When it comes to acoustic vents, size does matter!
An audiological engineer explains what we understand about vent size and how a new product improvement will help guarantee a good fitting for every custom earpiece.
Working as an audiological engineer at Phonak for 13 years I am always looking for new ways to improve our products. I am part of a team that regularly reviews and analyzes customer feedback to search for customer-driven ideas to develop improved solutions. I have a wonderful example of that. In our review of customer feedback, we noticed that HCPs were regularly asking for guidance on calculating vent size and improving the fitting of a custom hearing system.
A good fitting of a custom earpiece is dependent on a hearing care professional’s different inputs into the fitting software. When a hearing care professional communicates the precise size of the vent into the fitting software, the hearing performance improves for a custom shell. It sounds easy, but it is not always clear what we mean by vent size.
What do we mean by ‘vent size’?
The size of the vent consists of a measurement of the diameter, but it’s not as simple as it sounds. Looking at the different shapes and lengths of vent models today, a vent is not only always a straight cylinder – it can have different geometry.
That’s why it is important to take the shape and length of the vent into account.
There is a feature, developed more than 12 years ago, that contains all the information related to vent size – called the ‘coupling code’. It calculates the size of the vent taking into account all its geometrical parameters and computes an acoustic vent mass, AVM. This AVM is calculated with the highly sophisticated 3D modeling software (i.e., Rapid Shell Modelling software) that is used by our modelers during the process of modeling each individual custom shell. The calculated AVM is coded with 6 digits (i.e., the coupling code). Below is the formula of the acoustic vent mass:
where l is the length of the vent and A is the geometrical surface of the vent area.
In our fitting software, the length used for the fitting is based on an average that is l= 12mm. Looking at cavity vent styles, the length of the vent is l is 2mm. With a vent diameter size 1.5 mm, we can easily calculate the different of the AVM:
Knowing that 2 vents with the same AVM value do have the same acoustic performances, we can deduce that the vents below have the same performances:
Why the coupling code is so important
Without entering the coupling code (e.g. only entering the diameter of the vent), some important parameters of the vent are missing for a precise fitting. At this time, the coupling code is provided for orders with an acoustic optimized vent, AOV.
Need a refresher on AOV? Well, the AOV is a Phonak proprietary, multidimensional-weighted algorithm that sets an optimized target ‘acoustic mass’. By ‘optimized’ we mean the precise AVM needed for size and performance for a patient, based on their audiogram. The vent can then be modelled to reach the target AVM computed by the AOV algorithm.
AVMs are delivered (via coupling codes) for orders with an AOV:
For custom shell orders containing electronics, the coupling code is printed on the shell and sometimes additionally delivered with the special instructions included in the packaging of the earpiece. The hearing care professional enters the code manually into the fitting software.
For custom shells containing electronics (e.g., in-the-ear products), the coupling code is saved within the electronic piece and is automatically uploaded to the fitting software.
However, not all orders are an AOV. If a user-selected vent (e.g. 2mm vent) is ordered and cannot be modelled as is, the vent size could be changed to a custom large vent. This can happen if the ear canal is too small for the ordered vent to be modelled. And, with a custom large vent, it can be unclear to the hearing care professional which size of vent diameter to enter into the fitting software. This has been the case for hearing care professionals, until now!
New coupling code improvement released in February 2020
As of February 2020, all custom shells will be provided a coupling code, printed on the shell, if the vent ordered is an AOV or if the vent is a ‘user-selected vent’ (e.g. 2mm). That means that hearing care professionals can now input the exact vent size (AVM) for the custom earpiece into the fitting software by simply entering the coupling code created for the ordered vent. This change will improve the hearing performance of all custom earpiece fittings.
The coupling code is printed on every shell so that whenever hearing care professionals need it for the fitting, they can easily find it.
The coupling code written on the shell corresponds to the ordered vent, nevertheless, a Target update will soon be released to finalize this improvement so the vent displayed matches the vent size ordered.
Why is this big news for clinical practice?
Now that the code is both easy to access and available on all custom shells, I believe we will improve best practices when it comes to fitting because HCPs will automatically look for the coupling code on the shell and input it into the software.