Subwoofer placement vs. room modes (standing waves)

[Richard Greene]

I posted this at another website and then thought people here might need some scrap paper for the bottom of their bird cages, so let your birds read this. Also good for curing insomnia:

Room dimensions have nothing to do with how smoothly ("evenly") room modes
(standing waves) are excited.

Room dimensions do determine whether a listening room has "stacked" (same center frequency, as in a square room) or "adjacent" room modes (nearby center frequencies,
as in a near-square room). But the room mode center frequencies have nothing to do with how strongly each room mode is excited by a specific subwoofer location.

Subwoofer location is the key to how strongly room modes are excited
(or not excited much at all).

The location of your ears is the key to whether you hear a bass peak or null (trough) of a particular room mode. The nulls are quite narrow, but the peaks cover large areas of a room, so you are much more likely to hear one or more bass peaks (unless you sit in the geometric center of the room where there are several nulls).

A corner subwoofer in a rectangular room, for example, would evenly (fully) excite all room modes ... however that's no guarantee the sub-80Hz. bass will sound good at the listening seat.

There is no ideal subwoofer placement in most rooms.

Subwoofer(s) placed close to one (or both) of the main speakers will integrate best
... and this also prevents phase problems.

The bass frequency response will be uneven, primarily due to room modes, no matter where you place a subwoofer (or two). In my experience (and measurements) since 1980, this statement applies to about nine out of ten home listening rooms.

----------1/3 of the distance between opposing walls?
- There is no particular advantage to "1/3" Rules of Thumb for subwoofers (unless you have bass frequency response data from a number of rooms to prove me wrong.) The "Rule of Thirds" does work well for main speaker placement because odd fractions (such as 1/3 or 1/5) of the distance between opposing walls avoids placing speakers in room mode nulls.

For subwoofers, however, placing the subwoofer in or near a particular room mode null is a no-cost method to prevent, or at least reduce, a bass peak caused by that room mode.

In a typical rectangular listening room, here's what a listener located half way betwwen the side walls is likely to hear (axial room modes only -- and these generalizations are no substitute for bass frequency response measurements -- see linkwitzlabs.com for bass test CD):

--- Front-wall-to-back-wall first-order axial room mode
A bass peak is likely to be audible ... although perhaps at such a low frequency in rooms over 25 feet long that it's rarely excited by the program content (and the speakers may have significant roll-off at that low frequency so it may not be a problem even if the room mode frequency is frequently excited by the music)

--- Front-wall-to-back-wall second-order axial room mode
A bass peak is likely to be audible ... but can be reduced by placing the subwoofer at or near the 1/4 point and/or your ears at or near the 3/4 point between the walls.

--- Side-wall-to-side-wall first-order axial room mode:
A bass null is likely to be audible (nulls tend to be easier to overlook/ignore than bass peaks)
Left-right speakers or left-right subwoofers will be out of polarity for this room mode so will not excite it, assuming mono bass typical of two-channel recordings. Also, the typical two-channel listening seat located 1/2 way between the side walls will place the ears in or near a null for this room mode. That means the bass frequency response is likely to be weak at this room mode frequency and would be smoother if one uses one subwoofer located off center, or two subwoofers located on the same side of the room to excite this room mode.

--- Side-wall-to-side-wall second-order axial room mode:
A bass peak is likely to be audible:
Placing speakers or subwoofer(s) in or near the null at 1/4 or 3/4 of the distance between the side walls will reduce excitation of this room mode. The high pressure zone for this room mode is 1/2 way between the side walls where your ears are likely to be located for two-channel stereo = this mode will result in an audible bass peak if it is strongly excited.

--- Floor-to-ceiling first-order axial room mode:
A bass peak is likely to be audible -- this bass peak is so common that many audiophiles are used to it and don't notice it ... until it's eliminated with parametric EQ. Subwoofers are almost always placed on the floor where they strongly excite this room mode. The null is approximately 1/2 way between the floor and ceiling which is well above where your ears are going to be located ... unless you are about 8 feet tall


Background:
- Room mode nulls at approximately 1/4 of the distance between opposing walls: Subwoofer driver placement in a null located at or near "1/4" (or 3/4) of the distance between opposing walls can be used to reduce excitation of second-order front-wall-to-back-wall and second-order side-wall-to-side-wall axial room modes, assuming those room modes would otherwise cause a bass peak at your listening position.

Once again, the subwoofer should be located near one of the main speakers with it's driver approximately the same distance from your ears as the bass and midrange drivers in the main speakers, so integration is seamless. The attack of bass notes
(pluck of bass guitar string and slap of kick drum hammer are in the mid-range frequencies and do not come from the subwoofer. The transient response will be best if all the drivers involved with a bass note are located the same distance from your ears.
Many people will not notice a difference of up to three milliseconds
(approximately three feet).

Lots of bass traps and/or parametric EQ are more effective in reducing bass peaks heard at the listening position, but subwoofer placement can help with a few of the five or six axial room modes under 80Hz. in the typical home listening room ...
for cheapskates.


__________________
Richard BassNut Greene

[Worf101]

Why don't you post this in the "How To" section of the bulletien boards? Everybody could benefit from your knowledge. I'm serious!!!

Da Worfster

[Woochifer]

I would second Worfster's recommendation. This is good info, and well timed since we did a little bit of rearranging in the room and I need to reset some of the filters on my EQ.