# An Open Access Chamber Designed for the Acoustic Characterisation of Microbubbles
## 2 Materials and Methods  
### Microbubble Acoustic Characterisation Chamber Design  
The Microbubble Acoustic Characterisation Chamber (MACC) was designed for acoustic measurements of populations of microbubbles. An image of the MACC, manufactured using an Objet1000 Multi-Material Polylet 3D Printer (Stratasys, Minneapolis, MN, USA) in Vero(tm) material, is shown in Figure 1. The MACC consists of a central chamber that can accommodate up to six single-element transducer housings, an inlet housing, and outlet housing. The MACC chamber is an extruded octagon design, and 8 of the chamber's faces feature a 22 mm-diameter channel. Transducer housings, the inlet housing and the outlet housing can be inserted into these channels. The other 2 faces feature 28 mm-diameter channels to accommodate inspection window housings. The total dimensions of the central chamber are 70 \(\times\) 70 \(\times\) 28 mm. The dimensions of the chamber represent a trade-off between limiting the internal volume, keeping the transducers operating in the far-field and reducing reverberations inside the chamber. Inspection windows allow a limited view inside the chamber and consist of 32 mm-diameter, 3 mm-thick, and clear acrylic disks situated in the inspection window housings. The central chamber features a series of 4 mm holes that are populated with M3-threaded inserts. All housings are secured using M3 socket screws and 1.5 mm-thick neoprene gaskets. The design files for the chamber, various housings, and neoprene gaskets are included  
Figure 1: Microbubble Acoustic Characterisation Chamber populated with two 2.25 MHz transducers (V323-SU, Olympus Industrial, Essex, UK), four blank inserts, the inlet housing, and the outlet housing. in the repository. The list of parts and the assembly guide for the MACC can be found in the knowledge base ([[https://github.com/UARPGitHub/MACC/wiki/AssemblyGuide](https://github.com/UARPGitHub/MACC/wiki/AssemblyGuide)]([https://github.com/UARPGitHub/MACC/wiki/AssemblyGuide](https://github.com/UARPGitHub/MACC/wiki/AssemblyGuide))). The chamber was designed using Autodesk(r) Fusion 360. An exploded view of the MACC is shown in Figure 2.  
The MACC inlet housing consists of two coaxially aligned nozzles with 2 mm outlets. Careful control of a sheath flow and a sample flow at the two inlets in the inlet housing allow flow focusing of the microbubble sample. This constrains the microbubbles in the centre of the chamber and ensures they remain in the far-field of the transducers without the need for a barrier. The outlet housing is a funnel design with a waste outlet at the top of the chamber. Both of these housings have Luer lock adapters inserted for use with stopcocks and tubing. Syringe pumps are used to control the inlet flows and the outlet tube is positioned, so the waste sample is collected in a waste container placed below the chamber. Transducer housings have been designed for Videoscan(r) transducers (V323-SU, V384-SU, V310-SU, Olympus Industrial, Essex, UK). The transducer housings secure these transducers in a fixed position. A transducer is secured in the transducer housing by connecting a UHF plug to the UHF socket of the transducer. The chamber has been designed for use with pairs of unfocused 2.25 MHz, 3.5 MHz, and 5 MHz transducers. The pairs of transducers are placed inline with each other in the chamber for reference measurements. Backscatter is measured by the transmit transducer. All of the transducers are at a fixed distance of 35-37 mm from the centre of the chamber. Slight variations might be found due to the compressible neoprene gaskets. In addition to these housings, a design for a blank insert has also been provided that can be used to create acoustic absorbers or acoustic reflectors with the addition of an appropriate modification. These inserts can be used to create more flexible experimental setups. When less than the full complement of transducers are used with the chamber, these inserts can be used to make the chamber functional.