The Influence of room acoustics on the listening experience

[MicroMara]

Hello Klipsch brothers and sisters.

 

I have been dealing with  Hifi / High End for over 40 years and have also been active in German forums for years.  You buy good and expensive audio equipment, get it demonstrated at your dealer and wonder why the components you bought sound different at home than in the showrooms of your hifi dealer.

 

For this reason I have once summarized the subject of room acoustics and make this information available. For some people this is nothing new and generally known. For others however interesting because they have never thought about it before.

 

I hope that I have described it so that it is generally understandable. I apologize in advance for any orthographic or translation mistakes.

 

The listening room has a decisive influence on the sound experience.

 

Even the ancient world knew about the influence of acoustic laws on sound and speech intelligibility. The shape of the amphitheatre did not arise from arbitrariness. Our cuboid living rooms with their parallel walls, on the other hand, are not ideal and produce unwanted acoustic artefacts in the form of resonances and reflections, which have a decisive influence on the room acoustics and thus the sound experience.

 

 

 

The room acoustics determine the reproduction quality

 

Impulse response in a room. Clearly, the first reflections arriving with a delay can be seen.

 

Sonic propagates in free space in a straight line, similar to light. The situation is different for sound propagation in partially or completely enclosed rooms, where the room boundaries reflect the sound to a greater or lesser extent. In a room not only the direct sonic of a sound source reaches the ear of the listener, but also manifold reflections from walls, ceiling and floor. If sonic waves hit a boundary surface, they are partly absorbed and partly reflected back into the room with a changed phase position and direction. These reflections create the impression of the room, which varies greatly from room to room and is not contained in the original music or sound recording. The acoustics of the room are defined by the direction, strength and frequency with which reflections reach the listener compared to direct sound.

The mix of direct loudspeaker sonic and room acoustics results in the actual sound experience at the listening position. Due to the distortions of the room acoustics, the resulting reproduction pattern has a significant deviation from the reproduction pattern of the loudspeaker. The measured amplitude curves (a term from mathematics as well as physics and technology for the description of vibrations) of loudspeakers usually represent the sound pressure frequency response in free field and do not take into account reflections from walls, floor and ceiling. This makes sense, since there is little knowledge about the acoustic environment in which the loudspeaker is operated. The amplitude response therefore provides only unreliable information about how the loudspeaker will sound, since the important component of room acoustics is missing.

 

 

Superposition of two oscillations with equal amplitude and frequency. At 180 degrees of phase difference a complete cancellation occurs.

 

First reflections - also known as early reflections - determine what is commonly understood by the acoustics of a room. Reflections always occur when the propagating sound wave hits a surface in the living room and is reflected back to the listener. In general, the first reflections arrive relatively sporadically at the listening position in the first 50 ms after direct sound. The level of these first reflections strongly determines the primary sound impression of the room. With increasing time, the individual reflections coincide more and more and blur to a decaying reverberation.The problem with the reflections is that the reflected sound wave experiences a time delay due to the longer distance. The time delays compared to direct sound lead to a phase shift between the two sound components.

The components of reflected and direct sound therefore overlap with different phase positions. When the two sound components are added together, mixed products are created in the form of new harmonic oscillations, which manifest themselves as cancellations or level increases in the amplitude curve. One also speaks of the so-called comb filter effect. In extreme cases in rooms with low attenuation, the reflection component can significantly exceed the direct sound from the loudspeakers and thus massively impair the quality of the reproduction. You can prevent too much reflection by moving the sitting position closer to the speaker or by taking the complicated route of damping highly reflective surfaces. It is also important that the loudspeaker has a radiation pattern that is as homogeneous as possible over the entire frequency band, so that the diffuse sound components are distributed more evenly over the frequency band.

 

 

Direct & diffuse sonic field

 

As the distance to the sound source increases, the location becomes worse.

 

Direct sonic is the sound that reaches the listener in the direct path from the loudspeaker. In this context one speaks of the first wave front. Diffuse sonic  with increasing distance from the sound source, the direct sound component decreases, while the diffuse sonic components increase. If practically only diffuse sound components are present, one is in the so-called diffuse sonic field. The incidence of the sound components is then equally probable and equally strong from all directions in space. A localization of the sonic source is practically no longer possible.

 

In contrast to direct sonic, the level of diffuse sonic remains constant throughout the room. The sound level, however, is inversely proportional to the distance. With increasing distance, the direct sonic level becomes increasingly smaller. The point where direct and diffuse sonic are equally large is called the reverberation radius. A too large distance from the loudspeaker always results in a blurred sound image due to the high diffuse sonic components. The ideal seating position is therefore within the reverberation radius, which is calculated from the volume and the reverberation time of the room. It increases with larger volume, but decreases with longer reverberation time.

If a sonic wave is reflected from one or more room boundary surfaces back to the starting point, a flutter echo can occur. The sound is thrown back and forth between the walls in rapid succession. This creates a kind of sound cycle that slowly decays only because of the partial absorption of the walls. At greater distances from the wall, a sequence of fast echoes with decaying level can be heard, at smaller distances from the wall a so-called sound echo is created, i.e. the sound impulse receives a kind of reverberation that is not contained in the original signal. Flutter echoes are generally perceived as disturbing and disguising.

 

A flutter echo occurs especially when two reflecting walls are parallel to each other and the other directions in the room are more attenuated. To eliminate a flutter echo, one should either try to increase the absorption of one of the two opposite walls, e.g. by placing a curtain, or deflect the reflections in other directions with the help of diffusers and large partitions. Often simply changing the placement of the speakers or the sitting position helps.

 

 

Standing waves

Standing waves occur when half the wavelength or multiples thereof correspond to the width of the room.

 

A stubborn problem that can be difficult to solve is the occurrence of so-called standing waves. Standing waves can also only occur between parallel walls. If the sound wave propagates between two parallel walls and is reflected vertically, a superposition of sound waves and their own reflection occurs. Standing waves can lead to complete cancellation or to pronounced resonance peaks, which cause an actual booming in the bass range. The wavelength of a standing wave is always an integral multiple of the room dimensions. In a five meter wide room a standing wave with a basic resonance of 35Hz will be created.

 

In contrast to the advancing wave, however, there are stationary cancellations, i.e. a standing wave makes itself felt differently depending on the position in the room. However, if you use a weak horn loudspeaker, you may be able to benefit from this. One should avoid a sitting position exactly in the middle of the room, since this is where the bass usually disappears and one suddenly wonders why the Superbox, which just sounded so great in the shop, simply no longer has any low bass.

 

A massive sofa not placed directly against the wall is still the simplest bass absorber and a practical countermeasure for standing waves. Even old-fashioned chests of drawers are quite effective. A systematic reduction of the resonance modes is possible with the so-called bass absorbers offered by various manufacturers. Bass absorbers can be adjusted to a specific frequency. However, this requires an acoustic analysis of the room to determine the frequency of the resonance.

 

Improve acoustics

 

Elaborate but effective - the suspended ceiling in pyramid shape

 

The efficient means is to bypass parallel walls whenever possible. Sloping ceilings in attics or maisonettes are therefore traditionally very suitable for listening rooms. Otherwise, a tapestry can work wonders under certain circumstances. A sofa usually has a positive effect on the bass reproduction and acts as a real absorber for standing waves. However, better results are achieved with commercially available bass absorbers that are specifically adapted to the disturbing resonance range. Wall-mounted diffusers do not usually look very attractive, but in listening rooms they are justified and produce a more balanced sound thanks to the reduction of flutter echoes.

 

 

Positioning the speakers

 

If the loudspeakers are placed in a fair parallel position, the listener is reached by a lateral radiation that carries less direct energy. In addition, the reflection takes a shorter path than with direct response, so it reaches the ear earlier. This makes it harder for the ear to distinguish between sound source and reflection, which makes what is heard sound less precise.

The loudspeakers are correctly set up when the Loudspeaker as acoustical source can no longer be localized. This means that the stage front ,depth and width can be clearly located between the L's and even mixing effects can be heard to the left and right of the speakers.

 

The singer always stands in front of the band, at least during studio productions or on the same level with the band. The solos of certain instruments must be clearly locatable and delimitable.The whole sound spectrum should be balanced at the listening position, from brilliance to sub, and should not have any emphasis in certain FQ spectra, except for the sonic virtues attributed to them, e.g. ... "especially strong bass " . But drums and the bass, the instruments themselves, never tend to hum.

 

 

Two-channel stereo system:

 

(valid for slimline floorstanding Speakers like e.g. Klipsch RF7´s, RF 82, RP 280F , RP 8000 and others not for Klipsch Heritage Line like Forte , CW , LaScala, KHorn )

 

Figure 1 shows a typical speaker arrangement for a two-channel stereo system.a good starting point would be to form an equilateral triangle of the speakers and your listening position.depending on the size of the room and the distribution arrangement of the furniture,it may be advisable to increase or decrease the distance between the listening position and the line between the two speakers,but maintain a center position with approximately equal distance to both speakers.this will ensure the best stereo effect.Turning the speakers far away from each other slightly inwards towards the listening position will still give a good stereo effect.

 

Stereo placement for wide sound reinforcement :

If you are primarily interested in hearing the sound from any position in the room, rather than just from a specific seating area, you can move the speakers on adjacent walls far apart to form a large "L", which does not produce a very good stereo effect, but gives you a room-filling, well-balanced sound.If you now draw an imaginary line from the center of the two horns, the axes meet behind the head.This also requires that the listening point  (sofa, couch, or easy chair) in the back area must have at least 3 feet or more distance from the imaginary back wall. Accordingly, the sound absorbers, if still necessary, are to be positioned differently.

 

Digital room correction

 

The foundations for so-called RCS (Room Correction System) were already developed in the eighties. However, only modern chip technology allows practical application. Meanwhile, there are a large number of suppliers of RCS. A number of loudspeaker integrate an RCS directly into their digital loudspeakers. Such solutions can lead to satisfying results especially in acoustically unfavorable living situations.

 

Common to all variants is that the acoustic behavior of the room must first be determined with a measuring system and analyzed in the computer. Software then calculates a high-quality and very accurate correction signal for the room in question. During sound reproduction, the correction signal is included in the playback and the influence of the room acoustics is significantly reduced.

 

However, the use of graphic equalizers, as was still common in the past, does not prove to be a good service. Even digital equalizers can only make level adjustments. Room acoustics, however, take place on the time level. Therefore, equalizers are not an effective way to correct room acoustic phenomena and lead to more problems than they solve.

 

Conclusion

 

The acoustic phenomena that result from the reproduction in rooms are manifold and have a decisive influence on the quality of the audio reproduction. Even excellent HiFi components cannot show their capabilities if the room acoustics destroy a high-quality reproduction by flutter echoes, comb filter effects and standing waves.

In fact, room acoustics is the most important parameter of all. A circumstance that is often ignored by equipment manufacturers and specialists. After all, no one wants to hear that only in an adequate listening room do the high-end power amplifier and high-gloss loudspeakers really make sense and can develop their qualities appropriately. If a specialist dealer addresses the difficult subject of room acoustics in the home and can perhaps even offer some possible solutions, one can certainly have confidence in his seriousness.

However, realizing an improvement in room acoustics by means of structural measures will often quickly reach its limits or come into conflict with other living requirements.

As mentioned above, modern room correction systems with sophisticated software can help to solve difficult room problems.

 

I hope to have provided the Klipsch Community  with a factual basis of information.

 

 

 

[codewritinfool]

Thank you for posting this.

[Chris A]

  

On 8/16/2020 at 9:31 AM, MicroMara said:

The efficient means is to bypass parallel walls whenever possible. Sloping ceilings in attics or maisonettes are therefore traditionally very suitable for listening rooms.

This is actually not very correct--in fact it creates severe issues if not addressed via lots of acoustic absorption (i.e., sloping walls/ceiling) .  Think of the world's most acoustically acclaimed orchestra halls.  They are all shoebox-shaped:

MusikVerein Vienna (1812)

Concertgebouw Amsterdam (1888)

 

Boston Symphony Hall

 

 

I believe that the issue is avoidance of infrasonic bass cancellations.  The best way that I've seen to address that is not "bypassing parallel walls", but to use a double bass array to remove the longitudinal standing waves:  https://en.wikipedia.org/wiki/Double_bass_array

 

On 8/16/2020 at 9:31 AM, MicroMara said:

Otherwise, a tapestry can work wonders under certain circumstances.

Agreed.

 

On 8/16/2020 at 9:31 AM, MicroMara said:

A sofa usually has a positive effect on the bass reproduction and acts as a real absorber for standing waves.

Only if the sofa/chairs are not leather or vinyl covered (as most are nowadays).  These are acoustic reflectors, and must be covered with thick soft cloth in order to reduce nearfield reflections around the listener's ears.  Soft cloth sofas can help, but usually very little.  The effect of absorption in-room is governed by the absorption area presented and the absorption characteristic of the material itself.  For frequencies below the room's Schroeder frequency, it is usually only controlled by modified Helmholtz resonators (bass traps) that enclose a space interior or behind them and the walls/floor/ceiling. 

 

I recommend using REW to help measure the nearfield reflections (which can include a lot more than "first reflection points"), in-room decays and overall room reverberation times.

 

On 8/16/2020 at 9:31 AM, MicroMara said:

Wall-mounted diffusers do not usually look very attractive, but in listening rooms they are justified and produce a more balanced sound thanks to the reduction of flutter echoes.

 

There are diffusers that are nicer looking, but probably not very effective below 1 kHz.  The problem, however, is usually around 800-2000 Hz (arrived at heuristically), and most diffuser panels will work pretty well in this frequency band.  Why it's in the 800-2000 Hz band, however, is another issue.  Having diffusers that work below 800 Hz is a challenge, and will be quite deep as compared to your garden variety diffusers. Blackbird Studio in Nashville used this approach (which is not "female friendly" by any stretch of the imagination, and a dusting nightmare, as well as an unmitigated personnel safety issue):

 

 

On 8/16/2020 at 9:31 AM, MicroMara said:

Digital room correction

I find this to be a misnomer:  it's really in-room re-EQing.  You're not really "correcting the room", but rather re--EQing the loudspeakers for a specific listening spot. 

 

The problem is that very little can be done about these room reflections (especially at low frequencies) except at specific listening positions ± a few centimeters of ear/head placement.  I strongly recommend taking the in-room acoustic measurements at 1 metre in front of each loudspeaker, and if any other measurements are done at the listening position, to only attenuate response peaks. 

 

I'd instead recommend moving the listening position instead of re-EQing, since the re-EQ differentials based on the listening positions (vs. 1-metre measurements) will screw up the response in the balance of the room--thus making the room into a "head-in-a-vise" listening room.  I think headphones work a lot better than trying to correct for non-minimum-phase reflections in-room. 

 

JMTC.  I'll stop there. 😉

 

Chris

[Edgar]

Have any of you seen Jeff Borish's Ph.D. dissertation on this subject? https://asa.scitation.org/doi/abs/10.1121/1.392353 I have a copy that I got directly from Dr. Borish, but it's 41MB so I can't post it. Some interesting conclusions there as to the best shape for a hall (and, by extension, for a room), as well as acoustical treatments.

[MicroMara]

@Chris A

 

With these basics I want to give the simple HiFi friend, who is sitting in his living room, some information to explain this topic in a simple and easy way.That the room acoustics is a much more complex subject, is also clear to me, actually it is a science of its own. But many people buy an audio system, simply set it up and are first of all enthusiastic . After listening for a while, the one or other question comes up why, why it does not sound so good after all. Room modes, bass resonances, sharp high frequencies etc. are only recognized much later. Only a professional is able to filter disturbing frequencies immediately by means of an EQ, but if you want to use an EQ with the just bought equipment to optimize the sound, you have to do so immediately. Many do not know at all.The same with the REW, as easy as it is with a UMIK MK I / II measuring microphone and the REW software. You have to be able to understand the waterfall diagrams and FQ measurements.  Sure, a fabric sofa is absorbent while a leather sofa is reflective, and so on.And you must also not forget that it costs a lot of money to optimize the room acoustics. Without Know How these are mostly used senselessly.  I am glad that you are taking part in this discussion because it helps others as well.

 

YMTC

 

George

 

[Chris A]

George, thanks for the response.  Greg, too.  The real problem is that music halls are not usually home hi-fi listening rooms. The dimensions of home-sized listening rooms vary considerably but are much smaller, and are also influenced by where we live.  These very small dimensions are the problem.  Nonparallel walls are not going to do very much for such small-dimensioned listening spaces, I'm sorry to say.  If you live in the middle of a big city, you're usually living in minimal-sized rooms.  It becomes cost-prohibitive to do much about that.  I recommend headphones--good ones that avoid eardrum-bounce issues in those cases.

 

In places like the US south, southwest, and west, the size of the rooms can easily be double the size (or more) of an average-sized European listening room.  The reason for stating this is because I think many forget this when talking on forums. 

 

I do believe that there is such a thing as a minimal-sized listening room.  In my experience, that's smaller than ~4m horizontally (both directions) and ~2.5m vertically.  It's not a very popular subject on audio forums, but it exists in my experience.  I've done a lot to mitigate one small room in my house that's 3.5 x 4 x 2.4 room--to no avail.  Good luck if you have a very small listening room, even using full-range-directivity loudspeakers (like Jubilees, K-horns, La Scalas, or Belles), there are still issues with opposite-wall reflections, and short of covering the entire exposed surface of the room with absorption, there's very little that you can do to increase the listening performance of the room. It is what it is.

 

Chris

[MicroMara]

YES I agree with you, I have often seen pictures of apartments in this forum that are very small but there are nevertheless very large speakers such as CW / RF7`s / Fortes and so on completely oversized. I have a music room with a volume of approx. 8.500 kubicfeet, a room height up to 17 feet,  on a base of 22 feet length by 18 feet wide. An RF 7 / CW / Forte is suitable for this. When we were rebuilding the house, acoustic engineers built my music room. The result is a reverberation time of only 0.2 seconds.But that's not the point here, I just wanted to clarify the basics of room acoustics and the associated explanations of terms.Many people don´t know these terms, like direct sonic, diffuse sonic, standing waves, reverberation room and so on .

 

George

 

 

[Peter P.]

There are many valid observations in this thread, and certainly from MicroMara.

 

I feel the room is more of an influence to the sound than the speaker. I'm inclined to believe even speakers of modest cost will produce incredible music in the right room acoustics.

But for many reasons, we live under considerable limitations in room size, shape, and construction.

 

Some observations:

 

Notice that the typical amphitheater is shaped LIKE A HORN. Hmmm...

Notice that the typical well regarded acoustic hall like those in Chris A's photos above, have the music source along the short wall.

Also notice the many uneven surfaces from moldings, coffered ceilings, or drapes hiding the windows.

All contribute to improvements in the room acoustics. Try opening the windows and doors not just in your room, but in other rooms and see how it affects the sound.

 

Pack an empty music hall with bodies (live ones, that is!) and you absorb a lot of reflections.

 

I remember once reading in Stereo Review I believe, someone constructed a listening room with Klipschorns in all four corners. The walls were tilted 2 degrees to help reduce acoustic problems.

 

And yes; some speakers can be too large, whether physically or acoustically, for a room.

 

I remember once, when I was in high school in the 70's, some friends and I put on a little music show in a very large church gathering hall which was probaby 10x or more the size of my bedroom. There were only a handful of people present. I used my B.I.C. Formula 1 speakers, with an 8" woofer and 1" horn tweeter. They sounded spectacular in that room, even sitting on the hard tile floor, which is not how they sounded in my bedroom. Acoustics, for sure...

 

Just as MicroMara mentions, I always wondered why the speakers always sound so great at the dealer, but not the same at home. But the decor and dimensions are never the same.

 

[MicroMara]

On 8/16/2020 at 6:00 PM, Chris A said:

 

 

I think headphones work a lot better than trying to correct for non-minimum-phase reflections in-room. 

 

JMTC.  I'll stop there. 😉

 

Chris

 

I´ll try but got always problems

 

[MechEngVic]

You want to eliminate the influence of room on your listening experience? Sit right in front of your speakers!🤣

[tube fanatic]

You may find this quite interesting George:

 

http://www.cardas.com/room_setup_rectangular_room.php

 

I found the experience with this method to be wonderful. 
 

 

Maynard

[MicroMara]

4 hours ago, tube fanatic said:

You may find this quite interesting George:

 

http://www.cardas.com/room_setup_rectangular_room.php

 

I found the experience with this method to be wonderful. 
 

 

Maynard

Thanks Maynard, I find the information very informative and also applicable for listeners who use speakers such as 2 way compact speakers on stands or narrow 3 way floorstanding speakers with dome tweeter, ribbon tweeter or Airmotion Transformers.For Klipsch horn speakers such as RP-Line, RF7-Line, Forte, Cw, LaScala, AK6, or Jubilee, these tips cannot be used because they simply have completely different efficiency and sonic pressure characteristics. Basically a lot of things are described correctly and can be used as a guide for setting up your speakers. The important thing is that the listeners themselves experiment and don't buy great loudspeakers, just put them down and believe they will get the great sound experience. This is rarely the case.

 

[MicroMara]

12 hours ago, MechEngVic said:

You want to eliminate the influence of room on your listening experience? Sit right in front of your speakers!🤣

 

@tube fanatic  This is an example of how you should certainly not do it, but I think that @MechEngVic just wanted to make fun. He only has to move the office chair backwards.

 

Maynard .....you are also of the hearing type a fine delicated listener. A few hundred milliwatts are already enough for you. On average I  hear only at 55 - 65 dB volume, taht is below 1 Watt as well, but sometimes I´m turn up to 115 dB sonic pressure.  I have a music room with a volume of approx. 8.500 kubicfeet, a room height up to 17 feet,  on a base of 22 feet length by 18 feet wide. When we were rebuilding the house, acoustic engineers built my music room. The result is a reverberation time of only 0.2 seconds, that references to Studio Quality. 

 

 

 

[Oicu812]

On 8/25/2020 at 12:23 AM, MechEngVic said:

You want to eliminate the influence of room on your listening experience? Sit right in front of your speakers!🤣

 

These are KP-3002's, and the monitor is 55":

 

 

 

 

 

[babadono]

On 8/24/2020 at 9:23 PM, MechEngVic said:

You want to eliminate the influence of room on your listening experience? Sit right in front of your speakers!🤣

Yes its called "near field monitoring"

[MicroMara]

@MechEngVic  @babadono            What have you said ? Can´t hear ya

 

                                                                                       

[Shakeydeal]

Nearfield works for some designs. But IMHO, with horns you need some distance for correct integration. Things are certainly more "vivid" up close, but often times it's just a discombobulated mess.........

[babadono]

9 minutes ago, Shakeydeal said:

Nearfield works for some designs. But IMHO, with horns you need some distance for correct integration. Things are certainly more "vivid" up close, but often times it's just a discombobulated mess.........

yes near field monitoring is AKA "mixing for AM radio"

[mikebse2a3]

I wish anyone who thinks large horn systems can’t do imaging with depth as specific as a near field small monitor setup or that a near field small monitor setup can’t do life size imaging could have heard this comparison... 🙂

 

 

[artto]

On 8/16/2020 at 11:00 AM, Chris A said:

  

This is actually not very correct--in fact it creates severe issues if not addressed via lots of acoustic absorption (i.e., sloping walls/ceiling) .  Think of the world's most acoustically acclaimed orchestra halls.  They are all shoebox-shaped:

MusikVerein Vienna (1812)

 

Concertgebouw Amsterdam (1888)

 

Boston Symphony Hall

 

 

I believe that the issue is avoidance of infrasonic bass cancellations.  The best way that I've seen to address that is not "bypassing parallel walls", but to use a double bass array to remove the longitudinal standing waves:  https://en.wikipedia.org/wiki/Double_bass_array

 

Agreed.

 

Only if the sofa/chairs are not leather or vinyl covered (as most are nowadays).  These are acoustic reflectors, and must be covered with thick soft cloth in order to reduce nearfield reflections around the listener's ears.  Soft cloth sofas can help, but usually very little.  The effect of absorption in-room is governed by the absorption area presented and the absorption characteristic of the material itself.  For frequencies below the room's Schroeder frequency, it is usually only controlled by modified Helmholtz resonators (bass traps) that enclose a space interior or behind them and the walls/floor/ceiling. 

 

I recommend using REW to help measure the nearfield reflections (which can include a lot more than "first reflection points"), in-room decays and overall room reverberation times.

 

 

There are diffusers that are nicer looking, but probably not very effective below 1 kHz.  The problem, however, is usually around 800-2000 Hz (arrived at heuristically), and most diffuser panels will work pretty well in this frequency band.  Why it's in the 800-2000 Hz band, however, is another issue.  Having diffusers that work below 800 Hz is a challenge, and will be quite deep as compared to your garden variety diffusers. Blackbird Studio in Nashville used this approach (which is not "female friendly" by any stretch of the imagination, and a dusting nightmare, as well as an unmitigated personnel safety issue):

 

 

I find this to be a misnomer:  it's really in-room re-EQing.  You're not really "correcting the room", but rather re--EQing the loudspeakers for a specific listening spot. 

 

The problem is that very little can be done about these room reflections (especially at low frequencies) except at specific listening positions ± a few centimeters of ear/head placement.  I strongly recommend taking the in-room acoustic measurements at 1 metre in front of each loudspeaker, and if any other measurements are done at the listening position, to only attenuate response peaks. 

 

I'd instead recommend moving the listening position instead of re-EQing, since the re-EQ differentials based on the listening positions (vs. 1-metre measurements) will screw up the response in the balance of the room--thus making the room into a "head-in-a-vise" listening room.  I think headphones work a lot better than trying to correct for non-minimum-phase reflections in-room. 

 

JMTC.  I'll stop there. 😉

 

Chris

Chris, interesting which three concert halls you chose as examples.

 

The "shoe box" shape, as well as the actual dimensions, more specifically, the width of these halls, were in fact determined by the architectural/structural limitations of the time. It actually has nothing to do with what they were trying to achieve acoustically. They didn't know that much yet. And, in fact, music itself, was composed, quite literally, to "sound it's best" in these concert halls (or whatever kind of venue it was intended for). At the time, the width of these halls was determined by the longest practical unobstructed span that we knew how to build. The use of steel in building construction didn't come about till later in the 1800's.