By enabling a small number of conventional speakers to be effective in controlling the sound field without surrounding the space with a group of speakers, a range of sound applications could be broadened. In addition, by controlling the sound distribution of the areas with a range of roughly one meter, this technology could be used for the cases where the listener is one or so and not wearing wearable speakers.
Furthermore, minimizing the number of elements that must be custom-tailored to the usage application, and using general purposes devices, makes it possible for anyone to use these technologies at little cost. Controlling the directionality of sound including long wavelengths (low frequencies), which is difficult when using conventional phase delay control, expands the range of frequencies that can be used in controlled sound sources. This makes it possible to use various types of sounds. These concepts all contribute to enhance freedom in use cases.
To make these concepts a reality, we began by establishing a technical approach for controlling sound pressure using speakers of N numbers, and then we worked to reduce the value of N.
In general, the smaller N is, the less the freedom of the controllability of sound pressure is, and the greater the technical difficulty is. We created opposite phases between speakers even when small numbers of speakers were used. As a result, we currently can produce sound field controls with an N value of just three -- in other words, just three speakers.
And now, creating a sound pressure distribution within a small, roughly one meter space requires additional ingenuity. Creating a steep sound pressure gradation over a small area is difficult, because sounds do not attenuate much with distance.
In situations, such as when providing automated voice guidance at a small device like an ATM or ticket machine, when speaking with a customer contact point, or when taking part in an online meeting, the sound is often transmitted to other people, besides just the person to whom the sound is directed. Therefore, when considering the transmission of sounds to the people who need it, it is important to realize “small space control” that enables the creation of sound pressure distributions not only in larger 5 to 10-meter areas but also over compact areas of a meter or less.
To make this “small space control” a reality, we decided to combine two types of control low: control that reduces acoustic power*1, a characteristic value that indicates the energy level of a sound source, and control that maximizes sound pressure within an area. We have balanced the use of these two types of control low in our method what we call the “Combination method” (which we refer to hereinafter as the “C method”).
The C method, which makes it possible to realize both types of control with three speakers, has succeeded in creating a sound pressure gradient over a small space.
*1 Acoustic power: The rate at which sound energy E[J] is emitted by a sound source per unit of time (1 sec). Acoustic power is represented in units of watts (W=[J/s]). The acoustic power level is 10 times of the logarithmic value of W/W0, where W0 is the reference value of acoustic power. It is represented in units of decibels (dB). It is a characteristic value that indicates the level of sound energy of an audio source, and differs from the acoustic pressure level which changes with measurement distance.
Considering actual usage situations, the locations and sizes of areas where sound is meant to be heard might differ even within the same space. It would be extremely time- and labor-intensive to rearrange speakers or change speaker types to produce the desired characteristics every time. Furthermore, an approach that required optimization by engineers every time when use case changed would cause obstacles to the use of these sound field control technologies in general-purpose applications.
That is why we worked to design a technology based on the concept of using software control, without needing to change hardware configurations. This technology would optimize and control the distribution of areas with high sound pressures and areas with low sound pressures by software control, once the number of speakers and their locations had been decided. We were also able to include the sound with low frequencies which are hard to control using conventional approaches, into the controlled objects, by adjusting “virtual boundaries,” which are set when adjusting phases.
To make it clear that Toshiba’s sound field control technology could be used to select areas and control the sound pressure distributions within them using simple systems, we named the technology “sound field control for region separation technology.”