Room Acoustics - Large Room and Small Room

[mas]

{Gee, its interesting to see that a bit of the personal attack above has since been edited...}

I would suggest more fiber.

Let's see, another post decrying less math to balance the plethora of posts decrying any math, despite the inclusion of LOTS of hyperbole that you have missed, just as I suspect this jest and play on words will be as well. (...I guess I have to start pointing this stuff out [*-)] - hyperbolE, hyperbolA...get it? Probably not... Look it up...)

"Anybody can do that stuff". Congrats! ...Had I only known that Jethro had arrived to help those with their cypherin'....

Just to save those who already know it all a bit of time...this thread is not for you. You will be very bored here. Nor will we be discussing autotransformers. I know there are many folks here with varying degrees of experience and knowledge. I apologize if I have not acknowledged that some already know everything.

Some will be anxiously awaiting your original research. We are not attempting to present any here...but again, it is always entertaining to read of allegations based upon information not in evidence. Neither will I be attempting to present any original cookie recipes nor novel approaches for BBQ sauces.

And as my warped sense of humor simply can't resist, I hope we didn't confuse the really erudite folks with the "circle thingies".

May I suggest that you take your issues and discuss them with the those who have complained about the few math examples that were included. My tongue in cheek response was for them, not for the more 'erudite' folks such as yourself. But thanks, as you have indeed added allot to our understanding of the topic.

BTW, any time you might wish to talk and actually discover what I know, you are glad to PM me. But be careful, we wouldn't want to ruin the the speculation game based upon ignorance. After all, there is more to tuning a room than simply installing polycylinders that simply 'scattering' focused specular reflections rather than actually diffusing them. []

Have a nice holiday.

(PS...I suspect you will be in a better humor after you have found a few kids to whom you can tell that there is indeed no Santa Claus.

Oh, and for the rest of you kids on the forum who may have just read this..... I was just joshin'...there really IS a Santa!

...Geesh! Why do I have the feeling that I am going to hear from Coytee again and that Chloe is going to be really pissed now!?)

[Coytee]

She cornered him for 3 hours last year in our fireplace. Our fireplace is a ventless type.

[&]

[westcott]

I would suggest more fiber.

Let's see, another post decrying less math to balance the plethora of posts decrying any math, despite the inclusion of LOTS of hyperbole that you have missed, just as I suspect this jest and play on words will be as well. (...I guess I have to start pointing this stuff out [*-)] - hyperbolE, hyperbolA...get it? Probably not... Look it up...)

"Anybody can do that stuff". Congrats! ...Had I only known that Jethro had arrived to help those with their cypherin'....

Just to save those who already know it all a bit of time...this thread is not for you. You will be very bored here. Nor will we be discussing autotransformers. I know there are many folks here with varying degrees of experience and knowledge. I apologize if I have not acknowledged that some already know everything.

Some will be anxiously awaiting your original research. We are not attempting to present any here...but again, it is always entertaining to read of allegations based upon information not in evidence. Neither will I be attempting to present any original cookie recipes nor novel approaches for BBQ sauces.

And as my warped sense of humor simply can't resist, I hope we didn't confuse the really erudite folks with the "circle thingies".

May I suggest that you take your issues and discuss them with the those who have complained about the few math examples that were included. My tongue in cheek response was for them, not for the more 'erudite' folks such as yourself. But thanks, as you have indeed added allot to our understanding of the topic.

BTW, any time you might wish to talk and actually discover what I know, you are glad to PM me. But be careful, we wouldn't want to ruin the the speculation game based upon ignorance. After all, there is more to tuning a room than simply installing polycylinders that simply 'scattering' focused specular reflections rather than actually diffusing them. []

Have a nice holiday.

(PS...I suspect you will be in a better humor after you have found a few kids to whom you can tell that there is indeed no Santa Claus.

Oh, and for the rest of you kids on the forum who may have just read this..... I was just joshin'...there really IS a Santa!

...Geesh! Why do I have the feeling that I am going to hear from Coytee again and that Chloe is going to be really pissed now!?)

I have a question. I know it is probably not answerable but if the control room is manipuling the ITD and the Studio is manipulationg the ITD, how are we supposed to treat a room to address all the various manipulations that have been already implemented?

I also want to make sure I got this. Lpowerlevel or sensitivity and directivityQ equals Lpressurelevel?

[mas]

I have a question. I know it is probably not answerable but if the control room is manipuling the ITD and the Studio is manipulationg the ITD, how are we supposed to treat a room to address all the various manipulations that have been already implemented?

I also want to make sure I got this. Lpowerlevel or sensitivity and directivityQ equals Lpressurelevel?

The answer is actually quite simple. You don't. Your job is not to try to re-engineer the source.

The purpose of the ITD/ISD is essentially to create a small period of time where your experience is anechoic - you are only experiencing the direct signal without any reflected room effects. The rest of the room treatment is to break up specular reflections and to redistribute the acoustic energy in a well behaved logarithmic decay providing a semblance of 'space' that provides a semblance of a reverberant space (which is not actually present in a small room).

The purpose of a mix environment is to focus entirely on this direct signal component, as they have no control over the playback environment. Hence the use of RFZ (reflection free zones) and other techniques useful in a studio mix environment, but not appropriate fro a home listening anvironment.

And to digress...a major source of confusion for many is exactly this confusion of studio designs with 'home' designs. Various techniques and aspects are applicable to both, but you do not want to create a home listening environment copying a studio mix environment!

And "Lpowerlevel or sensitivity and directivityQ equals Lpressurelevel? " Ah...I am not sure what you are looking for here.

Q is a 'measure' (a ration of 'in to out') of the polar dispersion of the speaker - the coverage area. I guess that you are trying to get to an intensity distribution by saying, for example, if a given sound energy is distributed within a high Q distribution, the SPL will be higher as opposed to a given sound energy distributed in a low Q distribution, thus resulting in the same energy being spread over a greater area...But I am not exactly sure what you are trying to do with this. But, to state this from a 'position' that I am more familiar, yes, the smaller the volume of space a given amount of energy is dispersed, the higher the effective intensity. Hence, for example, corner placement is more 'effeicient' than free space placement. Please see the attached graphic that summarizes this.

[mas]

deleted dupl post

[mas]

hahaha! NOW ithe system gets religion and says my message in the post cannot be a duplicate when I try to replace the post with a "deleted dupl post" message! [:S]

This can only be running on a MS system!

[mas]

deleted duplicate post

[mas]

Neat!!!! ...as the Twilight Zone theme plays in the background with a small MS emblem incorporated in it...

I submitted the post, received an error, went back and looked - no post, so I then I resubmitted the post. And the result is 4 posts, 2 with the attachment and 2 without!

[*-)][^o)][:|][]

[DrWho]

I'm sure mas can answer this better than I, but I'll give it a shot...

Ultimately, if you're asking the question, then I don't think you quite understand how the audible effects of the ITD? You don't want the ITD to be infinitely long because that's what an anechoic chamber sounds like. You don't want it infinitely short either though unless you dig the sound of comb-filtering. The way I understand it, you want the ITD just long enough to clear the Haas window so that any reflections are not percieved as influencing the output of the direct sound...basically, achieving the effect of no percieved comb-filtering (since that always sounds bad).

Once the ITD is established, it's up to the end user to determine what kind of semi-reverberant field they prefer. In fact, it's up to the end user to determine how long they want the ITD to be too. It's all just personal preference really - the art of acoustics so to speak? I'm sure there's plenty of research going on to try and describe generally ideal scenarios, but no room is ever going to be close to the ideal. The purpose of the science then becomes one of understanding the systems at play so that the end user can pick a set of least compromises for the dollar.

Right now, my understanding is that you want the ITD to be right at the edge of the Haas window....so around 20-40ms depending on the frequency range. Beyond that, you want a strong Haas kicker (strong first reflection), say about 6-10dB down from the direct sound and then as closely as possible approximating an exponential decay with the ensuing reflections. The most important criteria being that spikes in the semi-reverberant field need to be non-existant since they distract your attention on the direct sound. You'll rarely be able to achieve a long enough decay to be "ideal" so it's usually a question of how long can you make it...

As far as differences between the studio and the home....in the studio, you want to hear exactly what is on the recording and then you use your mixing experience to predict what it will sound like in the environment of your target audience. This often means mixing only the direct signal and trying to ignore the acoustics of your control room. You need to be able to differentiate between ambience and spatial cues that need to be in the record from ambience and spatial cues of your own room, since your control room acoustics rarely exist in the target audience scenario.

The home listener then takes this recording mixed for the direct sound, and puts it into their listening room where they still get to hear the initial direct sound....followed by whatever semi-reverberant field they have determined they feel sounds most natural. Pretty much any amount of reverberant field sounds the same (provided it's dense enough), so there's really nothing the end-user can do to distroy the intended sound....they just get to pick the flavor they want. But to bring it back to the ITD....pretty much any amount of percieved comb-filtering is going to destroy the image. At least I'm yet to hear a situation where it sounds good.

Anyways, that's just my take on the issue...if I'm completely wrong, then perhaps it'll spur some conversation to guide us in the right direction.

[DrWho]

ahhhhh, overlapping posts! lol

You're outta control Mark....it's prob cuz you're not using a windows machine on the forum []

[westcott]

thus resulting in the same energy being spread over a greater area...But I am not exactly sure what you are trying to do with this. But, to state this from a 'position' that I am more familiar, yes, the smaller the volume of space a given amount of energy is dispersed, the higher the effective intensity. Hence, for example, corner placement is more 'effeicient' than free space placement. Please see the attached graphic that summarizes this.

THAT pretty much clarifies what I was asking.

I will keep reading to better understand how a room should be treated to avoid a studio like listenting room.

I do not have the luxury of randomly placing solutions so I want to make sure that I do what is best before making the pitch to the wife in our shared space. I have a large "small" room and I will have to be very judicious in my approach.

So, if you were going to pay a professional to come in and make in room measurements, who would you suggest,what questions would a prudent customer ask, and what would one expect to pay (per cubic foot?).

Oops. I should have PMed you. I will next time, I promise.

[mas]

This is just another overview. I will not be going onto the actual
specifics of particular diffusor or absober design in the piece.
Besides, it is not the time to be doing so anyway if we do not have
actual measurements identifying the required characteristics. That
comes later.

The Masonite/hardboard polycylinders are more properly labeled
'scatterers', in that they reflect sound in lower intensity specular
waves - meaning that the 'smaller' reflections have less intensity than
the incident waveform, but are still specular (focused) in nature.

They
were the early precursors to more effective diffusion methods which
were ushered in with the research performed by Manfred Schroeder.

Peter
D'Antonio was the first to transform Schroeder's ideas into a product
in the the form of his QRD (quadratic residue diffusers) - which along
with many advances since, more accurately and completely diffuse the
reflected sound.

But back to the absorption/diffusion conundrum.
All materials are partially both. All materials have an acoustic
impedance. And just like in electronics (where some think they can make
all sorts of definitive statements as opposed to that oh so confusing
and imprecise acoustic realm), materials will both absorb and reflect
sound waves in a frequency dependent manner.

Those which tend to
be more reflective at the frequencies of interest are generally
referred to as reflectors or possibly diffusers(depending upon the
reflected wave behavior) while those that tend to be more absorptive at
the frequencies of interest tend to be referred to as absorbers. Of
course there have been developed products that exhibit both
characteristics in a desirable way and hence their being referred to as
abfusors, diffsorbers, etc.

With measurements the pattern and
behavior of sound at a given spot is relatively easy to identify and to
ascertain the effect of applied treatments. But stated in words, a room
has a finite amount of energy upon stimulation. It acts much like a
capacitor storing energy and releasing it over time. The tendency is to
attempt to solve the acoustical problems by the use of absorption,
which at first seems a pretty reasonable solution. But in a small
acoustical space this quickly proves to be a problem, resulting in an
acoustically 'dead' room. And if all you want to listen to is the
direct sound, you are in heaven...a very 'dead' sounding heaven.

Most people on the other hand prefer a sense of space, and this space is provided by a semi-diffuse sound field.

So
the way that this is achieved while also maximizing the intelligibility
and imaging of the direct signal is to provide surgically applied
absorption at the first order reflection points, providing for what is
referred to as the ISD or ITD, an initial signal delay gap. During this
period where perception is dominated by the psychoacoustics of Henry
Precedence and Haas Effects, we establish an anechoic response, whereby
for a short period you experience only the direct sound, without the
reflections which are detrimental to intelligibility.

But we want to absorb only
that energy that arrives within that time window that we cannot diffuse
effectively to remain in the room system to be used to 'construct' and
'shape' a well behaved semi-diffuse field that will provide the sense
of space - the sense of being in a large acoustic space. And since this
remaining sound energy in a small acoustic space consists of specular
(focused) reflections which are detrimental to perception in this form,
our goal is to create a well behaved diffuse sound field that decays in
intensity in a predictable manner with respect to time. This is done
via diffusion.

Oh, and all of this is done in addition to
addressing the fundamental issue of room modes - LF standing waves.
These wavelengths are sufficiently long that they establish resonant
patterns based upon the room geometry, just like a pipe organ relies on
tuned pipes to establish resonances that correspond to the frequencies
of the various LF notes. And while in an organ, their goal is to create
and to utilize this resonance, in a room, we are trying to accurately
support the reproduction of a source signal rather than create music.
And just as amplifier distortion can be a valuable aid to a performer
in creating a particular sound or effect, such distortion is an enemy
in the reproduction of the recorded signal; and in a similar manner, LF
standing waves that reinforce certain LF modal frequencies are
detrimental to the reproduction of source in an acoustic environment.

And
these are addressed in much the same way as the higher order sound
waves are addressed as mentioned above. We first desire to distribute
the resonant peaks in order to minimize their summation, which serves
to amplify the problem frequencies. And as this is determined primarily
by the room dimensions, our next line of 'defense' is to then attempt
to address them via room treatment.

And the treatment of room
modes is done via the two methods mentioned above as well. The most
commonly mentioned treatment is via 'bass traps' - LF absorption. And
there are many types and configurations. Most may only be
familiar with one or two popular commercial products, but the reality
is that these are simply some of the simpler designs available.

Most
LF absorption takes advantage of the Helmholtz configuration, of which
there are many styles. It must be remembered that the LF standing waves
exhibit ALLOT of energy. And in order to decrease the signal strength
of such resonances, it requires 'big' tools capable of addressing and
acting as an effective sink for such energies. And just like electrical
sinks, the acoustical impedance of the sink must match the impedance of
the acoustical source very closely in order to increase the
effectiveness of the device in order to effectively reduce the
'offending' resonance.

Considering the large amount of energy and
the generally moderate to high Q of the offending resonance, typically
a tuned Helmholtz resonator is used. And while the various types are
beyond the scope of this entry, suffice it to say that many options are
available for a given application. These can take the form of wall
segments, applied panels, tubes, 'jars', columns, porous or slotted
sections...in other words, just about any shape that one has the
knowledge to design. And they can be designed to have a very high Q or
a lower Q with a wider bandstop. Of course, with the wider effective
frequency band, the efficiency of the absorption decreases.

But
absorption is not the only tool. Diffusion can be effective aid as well,
although the wavelengths of the frequencies involved require that the
effective size (depth more so than length and width) of the diffusers
be correspondingly large as well. And given the size of the space, and
whether or not you are dealing with a limiting WAF, these may or may
not be an appropriate alternative.

The primary measurements used for this are cumulative spectral plots (waterfalls) for the LF standing waves, and although they are not necessary, they provide a wonderful visualization of the relative behavior of frequency and the waveforms persistence with regards to time. In the higher frequencies, ETC (Envelope Time Curves) are of particular usefulness. Again, many other measurements can provide additional detail, but these are the most generally useful.

[gaspr]

Good stuff. Glad to see this thread back on top here.

[gaspr]

Mas...Could you please tell us more about how we should go about measuring a room and interpreting the data.

[@silverfox@]

from gaspr post this would be most benifical to those who would like
tohave some type of real life situation to get a clearer view ,there is
a large amount of info to sift through, & focusing on a
particular project would definately benifit some one like me .

excelent means of bringing this topic into focus.

[DrWho]

If I might jump in, it's actually really easy to measure the room - provided you have all the right equipment. If you have a laptop handy, you could probably put together a decent measuring rig for around $250. If I'm understanding correctly, all of the analysis of the fancier toys out there revolves around obtaining the impulse response of the room and then working some numbers to see whatever it is you want to see (like the frequency response, waterfalls, energy time curves, etc...). But I too think it would be beneficial to look at some measurements of a real room and then get walked through the treatment process, seeing how things change and improve the sound in the room rather than imagining it. While it won't be completely exhaustive, I think it would make more real a lot of the concepts at play here.

On a related note, I've finally finished putting together a portable pre-configured measurement system, which I was intending to ship around to those unable to obtain their own (too expensive, too difficult, whatever). There would be a small fee associated with it intended to cover the cost of wear and tear and to fund the purchase of more advanced software (so not trying to make a profit by any means). I would encourage everyone to build their own measurement rigs, but just thought I'd offer an alternative since most people probably will never need to measure more than once. I've only just begun to dabble with the measurements and it's absolutely fascinating how everything correlates to what you hear and even correlates with all the cool info in this thread.

And if it hasn't been said already, thanks Mark for all the time you're putting into this (despite how slow you're moving) [][]

[gaspr]

Hi Doc. I really find it truly amazing how much info can
be convolved from one little swept sine wave chirp....boggles the
mind. Good to hear about your roving measurement kit idea.
Hope some people a little closer to you can take you up on this.

Thanks for weighing in here...keep us up to date on your measurement ventures.

[mas]

{Please Note: Associated graphics in next frame!}

(In partial response to the
Speaker Placement thread in the Architectural forum begun on 4/26/07 http://forums.klipsch.com/forums/1/906285/ShowThread.aspx

Only two walls of the room have
been sheet rocked, still have two walls left and the ceiling. No flooring has
been laid (still concrete). So I should be taking measurements in the room even
though the construction phase of the room is not complete?

First, you will please pardon me,
when you mentioned the state of your room, I mistakenly thought that you were
saying that because it was small and only partially had sheetrock that perhaps
measurements didnt matter or were not useful!
Oops! Pardon my misunderstanding...

A few thoughts and general comments
some of which do not specifically apply in your specific situation

There still seems to be a
general/common confusion among some folks between sound transmission through walls (and other flanking path
vectors) with the behavior of the sound fields and component reflections within the room and the role of
measurements. For the most part, they are two separate and distinct topics,
despite the fact that they both involve walls. Aside from the potential for
building some traps, etc. into wall surfaces, the easy way to think of this is
that transmission from one area into another has to do with wall mass and
construction techniques, while the rooms sonic character is a function solely
of its surface (albeit conditioned by the previous qualification).

Techniques for minimizing sound
transmission are actually rather well understood, despite our penchant for
reinventing the wheel. I think what causes the most hassle for most is that
such considerations are entered after a portion or a majority of the project is
complete, rather than planning from inception which BTW is MUCH easier folks!
;-) And as such they are not the focus of this post.

Instead lets agree to focus on the
in room response, characterized by the geometry and by the room surface.

Once the skin of the room is
installed (as this is not a modeled room, meaning its behavior and response
characteristics were not first modeled and anticipated in software) we can take
preliminary measurements. The effect of embellishments such as carpet can be predicted
relatively easily. This can be done in a very cursory manner by calculating the
thickness of the pad and carpet calculated at a 45 degree incident angle. And
then by allowing this distance to correlate to ¼ wavelength, determining the
frequency to which this correlates. (Note: This will vary from the actual, as the two desperate densities and
acoustic impedances of the carpet and the pad will not act as a single
boundary. And the pad will also act as a reflective layerthus reinforcing the
much more complex real world of non-linear surface behavior.)

Thus a ¾ inch carpet and pad would
correlate to a 45 degree incident thickness of 1.31 inches which correlates to
a ¼ wavelength of 10,308 Hz. Well above the frequencies where we will be
worried about dense specular reflections. Thus you can see that most carpet and
Spiderman beach towel tapestries, etc. will have little positive effect on the
acoustics. To amplify and qualify this, assuming that the carpet is ¼ inch and
the pad is ½ inch, the ¼ inch carpet would correlate to a frequency of 38,200
Hz. And the 1.2 inch pad to frequency of 19.097 Hz. So it should become pretty
apparent that we are dealing with frequencies that are not only extremely
easily controlled and damped, but we are also not dealing with frequencies that
really need control.

All of this is to show that some of
the more commonly accepted treatments will have effects, but they are not the
ones that we really want to focus upon. They will tend to damp frequencies that
we really do not need damped and ignore the lower frequencies containing much
more energy that constitute more problematic specular (focused) reflections.

But lets take a step back before
we get caught up in lots of minutia and lets look at the major divisions of
approaching a small acoustic space:

Generally speaking, lets summarize
the various groups of acoustic distortion:

Below 300Hz:

Problem 1: Room Modal Response

Solutions: Room Dimensions, Speaker
Placement, Tuned Absorption (Corner Bass traps, Helmholtz resonators)

Problem 2: Speaker Boundary
Interference;

Solutions: Speaker/Listener
position, Tuned Absorption

Above 300 Hz:

Problem 1: Specular reflections (1st order early reflections)

Solutions:Surgically placed
Absorption, Diffusion (Create an effectively anechoic Initial Signal Delay Gap)

Problem 2: Specular reflections (later arriving reflections)

Solutions: Diffusion, Selectively placed
Absorption (rare) (Create a well behaved diffuse semi-reverberant sound field)

Derivative Problem 3: Comb filtering/Polar
Anomalies (Superposition of direct and reflected signals)

Solutions: Signal alignment, Selectively placed
Absorption, Diffusion,

Solets start by looking at room modes and standing waves

The room calculator programs for
standing waves can give us a few ideas, allowing for their assumption of the
room as a perfect space consisting of 6 parallel surfaces with no doorways,
irregularities or dimensional variances. Not exactly any room that you or I can
get into or out of! Especially as they do not allow for doorways! But its a
good place to start!

From this, the average room will
have doorways, alcoves, open passageways connecting adjoining rooms, windows,
perhaps with inset frames, and a myriad other surface and structural
irregularities. The net effect of this will be to modify the modal structure of
the room.

The reason for this is pretty
simple, and a basic understanding of tuned spaces is helpful here.

The useful information illustrated
above is in the location of the nodes and antinodes, the peaks and the nulls of
the standing pressure wave. Remember, a standing wave is a wave that basically
reflects back and forth with the high and low pressure zones reinforcing itself
at particular resonance frequencies. Beginning with a fundamental frequency and
repeating at harmonic intervals (multiples of the fundamental frequency).

Please note. In a closed end of a resonant
space as illustrated in the diagrams, there is a node, a high pressure zone. If
the room is open, the open end ends in an antinode or a low pressure zone. If
the room is closed on both ends, you have high pressure zones at both ends,
with an antinode in the center (in the most basic form), or a series of
distributed nodes and antinodes.

If a room is not a perfect
rectangular space and exhibits the common asymmetrical anomalies mentioned
briefly above, these well behaved patterns are modified. And in the more
complex examples, alcoves, adjoining rooms, be they joined by large openings or
simply doorways act variously as multiple resonant spaces, summing in very
complex ways.

This phenomena is referred to as a
coupled space, and its behavior is anything but simple. Oh, some may be
simple, but most become very complex very quickly! The significance of this
fact being not so much that they are complex, but to reinforce the
understanding that predicting them from simple ideal calculators is almost impossible.
And thus the easiest way to address them is by the actual measurement of them.
Then the focus is not so much oh how they occur, as it is on simply what is
occurring!

Oh, and while we are here, I will
also mention that the modal response is not one dimensional. We are dealing
with a 3Space space, and as such, we have 3 primary modes. Room modes consist
of three different types of resonances; these are known as axial,
tangential & oblique modes. Axial modes consist of waves resonating
only along one dimension: the length, width or height of the room, Tangential
modes involve two dimensions, the length & width, length & height, or
width & height. Oblique modes involve all three dimensions in each mode of
resonance. Normally the axial modes have the most strength while the oblique
modes have the lowest strength.

The concept of dimensioning a room
is to attempt to more evenly distribute the frequency centers of the various
modes in an attempt to minimize their summation and hence greater peaks and
lower nulls, resulting in a more evenly distributed series of peaks ands nulls
exhibiting lower peak and higher null SPL levels.

A very simple example of an axial
mode would be that formed along the length an example room: L=25ft, W=16ft,
H=8ft. The first axial mode along the room length would be:
SpeedOfSound/(2*RoomLength) = 1130 ft/sec/(25.0 ft*2) = 22.15 Hz. Additional
modes would exist at integral multiples of 22.15 Hz : 44.3 Hz, 66.45 Hz, etc.
The width dimension gives a first axial mode at 1130/(16*2) = 35.31 Hz, 70.62
Hz, etc. The height dimension gives a first axial mode at 1130/(8*2) = 70.62
Hz, 141.25 Hz, etc.

You will quickly note that just for
this axial mode, that the frequency centered around 70 Hz will be a very
popular frequency for many of us! And common multiples of any dimension will
tend to create additional nodes at common frequencies that will sum! And just
as in a Dickens novel, these coincidences portend future issues that will
require addressing.

What all of this means is two things.
One, there will be zones in the room where the low frequencies will tend to be
much more pronounced than at others. And two, the bass will tend to overdrive
the small space and render it boomy and not very distinct and tight. And
this quality is the primary problem encountered in small acoustic spaces.
Higher order reflections, etc tend to get more attention, but room treatment
MUST begin with the fundamental treatment of room modes and Lf energy.

So, for most rooms that exhibit
closed ends, there will be a node at the rear corners. This is further
reinforced by the corner placement of speakers. Additional nodes may be spaces
at intervals in the room, usually ½ and 1/3 the distance of each lateral wall.
And for the additional modes, at the intersections of each of the lateral
surfaces walls, floors and ceiling in all of the corners.

We can address additional room boundary concerns which affect alternative speaker placement at a later date, but personally I try to avoid them if at all possible either via corner placement or via acoustically isolated soffet (in the wall) installation...

Generally speaking, corner traps
consisting of any of a variety of tuned absorptive traps are effective. These
are most commonly of a tuned tube or pipe, or a modified version thereof, or in
the form of panels. The fundamental physics model which is employed is that of
the Helmholtz resonator. This is a tuned enclosure that resonates at a
particular frequency. A very common example that most have experienced is that
of blowing across the top of a coke bottle or a jug. Likewise the use of a tuned pipe to create a
sound in a traditional pipe organ. The varying volume of the enclosed space
will result in a particular resonant frequency. A trait that is most helpful
when tuning a jug for use in the weekend jug band festivals that are fast
approaching!

And if we refer back to our
knowledge of impedance, a properly terminated source with a load of identical
impedance will result in the maximum power transfer, and all of the incident
energy will be absorbed and none will be reflected and returned back into the
system. And since a room exhibits an acoustic impedance, and each surface
likewise exhibits an acoustical impedance, they will absorb and reflect incident
energy. And since nothing is ever simple, each surface exhibits a complex
impedance, resulting in a non-linear absorption of some frequencies, and a
reflection of other frequencies. So each wall surface will absorb or reflect
energy in varying ways, and with varying frequency spectrums, in effect EQing
the reflected sound. (And this is a critical consideration, as many will
erroneously believe that an absorber simply absorbs! NOT TRUE!)

The Helmholtz absorber is a
precisely tuned enclosure, meaning that it tend to be a high Q device,
efficiently absorbing a very narrow band of frequencies and tending to ignore
others (although lower Q, broader absorptive band designs are possible, they
are correspondingly less efficient over the frequency band).

Again, many variations in the
design of Helmholtz enclosures

are possible, from tuned pipes, to
panels exhibiting various resonant/flexure, porous, or slotted designs.The
design of each of these is beyond the scope of this diatribe. But they are very
precise, and able to be tuned to quite a variety of frequencies. Often the most
critical aspect is not the design, but of obtaining materials with known
qualities (density, porosity, mass, etc) allowing the construction of a trap
that correlates closely with the design!

So, to make a very general
recommendation In a room that is generally closed, LF traps in all of the
corners consisting of a combination of corner and lateral flanking tuned LF
absorptive traps is almost always advantageous. The trick is to know what
frequencies are problematic and then responding by designing and building traps
sufficient to accurately and efficiently trap them. Additionally, room
furnishing can have an impact, both good and bad on the room response as well.
So as things become more complex and more variables are introduced, its nice
to know what is actually happening. Thus measurements knowing exactly what you are dealing with
comes in much more handy then a calculator that provides a very pretty graph
and a guess.

Oh, and since many are using
Heritage speakers, this means LF traps placed above the corner speakers in the front of the room as well!

Oh, and while we are here, and
since there is so much more to any of these topics, at the risk of introducing as many or more problems as the technique
actually solves in the complete overview of the topic, I might note that variable
subwoofer placement offers a mixture of both good and bad options! The good is
that dependent upon its placements about the room, the subwoofer will tend to
shift the primacy of the modes from axial to tangential to oblique. With this
shift, different modes will be reinforced differently, thus offering the
potential to reinforce different modes at differing fundamental and subsequent
harmonic frequencies and reinforcing various nodes and antinodes and their
placement according to the dynamics of the particular modes. Additionally, additional units can be placed
in positions so that certain modal peaks and nulls are cancelled via 180 degree
out of phase superposition. So, varying the position of the subwoofer can
potentially be used to your advantage. This is the focus of the oft cited
Harmon whitepaper on subwoofer placement.

But there is a cost! Despite the mantra that LFs are omnidirectional, the
notion that LFs are non-localizable is optimistic at best, and simply incorrect at worst. Oh sure, at sufficient distances
this becomes more valid; witness the attempt to localize a approaching low
flying helicopter. But in a small acoustical space, where the Henry precedence
effect is still valid, the difference in arrival times at each ear still allows
quite a bit of localizable cues, despite the frequencies at hand. Additionally,
the lack of signal alignment in the time domain also causes radical signal offsets (significant and
perverse group delay anomalies) and crossover anomalies manifested via
superposition, ALL of which are audible effects that ideally should be avoided.
But, I mention this as you do have a variety of variables at your disposal, but
it must be remembered that these have negative ramifications as well as
positive ones.

And generally speaking, aligning
signals in the time domain minimizes frequency response and intelligibility anomalies
and should be your first goal. After
all, once signals are aligned within the time domain in a minimum phase
relationship, many well behaved options, including EQ, are available for
adjusting the character of the direct signal.

There is much more than can be
addressed, but I prefer to avoid going into too much arcane detail if it is not
productive. So lets start with this and see where the needs are.

Oh, and let me make a request. At
this point we are still talking about the basic concept. And my goal is to try
to present an opportunity for folks to understand the basic issue at hand. I am
not going to go off on lots of tangents here chasing lots of how do I solve
thi or that at this point. After we get the basic concept down,
there will then be plenty of opportunity to pursue specific solutions. And
hopefully by then, most will have a basic idea of how to begin to go about
doing this.

Besides, I will have my hand full
trying to avoid getting off on tangents myself. Trust me, I dont need any help
finding tangents to chase! So help me stay focused and I will try to help
youwith lots of help from some very able folks here!

[mas]

Tuned pipe resonance and Mode diagrams

[Roc Rinaldi]

Tuned pipe resonance and Mode diagrams

Huh?