Multisub bass-systems and potential challenges

Multisub means to use several subwoofer units placed around the room, so that they together form a bass-system that eliminates room acoustic influence. The goal is perfect bass, with focus on achieving a flat frequency response.

Why use many subwoofers

Several publications of methods for set-up of such systems exist, where placement of subwoofer units and equalizer calibration are described.

Inside a room the bass is influenced by reflections between the surface boundaries of the room, so that the frequency response will have both resonances with too high level and dips with too low level due to cancellations.

Using 3 or 4 subwoofers it is possible to achieve good frequency response in virtually any room. This is achieved when the subwoofers fill in the dips and holes in the response, making it possible to use eq to smooth out the rest.

 

Using 2 subwoofers the response is rather unevenly distributed in the room, and there is a dip around 60Hz that can not be fixed using eq
With 4 subwoofers placed in corners a smooth response is achieved across most of the room

Two potential challenges

But even though the frequency response is corrected, two potential problems occur:

  • Imprecise impulse response in the time domain
  • Cancellation of intensity and particle velocity in the sound field

This causes transients to become soft and imprecise because timing is compromised, and the bass lacks tactile power because the sound field has too little intensity.

In a LTI – linear time-invariant system – the time domain follows and improves when the frequency response is fixed, frequency and time are connected. But multiple subwoofers inside a room is a system where several sources of sound are offset in time related to each other, so that the concept of LTI is no longer valid. Reflections add contributions of delayed signals, and the different suwoofers has different distance and thus different delay. Placement of subwoofers together with time delay adjustments in dsp (digital signal processor) is the solution to fix this.

It works

In The Moderate Cinema a simple bass system with four Compact Horn subwoofers in the corners is installed. This achieves smooth frequency response across the whole bass range, without resonances, and it is calibrated is such a way that much of the tactile energy is kept all the way down to the lowest subsonic bass. This system is crossed over to the main speakers at 120Hz.

With main speakers only to cover the whole frequency range the bass response is very rough and there are huge resonances. A hopeless starting point that can not be fixed only by using eq.

 

Decay with no bass-system

With four subwoofers the situation is very different.

The two subwoofers at the back fills in a hole in the response around 60Hz, the bass is also much more even across the whole room with four units.

Decay with multisub bass-system

The bass system also improves impulse reproduction. Behavior in the time domain is very good, everything starts and stops at the right time, the resonances are gone.

Tactile force and particle velocity

An experiment was performed to study the effects of calibration that maintains particle velocity in the sound field. Two calibrations were compared, where one is with all subwoofers adjusted in time so that the sound reaches the listening position at the same time from all units. It is then expected that the particle velocity is low, because the sound arriving from opposite directions cancel out. The other calibration is optimized to retain intensity and particle velocity. Frequency response is sufficiently similar for both calibrations so that observed differences in sound must have other causes.

Measurements of frequency response and particle velocity confirms that the time-aligned calibration has significantly reduced particle velocity in the bass range.

 

Frequency response and particle velocity – red curves for the tactile cancellation calibration shows reduction in particle velocity in the bass range.

The sound of the time-aligned calibration is experienced as lack of power in the bass, and the character of the sound is more like if the horns were replaced with something you could find in an ordinary shop. We do not want that.

Bass-system with multiple subwoofers gives better sound quality

With proper calibration the multisub system is experienced as a major improvement in sound quality, also when playing at low volume levels. Everything is better – smoother bass, resonances are gone, tactile force is much better, bass instruments keeps their characteristic sound.

This shows that it is possible to achieve good bass in a small room, with reasonable means. However, the subwoofers must be positioned and calibrated correctly to make it work.

Requirements for good results

  1. The quality of the subwoofers is important for sound.
  2. Placement of subwoofer units must be done in relation to the room, where dimensions, losses of boundary surfaces and any openings are taken into account.
  3. Calibration with dsp must be done so that the frequency response, time domain and tactile power are reasonably correct.