Servo Controlled Subwoofers: What Is It & How It Works?

Servo-controlled subwoofers have the potential to reproduce better bass quality in your favorite music notes.

The servo mechanism has been used for quite a long time in audio systems.

These particular units use feedback signal technology to make corrections in the audio signals.

The motive behind such a technique is to reduce distortion in sound and subwoofer clipping from a technical point of view.

Additionally, the servo-controlled mechanism is introduced to make compact subwoofers and speakers so that they can be accommodated in smaller enclosures.

Compact units will help you accommodate or place the subwoofers and speakers conveniently even in smaller spaces.

Let us learn about servo technology in brief.

What are Servo Subwoofers?

Servo controlled subwoofers are specialized speakers designed for low-frequency sounds. They use a feedback mechanism, called a servo, to monitor and correct the movement of the speaker cone. This ensures accurate sound reproduction and reduces distortion.

The comparison of the input signal to the amplifier and the actual motion of the cone typically derives the servo feedback signal.

The source of the feedback signal can be a microchip-based accelerometer placed on the cone or simply the movement of the voice coil attached to the cone.

The motive behind this design is reduced distortion making smaller enclosure sizes possible.

However, servo-controlled subwoofers can be costly and the setup can be more complex.

That being said, the advanced mechanism helps the manufacturers design compact enclosures without compromising the audio quality.

Servo-controlled subwoofers can be based on different mechanisms and a few common ones are listed below:

  • Accelerometer-based mechanism
  • Sensing coil technique
  • Motor based Designs
  • Positive Current Feedback

Let’s take a quick look into each of the different mechanisms below.

Servo-controlled Subwoofer mechanisms

Accelerometer-based servos

Accelerometer-based servos use accelerometers to measure the motion or vibration of an object.

An accelerometer is a device that detects and measures acceleration, often due to movement or changes in position.

In the context of servo-controlled subwoofers, an accelerometer-based servo system uses the accelerometer to monitor the movement of the speaker cone.

By comparing the actual cone movement to the intended movement, the system can make adjustments in real-time, reducing distortion and ensuring more accurate sound reproduction.

With all the good things that it brings into the table, it also has a few drawbacks.

The subs with this mechanism can exhibit servo-noise (hiss) as the feedback signal from the accelerometer gets amplified at a specified pace.

A mechanical assembly is needed to coordinate acceleration to a linear amount of deformation;, this assembly is commonly implemented using a mass coupled to a piezo film.

The mass usually presses against the piezo film as the accelerometer moves with the voice coil. This is how the acceleration signal is generated.

However, in some instances, the device can create a false acceleration signal due to variations in air pressure inside the enclosure.

The variation can act on the casing of the accelerometer and lead to an unwanted sound effect.

Apart from the variation of air pressure the accelerometers typically pick up acceleration details in all three axes which can also lead to incorrect feedback.

This is because the cone moves in one direction that can be corrected, but only in one direction.

When the accelerometer picks up acceleration in the remaining two directions it can deliver incorrect feedback.

Sensing coil technique

To detect the cone movement a sensing coil is used in this particular mechanism.

The sensing coil-based technique was used in most servo systems in the past to organize and generate feedback signals.

Most people use this technique while constructing a subwoofer as a DIY project for their own sound systems.

Typically, there are two paths used for feedback, one from the sensing coil and the second from the driver coil or from the power amp’s output.

However, there’s a possibility of frequency response changing with the voice coil resistance.

To use this mechanism, experts advise using one coil as a sensing coil in a dual voice coil driver so that one can easily get a decent excursion.

Motor based Designs

DC motors are used in motor-based designs with the shaft connecting the cone.

It helps in controlling the cone’s movement and these motors typically include few poles.

This mechanism is usually implemented in turntables and the force created by these motors at times lack uniformity.

Hence, heavy platters are used in turntables to smooth out the force.

It helps control the warbling on the pickup signals.

When it comes to subwoofers the movement of the cone is quite less with the mid-bass signal compared to low-bass signals.

Additionally, the modulation distortion between low and mid-bass signals is quite higher compared to non-servo subs.

Hence, they are ideal for PA systems.

Positive Current Feedback

One of the most popular techniques in the past was the positive current feedback mechanism.

This specific technique does not use any sensor, hence it is less effective.

In this mechanism, the frequency responses change more dramatically as the voice coil resistance changes. Which is even better in non-servo subwoofers.

What are Direct Servo subwoofers?

In direct servo subs, the feedback system ensures that the cone’s movement matches the input signal, reducing distortion and resulting in more accurate bass reproduction.

The “Direct Servo” technology involves sensors that measure the cone’s movement and send this information back to the amplifier.

The amplifier then adjusts the signal in real-time to correct any discrepancies between the input signal and the actual cone movement.

This continuous feedback loop allows for tighter, cleaner, and more precise bass performance.

Additionally, servo-controlled subs require extensive protection circuits that can further degrade the signals.

The simplicity of the technology defines the superiority of direct servos such as it uses a very thin sensing coil which is typically wound adjacent to the voice coil.

The coil acts as a special microphone and creates a signal. This particular signal is subsequently utilized to correct any variations from the original signal.

Additionally, there’s instant correction done to any non-linearities in the signal.

Other benefits of direct servo subs units include the following:

  • Reduced thermal-induced compression and distortion.
  • Reduction in distortion caused by the spider and surround by 6 to 9 dB.
  • A better excursion is utilized for higher output in sealed enclosures such as subwoofers.
  • Less dependent on T/S parameters due to flatter frequency response.
  • Articulated, tighter, and well-defined bass sound.

Also, this technology is applicable to all subwoofer configurations such as horns, IBs (Infinite Baffles), dipoles, and others.

Final Thoughts

Various technologies have been introduced to make sound systems even better and better for music lovers including servo and direct servo mechanisms.

When it comes to reproducing bass effects accurately, most subwoofers fail miserably however, some subs are capable enough to shake your room.

Servo-controlled subs are designed to deliver bass notes more accurately while sitting in compact enclosures.

Although, some high-end subwoofer models including standard and servo-based units perform at an acceptable level, direct servo subs can leave a dynamic impact comparatively.

Frequently Asked Questions

What is the active servo technology?

Active servo technology uses a unique mechanism that ensures the subwoofer and speaker have a perfectly linear motion. Using this technique the speaker in synch with the amplifier, cancels out the impedance and is quite popular with Yamaha audio systems termed as Advanced Yamaha Active Servo Technology or simply Advanced YST.

Are Servo Controlled Subwoofer same as SVC?

A servo-controlled subwoofer uses a feedback system to reduce distortion and ensure accurate sound reproduction. On the other hand, “SVC” stands for “Single Voice Coil,” referring to a type of subwoofer based on its coil configuration. SVC describes the subwoofer’s design, while servo-controlled refers to its feedback mechanism.

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) He's always ready to lend a helping hand!