"Invisible" Sound Experiences

Tactile Transducers Create Riveting Immersive "Invisible" Sound Experiences in Home Audio

Image Source: New Africa/stock.adobe.com

By Jean-Jacques DeLisle for Mouser Electronics

Published January 13, 2022

Introduction

Though “invisible sound” may seem like a nonsensical phrase, it is a common way of describing immersive sound or the sounds you can physically feel in response to media entertainment. Think Jurassic Park when the T. rex came along, heralded by a few mini earthquakes that caused vibrations in the iconic water cup. Large theaters create this riveting effect using high-power subwoofers that literally shake, rattle, and rock the room with pressure waves. This solution isn't always the best fit for a home theater or audio setup, but there are ways to replicate this effect without needing neighbor-enraging bass blasters. This solution is called a tactile transducer or "invisible" sound transducer. These transducers attach to objects, typically in contact with the viewer, and cause them to vibrate in response to low-frequency sound signals from audio devices.

As more consumers are looking to amp up their home theater/audio systems instead of seeking those thrills at the local cinema, the home audio market is swiftly expanding. This article aims to educate readers on the basics of audio transducers, specifically tactile transducers, and their role in the evolution of home audio.

Sound Transducer Primer

An audio or "sound" transducer is the bridge between sound waves a user senses or generates and the electronics that facilitate a modern audio experience. In essence, an audio transducer is a device that transforms sound waves in the frequency range of ~20Hz to ~20kHz—known as acoustic waves—to electrical signals and vice versa. In some cases, the terms sound waves and acoustic waves are used interchangeably; but more specifically, acoustic waves describe a subset of sound waves in the acoustic frequency range.

As a sound generator, an audio sound transducer (or, simply, an audio transducer) uses electrical energy to generate mechanical vibrations that stimulate the air at the surface of the transducer and result in sound wave emission. Devices that use electrical energy and output sound waves from electrical signals within the acoustic frequency range are typically called "speakers." In general, the original audio signal is relatively low power and must be fed to an audio pre-amplifier and then an audio amplifier to reach the desired power levels to drive a speaker system.

There are a variety of speakers and speaker styles and technologies, which are generally classified by their use and the audible frequency range that they cover. Bass speakers, known as woofers (Figure 1), typically cover frequencies from 40Hz to 1kHz.

Figure 1: Bass speaker, known as a woofer. (Source: surasak/stock.adobe.com)

A subwoofer, of course, covers an audio range below a woofer, generally from 20Hz to 200Hz. Midrange speakers exceed the audio frequency range of woofers and cover between 200Hz and 5kHz. Covering the higher audible range, are tweeters, which emit frequencies from around 2kHz to 20kHz. Other speakers, such as loud and full-range speakers, cover a wider frequency range. Still, they tend not to produce as impactful audio, given their limited reproduction of lower and higher frequency ranges or potential issues with fidelity.

But a speaker isn't the only type of audio transducer and certainly not the only type of sound transducer. A microphone serves the reverse function of a speaker. A microphone (or more generically, a sound detector) responds to the stimulus of mechanical vibrations at the surface of the transducer and generates proportional electrical signals. As these are typically very-low-power signals, an audio amplifier is often used to increase the gain of the microphone signal to achieve better reproduction or conversion.

What Are Invisible Audio Systems

Outside the acoustic frequency range, there are sounds below the audible range, known as infrasound, and sounds above the audible range, known as ultrasound. Infrasound generally refers to sounds between 0.1Hz and 20Hz, which most people cannot hear but can feel if powerful enough. Infrasound is also a beneficial range of frequencies for monitoring earthquakes, performing geological studies through vast amounts of earth, and for studying heart mechanics through ballistocardiography and seismocardiography. Another interesting note about infrasound is that elephants use infrasound to communicate over vast distances as the long wavelengths of sound in this frequency range travel more efficiently through dense atmospheres than higher-frequency sounds. The caveat is that natural and manufactured systems generally need to be rather large to produce such low sound frequencies.

On the opposite side of the spectrum, ultrasound is a range of frequencies beyond 20kHz with a shorter wavelength than most humans can hear. These very short sound frequencies tend to attenuate rapidly in the atmosphere, so they are limited in range. However, ultrasound frequencies also reflect from many surfaces, including human and animal tissues. This is why ultrasound transducers and sensor systems are used in medical ultrasound machines for non-invasive observation of internal body structures. For this same reason, bats use ultrasound for echolocation, as these high-frequency sound waves also reflect off insect body structures and most solid objects.

Most people cannot hear infrasound and ultrasound frequencies (Figure 2). However, infrasound, if powerful enough, can be felt. Many natural phenomena and human activities generate infrasound and audible sound simultaneously, which is a big part of the experience. Think of the feel of thunder from nearby lightning strikes, the sensation of being inside the cabin during take-off of a jet-powered plane, and the "roar" of an earthquake.

Figure 2: Sound wave diagram. (Source: designua/stock.adobe.com)

These invisible sounds are intrinsic to a visceral experience but are often neglected as part of the total experience of many entertainment systems. This is not the case in movie theaters, music concerts, and modern immersive virtual reality (VR) and augmented reality (AR) entertainment systems, which specifically include infrasound transducers or powerful subwoofers so that a user not only hears but also feels the sound experience.

Tactile Transducers in Home Audio/Entertainment

To some, this may sound like a highly exotic sound system setup. However, it is actually relatively straightforward, and there are plenty of options to integrate these invisible sound systems into a home audio/entertainment setup. In essence, an invisible sound transducer is merely an infrasound or audible sound transducer installed in the entertainment setup so that the users can feel the vibrations from the transducers. Large theaters and music performance venues use huge subwoofers to accomplish this effect, and gaming controllers have been equipped with vibration features for decades. Integrating a tactile transducer into a home audio/entertainment setup can also be done simply by mounting the transducer to the structural or support members of the user's seating. Using a suite of tactile and sound transducers may even be desirable to generate a richer feeling of physical stimulus, evolving beyond mere "bass shaker" audio systems.

This evolution of home entertainment is much like how many home audio systems in the past began incorporating additional speakers dispersed throughout the listening environment, with each speaker composed of a greater number of internal speaker elements. These multiple speaker elements, each designed to produce higher-fidelity sound more accurately and pleasurably in a given frequency range, are arranged around an environment to ensure greater immersion and a more seamless listening experience.

Incorporating infrasound and tactile transducers into this home audio/entertainment mix leads to a more lavish experience and can provide even deeper immersion. With the advent and growing accessibility of VR/AR setups in game rooms and home theaters, more users are looking for ways to enhance their gaming, viewing, or listening experience, making it more realistic or at least more exciting. There are even heavily electronically integrated chair systems for VR/AR setups designed with features like this in mind, making it easier for users to escape into the vastness of their VR/AR experiences.

To augment a home audio/entertainment system with invisible sound, users can upgrade the seating, entertainment fixtures, or user-worn equipment with electronics that are fed from the low-frequency range of the audio output of the entertainment system, such as the low-frequency effects (LFE) channel in Dolby systems. These systems operate much in the same way as subwoofers can but, in some cases, reach even lower frequencies than subwoofers to enhance the spectrum of invisible audio felt by the user.

Conclusion

Invisible audio systems are likely the next rage in home audio/entertainment, which is a big deal as people increasingly prefer to watch movies at home instead of theaters, and VR/AR system adoption is on the rise. Invisible audio technology has existed for many decades but is still relatively rare in home audio/entertainment systems. As home gaming and entertainment continue to boom, so will the desire for users' seating and interactive systems to boom along with the entertainment.