December 4, 2012
Bass Management for High-End?
Consumer systems often use bass management for convenience and reducing cost.
Introduction
Consumer systems often use bass management for convenience and reducing cost. Bass management enables the use of smaller main loudspeakers, by reproducing low frequencies over a subwoofer.
The use of bass management and a subwoofer has been more popular for home cinema systems than for stereo systems,because for most people it is simply not practical having five or more large full range loudspeakers.
For high-end consumer stereo systems, subwoofers are hardly used. Why use a subwoofer if the main loudspeakers are large and truly full band?
While the optimal solution certainly depends on the specific case, there are situations when also high-end systems with full range loudspeakers benefit from bass management.
In the following, we are discussing and explaining bass management and reproduction of low frequencies with subwoofers. Even when all loudspeakers are full range and bass management per se would not be necessary, bass management can have its benefits.
Background: Room resonances
In this discussion, we are concerned about the impact of relatively late reverberation. Each pair of parallel walls of a room generates room resonances, that is, standing waves between the walls. The frequencies at which these room resonances occur are usually also the problematic frequencies at which reverberation decays too slowly, causing annoying “temporal smearing” and “bass boom” effects.
The figure below illustrates the sound pressure for room resonances occurring between two walls. The lowest frequency room resonance (shown in blue) has a wavelength twice as large as the distance between the walls. Thus, its frequency is:
f0 = c / (2*d) ,
where c is the speed of sound in air (about 340 m/s) and d is the distance between the walls. The frequencies of the higher frequency room resonances f1, f2, f3, … are:
fn = n * c / (2*d) .
When one measures a room, with the measurement loudspeaker and microphone located at two opposing walls, the corresponding room resonances appear as peaks in the spectrum. Above the so-called Schroeder frequency, the room resonances are so closely spaced in frequency that they do not appear as distinct spectral peaks.
Loudspeaker (Subwoofer) Positioning
When loudspeaker and microphone measurement positions are at or near peaks of the standing room resonance waves, as illustrated in the example in the figure above, the resonances appear as peaks in the spectrum. For general loudspeaker and microphone measurement positions, a room resonance may also appear as a notch in the spectrum. This occurs when either loudspeaker or microphone are positioned at or near a zero position of the room resonance. The figure below illustrates measurement or the room with the loudspeaker at a zero position, resulting in a spectral notch at the corresponding resonance frequency. Generally speaking, for an arbitrary loudspeaker and microphone position, each room resonance may appear as a peak, notch, or something in between.
Note also, that the energy transferred between an omni-directional loudspeaker and microphone by means of a room resonance remains the same if loudspeaker and microphone positions are swapped.
Now consider subwoofer positioning. The previously discussed loudspeaker position corresponds to the subwoofer position and the microphone position to the listening position. Given a pre-determined listening position (e.g. sweet spot of a stereo system), different room resonances are excited with different strength depending on the subwoofer position.
A good subwoofer position is reached when the room impulse response measured from subwoofer to microphone is as frequency smooth as possible. This occurs at a position at which the energy flow from subwoofer through the room resonances to the microphone is as frequency independent as possible.
The above mentioned condition does not yet ensure that the overall gain of the subwoofer signal matches the main loudspeakers. Thus, once the subwoofer is positioned, a gain calibration should be carried out.
The described positioning and calibration procedure may be carried out based on room impulse response measurements (you can use for example the free Room EQ Wizard). Less effort but more skill may required to optmize subwoofer positioning and gain by simply trying out different positions and listening.
Another method which may help for finding a good subwoofer position is to place the subwoofer in the desired optimal listening position and listen at all possible subwoofer positions whether bass is reproduced well. The point at which bass is reproduced well may be a good subwoofer position. This method is based on the above mentioned insight that loudspeaker (subwoofer) and microphone (listening) position may be swapped, while the energy transfer through room modes remains approximately the same.
Subwoofer Positioning II
In the previous section, subwoofer position optimized for a single optimal listening position (“swee spot”), was discussed. Especially for home cinema, but also whenever one intends to have more than one listener at a time, one should not only optimize the reproduction system for a single sweet spot.
One extreme example, to illustrate the problem of single sweet spot optimization, is the following. If the sweet spot is near a zero position of a room resonance, the subwoofer is potentially placed at a position where it excites this room resonance maximally. This likely leads to annoying “temporal smearing” and “bass boom” effects, for listeners which are not exactly in the sweet spot.
A good strategy for full room (or large listeing area) optimization is to measure the room resonances not only at the sweet spot, but at multiple positions. One way to optimize subwoofer position then is to optimize it in terms of spectral flatness with respect to the average room impulse response (averaged over all measurement positions).
Again, a skilled listener may optimize subwoofer positioning for an extended listening area by simply trying a number of subwoofer positions and listening at different positions.
Putting It All Together
So what has the previous discussion to do with a high-end stereo (or high-end surround) system with full range loudspeakers?
Low frequencies, below at least 80Hz, can not be localized by a listener. Thus, for perception of the stereo/surround image, it does not matter where the loudspeaker is, reproducing such low frequencies. For higher frequencies, it is crucially important that the main loudspeakers are at their appropriate positions.
It may be that the position of your main loudspeakers, due to your specific room, is suboptimal to reproduce low frequencies. If yes, it indeed makes sense to use bass managment to remove bass from the main loudspeakers and reproduce it over a well placed subwoofer.
Since in this case full range loudspeakers are used, the cross over frequency is chosen as low as possible, but such that most or all of the bass reproduction problems (“time smearing”, “bass boom”) occur below it.
