How to distinguish low-frequency noise? (Original content)

Many people are confused about what low-frequency noise actually sounds like. Today, we'll explain this clearly by defining low-frequency noise and providing examples.

Low-frequency noise:

1. Wavelength Explanation: Low-frequency noise, also known as long-wave noise, is noise with a relatively long wavelength. The unit of wavelength is meters or centimeters. Long-wave noise refers to mechanical waves with a wavelength of 1.7 meters or more. Wavelengths of 1.7 to 17 meters are considered long waves (20 to 200 Hz), meaning that the distance between two adjacent peaks or troughs of such mechanical waves is between 1.7 and 17 meters.

The wavelengths of sound that ordinary people can hear are between 1.7 centimeters and 17 meters. Wavelengths above 17 meters are infrasound, and wavelengths below 1.7 centimeters are ultrasound.

2. Frequency Explanation: Low-frequency noise, with frequency measured in Hz. Low-frequency noise refers to sounds with frequencies below 500Hz (octave band). Within the range of the human ear, 20Hz to 500Hz is considered low frequency, meaning irregular sounds produced by vibrations of 20 to 500 times per second are called low-frequency noise. The frequency range of 3 to 50Hz has the most significant impact on the human body.

The relationship between low frequency and decibels:

We often use decibels to describe the loudness of a sound, so people often confuse low-decibel noise with high-decibel noise. In fact, high and low frequencies refer to the speed of sound wave vibration, while decibels refer to the amplitude of sound wave vibration. The two should not be confused.

Low-frequency acoustic characteristics

Long-wave noise differs from short-wave noise. Short-wave noise has poor penetrating power and attenuates rapidly with distance or when encountering obstacles; a point source of short-wave noise experiences a 6-decibel decrease every 10 meters. Long-wave noise, on the other hand, has extremely strong penetrating power, attenuates very slowly, and its longer wavelength allows it to easily pass through obstacles, traveling long distances and penetrating walls to reach the ear. Long-wave noise, like ordinary noise, is a mechanical wave. Mechanical waves, like electromagnetic waves, are a form of energy propagation. According to the laws of physics, whether mechanical or electromagnetic waves, the longer the wavelength, the stronger the penetrating power.

Low-frequency transmission pathways

Long-wave noise is mainly classified into structure-transmitted noise, airborne noise, and standing waves according to its propagation path, with standing waves being the most harmful.

①Structural acoustic transmission refers to the transmission of long-wave sound waves to each household through the building's foundation beams and load-bearing beams.

②Airborne sound refers to long-wave noise that travels directly through the air to the homes of residents in the community;

③Standing waves refer to long-wave noise that has been reflected multiple times during propagation. The amplitude of long-wave noise is strongest at the antinodes, and it poses the most obvious threat to human health.

Why do low frequencies cause discomfort?

The principle of resonance: Objects of the same frequency will resonate with each other. When the human body encounters vibrations or noises at a similar frequency, the body will also vibrate simultaneously, causing harm. Therefore, many sonic weapons exist in the military, possessing astonishing destructive power.

The normal resonant frequency of the human body is around 7.5Hz, with different parts having their own resonant frequencies. For example, internal organs resonate at 4-6Hz, and the head at 8-12Hz. Most low-frequency noise in daily life is around 250Hz.

What are some examples of low-frequency sounds?

Listening experience of low-frequency sounds:

The sound in the 150~300Hz frequency range is deep, heavy, soft, mellow, and has a booming quality.

To better understand low-frequency noise, let's use actual objects that emit low-frequency sounds as examples:

Compressors, exhaust fans, blowers, cooling towers, engines, water pumps, conveyor belts, boilers, air conditioners, transformers, helicopters, washing machines, refrigerators, automobiles, iron bridges, tunnels, blasting, earthquakes, lightning, wind, etc.

Elevators, transformers, central air conditioning (including cooling towers), and traffic noise (125~750Hz).

Low-frequency musical instruments:

The 200Hz~400Hz frequency range affects the fullness of the violin's tone;

The frequency range of 150Hz to 300Hz affects the dynamics of the viola's tone.

The 100Hz~250Hz frequency range affects the fullness of the cello's tone;

The 50Hz~150Hz frequency range affects the fullness of the bass tone.

Baritone 123–493 Hz, tenor 164–698 Hz.

Human speech frequency (normal speaking frequency):

The frequency range is mainly between 500 and 3000 Hz. According to measurements, most Chinese speech frequencies fall within the range of 500 to 3000 Hz, with sound intensities between 10 and 50 dB.

▶Frequency representation of different sounds:

A soft voice:

The wall clock/stopwatch has a frequency range of 500-1000Hz and a volume of approximately 30 decibels.

The distant thunder has a frequency range of 250-750Hz and a volume of approximately 40 decibels.

Computer host noise frequency range: 500-1000Hz; sound volume: 30-40 decibels.

The chicks' calls have a frequency range of 2000-4000Hz and a volume of 40-50 decibels.

The frequency range of the coin-clinking sound is 2000-4000Hz; the sound volume is 40-50 decibels.

Medium volume:

The range hood's sound frequency range is 200-750Hz; the sound level is 50-60 decibels.

Keyboard typing sound frequency range: 500-1000Hz; sound volume: 50-60 decibels.

The knocking sound has a frequency range of 500-1000Hz and a volume of approximately 60 decibels.

The doorbell's frequency range is 1000-2000Hz; the volume is approximately 60 decibels.

The key sound has a frequency range of 1000-2000Hz; the sound level is around 60 decibels.

The frequency range of the sound of rubbing paper is 1500-2000Hz; the sound level is about 60 decibels.

Whistling frequency range: 1000-2000Hz; sound volume: around 60 decibels.

The frequency range of the friction sound between fingers is 4000-6000Hz; the sound volume is about 50 decibels.

Loudly:

The frequency range of nearby thunder is 250-750Hz; the sound level is about 85 decibels.

The car horn's frequency range is 1500-3000Hz; the sound level is approximately 85 decibels.

An infant's cry has a frequency range of 1500-3000Hz and a volume of approximately 80 decibels.

The frequency range of the glass breaking sound is 2000-4000Hz; the sound level is about 80 decibels.

Very loud:

The frequency range of the bass drum is 100-500Hz; the sound volume is about 90 decibels.

The frequency range of the hair dryer sound is 1000-3000Hz; the sound level is about 90 decibels.

Train stopping frequency range: 2000-4000Hz; sound level: above 90 decibels.