The low frequency response id greatly influenced by the room. Since low frequencies are only marginally absorbed, so called standing waves can easily build up, possibly deterioration the reproduction and causing offsets of up to +-10 dB. With the help of clever placement of the subwoofer and the listening position, the problems can be reduced. Standing waves are particularly tricky using only one excitation point, ie one subwoofer, as no spatial averaging will be present.
There are several software programs that can calculate and predict the room response at listening position, but this assumes the room has the shape of a box, with no "soft" or absorbing walls such as windows, parquet flooring, plaster or wooden panels.
Measurements:
- We calibrated a DSP subwoofer (XTZ 99W12 DSP) to be linear down to 13 Hz and the frequency response at the listening position is almost non-affected by the excitation. This way, only the room influence itself was measured.
- The subwoofer has been placed in all different 76 positions.
- The frequency response has been measured at 6 different listening positions.
The result looks like this: (average energy level at all 6 different listening positions)
Exclusively the results for the standing waves at 29, 40, 55 and 83 Hz and the lower boundary frequency of 13 Hz:
(freq and energy is shown to the upper left)
Here are all 456 measurements put together:
-> the energy average (magenta) of all 456 measurements shows peaks at 30, 40, 47 and 55 Hz
-> below 60 Hz peaks are up to 10 dB high
The general trend from 30 to 55 Hz is 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, which would be somewhat controversial to most people.
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 and consequently for smaller rooms the problems are shifted upwards in the frequency range.
Even better would be using a subwoofer where the bass reflex ports can be plugged, thus offering different characters that can be used to adapt to the room:
For excitation point X=2, Y=2 and at the front center microphone the resulting frequency response is:
- From 35 to 150 Hz the frequency response is roughly linear
- At 55 Hz a 15 dB high peak is bothering!
- From 36 to 30 Hz the frequency response increases by more than 20 dB !
- Only a parametric equalizer or a DSP-subwoofer will help in this case!
Conclusion:
- The sound pressure distribution in a listening room with only one subwoofer fluctuates very much; thus a change in loudspeaker and/or listening position may have a very big influence. Itīs definitely worth trying different positions. We have proved the room can affect performance as significantly as 45 dB! The results from these measurements suggests that a larger gain in performance can be achieved by focusing on the listening room rather than spending large amounts of money on more subtle tweaks such as cables and similar.
- 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. That might be the reason why many HiFi-enthusiasts prefer subwoofers with a smooth low frequency roll-off (e.g. closed box). This is a controversial standing point, but still very true. Achieving a flat frequency response that includes the listening room must be the primary ambition, rather than optimizing each component only to end up using endless tweaks in the pursuit of the best possible performance.
- 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 boost and/or high pass filter) or mechanically (e.g. two or more bassreflex tubes that can be closed)
- Ideally parametric equalizers are used for a perfect integration of a subwoofer into a listening room as e.g. provided by DSP-based subwoofers! DSP technology has the advantage of high precision calibration allowing almost any listening room to be sufficiently tuned.
- In general it is recommended to use multiple subwoofers; by clever placement disturbing frequency ranges can be cured by means of spatial cancelling/averaging. This way, achieving a flat response is much easier.
* The contents of this page (Subwoofer in a listening room) is published with the courtesy of HiFi Selbstau
