Ultrasonic pest repellers are specifically designed to emit sound waves above the range of human hearing, making them an appealing, non-chemical pest control option. When a faint, high-pitched whine or clicking sound is heard, it suggests a complex interplay between the device’s technical flaws and the nuanced sensitivity of the listener’s hearing. Understanding the physics of sound and the physiology of the human ear reveals why these “silent” machines sometimes betray their promise.
Defining the Ultrasonic Range
Sound frequency is measured in Hertz (Hz), which represents the number of sound wave cycles per second, determining the pitch of a sound. The human ear’s audible range spans from approximately 20 Hz up to 20,000 Hz, or 20 kilohertz (kHz). Sounds below 20 Hz are known as infrasound, while those above 20 kHz are classified as ultrasound or ultrasonic sound.
The pest repellers are engineered to operate within the ultrasonic territory, often between 30 kHz and 60 kHz, a range inaccessible to the vast majority of human adults. This design is intended to produce a sound that is irritating to pests like rodents, which have much wider hearing ranges, without causing disturbance to the human occupants of the home.
Technical Reasons the Device is Making Audible Noise
The most common reason for hearing an ultrasonic device is that the sound perceived is not the intended ultrasonic wave itself, but an audible byproduct of its creation. The electronic components responsible for generating the high-frequency signal can produce secondary, lower-frequency sounds. These audible sounds are often a form of acoustic distortion called harmonics or sub-harmonics.
When the device’s internal transducer vibrates to create the 40 kHz ultrasonic wave, this primary vibration can also induce sympathetic, lower-frequency vibrations in the device’s components or housing. These secondary vibrations are acoustic energy that falls back into the human audible range, resulting in the characteristic buzzing, clicking, or high-pitched squeal. The noise may also be purely mechanical, such as the sound of the internal speaker cone or the plastic casing vibrating against the wall socket or its own housing.
The quality of the device’s electronics is another factor. Cheaper ultrasonic repellers may have poor frequency stability and cannot consistently maintain the high frequency they are designed for. This instability can cause the output frequency to “drift” downward, intermittently dropping into the upper band of the audible range, such as 18 kHz, which is perceptible to some people. The noise may also combine electrical noise from the circuit with the sonic output, particularly in poorly manufactured units.
The Role of Human Hearing Sensitivity
Beyond the device’s technical shortcomings, the ability to hear the sound is heavily influenced by the condition and sensitivity of the auditory system. Hearing sensitivity to high frequencies naturally decreases as a person ages, a condition known as presbycusis. This age-related hearing loss disproportionately affects the higher frequencies first.
Younger individuals, particularly children and teenagers, have a much greater capacity to perceive higher frequencies than adults over 40. For instance, a sound around 15,000 Hz is difficult for most people over the age of 40 to hear, while a frequency of 17,400 Hz is often only audible to teenagers. This means that a person with relatively young or undamaged ears can more easily detect the low-level, high-frequency leakage from the pest repeller than an older adult.
Individual biological differences also play a part, as the physical structure of the inner ear, including the cochlea, varies from person to person. Some individuals have a naturally more acute sensitivity to high-frequency sounds than others, even within the same age group. What is silent to one person in a household may be a persistent irritation to another.
Are Audible Pest Repellers Effective or Safe
If a pest repeller is producing audible noise, it suggests a technical flaw that may compromise its primary function. The presence of an audible byproduct often indicates that the device is inefficiently converting electrical energy into the intended high-frequency ultrasonic waves. If the device spends too much energy creating audible noise, it is likely not producing the high-intensity ultrasonic output necessary to deter pests effectively.
Scientific studies have often found that these devices have limited or mixed effectiveness against most pests, with some pests quickly becoming accustomed to the sound over time. The audible sound itself is not considered physically harmful to human hearing, but it can cause subjective symptoms in sensitive individuals. Exposure to this noise, even at low levels, can cause annoyance, headaches, tinnitus, fatigue, and dizziness.
The sound can also be stressful for household pets with a wider hearing range than humans, such as pet rodents, including hamsters and guinea pigs. While dogs and cats can also hear in the ultrasonic range, smaller pet rodents are more likely to be distressed by the output, especially if the device is placed near their living area.