Making a powerful sound is easy. Making one nobody can get used to takes research.
This page sets out what LIMA SHIELD is built on: eighty years of work on hearing, auditory pain and psychoacoustics, and what each finding demanded of the product's design. The references are cited, they are public, and you can check them.
The idea of using sound as a means of deterrence is neither recent nor marginal. It has been studied, funded and documented by institutions for whom it was no hobby.
During the Second World War, the psychological impact of extreme noise was analysed for military purposes. For the first time, measurements showed that a sound could make a person back away without a single shot being fired.
Peter J. Westervelt's work on parametric acoustic arrays established that it is possible to create narrow, directional sound beams. This is the theoretical foundation of every directional acoustic device that followed.
Laboratories, including those of NASA, pursued research on these parametric arrays for underwater, industrial and atmospheric applications. Directional acoustics left the laboratory.
DARPA funded research programmes on acoustic devices and sound projectors. The physiological and neurological effects of acoustic waves were studied systematically.
The LRAD (Long Range Acoustic Device) was officially deployed. It went on to protect ships and bases, and was used in theatres of operation. Acoustic deterrence became standard equipment for institutions.
We built on this foundation to design a device specifically conceived, limited and regulated for civilian use: adjustable intensity, advance warning beeps, adjustable emission duration, automatic shut-off. The technology draws on this research; the product itself is designed for a home, a shop or a warehouse.
The human ear does not hear all frequencies with the same intensity. At an identical sound pressure level, some are perceived as much louder than others. This is what equal-loudness contours describe, one of the best-established findings in all of acoustics.
The founding work. Fletcher and Munson (1933) established the first equal-loudness curves. Robinson and Dadson (1956) refined them. The ISO 226:2003 standard is today's reference.
What we can say with confidence: at an equal level, a sound in the 2 to 5 kHz band is perceived as louder, more urgent and more unpleasant than others.
Two anatomical reasons explain this peak sensitivity. First, this band corresponds to the resonance of the ear canal: the canal itself amplifies these frequencies before they even reach the eardrum. Second, it is the critical zone for understanding speech: the human brain gives it special processing priority, because that is where the intelligibility of language is decided.
The consequence for design: emitting in this band achieves maximum effect for a given electrical power. It is not brute force, it is targeting.
The threshold at which a sound stops being unpleasant and becomes unbearable is documented by public health bodies and by hearing-risk standards.
The sources. World Health Organization, Environmental Noise Guidelines. NIOSH and CDC, Occupational Noise Exposure. The ISO 1999 standard on estimating hearing risk.
Established finding: the average threshold of auditory pain lies between 120 and 130 dB SPL.
At these levels, three phenomena appear: a reflex to flee, an inability to remain voluntarily exposed, and acute stress accompanied by functional disorientation. The key point, often misunderstood: this is not conscious reasoning. Nobody decides to endure such a sound level. The reaction stems from primitive neurosensory reflexes, governed by the brainstem, long before willpower comes into play.
That is why LIMA SHIELD can be set from 86 to 138 dB at one metre. Even at 86 dB, the lowest setting, the sound is already unpleasant enough to make staying hard to bear; the high settings, from 120 to 138 dB, reach the pain threshold. Depending on the application, a range of 128 to 133 dB is enough to achieve the intended effect without reaching the thresholds associated with permanent damage during short exposure. The goal is to interrupt the intrusion, not to cause injury.
This is what separates a very powerful siren from a sound shield. Building a 130-decibel siren is technically simple. Building a signal the brain cannot get used to is a different craft altogether.
The standard reference. Zwicker and Fastl, Psychoacoustics: Facts and Models. The book establishes that perceived annoyance depends not only on the decibel level, but on several psychoacoustic quantities: loudness, sharpness, roughness, fluctuation strength and tonality.
Demonstrated effect: a tonal, modulated sound is judged significantly more unbearable than a continuous noise at the same level.
The mechanism is perceptual adaptation. Faced with a constant stimulus, the auditory system adjusts: perception dulls, attention drifts, the annoyance becomes bearable. That is what happens with a conventional siren in under a minute, and it is precisely what experienced burglars know how to exploit.
Research on habituation and modulation. Hellbrück et al. (1999), Fastl (2007), and the DIN 45692 standard for measuring roughness.
What these studies show: faced with a constant sound, the brain adapts. Faced with a modulated, fluctuating sound, adaptation is severely limited. The result is cognitive fatigue, stress, and the inability to tune the signal out.
Stated precisely: temporal and spectral variations reduce perceptual adaptation and increase subjective annoyance. The LIMA SHIELD signal varies several times per second, following a controlled, evolving progression. The brain can neither predict it, nor get used to it, nor push it into the background. Instead of decreasing over time, the annoyance stays constant.
A very high-intensity sound is not simply heard. It affects balance and the perception of space.
The sources. Brian C. J. Moore, An Introduction to the Psychology of Hearing, the reference work on the psychology of hearing. NIOSH studies on the relationship between noise and balance.
Established data: very intense sounds disrupt the vestibular system, with reported effects of spatial disorientation, difficulty moving, temporary confusion and acute stress.
In technical terms: the level and character of the sound stimulus cause sensory overload leading to functional disorientation. In practice, an exposed person does not just want to leave: they struggle to find their bearings, to communicate, to carry out a coordinated action. That is why an intruder, even a determined one, stops what they are doing.
It is also why covering your ears is not enough. Sound pressure is felt physically, particularly as vibrations in the skull and chest. Even with proper hearing protection, the intruder is left without any useful perception of their surroundings: they can no longer hear what is happening around them, or anyone else arriving.
A deterrent device that can be spotted is a device that can be disabled. Indoors, physics works in our favour.
The sources. Heinrich Kuttruff, Room Acoustics, the reference work on room acoustics. The ISO 3382 standard on measuring reverberation.
Known effect: at high pressure levels and in the presence of reverberation, reflections multiply, directional cues are lost, and the result is the sense of a sound coming from everywhere.
The phenomenon is particularly marked in hard-walled environments: tiled rooms, stairwells, warehouses, industrial spaces, technical rooms. The sound reflects off surfaces, gradually fills the space, and the ear loses the timing cues that normally let it locate a source.
For the intruder, the consequence is direct: they cannot identify where the signal is coming from in order to go after the device. On top of that, the unit's tamper protection triggers the deterrent if anyone tries to tear it off. The only way left to go is out.
Every scientific finding on this page has a concrete counterpart in the machine. No design choice is cosmetic.
| The scientific finding | How it shows up in LIMA SHIELD |
|---|---|
| Peak hearing sensitivity between 2 and 5 kHz (equal loudness, ISO 226) | Emission calibrated to the 2 to 5 kHz band |
| Pain threshold between 120 and 130 dB SPL (WHO, NIOSH, ISO 1999) | Adjustable range from 86 to 138 dB at 1 metre, typical use between 128 and 133 dB |
| Perceptual adaptation sets in with a constant sound (Zwicker & Fastl) | Signal modulated several times per second, controlled and evolving progression |
| Sensory overload and functional disorientation (Moore, NIOSH) | Even distribution that saturates the space rather than a single point |
| Loss of directional cues in reverberant spaces (Kuttruff, ISO 3382) | Positioning and distribution designed to exploit reflections, source hard to locate |
| Hearing risks arise with prolonged exposure (ISO 1999) | Advance warning beeps, time delay, emission in sequences alternating sound and pauses, automatic shut-off |
On top of this come the tests carried out on the product itself: acoustic measurements to EN 61672-1 (class 2) and EN 50332-2, compliance with Directive 2003/10/EC on noise exposure, and an individual check of the emission cycle before each unit is shipped.
Scientific rigour also means setting out what the cited work allows us to state, and what it does not.
Finally, a useful point about exposure: as with any high-intensity acoustic device, very close and prolonged exposure can cause lasting hearing damage. That is exactly what the framework of use prevents, with the prior warning phase and the installation rules. The full regulatory framework is detailed here.
| Area | References |
|---|---|
| Hearing sensitivity and equal loudness | Fletcher & Munson (1933) · Robinson & Dadson (1956) · ISO 226:2003 |
| Auditory pain and risk | WHO, Environmental Noise Guidelines · NIOSH / CDC, Occupational Noise Exposure · ISO 1999 |
| Psychoacoustics of annoyance | Zwicker & Fastl, Psychoacoustics: Facts and Models · Hellbrück et al. (1999) · Fastl (2007) · DIN 45692 |
| Hearing, balance and disorientation | Moore, An Introduction to the Psychology of Hearing · NIOSH studies on noise and balance |
| Acoustics of enclosed spaces | Kuttruff, Room Acoustics · ISO 3382 |
| Directional acoustics | Westervelt's work on parametric acoustic arrays · NASA and DARPA programmes · LRAD (2003) |
| Product measurement and compliance | EN 61672-1 (class 2) · EN 50332-2 · Directive 2003/10/EC |
This research is public. We cite it because it forms the foundation of the design, and because an informed buyer should be able to check what they are told.
A demonstration with a security expert is the most direct way to understand the difference between a loud sound and an engineered signal. LIMA SHIELD is produced in limited runs and is regularly out of stock.
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