Hearing a persistent low-frequency sound, often described as a hum, drone, or distant engine noise, can be a frustrating and mysterious experience. This phenomenon, which can be perceived both indoors and outdoors, is typically low-pitched, falling below 100 Hertz (Hz). The nature of this sound makes it difficult to pinpoint, leading many to question whether the noise is physically real or generated internally within their auditory system. Investigating the source requires distinguishing between true environmental sound waves and a physiological response.
Investigating External Environmental Sources
A persistent, low-frequency hum can stem from various environmental and industrial sources. These low-frequency noises (LFN) attenuate less over distance and penetrate standard building materials like walls and windows with greater ease, sometimes sounding louder indoors than outside.
Common sources include infrastructure noise from large, constantly operating mechanical systems, such as industrial machinery, power generation plants, and large building HVAC systems. Electrical grid components, like substation transformers, produce a humming noise (often near 50 or 60 Hz) caused by magnetostriction in the core. Heavy traffic and vibrations from underground gas pipelines also transmit sound over vast distances.
This environmental sound is sometimes referred to globally as “The Hum,” with localized reports like the “Taos Hum” or “Windsor Hum.” This phenomenon is a persistent, low-frequency droning (30 to 80 Hz), audible only to a small percentage of the population. While some instances are traced to specific industrial sources, many cases remain unexplained. The sound is often perceived more intensely at night when background noises subside.
Low-Frequency Tinnitus and Subjective Hearing
If the persistent hum lacks a measurable external source, it may be categorized as low-frequency tinnitus, a subjective auditory perception. Tinnitus is the hearing of sound when no external sound is present. While often described as a high-pitched ringing, it can manifest as a deep humming, roaring, or rumbling. This subjective noise is generated internally, typically resulting from changes within the auditory system.
Low-frequency tinnitus is confusing because its deep tone is easily mistaken for an external environmental hum. The sound is only heard by the individual and is often associated with hearing loss in the low-frequency range. Physiological factors that can trigger this subjective hum include earwax buildup or certain medications.
More specific causes involve inner ear disorders, such as Meniere’s disease, characterized by fluctuating low-frequency hearing loss, dizziness, and ear fullness. Pulsatile tinnitus is a less common internal source, where the sound is a rhythmic whooshing or throbbing synchronized with the heartbeat. This often indicates a vascular or circulatory issue near the ear.
Practical Steps to Identify the Source
To determine if the hum is external or internal, start with the “Location Test.” If the sound persists with the same characteristics when you move to a different room or town, it suggests an internal source like tinnitus. If the noise disappears or changes significantly when you leave your immediate environment, the source is likely external.
A simple acoustic check provides quick insight. Temporarily turning off all household appliances, such as the furnace and refrigerator, eliminates immediate internal sources within the home. Placing your palms firmly over your ears will decrease the volume of a true external hum, whereas a subjective sound like tinnitus will typically remain the same or increase.
If the investigation points toward a localized external source, contact environmental health authorities who measure LFN. If the hum suggests a physiological cause, consult a general practitioner or an audiologist. They can perform a comprehensive audiological exam to check for hearing loss, rule out treatable conditions, or refer you for imaging tests if a vascular issue is suspected.
Strategies for Managing Persistent Hum
Once the source of the persistent hum is identified, specific strategies can be employed for mitigation and management. For an external hum, the primary goal is reducing its transmission and perception within the living space. This involves physical measures such as soundproofing, including sealing air gaps around windows and doors, or introducing mass to walls to dampen the long sound waves.
Vibration dampening is useful if the hum is felt as a physical vibration through the floor or walls; this is addressed by isolating appliances or machinery. Since the hum is most noticeable in quiet environments, low-level background sound, such as pink noise or nature sounds, can help mask the unwanted frequency. This technique increases the ambient noise floor, making the hum less distinct.
For internal hums or low-frequency tinnitus, management focuses on retraining the brain’s reaction to the sound. Tinnitus Retraining Therapy (TRT) combines counseling with sound therapy to help the brain reclassify the tinnitus signal as neutral. Cognitive Behavioral Therapy (CBT) is also effective, providing coping techniques to reduce the anxiety and emotional distress associated with the constant perception of internal sound.