What is Frequency?

What is frequency?

Frequency describes the pitch of a sound — whether it sounds low and deep or high and sharp. Low-frequency sounds include things like:

• Thunder
• Truck engines
• Bass guitars
• Subwoofers at concerts

High-frequency sounds include:

• Birds chirping
• Reversing alarms
• Whistles
• The hiss of compressed air

Frequency is measured in Hertz (Hz), which simply means the number of sound vibrations occurring each second.

For example:

• A deep bass note may have a frequency of around 50 Hz.
• Normal speech typically occurs between 250 Hz and 4,000 Hz.
• A smoke alarm may produce frequencies above 3,000 Hz.

The human ear can generally hear frequencies between approximately 20 Hz and 20,000 Hz, although our hearing range gradually reduces as we get older.

Why frequency changes how noise behaves

Understanding frequency is important because different frequencies behave differently:

• Low-frequency noise travels much further than high-frequency noise.
• High-frequency noise is generally easier to block with walls and barriers.
• Low-frequency noise is more likely to pass through buildings and be felt as vibration or pressure.

Understanding frequency is one of the keys to understanding how sound behaves and how noise can be effectively controlled.

What is low frequency noise?

Low frequency noise is exactly what it sounds like — sound that occurs at the lower end of the frequency spectrum.

These sounds are often described as:

• A hum
• A rumble
• A drone
• A thump
• Vibration or pressure rather than a traditional “noise”

Common examples of low frequency noise include:

• Truck engines
• Diesel generators
• Large air-conditioning systems
• Mining equipment
• Wind turbines
• Industrial fans
• Bass from concerts and entertainment venues

Low frequency noise is generally considered to occur below approximately 200 Hertz (Hz), although there is no universally agreed cut-off.

Why is low frequency noise different?

Low frequency sound behaves very differently to higher-frequency sound. It tends to:

• Travel much greater distances
• Bend around buildings and barriers more easily
• Penetrate walls and windows more effectively
• Remain noticeable even when other sounds have disappeared

This is why you may still hear or feel the bass from a concert kilometres away, even though the vocals and higher-pitched sounds are no longer audible.

Why can I feel it more than hear it?

The human ear is less sensitive to low frequency sound than mid-frequency sounds such as speech.

As a result, low frequency noise is often experienced as:

• Pressure in the room
• Rattling windows
• Vibrations in walls or furniture
• The sensation that “something is there” even though it is difficult to describe the sound itself

Many people describe low frequency noise as something they can feel rather than hear.

Why is low frequency noise often associated with complaints?

Low frequency sound can be particularly frustrating because:

• It travels long distances
• It can be difficult to identify the source
• It penetrates buildings easily
• It is more difficult to block using conventional construction methods

A standard wall or window that performs well for traffic noise may provide much less reduction at very low frequencies.

How is low frequency noise measured?

Low frequency noise is often assessed using both:

• A-weighted measurements, which represent how humans hear sound
• C-weighted measurements, which capture much more low frequency energy

Large differences between A-weighted and C-weighted levels can indicate that low frequency noise may be significant.

This is one reason why many concerts and music events include both L_Aeq and L_Ceq noise limits.

Can low frequency noise be controlled?

Yes, but it is often more challenging than controlling higher-frequency noise. Effective solutions may include:

• Selecting quieter equipment
• Reducing fan or engine speeds
• Improving building design
• Increasing separation distances
• Using acoustic enclosures
• Designing noise barriers specifically for low frequency performance

The most successful projects consider low frequency noise early in the design process, when solutions are usually simpler and less expensive.

The EcoAcoustics Approach

Low frequency noise is one of the most misunderstood aspects of environmental acoustics.

Because it behaves differently to other types of sound, solutions that work well for traffic noise or general environmental noise do not always work for bass or industrial hum.

Understanding how low frequency sound behaves is often the key to solving noise problems that have frustrated communities, operators and regulators for years.