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Enhancing Audio Experiences through Equal Loudness Curve Correction, Powered by Smartphone

Updated: Apr 15, 2023


Understanding the way your listener perceives sound is a key part of creating a memorable listening experience, whether you are a writer, producer, engineer, acoustician, or even for the home-audio enthusiast. One of the key steps in this journey is understanding the impact of equal loudness contours on the listening experience.


In this blog post, the Oikla team explore how equal loudness impacts the listening experience, and why Perception will revolutionise the way you listen. Perception, our revolutionary audio technology, utilises your smartphone’s microphone to understand your listening space. It builds a bespoke solution for the way you listen; in the space you listen.





Equal Loudness and the Human Ear


Not all sounds are equal: or at least, not to us! Our ears, in fact our whole auditory system perceive the loudness of different frequencies at varying sound pressure levels (SPLs) very differently. Technically, this phenomenon is known as nonlinear behaviour in our auditory system. Our auditory system is more sensitive to mid-range frequencies (between 1 kHz and 5 kHz) than low and high frequencies. The result is that bass and extreme treble frequencies seem comparatively quieter than mid frequencies. As SPL increases, our sensitivity to low and high frequencies also increases, causing them to appear louder relative to mid frequencies.


Equal loudness curves, or equal loudness contours, depict the relationship between frequency and our perception of loudness. These are key in understanding how we experience any sort of complex sound, from dance music to an explosion in a video game. Loudness levels are measured in phons, with one phon being equal to one decibel (dB) of sound pressure level at 1 kHz. This phenomenon is surprisingly universal.


An understanding of these dynamics is essential for anyone trying to understand the listening experience; what makes for a balanced and natural-sounding audio experience can change dramatically across various listening volumes. Whilst the measured SPL remains constant across all frequencies, our ears' varying sensitivity is what causes certain frequencies to be perceived as louder or quieter than others at different SPLs.


Our auditory system has evolved to be most sensitive to particular frequencies; the range between 1 kHz and 5 kHz. This sensitivity is essential to human communication and detecting of crucial auditory cues in our environment. It’s part of how we can filter out speech on a noisy street, or how our ancestors could locate their lunch! The result is that our perception of loudness is not uniform across the entire frequency spectrum. In an aesthetic based on extremes of high and low – the bass of a dance track or the ear candy of a movie sound effect – this can create huge problems for designing or playing back audio.


below how your sound perception changes as sound level changes


SPATIALIZATION

Another less frequently discussed effect of this equal loudness phenomenon is the influence it has on the spatialization or the perception of sound location. Spatialization is a critical aspect of audio experiences; it creates a sense of depth, direction, and immersion whether in music production, movie soundtracks, or virtual reality environments.


Ready for the science? Our auditory system relies on several cues; mainly interaural time differences (ITD) and interaural level differences (ILD), to locate sounds in space. Try closing your eyes on a train – you still have a strong sense of not only where each sound is coming from but also how far away it is. This ability to locate sounds is space is easily affected by sound pressure levels; put simply, we’re good at locating the sounds that are important to us. Sounds far away (or unusually quiet) don’t represent either danger or food, so we’re simply not designed to give them the same level of attention.

When you factor in the non-linear response of our ears to different frequencies, it becomes clear that this issue can be a minefield for sound designers. At lower SPLs, our hearing is less sensitive to extreme low and high.

Consequently, our ability to localize these sounds may be diminished. Much of the information which allows us to localize a sound is included in the low and high frequency information in a sound.


This relationship between SPL, equal loudness curves, and spatialization is central to the understanding of how an audio event is reproduced. By taking into account the influence of SPL and frequency on sound localization, sound designers can create more immersive and realistic audio experiences. For example, in music production, an awareness of these factors can help engineers balance the stereo image and create a sense of depth and space within the mix. In movie soundtracks and virtual reality environments, understanding the impact of sound level and equal loudness curves on spatialization can lead to more engaging and believable soundscapes.


To better understand the influence of sound levels and equal loudness curves on spatialization, let's consider an example from the gaming industry. Imagine a scene in a video game where a sudden explosion occurs. Explosions typically have a significant amount of energy in both low and high-frequency ranges, creating a powerful and impactful sound.


At lower SPLs, the player's perception of the location of the explosion is less accurate due to our ear’s natural roll-off of high low and frequencies. The low rumble and high crackle of the explosion might appear less prominent, which can affect the overall realism and immersiveness of the gaming experience.


As the SPL increases, our perception of low and high frequencies improves. In this case, equal loudness curves demonstrate that the perceived loudness of the explosion's low-frequency rumble and high-frequency crackle becomes more balanced with the mid frequencies. As a result, the player's ability to localize the explosion's source improves, and the overall spatialization becomes more accurate.


By considering the relationship between sound levels, equal loudness curves, and spatialization, game audio designers can create more realistic and immersive soundscapes for players. This can be achieved through careful mixing, ensuring that the game's audio is well balanced across all frequency ranges, and using advanced audio spatialization techniques that account for the human auditory system's non-linear response to different frequencies. In doing so, the gaming experience becomes more engaging, drawing players deeper into the virtual world.



The History of Equal Loudness Curves


The journey to understanding equal loudness curves began with early research by Harvey Fletcher and Wilden A. Munson in the 1930s. Working for the Bell Telephone Laboratory, Fletcher and Munson realised that not all frequencies are perceived equally. Their work led to the development of the Fletcher-Munson curves, which were the first attempt to map equal loudness contours. They also contributed to the first commercial hearing aids, and the development of telephone microphones. Since then, the International Organization for Standardization (ISO) has built on their prioneering work, developing the ISO 226:2003 standard, which includes the most recent and accurate equal loudness contours.


How Equal Loudness Contours are Measured


Let’s start from the idea that no one has “20/20” hearing. In the same way that very few of us have perfect eyesight, with some of us resorting to corrective lenses, very few of us have textbook ‘perfect’ hearing. Equal loudness contours are measured in the same way as an optician’s chart; test subjects listen to sine waves at different frequencies and adjust the volume of a comparison tone until they perceive both tones to be equally loud. This process is repeated for various frequencies and sound pressure levels, creating a comprehensive map of frequency sensitivity across the subject’s hearing range. It is important to note that individual variability plays a role in loudness perception, which means that equal loudness contours represent an average across a large sample set.



Applications of Equal Loudness Curves


We have already seen the roll this data plays in mixing and mastering music, and sound design for game and cinema. Equal loudness curves are also essential in headphone and speaker design, ensuring that the frequency response accurately responds to how humans perceive sound. This data is also to developing audio codecs and compression algorithms, optimizing audio quality and file sizes; understanding what information is necessary to accurately recreate an audio event has made streaming music a reality.



Real-world Case Studies


Equal loudness creeps into many fields disparate fields; in airplane design, where the noise levels at take-off landing and are heavily regulated by law, understanding how high and low frequency sound will propagate across a city, and be perceived by the population can mean the difference between the success and failure of a design. The Benefield Anechoic Facility in Los Angeles is currently the world’s largest sound isolated space; used for testing new aircraft prototypes to understand their impact on the world. With the Boeing 737 capable of producing an ear-rupturing 122 dB on take off, it’s hardly surprising that this issue is so heavily regulated.

The design of a room can easily colour the results of any industrial testing; the shape of room, the materials covering the walls, ceiling, and even floor can reflect specific frequencies, whilst absorbing others, dramatically changing the frequency profile of a sound event. An anechoic chamber eliminates all forms of reflection or reverberation, producing the most accurate test results, and also the quietest places in the world.

At the other end of the spectrum, I recently spoke to manufacturers Read Audio, who build three tiny audio drivers into their custom-moulded R303 In Ear Monitors for live musicians; replicating the acoustic event that their audience is getting to a level of precision that the average air pod can only dream of! For Read, and for their customers, understanding the ear for which their IEMs are designed is as important as an optician crafting my latest prescription. Just as a room can colour and influence the sound, so the shape of the individual ear can adjust the results from a set of earphones. Moulding the unit to the contours of my ear doesn’t just produce a comfortable fit, it ensures the most accurate sound possible.



Introducing Perception – The Solution to Equal Loudness Curve Challenges


Perception is an innovative audio technology designed to address the challenges posed by equal loudness curves and enhance the listening experience in real-world environments. By connecting to your Smartphone's microphone, Perception captures the audio you are listening to and applies equal loudness contour correction, delivering the most balanced and immersive audio experience available. Here's how Perception works:


  1. Perception connects to your Smartphone's microphone, streaming the audio you are listening to in real-time.

  2. Perception filters the captured audio input through an inverted equal loudness curve phone to estimate the loudness of the incoming signal, producing a dynamic dB A value.

  3. Perception uses the dB A value obtained from the inverted equal loudness curve to estimate the loudness of the incoming signal, utilizing the ISO:226:2003 standard.

  4. The user selects a desired listening level using a knob, which calculates the corresponding phone curve.

  5. The phone curve generated in Step 3 and the selected phone curve from Step 4 are compared.

  6. The resulting difference represents the necessary EQ gain to be applied to the audio signal to match the desired sound perception.

  7. Perception equalizes the audio signal using Fast Fourier Transform (FFT), allowing for specific equalization that would not be achievable with other EQ methods.

  8. The left and right audio channels are processed independently, ensuring the best possible stereo image.

  9. Perception applies the calculated EQ gain to the audio signal, adjusting its equalization to match the user's preferred sound perception.

The result is a listening experience that will seem louder, wider and deeper than the actual sound pressure level in your room, offering a more immersive and engaging listening experience in real-world environments.



Benefits of Using Perception

  1. Hearing Protection: Protecting your hearing is the most important thing you can do; noise-induced hearing loss (NIHL) is irreversible. Together with other hearing-related issues, NIHL can have a huge impact on your quality of life. Perception allows you to listen at safer levels. You can reduce your risk of hearing damage without compromising your listening experience.

  2. Reduced Listening Fatigue: Listening at high volumes for extended periods doesn’t just lead to discomfort. Over time, you become acclimatized to the frequency response, leading to ear fatigue, and loss of judgement, and potentially even permanent hearing damage. By offering a louder sensation at safer levels, Perception enables you to enjoy the same audio content for extended periods without experiencing listening fatigue, ensuring a comfortable and enjoyable listening experience.

  3. Enhanced Audio Clarity & Reduced Total Harmonic Distortion (THD): Audio clarity is a crucial aspect of any listening experience. At lower listening levels, loudspeakers typically exhibit reduced THD, resulting in a more accurate representation of the original audio signal. By allowing you to listen at more favourable levels for your speakers, Perception allows you to get the best out of your set up, leading to improved audio clarity and a cleaner, more accurate audio experience.

  4. Reduced Reflections, Improved Sound Localization & Enhanced Immersion: Lower listening levels will also give you a clearer stereo image, more precise sound localization, and reduce room reflections. The result is a more immersive and realistic audio experience. At lower sound pressure levels, room reflections and reverberations are diminished. The result is an improved audio environment without spending thousands of pounds on acoustic design and treatment. As SPL increases, our sensitivity to low and high frequencies improves, resulting in a more balanced perception of sound across the frequency spectrum. This in turn, results in enhanced sound localization and more accurate spatialization, as illustrated by our video game explosion example. By using Perception to maintain a louder perceived experience at lower levels, listeners can experience a more immersive overall audio experience.

  5. Improved Speech Intelligibility: Clear and intelligible speech is essential for understanding conversations, podcasts, and other vocal content. Dynamic compression was originally pioneered to improve radio reception for soldiers in the field. Perception's equal loudness curve correction improves the clarity of speech frequencies, making it easier for listeners to comprehend vocal content even at lower listening levels.

  6. Increased Immersion: In the real world, sound comes from an infinite number of points around you. In recreating that audio, the engineer has to recreate that illusion using two, five, or eleven speakers. Immersion is the measure of how well that magic trick has worked, creating a universe of sound for you. Allowing you to experience the full depth and dynamics of audio content without the need to push the volume to potentially harmful levels, will help bring the magic of immersive audio to life.

  7. Optimized Headphone and Speaker Performance: Your auditory system's innate non-linear response to different frequencies plays a part in the performance of your headphones and speakers. By accounting for these perceived differences, Perception enables you to get the most out of your audio equipment, ensuring an enjoyable and accurate listening experience at any volume level.

  8. Personalized Listening Experience: Your individual preferences and hearing sensitivity are personal to you. My ears work differently – in just the same way as my glasses won’t help you see better. Perception allows you to customize your preferred listening levels, catering to individual preferences and delivering a satisfying audio experience tailored to each listener's needs.

  9. Increased Accessibility: Providing a louder perception at safer listening levels makes audio content more accessible to individuals with varying degrees of hearing sensitivity. By optimizing your listening experience with Perception, everyone can enjoy audio experiences without risking hearing damage, ensuring that audio content remains accessible and enjoyable for all.


Conclusion


Perception offers a ground-breaking solution for enhancing real-world audio experiences by addressing the challenges posed by equal loudness contours. With the benefits of reduced THD, enhanced audio clarity, hearing protection, reduced listening fatigue, increased immersion, better localization, and optimized headphone and speaker performance, Perception brings the power of equal loudness curve correction to you, however you listen. Explore and apply the power of equal loudness curves and leverage the power of Perception. You won’t believe the magic of the results.

We need you to be part of Perception’s beta testing. Your ears are invaluable in changing the way the world listens. Please sign up [here](insert link) to join our team. Your feedback and contributions will help us refine and improve this ground-breaking audio technology, brings the power of equal loudness curve correction to all.


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