Subwoofers can add a lot to the credibility
of a stereo system when placed well, using the right cross-over frequency and set at
the appropriate output level. Let’s see how that is done. Let’s first start by defining what I consider
to be a subwoofer. Sub means ‘below’ and ‘woofer’ is
the term we use for low frequency loudspeaker drivers. So a subwoofer is designed to extend the frequency
response on the low end. But there are two ways of working with a subwoofer:
used as effect channel as with 5.1 video and film sound or used in a stereo setup for the
reproduction of music. It is the latter application that I will discuss
here. The best known application facilitates the
use of smaller ‘satellite speakers’ for the left and right channels and have the deep
lows added by a subwoofer, like I have in my setup 2 and 3. Apart from the obvious reason - the use of
smaller loudspeakers for left and right - it also offers another benefit: you can optimise
the low end by placing the subwoofer at the best spot for lows while you can place the
left and right speakers for optimal stereo imaging. Especially in smaller rooms this is a very
good solution. But be careful, if the left and right speakers
get too small, the cross-over frequency needs to be too high for proper integration of lows
and highs. You might still measure a decent frequency
response but not a flat energy response. If the cross-over frequency needs to be above
100 Hz, things easily go wrong in my experience. You might think there is no use for a subwoofer
if you have loudspeakers that by themselves go very deep. But there is. The Audio Physic Scorpio’s in my Setup 1
go down to 32 Hz but I still use the Rel Britannia B2 sub. This sub goes down to 16 Hz, but that’s
not the point as we will see later on. The web is full of acoustic knowledge but
some caution is in order. General acoustics science is about hall acoustics,
acoustics for large venues like concert halls, houses of worship, conference halls and the
like. In large halls the wavelength of low frequencies
is only a fraction of the distance between two walls. A tone of 50 Hz, for instance, has a wave
length of 6.8 meters - 22.3 feet. So the room mode problem is quite different
from normal living rooms where walls are likely to be somewhere between 4 and 10 meters - 13
and 33 feet - apart. These lengths correspond with 85 and 34 Hz. Now you see why in surround systems for the
home the cross-over frequency for the subwoofer channel is 80 Hz. For if the wavelength of a sound wave is half
the length of the distance between two walls or integer multiples of that, that wave starts
to behave particular. To be more precise, sound waves are variations
in air pressure that we like to visualise as waves for if you plot the variation in
air pressure moving through a room, it looks like waves. In this simulation the maximum sound pressure
level is 100%. Although normally the level would be expressed
in dB’s, I use a percentage since that is easier for the average viewer. If we look at a problem frequency - in this
case with the wave length equal to the distance between the two brown walls, - you see that
there is no wave moving from left to right, it’s a stationary wave. A standing wave. The problem with standing waves is that at
the null - here in the middle - there is no varying pressure - so 0% sound - while - with
this wave - at a quarter of the room the pressure variation is much higher - for this frequency
- than with the propagating wave we saw earlier. This means that - at this frequency - the
loudness at this position will be higher than at neighbouring frequencies while it will
be zero in the middle of the room. The same goes for half this frequency and
multiples only at different positions. And that for all sound traveling in all directions. Now you will understand that there is an interaction
between the room and the subwoofer, thus placement will be of importance. To make things worse, this problem is far
easier to hear than comparable problems at mid and high frequencies. Most types of music have little complexity
below 100 Hz when compared with midrange and higher. Percussive instruments don’t generate sustained
notes and therefore are less of a problem but tonal instruments like double bass, euphonium,
organ and bass guitar do. Still there seldom are more than one of those
instruments playing notes at the same time. If there is a bass guitar, there usually is
no double bass. And if there is an organ playing the low registers,
there rarely will be a double bass, bass guitar or euphonium You might think that this makes
things simpler but the opposite is true for there is little chance amplitude peaks or
dips caused by room modes are masked by other tones as is the case in the midrange where
several instruments play at the same time. So it is likely that problems on the low end
of the spectrum are easily detected and thus will have a negative effect on the perceived
sound quality. The longer the sound wave in relation to the
size of the speaker, the more omnidirectional it is radiated. Subwoofers are made for very low frequencies
so they effectively radiate sound in all directions. Compare it to a bare lamp bulb that radiates
in all directions too. If you hang the light in the middle of the
room, the light will be distributed quite evenly unless the room has an irregular shape. Basically the same thing will happen with
the output of a subwoofer when played in the middle of the room. The sound waves will be sent in all directions
with equal energy. That is, if we for now ignore room modes that
cause standing waves. Most people will not place the subwoofer in
the middle of the room but rather place it, for instance, in front of a wall. Now the energy that is radiated towards the
wall is bounced back into the room. The same energy is radiated over only 180
degrees in stead of 360 degrees. This causes a louder signal in the room. It’s like having a lightbulb in front of
a mirror. It will increase the light radiated in one
direction. That is why headlight of most cars have reflectors
behind the lightbulb. These are no flat mirror but conical to narrow
the light ray further. And that is what also happens when you place
a subwoofer in a corner of the room. For now the same energy is bounced by two
walls the output into the room will be even higher. What about the floor then, that mirrors the
energy too. Yes, but since it is rather cumbersome to
have the subwoofer floating in mid air, that is a given. Now, you might think that the corner might
be the best place since it gives the highest energy. But that depends on the room modes and the
place you want to optimise for listening. That can be calculated but that’s rather
complex unless you oversimplify the problem. There are many spreadsheets and applications
that claim to be able to calculate the best position. But these are mostly based on the assumption
that the room is boxed shaped, often ignore the presence of furniture and the properties
of the walls. If your living has concrete walls, low frequency
energy gets bounced back into the room while drywalls can absorb low frequency energy to
a certain extend. There is no need for these kinds of calculations,
just use and old and proven method: place the subwoofer at the listening position, play
music containing rich lows and start listening at positions where you can place the subwoofer. Do listen just above the floor, exactly where
you would place it. Where the lows are the strongest is the best
place. It really is that simple. This approach does give the best performance
possible, but that doesn’t mean it will be optimal. There will be spots in the room where the
bass is less optimal and the best way to further optimise is to use more than one subwoofer,
of course placed at different positions. This way the position that has the nulls of
one subwoofer will be compensated by the other subwoofer, provided they are placed strategically. So you place the subwoofer at the listening
position and define the place for subwoofer 1. Then move the subwoofer to a second position
you might want to sound good and define the place for subwoofer 2. Now move the subwoofer to place one and place
the second subwoofer to position 2. There are reports of rooms with four subwoofers
that have impressively even distribution of lows. They didn’t tell how to bring that to your
partner, though. It is why I use a subwoofer with two large
speakers for left and right. For now the lows that are produced by three
speakers are at different positions more or less as if there are three subwoofers. Of course the placement of the left and right
speakers is limited since I also want a decent stereo image. There are several ways to hook up a subwoofer. Some brands offer the option to connect it
to the speaker terminals on the amplifier. I work that way for it makes it easier to
get good integration of the lows into the stereo. But the sub must have a specific input for
that and you must check if your amplifier allows for connecting it that way. There are amps with floating outputs that
need a different way of connecting. Here you connect the plusses to the red speaker
terminals on the amp while the minus is connected to the chassis of the amp. Check with your supplier if you need to go
this route. When connecting the sub to the speaker terminals
on the amp, the left and right speakers just are fed the full spectrum while the sub is
set at a low cross-over point. A second way is to connect the line inputs
of the sub to a special subwoofer output on the amplifier, provided the amp has this output. The sub output on amplifiers is normally filtered
and depending on the amp there might be a fixed cross-over frequency or you might set
the cross-over frequency of your choice. Check the manual of your amp for more info. In either case you want to disable the cross-over
in the subwoofer for two filters in cascade does degrade the sound unnecessarily. There are subwoofers that can adjust themselves
to the room - often via an app. Since these might have different approaches
from brand to brand, you best follow the adjustment instructions in the manual after you have
found the best place as described before. If the subwoofer has been used before, reset
all settings to neutral before finding the best placement and then let it adjust itself. You’re only left with setting the cross
over frequency - when needed - and the output level. For all other subwoofers the following approach
applies. If you use the subwoofer to only extend the
low end of your speakers, look up the specifications of your speakers and look for the specified
frequency response. That might specify for instance 52 - 22,000
Hz ±3 dB. In that case 52 Hz is where the speaker starts
losing output by 3 dB. Then start with setting the cross-over frequency
on the subwoofer at 52 Hz. If you want to use the subwoofer to have another
low frequency source to have a more even low end throughout the room, set the cross-over
frequency to 80 Hz. The next step is to set the phase. This can be a switch with 0° and 180° as
option or a continue or stepped arrangement to vary phase over that range. Play music with rich lows, try to set the
volume of the left and right speakers and the sub about equal and listen at the listening
position for most powerful lows. You best have someone vary the setting for
you while you listen. The next step is to tweak the volume. It’s best to adjust it over time for each
and every record might sound different. Simply start with the subwoofer volume too
high and reduce it every time you hear it work. Keep doing that until you think you don’t
hear the sub anymore. Then switch it off and you will most likely
hear that it was loud enough but since is was well integrated, it didn’t stand out
anymore. If this doesn’t happen, you might vary the
cross-over frequency slightly. If the sound remains too boomy, lower the
cross-over frequency and if it’s too lean, increase the cross-over frequency somewhat. Use the same tactics as with the volume; play
many different records over several sessions. It took me several weeks before I was happy. Not that I continuously played music for several
weeks, it were many short sessions with a lot of time in-between to reset my hearing. Remember to start at a high subwoofer level,
lower it in small steps and try not to increase the volume again. There will always be albums that sound thin
on the low end, a subwoofer can’t really compensate for that. It can’t reproduce sound that isn’t there. Once you’re happy, take a photo of the settings
and fixate the settings with tape so they can’t accidentally be changed during cleaning. A good subwoofer doesn’t make your stereo
sound cheap. When done well you won’t hear the subwoofer
until you switch it off. It doesn’t need to be a very expensive sub,
the REL I use in my setups 2 and 3 was only € 450 but makes both setups sound so much
richer. Don’t fall for the cheap systems with very
small left and right speakers and a central woofer. If the left and right speaker can’t go deeper
than 100 Hz, you will never get decent energy in the 80 to 250 Hz region. You will miss the bite of a snare, for instance. Note that I used the name central woofer in
stead of subwoofer for since the left and right speakers really have no real woofer,
the central woofer can’t be called subwoofer. Another thing to mention is that a subwoofer
doesn’t need to be placed near the left and right speakers, a side wall placement
or even at the wall opposite the left and right speakers might work. Just try it. Adding a subwoofer to your system the good
way does admittedly takes time and effort. But the reward will almost always be rather
rewarding. And that brings you to the end of this video. Next week there will be another video, as
always at 5 PM central European time. If you don’t want to miss that, subscribe
to this channel or following me on the social media so you’ll be warned when new videos
are out. If you liked this video, give it a thumbs
up. Many thanks to all that support this channel
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the links are in the comments below this video on Youtube. I am Hans Beekhuyzen, thank you for watching
and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music.
