Nonlinear Distortion in Subwoofers (Explained)

Wherever there’s production of audio signals, distortion are quite common, especially in sound systems.

Distortions are categorized into linear and nonlinear forms, here, linear distortion occurs due to a change in amplitude or phase and there’s nothing to do with frequency change.

On the other hand, nonlinear distortion occurs when there’s a change in frequency such as when the output audio signal isn’t similar to the input signal.

In this article, we will try to understand nonlinear distortion in audio signals and their causes.

What is a nonlinear distortion in a subwoofer?

Non-linear distortion occurs when an audio or electronic system produces an output that isn’t a direct proportional representation of its input. This results in unwanted harmonics or changes in the original signal, leading to a distorted sound or output.

In other words, Nonlinear distortions are essentially physical sources of distortions in audio frequency output waveforms.

For instance, if you play a single frequency as a test tone into an audio system, and the output consists of multiple frequencies instead of a single audio waveform, there will be nonlinear distortion.

In simple words, the output signal contains additional frequency components that weren’t present or fed in the original input signal by the amplifier.

In most cases, the moving components including the magnetic elements of electroacoustic transducers (such as loudspeakers and subwoofers) cause nonlinear distortion in sound.

It is typically when they are forced to perform outside of a narrow operating range where they become non-linear.

There can be many causes of nonlinear distortion in sound, but in general, they are divided into harmonic and non-harmonic distortions.

Harmonic distortion is applicable to the modified frequencies in the output signals from a subwoofer or speaker which do not match with the original input signals sent by amplifiers.

On the other hand, non-harmonic distortion has a negative impact on acoustic perception where signal elements are formed whose frequencies are not equivalent to the integer multiples of the actual tones.

These audio signals will sound unfamiliar to our ears and be sensed as distortion.

It is considerably sensitive to acoustic perception, especially at the mid frequencies, which can range anywhere between 1,000 Hz and 4,000 Hz.

Additionally, if the Total Harmonic Distortion is related to the mid-frequency range, the human ear is able to sense even the finest non-linear distortion which can be about as low as 0.5%.

However, in some cases, the low-frequency bass that ranges around 150 Hz is not sensitive enough for the human ear to sense harmonic distortions below 5%.

Nonetheless, there’s more to do with the harmonic aspect when it comes to non-linear distortion in subwoofers and speakers.

Let us understand harmonic distortion in brief:

Harmonic Distortion in Subwoofers and Speakers

Harmonic distortion in subwoofers

Harmonic distortion occurs when an audio or electronic system introduces harmonics (frequencies that are multiples of the original signal) that were not present in the original input signal.

For example, if the input signal is a pure 100 Hz tone and the system produces additional tones at 200 Hz, 300 Hz, etc., these additional tones are harmonics.

The severity of harmonic distortion is often expressed as a percentage, indicating the ratio of the distorted signal to the original signal.

It can affect the clarity and fidelity of audio playback.

Harmonic Distortions are typically caused by non-linear distortions where unwanted frequencies can be noticed in the actual output signal which were not fed in the original input signal.

In general, distortion in sound occurs in subwoofers and speakers due to the involvement of non-linear components and is particularly in cases where semiconductors and electron tubes are non-linear.

It changes the sine wave of the audio signal at the input of a subwoofer and speaker thus impacting the sound quality.

When it comes to bass output there are typically a few factors responsible for harmonic distortion.

However, any nonlinearity in the components can create harmonics by altering the output frequencies in multiple ways such as through nonlinearity in the movement of the cone.

Subwoofers and speakers typically rely on a suspension system attached to center the coil in a gap (voice-coil gap).

Additionally, the suspension system helps generate a force to move the voice coil back to its resting position or to simply re-center it.

Similarly, most woofers have a suspension that includes a spider and surround which surpasses the rocking mode by allowing the movements in only one direction.

Any deviation in the movement can lead to output audio signals with harmonic distortion.

The mechanisms of non-linear distortion can also be caused by excursions that can further cause variation in the magnetic field.

That being said, inductance can also be responsible for altering the magnetic field which can further cause harmonic distortion.

Likewise, when we talk about excursions, it can also lead to modifying the compliance of the speakers and subwoofer suspension system.

Any improper electrical wires and cables can also be responsible for harmonic distortions, hence, make sure to use cables of higher quality and keep the required wire gauge in mind.

The level of harmonic which is nonlinear distortion is measured using THD or THDi.

On the other hand, Intermodulation distortion (IMD) measures the non-harmonic (frequencies) distortions added to the input (original) signal.

Both THD and IMD represent the values in percentage of the total output signal.

As a general rule, lower numbers mean better performance in the above measurements.

THD in Subwoofers: Total Harmonic Distortion

THD stands for Total Harmonic Distortion and is used to measure the level of harmonic distortion present in an audio signal.

Total Harmonic Distortion is a measure used to describe the extent to which an audio system, such as a subwoofer, introduces harmonic distortion to the input signal it receives. In simpler terms, it quantifies how much the output of the device differs from the original input due to the introduction of harmonics (frequencies that are multiples of the original signal).

Why It Matters in Subwoofers

Subwoofers are designed to reproduce low-frequency sounds, typically ranging from 20 Hz to about 200 Hz. When a subwoofer introduces harmonics to these low frequencies, it can interfere with the higher frequency sounds played by other speakers in a system. Since a subwoofer’s primary job is to provide clear, powerful bass, minimizing THD is crucial to achieving accurate sound reproduction.

Measuring THD

THD is typically expressed as a percentage.

A THD rating of 1% means that 1% of the total output signal consists of unwanted harmonic distortion, and 99% remains true to the input.

For instance, if you input a pure 50 Hz tone into a subwoofer and measure the output, in an ideal world, you’d only see a 50 Hz tone.

However, due to various imperfections and non-linearities in the subwoofer’s design and components, you might also detect tones at 100 Hz, 150 Hz, 200 Hz, and so on.

These additional tones are the harmonics introduced by the subwoofer.

Additionally, distortion is sometimes defined as the proportion of signal voltage to voltage distortion wherein the outcome value is usually denoted in percentage or in dB (decibel).

In most cases, you can use THD analyzers that are easily available in the market which will simplify your process.

The THD analyzer will divide the ratio of the equivalent RMS (Root Mean Square) voltage of all harmonic frequencies by the fundamental frequency’s RMS voltage.

THD in Practical Terms

While low THD is generally desired, it’s essential to understand that small amounts of harmonic distortion (e.g., under 1%) are often imperceptible to the human ear, especially at higher volumes.

However, as the THD percentage increases, the likelihood of the distortion being audible also rises.

Factors Affecting THD in Subwoofers

Several factors can influence the amount of harmonic distortion a subwoofer introduces:

  1. Driver Design: The physical design and materials of the subwoofer’s driver can impact its performance and THD.
  2. Amplifier Quality: A subwoofer’s built-in amplifier can introduce distortion, especially if pushed beyond its limits.
  3. Enclosure Design: The subwoofer’s enclosure can affect how the driver moves, potentially introducing distortion.
  4. Overdriving: Pushing a subwoofer to play louder than its design allows can result in increased distortion.

What causes high THD?

Small amounts of THD are quite common in audio systems and aren’t noticeable or audible in most cases. Typical electrical problems with the audio system are responsible for high THD. However, non-linear moving parts equally contribute to an increase in THD.

Final Thoughts

Non-harmonic distortions are typically a concern with the linearity of the moving parts in a speaker and subwoofer system.

Total harmonic distortion or THD should be on the lower side for better sound quality hence the lower THD value indicates the unit is of high quality.

Most manufacturers make sure that the sound system meets all the parameters to decrease distortions of any kind.

Hence make sure to add subwoofers in your sound system with lower THD values to obtain the best bass quality.

Armed with a Diploma in electrical engineering and a remarkable 12 years of expertise, he excels in rejuvenating music systems, washing machines, dryers, and laundry-related appliances. Manish's profound insights and practical know-how establish him as a credible authority in appliance and music system repairs. Contact: manish.singh (at) subwooferblog.com. He's always ready to lend a helping hand!