02_The Hardware Project

Working in Ambisonics is known to be particularly demanding in terms of hardware. Typical Ambisonics rigs in a studio context are set up with a number of loudspeakers ranging from 8 to 16, in concert venues this is usually between 24 and 48 loudspeakers, but can go up to 140 for The Cube at Virginia Tech. These loudspeakers are usually high quality studio monitors and require powerful computers and audio interfaces to operate. It goes without saying that such systems are beyond the reach of most students and young artists, who rely on access to schools and other facilities equipped with such equipment to work on their projects. Such facilities are not always available everywhere or have strict access requirements.

An alternative is certainly to decode Ambisonics sounds into binaural stereo and use headphones, but to ensure accurate spatial rendering in binaural, users should use individualised Head Related Impulse Responses (HRIR's), which need to be generated [1].

For these reasons, we imagine that the construction of a small, portable and relatively inexpensive multichannel loudspeaker setup can definitely become an asset for students and artists who want to emancipate their workflow and explore ambisonics on a low budget. In addition, such a solution can also be useful for installation purposes, when you need to design and set up a multi-channel diffusion rig in a public space without having to leave your computer to control the installation.

The Loudspeakers:
Concerning loudspeakers, the choice fell on car systems. The reason for this is that modern loudspeakers for cars are often coaxial, which means that their tweeter is allocated inside the woofer, packing a two-way system in a single loudspeaker unit hence saving space. Wanting to build a compact system, we consider using these loudspeakers an advantageous solution. Moreover, being actually able to reproduce a proper full range, a two-way loudspeaker unit would offer better response on high frequencies compared to one-way woofer-only loudspeaker equivalents.

Another advantage in the use of coaxial loudspeakers in spatial audio application is that their behavior is a closer approximation of an audio point source than their non-coaxial two-ways counterparts, which increases directivity[2]. It is true that some coaxial loudspeakers generally present a slight time misalignment between the tweeter and the woofer, due to the tweeter usually protruding over the woofer, resulting in the sound from the tweeter arriving slightly earlier to the listener than the sound from the woofer. Generally, loudspeakers for cars do not provide any time-alignment correction, mainly because these loudspeakers are used in a nearfield context where this misalignment is not compromising the quality of the sound field. We are expecting to use our setup in a pretty similar context, employing either 4 or 6.5” inches loudspeakers, able to provide a reasonably defined soundfield.

The loudspeakers we have been testing for the project were:

Pioneer TS-F1634R 6.5”

Kenwood KFC-S1066 4”

These two speakers are very similar in terms of performance, with similar peak and RMS power, 4 ohm impedance and sensitivity. The only difference is in the frequency range, with the Pioneer claiming to go down to 31Hz, as opposed to the Kenwood's 45Hz. In our tests we found that the Kenwood was the better choice for this project. Details of the tests will follow.

The Amplifiers: 
For the amplifiers we decided to use pre-assembled aftermarket digital stereo amplifiers from a local electronics shop. They operate on 12 volts and deliver 20 watts per channel at 4 ohms, which does not exceed the RMS of the speakers we have chosen. They are also Class D amplifiers, the most efficient of the amplifiers commonly used in car audio, meaning that theoretically 100% of the input power is converted to output power and none of it is converted to heat and dissipated[3]. An alternative would be to buy small amplifier boards and mount them in a box, which could result in a more space-efficient solution.

The Power Supply:
As for the power supply, we are using a Mean Well LRS-350-12, which is pretty standard and delivers up to 350W at 12V. This allows us to comfortably power four 50W amps, taking into account any power dissipation.

Raspberry Pi:
We decided to use Raspberry Pi to control our system. Running 64x Raspberry OS, it seemed a good solution to implement software tools in Pure Data, Octave and Supercollider. In addition, with the release of RNBO by Cycling '74, it is now possible to run MaxMSP patches as standalone applications on Raspberry Pi, further extending the capabilities of this microcomputer.

Audio Interface:
There are several options for audio interfaces that work with Raspberry Pi. Typically, most class-compliant USB audio interfaces will work, but not many offer official Linux support, and even fewer have control software available. It is very difficult to keep track of all the hardware devices and their compatibility with different versions of Linux, and most websites that offer lists of compatible devices are not always updated[4, 5, 6]. One solution is to use dedicated forums and repositories, such as the reddit subgroup r/linuxaudio, which has a very active and helpful community. For our project, we decided to use a multi-channel audio interface designed specifically for Raspberry Pi called the Audioinjector Octo. It offers six channels of input and eight channels of output in a very compact setup. The Esi Gigaport eX and the Behringer UMC1820 are other affordable and reliable options.

Setting up the system:
One possible challenge that one might encounter when installing an ambisonics setup is to measure loudspeaker coordinates and positions. One of the easiest solutions to do so is using a laser meter and a protractor. for practical purposes I have made a printable protractor layout that can be used with a laser meter to calculate coordinates precisely. It can be downloaded here.

References:

[1] Guezenoc, Corentin, and Renaud Seguier. "Hrtf Individualization: A Survey." arXiv preprint arXiv:2003.06183  (2020).

[2] Kessling, Philipp, and Thomas Görne. "Studio for Immersive Media Research and Production: Immersive Audio Lab at Haw Hamburg." Paper presented at the Audio Engineering Society Convention 145, 2018.

[3] Jiang, Xicheng. "Fundamentals of Audio Class D Amplifier Design: A Review of Schemes and Architectures." IEEE Solid-State Circuits Magazine 9, no. 3 (2017): 14-25.

[4] https://discourse.ardour.org/t/multichannel-audio-interface-for-linux/89706

[5] https://wiki.linuxaudio.org/wiki/hardware_support

[6] https://alsa-project.org/wiki/Matrix:Main