Subwoofer in a listening room
The low frequency reproduction of a loudspeaker is massively affected by the listening room. As low frequencies are absorbed only marginally so called "standing waves" can build up and often cause deterioration of the frequency reponse by up to +/- 10 dB. By clever choice of excitation and listening position this effect can be reduced. Standing waves are especially critical with only one excitation position (e.g. for a single subwoofer) because then no spatial averaging may occur.
Several programs are available that can predict the room frequency response at the listening position (e.g. this ONLINE-calculator) - assumed that the listening room is box-shaped and does not have "soft" walls like big window areas, parquet/laminate floor, gipsum board or wooden panels etc.
In order to understand our listening room (width 4.43m, length 6.21m., heigth 2.60m) we have performed a series of measurements:
- we have equalized a DSP-Subwoofer (XTZ 99W12DSP) to be linear down to 13 Hz so that the frequency response at the listening position is mostly independant of the excitation and just shows the characteristics of the listening room
- then we have positioned it at 76 floor locations in total (50 cm grid, red points) and
- measured the frequency response at 6 listening positions (blue points)
The result looks like that (energy average of all 6 listening positions). Only shown are the results for the standing waves at 29, 40, 55 and 83 Hz and for the lower cut-off frequency of 13 Hz (the frequency and the average room energie is displayed on the top left):
Here you see all 456 frequency response displayed at the same time::
-> the energy average (magenta) of all 456 measurementsa shows peaks at 30, 40, 47 und 55 Hz
-> below 60 Hz peaks are up to 10 dB high
All this was measured - as already said - with an "ideal" subwoofer that works lineary down to 13 Hz! The general trend from 30 to 55 Hz ist a continous increase of 6 dB/octave towards lower frequencies. In order to achieve a linear frequency response at the listening position the subwoofer should have a mirror-inverted frequency response, thus a 6 dB/octave decrease below 55 Hz.
Remark: the mentioned frequencies are valid for rooms of similar size (4.43 x 6.21 m). For bigger rooms the problems are shifted towards lower frequencies.
A subwoofer that works linear down to 20 Hz is therefore in general not desired in rooms of similar size because it emphasizes the standing wave problems. That might be the reason why many HiFi-enthusiasts prefer subwoofers with a smooth low frequency roll-off (e.g. closed box).
Even more flexible (= better) is a subwoofer that can be switched between a closed box and different bass reflex modes(linear or too high):
For excitation point X=2, Y=2 and at the front center microphone the resulting frequency response is:
-> here only a parametric equalizer or a DSP-subwoofer will help !
- the sound pressure distribution in a listening room with only one subwoofer is fluctuating very much; thus a change in loudspeaker and/or listening position may have a very big influence. Therefore its definitely worth to try different positions !
- a linear frequency response down to 20 Hz is not desirable in general - at least not in rooms of similar size - because this often results in boomy low frequency response
- depending on the subwoofer location either a soft low frequency roll-off (closed box) or a peak at e.g. 50 Hz combined with a steep decrease below 40 Hz (bassreflex enclousure tuned too high) may be desired. Therefore a subwoofer is preferable that allows a manipulation of the frequency response either electrically (switchable bass bost and/or high pass filter) or mechanically (e.g. two or more bassreflex tubes that can be closed on demand)
- ideally parametric equalizers are used for a perfect integration of a subwoofer into a listening room as e.g. provided by DSP-based subwoofers !
- in general it is recommended to use multiple subwoofers; by clever placement disturbing frequency ranges can cured by means of spatial averaging