decay time decreased when we used the Vari Bass. We can also see that the energy of the first harmonic decreased. This is clearly shown in the small top line in the images. 4

5 Image 4 e 5 - The images above show a Vari Bass spectrogram, with the results before and after using the Vari Bass (for 62Hz and 66Hz). Image 6 - The images above show a Vari Bass spectrogram, with the results before and after using the Vari Bass (for 91Hz). Image 6 - The images above show a Vari Bass spectrogram, with the results before and after using the Vari Bass (for 91Hz). 5

6 In the acoustic modes of 66Hz and 91Hz we can clearly see a change in the color of the line, from a pinkish color to a more reddish one, indicating a substantial decrement if the mode s energy. We can also see a reduction of the sound decay (a reduction of approximately 6 to 9 db). The images clearly show extremely interesting results - the Vari Bass does not only decrease the energy of the acoustic mode, it also spreads out the energy evenly to the neighboring frequencies, modifying the Q-factor of the corresponding resonance frequency. This leads to more energy distribution in this range of frequencies, which is crucial to the energy gap between the resonance frequencies acting upon the acoustic modes of the room. The green color is the room s response - sweep sine; the blue color shows the corresponding harmonics without Vari Bass in the room; the red color shows the corresponding harmonics this time using the Vari Bass in the room. Images 7, 8, 9 - the graphics above show the results for the acoustic modes of 62Hz, 66Hz, and 91Hz 6

7 The reason why we chose to use a sine wave signal in each of the acoustic modes of the room is because it is important to evaluate the harmonics generated in the room and how it influences the room s response. If we look closely, we can clearly see that harmonics can influence the room s response. The graphics above show the efficiency of the device. For the frequencies of 62Hz and 66Hz, the results show decrements of about 6 to 8 db and for 91Hz we can see a reduction of 9dB s. These results are extremely impressive. It s important to note that we are only using 1 unit. And this unit does not only influence the fundamental frequency, it also acts upon the harmonics extremely efficiently. The results shown by the Vari Bass reveal that it can be efficient between the 60Hz and 120Hz, clearly indicating a wide frequency range. We cannot forget the importance of the foam involving the tubes that lead to the air chamber. The foam plays an important role in the device. The foam is an absorbent material, and with 35cm of diameter it definitely plays a major role absorbing frequencies above 100Hz, helping to reduce the energy of the harmonic s mode. To summarize how the Vari Bass works, we can briefly say that the device uses 3 important principles: absorption by a Helmholtz resonator, absorption by a piston system (isolating rubber used on the device), and absorbing foam. Jorge Castro [email protected] Vicoustic Project Manager and Research & Development University of Evora - Lecture Degree Music Audio Technologies IPP (PT) Eng. Electroacoustic & Acoustic (MSc/Pg) University of Salford (UK) Tiago Silva Vicoustic Civil Engineer Degree Civil Engineer ISEL (PT) 7