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Knowles Corporation (NYSE: KN) is probably the leading manufacturer of advanced micro-acoustic microphones and speakers as well as high-performance capacitors and RF products. The company makes the incredibly intricate balanced armature drivers that use to be used exclusively in hearing aids but are now being used more widely in higher-end earphones.

To ensure its products are always at the cutting edge of the technology envelope, Knowles carries out regular research to find out what sort of sound consumers want. The latest batch of research from the company is looking to nail down what’s known as a “Preferred Listening Response Curve”. That’s the tonal response curve that we humans enjoy most when listening to music.

Sound quality is at the top of our listening wish list. However, a lot of companies are still designing their TWS (true wireless) earphones to a curve based on out-of-date research. Many have also been slow to take advantage of the developing technology available for hi-res audio.

The drive towards hi-res audio is being boosted by a growing number of lossless music streaming services like Tidal, Amazon Music HD and Apple Lossless. These high-end music services and the availability of better Bluetooth audio codecs, mean the earphone industry can now deliver hi-res audio without the need for wires, just so long as they are using the latest hardware and tuning.

The new research from Knowles provides some valuable insights into how earphone makers can create the very best-sounding audio via wired or wireless connections that their customers will love listening to.

To carry out the research, Knowles analyzed more than 200 sound recordings from the last 20 years of the Billboard Hot 100. The selection was deliberately chosen to ensure the music reflected the tunes that headphone wearers were likely to be streaming. The research revealed that many of the tunes included high-frequency content above 10 kHz. It’s this part of the frequency range that so many earphones currently on sale struggle to reproduce properly.

The Knowles team carried out a series of carefully controlled blind tests using a wide variety of listeners from a range of ages and hearing abilities. The purpose was to find out what the listeners preferred to listen to.

It turns out that optimum high-frequency response is essential when designing hi-res earphones. Using the latest technology, Knowles discovered the user preference for sounds beyond 10 kHz and this was used to create the new Knowles Preferred Listening Response Curve. By focusing on the high-frequency response, the new curve is uniquely suited to give earphone manufacturers the insight they need to create better products for hi-res listening.

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Key findings from the research showed that earphones tuned to an earlier concept of what sounds good, severely understated the amount of high-frequency energy that listeners liked. According to the Knowles Preferred Listening Response Curve, listeners consistently favored a boost of between 12 and 21 decibels (dB) at frequencies beyond 10KHz, depending on the listener’s age and hearing ability.

Designing and tuning earphone designs so that they match the high-frequency boost identified using the new Knowles Preferred Listening Response Curve should provide a more satisfactory end-user experience that ought to receive high satisfaction ratings from listeners.

Knowles claims that its new Curve is an especially powerful tool when used with hearing personalization algorithms. These are software tools that can shape the sound of a pair of earphones to suit the listener’s personal preference. Knowles included listeners with various levels of reduced high-frequency hearing response and found their preferred amount of boost. With this data, designers of TWS earphones that include hearing personalization can configure their algorithm to produce the optimum sound quality across the range of hearing abilities experienced most often by consumers.

Shehab Albahri Sr is the director of Knowles’ Hearing Health Technologies R&D. He says: “Consumers want hi-res, premium sound through their TWS earphones. This has been a challenge for OEMs who have had no clear guideline for what consumers prefer across the full spectrum. Now we know exactly how to design and tune a TWS earphone to create the best sounding audio available. Brands that design with hi-res capable hardware and tune their earphones to the Knowles Curve will unlock the true potential of lossless streaming audio.”

So, there we have it. Knowles has discovered the perfect recipe for making and tuning earphones and earbuds that listeners will find irresistible. I think it’s fair to say that the number of poor-quality earphones on sale is falling. However, by using this sort of high-level research, Knowles claims that earphone makers will be able to fine-tune their products to provide a sound that fits the sweet spot of human hearing, even for people who have hearing difficulties.

Notes: This research was first presented at AES Chicago Chapter Meeting on May 17, 2022. Knowles will also release a new TWS reference design tuned to the Knowles Curve, available in Q3 2022. A Knowles Preferred Listening Response Curve whitepaper with suggestions on the hardware needed to meet the new curve can be downloaded from Knowles.

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