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Old 27th February 2004, 08:57 AM   #1
Yuihb is offline Yuihb  Germany
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Default Room modes caused by omnidirectional speakers

The picture below shows a woofer in the lower left corner of a room. There will be standing waves along the black lines whereas along the blue lines the reflected waves will be sufficiently attenuated to inhibit resonance.

If additionally the wall is damped where the black lines end, this room will exhibit little sound coloring due to room modes. A bass boost that may be caused by the corner the woofer is placed in can be equalized electronically.

Opinions?
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Old 27th February 2004, 02:18 PM   #2
sreten is offline sreten  United Kingdom
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Default Re: Room modes caused by omnidirectional speakers

Quote:
Originally posted by Yuihb
The picture below shows a woofer in the lower left corner of a room. There will be standing waves along the black lines whereas along the blue lines the reflected waves will be sufficiently attenuated to inhibit resonance.

If additionally the wall is damped where the black lines end, this room will exhibit little sound coloring due to room modes. A bass boost that may be caused by the corner the woofer is placed in can be equalized electronically.

Opinions?
I'm sorry but I can't agree with any of your surmisations.

Its impossible to damp a wall at the bass frequencies described,
however bass traps can be used for a particular frequency.

Your modeling is very simplistic. Basically a spherical wave
front will resolve into parallel wave fronts traveling along the
dimensions for the basic room modes and their harmonics.

More complex modes that are not parallel to walls can be
deduced from the diagonals, for two walls the hypotenuse
formed, for all three walls the bottom corner to far top corner
dimensions. These modes also have harmonics.

Damping of the lower frequency modes is low in most rooms,
consequently excitation of modes is unavoidable. For smooth
excitation of these modes nice room dimensions are required
with placement of the sub-woofer ideally 1/3 along the main
diagonal. This is generally impractical, and usually 1/3 along
the floor diagonal is also difficult - leading to the standard
recommendation for one sub of 1/3 along one of the walls.

sreten.
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Old 27th February 2004, 02:25 PM   #3
Rudolf is offline Rudolf  Germany
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Donīt think of sound as "beams". It radiates in spheres. So room modes will not be excited along the black lines only. There will be standing waves between the complete surface of opposite walls.
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Old 27th February 2004, 04:30 PM   #4
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Default Room modes - some facts

YOU WROTE, and I disagree with:
" For smooth excitation of these modes nice room dimensions are required with placement of the sub-woofer ideally 1/3 along the main diagonal. This is generally impractical, and usually 1/3 along
the floor diagonal is also difficult - leading to the standard
recommendation for one sub of 1/3 along one of the walls."

RG comments:
Room dimensions have nothing to do with how smoothly
(I assume you mean "evenly") room modes 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 at all)

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.
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Old 27th February 2004, 06:14 PM   #5
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Very nice. Let's hear applause for someone who knows what they are talking about.
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Old 27th February 2004, 07:05 PM   #6
sreten is offline sreten  United Kingdom
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Default Re: Room modes - some facts

Quote:
Originally posted by Richard Greene
YOU WROTE, and I disagree with:
" For smooth excitation of these modes nice room dimensions are required with placement of the sub-woofer ideally 1/3 along the main diagonal. This is generally impractical, and usually 1/3 along
the floor diagonal is also difficult - leading to the standard
recommendation for one sub of 1/3 along one of the walls."

RG comments:
Room dimensions have nothing to do with how smoothly
(I assume you mean "evenly") room modes are excited.

etc.........

Pedantically you are correct, but you also know what I meant -
for the smoothest room response good dimensions are required.

I disagree that the point of sub woofer placement is to place
it in a null position to cancel a mode - unless the room has bad
dimensions (causing stacking) and then it is completely valid.

If the room has good dimensions then even excitation of modes,
and specifically avoiding any null dips is the point of placement.
(Nulling a mode causes exaggeration of a higher mode)

Quote:
Subwoofer location is the key to how strongly room modes are excited (or not excited at all)
We completely agree on this, given we are talking low modes.

I generally agree with your other observations in terms of
cause and effect but again disagree than subwoofer or
listener placement to cause or be in a null is a good thing.
(except to deal with bad room dimensions)

In the absence of any further information and the acceptible
postioning of a subwoofer 1/4 to 1/3 along a wall is a good
place to start as any.

For most situations this is the only range of adjustment
acceptable to most people. And better than the corner.

sreten.
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Old 27th February 2004, 08:36 PM   #7
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Cool Hope I don't have to disagree on EVERY post here !

"Pedantically you are correct, but you also know what I meant -
for the smoothest room response good dimensions are required."

RG:
"Pedantically" ?
Please use four-letter words when insulting me as I was edumacated in NY public skools and don't know what pedantically means. I read your prior post, tried to understand it, and disagreed in writing with it. I disagree even more with your latest post and here's why:

Your words "smoothest room response" are not very relevant for subwoofer bass frequencies because in about nine out of ten home listening rooms, the bass frequency response from monopole non-equalized subwoofers is either bad ...
or very bad when there are stacked or adjacent room modes causing large bass frequency peaks. The words "smooth" and "bass" do not belong in the same sentence to describe most rooms! This even applies to rooms with "golden ratio dimensions" with no stacked modes or adjacent modes ...
and is easily proven with measurements at the listening position using slow sinewave sweeps and the more demanding repeating bass tone bursts.

In some very large rooms, where room modes are much more dense in the critical octave from 40-80Hz. (where the room modes are frequently excited by bass guitar and kick drum fundamentals), our ear's one-third octave smoothing ability
can make well-distributed room modes sound like a reasonably smooth bass frequency response.

In a very large room with good dimensions, the listener also has much more flexibility in where he places his stereo equipment. He doesn't have to sit in the null half way between the side walls -- placing the main speakers a little off center and sitting a little off center to obtain a better bass frequency response is an option in a very large room.
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"I disagree that the point of sub woofer placement is to place
it in a null position to cancel a mode - unless the room has bad
dimensions (causing stacking) and then it is completely valid."

RG:
The main point of subwoofer placement is perfect or near perfect integration with the main speakers.

The second point is a reasonably smooth bass frequency response -- I personally prefer +/-5dB or less which I have never measured in any room that did not have lots of bass traps
(12 to 16 tubular traps filling 2 to 3% of the room's volume in one room) or parametric EQ.

Most rooms are +/-10dB or worse for bass under 80Hz. measured at the listening position using a slow sinewave sweep.

For cheapskates who are unwilling to invest in bass traps and/or parametric EQ (which I believe is a serious mistake as DIY bass traps and many parametric EQs are so cheap), sub placement can be used to reduce excitation of one or two of the axial room modes that would otherwise cause an annoying bass peak at the listening position.
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"... even excitation of modes, and specifically avoiding any null dips is the point of placement. (Nulling a mode causes exaggeration of a higher mode)."

RG:
Well ... a corner subwoofer would evenly excite all room modes but I don't recall you recommending that!

While placing a subwoofer in or near a null would mean it would be in a high pressure zone for the next order room mode one octave higher, that octave higher mode is going to be outside the subwoofer's frequency range (when you address the second order room modes as described in my previous post).
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"I generally agree with your other observations in terms of
cause and effect but again disagree than subwoofer or
listener placement to cause or be in a null is a good thing.
(except to deal with bad room dimensions)"

RG
Using subwoofer position to reduce excitation of a room mode is especially useful for stacked or adjacent room modes.
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"In the absence of any further information and the acceptible
postioning of a subwoofer 1/4 to 1/3 along a wall is a good
place to start as any."

RG:
Well, as you know, I disagree with the 1/3 position as being anything special. The 1/4 position can be useful for a specific room mode in the absense of bass traps and/or parametric EQ, but only if the subwoofer is also close to one of the main speakers.

Experimentation and measurement is the key for a decent bass frequency response along with bass traps and/or parametric EQ
in most rooms ... but the further the sub driver voice coil is located from the main speaker's voice coils, the more likely subwoofer-satellite speaker integration will suffer as a result.
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"For most situations this is the only range of adjustment
acceptable to most people. And better than the corner."

RG:
There's nothing inherently wrong with a subwoofer in a room corner if that happens to be near one of the main speakers.

I personally try to place my main speakers quite far from room corners, but some people are unable to do that (i.e.; they have
a wife to share the room with!)

The change in the bass frequency response as you slide a subwoofer out of a corner is not very large until you approach the
1/4 point between opposing walls where there is a narrow null
(narrower than a subwoofer driver!). Remember that these are very long wavelengths involved under 80Hz (if there is significant output over 80Hz. and male voices can be heard through the
"subwoofer" when all other speakers are turned off, different Rules of Thumb apply to subwoofer placement because the subwoofer's output will be directional and affect the stereo soundstage).

If one happens to be using the Rule of Thirds placement for the main speakers and listening seat as I often do, there's a good chance the subwoofer will end up somewhere near 1/3 of the way between the front and rear wall.

I personally build subwoofers as a hobby and currently use a mono low Qtc sealed subwoofer I built a few years ago:
- 15" Adire Audio Tempest driver
- 48" by 18" tube stuffed with 8 lbs. polyester fill (laying on it's side in my living room believe it or not = nice wifey)
- Onkyo M501 stereo basic amplifier (150 watts to each voice coil)
- Marchand XM9 70Hz. 24dB/octave active crossover
- EPOS ES11 main speakers on 26" Premier stands


The subwoofer is laying on the carpet right next to my left speaker, equalized to within +/-5dB at my listening position using four or five bands of a Behringer Feedback Destroyer (digital 12 band stereo parametric EQ).

This seems like a lot of work to obtain good bass down to 20Hz.
from a sonically invisible subwoofer, but then I'm a BassNut who goes berserk when bass frequency response peaks interfere with my music.
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Old 27th February 2004, 09:16 PM   #8
tiroth is offline tiroth  United States
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If you like the FBD you'd love using a DEQ2496 for this purpose. The parametric module can nail down any peaks (within reason) and the graphic EQ can tweak the final result within a few DB from 20-80Hz...not to mention seeing the filters visually makes the data-entry a bit easier.
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Old 27th February 2004, 10:42 PM   #9
sreten is offline sreten  United Kingdom
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RG,

you seem to be ignoring the points where I agree with you,
i.e. null positioning is useful against stacked modes just to
disagree with some of my other points.

Bass traps etc. are distinctly beyond the realm of most people,
and way beyond what the starter of this thread was considering.

Personally I'd go for fullrange speakers any day of the week,
for lots of reasons, mainly freespace positioning and optimum
use of room gain for bass, (which agrees with your positioning
next to the loudspeakers) but for what its worth I agree flat
low bass response is not realistic for "cheapskates", we have
to make the best of rather simplistic measures.

Corner mounting of the subwoofer will maximaly excite the room
modes, which is not condusive to "smoothest" bass, it also causes
deep nulls, a point you neglected to mention.

I don't think this discussion is going anywhere, usually when told
I'm being pedantic, generally on reflection I have to agree, most
of the time it isn't important but sometimes it is.

But to be pendantic "smoothest" can be defined as the the peak
to null ratio, weighted somewhat against the lowest frequencies
influence (as amplitude errors higher in frequency are more
audible), and the position that gives that, the actual dB ratio is
irrelevant. The Db ratio's can be improved by traps for specific
frequencies which may allow more convenient positioning.

sreten.
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Old 1st March 2004, 04:34 PM   #10
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Talking I agree with everything ... except

you seem to be ignoring the points where I agree with you,
i.e. null positioning is useful against stacked modes just to
disagree with some of my other points.

RG
Sorry that I focused on disagreements as we had many points to agree on, including most of your post I'm now reponding to.

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Corner mounting of the subwoofer will maximaly excite the room
modes, which is not condusive to "smoothest" bass, it also causes deep nulls, a point you neglected to mention.

RG:
In a 24 foot long room, for example, peaks and nulls measure almost the same whether the sub is "in the corner" or 3 feet from the corner.

It isn't until you approach 1/4 of the distance between the front and rear walls (6 feet) with the subwoofer that differences versus a corner subwoofer are likely to be audible while listening to music.

Any subwoofer located next to a side wall and on the floor will excite side-wall-to-side-wall and floor-to-ceiling standing waves just as strongly as a subwoofer located in a corner.

.
.
.

But to be pendantic "smoothest" can be defined as the the peak
to null ratio, weighted somewhat against the lowest frequencies
influence (as amplitude errors higher in frequency are more
audible), and the position that gives that, the actual dB ratio is
irrelevant. The Db ratio's can be improved by traps for specific
frequencies which may allow more convenient positioning

RG
That's an interesting way of analyzing bass frequencies,
but I doubt if that ratio correlates well to subjective evaluations
of bass frequency response, for two reasons.

First, the only peaks and nulls that matter are those heard at the listening position.

It's actually tough to place both your ears in a standing wave null because they are very narrow.


Second, while listening to full-range music, not bass solos,
peaks will be considerably more audible than nulls.

It's tough to overlook loud bass booms -- much easier to overlook weak bass at specific frequencies.

Frequencies too loud are much easier to notice than what's weak or missing.
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