Meeting the Demands of Modern EV Sound Measurement:
The Rise of the ¼” Microphones

In the rapidly evolving landscape of electric vehicle (EV) technology, a recent GRAS whitepaper sheds light on the critical role of ¼” microphones in acoustic testing. They represent a significant step forward in ensuring consistent, high-quality acoustic testing across the industry that aligns with recent recommendations from the AES.

From one dedicated family to another: The GRAS story continues

In 2024, it is 30 years since visionary acoustics pioneer, Gunnar Rasmussen, founded Gunnar Rasmussen Acoustic Services – today known as GRAS Sound & Vibration. What has followed since then is a fairy tale story of growth and acoustic achievements, all built on Gunnar Rasmussen’s guiding vision of innovation and customization. Today, GRAS is part of the Axiometrix family of measurement solution providers. But Gunnar’s vision and values still guide the company.

Pass-fail windows

On a production line, the devices under test (DUTs) have a set of specifications from R&D or the customer that they must comply with. This can be viewed as a window on a graph.

The shaded red areas indicate a simplified hypothetical pass-fail window.

A day without EQset versus one with EQset™

There are many factors involved with running a production line that can affect its efficiency. Some of them are controllable and others are just facts of life.  Calibration (particularly calibration frequency) is one area that can be adjusted and have a considerable impact on the performance of the line. Of course, there is a trade-off. The trade-off for traditional microphones on a production line is increased uncertainty. Reduced calibration is more uptime on the line, but it results in an increase of good units being tagged bad and bad units making their way into the market, damaging customer satisfaction and adding the expense of returned products. Another option is a microphone that is stable and accurate without frequent calibration.

Electroacoustic Engineer, Brian Johansen, has tested sound in everything from airplanes and cars to refrigerators and satellite launches.

It took over 40 years for someone to come up with a better ear simulator than the 711 coupler originally developed by acoustic engineering pioneers Per Brüel and Gunnar Rasmussen. That someone was Brian Johansen.

What's the connection?

They are not directly related, as such. An initial guess might involve how much ash plane engines can take. While that would be on the right track, it is not the connection in this particular case. 

Understanding laminar flow, turbulent flow and the transition region

As examined in the What is the difference between acoustic and aeroacoustics? article, the studies of aerodynamics and aeroacoustics are bound together. Using microphones, surface- or flush-mounted, in a wind tunnel focuses on measuring the characteristics of air pressure in the boundary layer of an object in flow through a medium (or the medium around the object).

The answer is…

Aeroacoustics is a subset of the field of acoustics. The end...

But before you go, it’s not quite so simple as it may seem, and it is much more interesting.

Remove the grid for better data

Measurement microphones are highly specialized and fine-tuned devices designed for sensing minute dynamic variations in the ambient pressure. These variations are so small that measurement microphones use a very thin (only a few micrometers thick) metal foil as a diaphragm to be able to sense those variations. The thinness of the diaphragm allows it to deform even when the excitation is extremely small, making the detection of pressure variation possible even at very low sound pressure levels.

Why so small?

The Accuracy at high frequencies is vital, and... post, deals with the benefits of using a ¼″ microphone compared to the more commonly used ½″ mics and measurement scenarios where those benefits outweighed the drawbacks, or where the drawbacks weren’t relevant. The obvious question, of course, is why stop at the ¼″ microphone? If there are benefits to smaller microphones, and the areas of interest in noise mitigation and sound quality trending to hi-res and higher frequencies, how small can microphones get and still be viable?

Microphone size matters

In any measurement scenario, the measurement equipment and the measurement environment interact in two ways. The first and simplest way is how the microphone ‘adds’ data to the environment, and the second is how the microphone ‘extracts’ data from the environment. And size has a great impact in both directions.

Where does your noise come from?

Our perception of what is and is not an acceptable noise level has changed a great deal in the past 100, or even 25 years. The change in designs for improvements in transit times, fuel economy and even energy production are visible all around us and impact our daily lives. Vehicles on the motorway, local commuter and high-speed trains, aircraft, and wind turbines all impact our lives in various ways, including the addition of noise.