Here's some of my thoughts on rooms and subs.
Right now, I'm
figuring out the best location for my system in my room. It's about
time. My room is 14.5' x 19.5' x 8' high. I listen across the 14.5"
dimension out of necessity. I am using a readily available Room Analyzer
package. This hardware/software generates audio sweeps, records the
sound pressure with a mic, and calculates freq response, reverberation
time, room modes, phasing and delay times. I am using a DIY sub. It has a
10" Scanspeak woofer in an acoustic suspension design. The main
speakers are B&W 805s on the stands provided by B&W.
With
the listening position against the wall opposite the speakers and the
sub to the left of the left speaker, I've found several frequencies (40,
62, 125, 175 Hz and others) where there is a peak or null with a long
delay time, much longer than other frequencies. Hand calculation of the
expected room modes fits this observation. At 40 Hz, the sound
wavelength is about 28'. Half wave is 14', which is nearly exactly the
length I'm listening across. So room modes are important for an average
sized room. The 40Hz peak is +10dB (above mean). the others have similar
decay times but are not as strong, although the dip at 63Hz is nearly
-10dB.
I moved my listening position (sofa) out into the room away
from the wall. I placed it 1/3 of the length of the room (recommended).
New measurements are now quite different. All modes are suppressed to
different degrees, but are still there. The software cannot find any
modes, although I do have irregularities in freq response.
Listening
position is a major factor in suppressing room modes. The explanation
is that the new listening position is near a null in the 40Hz room mode.
The conclusion is that room modes are important for bass. If you want
good sounding bass from 30Hz upward, room modes must be considered.
Now,
imagine I were to use the DARO system in my previous listening
location. The DARO would call for 10dB suppression at 40Hz, but also for
a approx +10dB increase in SPL at 63 Hz. This means that DARO would be
calling for 100x more power from the sub amp (see
https://geoffthegreygeek.com/amplifier-power/). This is very difficult to do for most amps. The DARO can reduce peaks, but cannot be used effectively to remove dips.
In order to effectively suppress modes, several things are needed prior to using DSP.
.First,
get the sub away from the walls and corners. Get it (them) out into the
room (unless you want your bass to overpower everything else).
Second, use the recommended listening position.
Third,
after the first two are done and characterized, add room treatments
that includes corner traps and bass traps for the low frequencies. These
absorb the appropriate frequency bands so the standing waves are
absorbed, not reflected. Additionally, the long reverberation will be
dramatically reduced.
The first two modifications will get you a
long way, but room treatment is absolutely needed to suppress the
remaining modes, unless you are fortunate or the modes are sufficiently
suppressed for your taste.
After the above three things are done,
then the DSP (such as DARO) can be used as the peaks and nulls will be
of lower amplitude and therefore the amp will not be stressed as
severely.
In conclusion, room modes determine the lower frequency
response of an audio system. The exact modes depend on the room size and
geometry. Room modes cannot be corrected entirely with DSP software,
especially nulls (dips).
The modes are there whether DSP is used or not.
Optimal location of the sub(s) and the listening position is vitally important.
To avoid excessive use of amplifier power, add room treatments prior to adding DSP.
Please note that there are other considerations if two or more subs are
used as they can be placed such that certain room modes can be
suppressed. But room modes are still present since these are
characteristics of the room only . One person's solution may not work
for someone else.
Other modes may not be.suppressed. Room modes are always present in a
room since these are characteristics of the room only . It doesn't
matter what is in the room, the modes still exist. Any item added to the
room can only affect the relative amplitudes of the room modes (this
goes for treatments also). One person's solution may not work well for
someone else.
Even without a room analyzer, one can improve the performance of
an audio system. Searches will provide guidelines, such as placing the
listening position about 1/3 (actually 0.38) of the distance between the
walls behind the speakers and the listener..
Some boilerplate - I
am not affiliated with any manufacturer of audio equipment, room
treatments or DSP. I have no interest in selling you anything. I have
an MS in Physics and over two decades designing and building optical
systems (telescopes for example). Wave phenomena are very familiar to
me.
Constructive criticism is always welcomed.
