Why are closed cabinet speakers required to be away from the back wall?

I've had rear ported and front ported, both transmission line, in the same room, and I can confirm it has little to no effect of the natural room modes. It's striking how similar both sets performed in terms of measurement in the same space. One set is stand mount, the other floor standing, FWIW. Now if someone has a magic way to absorb 48 Hz frequencies, I'd be interested, but I honestly don't think you can fit enough bass traps into the area to make much difference. It's down to placement of the listening position to avoid nulls.

 

Not magic, use tuned resonators tuned to 48 Hz or around that. This one is tuned to 40 Hz, you might be better off with one centered on 50 Hz, but you can treat bass frequency problems in that sort of frequency range without giant porous absorbers. GIK Acoustics Scopus Tuned Membrane Bass Trap (T40)

 

This issue of placement and room boundaries is a little more complicated than just how far a speaker is from the wall behind it.

Every boundary in the room will affect a speaker's in-room frequency response because of speaker boundary interference, particularly below around 150-200 Hz where the lengths of the waves are so big that they just kind of fill and pressurize the whole room (HF waves are smaller, and they dissipate more over space, so really the space behind the speakers is not all that critical in terms of HF performance) -- that's because sound is bouncing off every boundary, and crossing other sound, and the bouncing sound is bouncing again off other boundaries, sometimes at acute angles, sometimes at oblique angles, and creating an array of reinforcements and cancellations at the listening position based on the mix of distances between the speaker and the listener and the walls and the floor and the ceiling. The frequencies at will those effects take place will change as you move speakers and listener around but it's not a single, non-dynamic calculation. If you move the speakers farther away from the wall behind them, you're moving them closer to the wall in front of them and so you'll get a mix of changing frequencies at which you're having boundary reinforcements and cancellations based on both dimensions.

What frequencies are affected and which by cancellations and which by reinforcements will depend on the lengths of the waves. A 40 Hz wave is 28 feet long, most of us don't have speakers more than 28 feet from any room boundaries, in fact most of our room boundaries are all significantly nearer to the speakers and the listener than 28 feet, so we're just hearing that 40 Hz wave immediately filling the whole room and bouncing back on itself before a single cycle. But at around 7 feet from a room boundary, which is the 1/4 wavelength distance of the 40 Hz/28-foot wave, you'll have cancellations. There's not much you can do about it except stay close or farther than 7 feet from boundaries, except if you move to 5 feet, not you might be having problems not at 40 Hz, but at 55 Hz. And these kinds of problems are happening with respect to all the room boundaries.

You might be able to EQ around some of the reinforcement, but you won't be able to EQ around nulls -- you can throw more and more energy at the frequency but if the bouncing waves are out of phase, they'll cancel. And you won't be able to EQ around the different decay times you'll have at different frequencies. It'll take a mix of tweaking speaker and listener placement, room treatment (including if you have the space to do it, diffusion), and maybe, particularly if you're already in the digital realm anyway, frequency dependent time and phase processing to get things flat and even at the listening position with respect to all this stuff. And certain things are inevitably going to be hard to work around -- a floor bounce cancellation related to speaker height and cancellations relating to the fact that we tend to be sitting symmetrically in the center of the room on at least one axis in order to have proper stereo imaging.

Zero inches is great -- in wall speakers -- for eliminating SBIR. But it's tough to actually implement that so that you're listening on axis in a stereo arrangement for best imaging. Near to a wall tends to move the SBIR cancellations up into higher frequencies that are easier to treat. But like I said, unless your moving your speakers into a corner, where not only do you wind up exciting room modes excessively, but where the sound bounding around the speaker causes all kinds of filtering, usually when you're moving the speaker nearer to one boundary, you're moving it farther away from another, and both distances are going to have an impact on the frequencies at which there's interference and reinforcement. If you're in a 15 foot room and the speakers are 2 feet from the wall behind them, they're 13 feet from the wall behind you. At 13 feet you're going to have a reinforcement at around 43 Hz. If you move the speakers out to 5 feet from the wall behind them, they'll be 10 feet from the wall behind you. That reinforcement will now be at around 56 Hz.

It really is a matter of room tuning, almost kinda literally.

 

Yes, thanks for mentioning. I have a deficit of wall space, and could place one behind the listening position and one behind one speaker or stack two of their soffit traps in a corner. Beyond that it would be the ceiling or trying to turn a sofa into a tuned trap. IDK.

 

I haven't played around with tuned resonantors, but since they're pressure based (vs. porous absorption which is velocity based), I think best placement for them is at points where the pressure at the target frequencies is greatest.

 

No doubt, but a big bay of windows makes that impossible. I'm always looking for a way I might turn the couch into one as it sits against the back wall, generally.

 

I think it is curious that few loudspeaker manufacturer even try to make speakers that work well against a wall. For a lot of people they don’t have space in there room to have the speakers out into the room and some people even need to put them on wall mounts.
In the past Linn Kans were designed to be back to wall, and 20years ago I had some on wall brackets made specifically for Kans and they were great.
Fortunately I now have space to have floor standers which can be out into the room a bit
I think with the thousands of different designs they ought to be some model that will work well on shelves or flat to the wall, but there doesn’t seem to be.

 

I recall a few vintage speakers designed specifically for on a wall, the infamous Radio Shack Minimus speaker (the early serials sound better than the later ones) and the AR 10(pi) had different settings for free standing, on wall, or in corner. The famous Klipshorn must be in a corner. I recall a certain KEF transmission line column designed exclusively for back wall location. (oval woofer) My first system I saved for, paycheck to paycheck, the Pioneer CS-66 bookshelf speaker sounded best against the back wall, but was about 30 inches off the floor on the bookshelf.

Remember, if a speaker is far enough off the floor, more than approx 20 to 30 inches, it should be very near or right on the back wall. Tower speakers are generally designed to be a little away from the back wall. These are generalities, specific recommendations by the manufacturer should be considered.

Here we go, found this: Speakers that work close to back wall

 

good question!
most speakers, regardless of port position (front, rear, down-firing, etc.) sound best away from the wall- unless they are designed / voiced to be placed near the wall or corner.
when you pull speakers away from the wall (5 feet wall to baffle seems best if you can do it) three things happen-
1) bass response is optimized because wall reflection cancellations and reflections are minimized.
2) midrange and treble response is smoother because you have minimized wall bounce reflection / amplification of these frequencies (that radiate behind the speaker) that otherwise would create peaks and comb filtering distortion (which can also be minimized by wall diffusion and acoustic panels)
3) sound stage improvements- depth and focus can happen behind the speakers which allows them to "disappear".

 

Agreed about side wall reflections but rear wall reflections do matter- especially in the center between the speakers where their sound blends together.
Imagine the 360 ripple waves of the mids and highs coming together from each speaker at the center between the speakers. A zone in space dead center directly between each speaker at ~ tweeter height. This area where the right and left channels mix together in the center is critical for a clean, focused sound. With the wall reflecting this "blend" it upsets the decay of the ripple sound waves and results in summing of certain frequencies (which sounds hot or harsh), cancelling of certain frequencies and slight overlap of many frequencies. Overlapping is especially harmful to the sound because of comb filtering, the distortion cause by reflected sounds slightly out of phase. Sounds like the music is all being fed through a low volume flanger.
If your speakers are near a wall and the treble sounds kind of edgy / breaking up slightly when you stand away from the speakers, this is from rear wall reflections.

 

Yes, I deleted my comment too late On second thought, you're right! The speaker against the back wall does reflect comparatively enough energy in the midrange and treble to cause irregular response, and affect also the depth of image. Irregular response in the worst way, as you say a light flanging effect, this location produces an edgy character that can not be eq'd out.

 

Mostly it just depends on what the designers of the loudspeaker intended. A speaker designed for optimum anechoic measurements with full baffle step compensation will require free space placement, regardless of the type of low-frequency loading. A speaker can be designed to require boundary reinforcement, regardless of whether it is sealed or ported, or where the port is.

 

While room boundaries seemingly always play a role, I'm certain that each individual room presents it's own unique set of issues when it comes to this topic. In the room where I have my system set up, any speakers that were designed to vent on the backside of the speaker cabinet (or at the bottom of some floorstanding designs) interacted with my room in such a way that they presented numerous problems when it come to low frequency reproduction. In those cases, boomy and bloated bass with lots of overhang resulted and it was incredibly irritating. While room boundary effects can be detected with almost any speaker, in my room the difference between speakers that used rear-ported or bottom-ported designs and speakers that used a sealed or front-ported design was very notable. Sealed designs seemed to offer the safest bet in my room, but some (certainly not all) bass reflex and transmission line designs also worked well when the resulting exit for bass tuning was located on the front wall of the speaker cabinet. In my particular case, I've always considered this issue to be more of a room-based problem as opposed to a speaker-based problem. However, in my case since this particular room happens to be my living room, I preferred to address the issue by way of the speakers I purchased as opposed to special acoustic room treatments (in this case nothing beyond rugs/furniture/drapes was acceptable).

 

How about mortals like me who have a TV between the speakers then ? And all we can do is cover it with a blanket while listening to audio and, obviously, removing it for concert DVD watching ?

 

Not necessarily...

There is really no point of measuring a speaker in a chamber with no boundaries. This would only be relevant if a speaker would be listened to hanging inside the center of a very large sphere.

In reality, a speaker is usually designed to either be placed on a floor (half space) or on speaker stands (referring mostly to bookshelf speakers, in which case, the speaker should be measured on a stand, because that would be its intended use within a home listening environment.

There are many companies out there who manufacture speakers. A true anechoic chamber is expensive to construct, takes up valuable space and is unneeded after measuring a speaker's efficiency and/or response curve.

While the BIG speaker manufactures might have and use an anechoic chamber, most small speaker manufacturer's don't have or use them.

What many will do instead, is to take a speaker outside, place it on a flat open ground, place a measuring device 1-meter from the speaker and take a measurement, which would be in half space.

Most speaker measurements are measured in half space, not "whole space". Sitting on the floor in a home is half a half space environment, to if the speaker is not placed near other room boundaries, than a manufacturer's half space measurements should be similar to what you would get inside a home environment.

Still, as most home that I am familiar with do have walls and a ceiling, then the sound would still usually become more "efficient" because of the sound waves reflecting off of these surfaces, which would not occur within a true half space environment.

Speaker manufacturer's were not stupid all of these years, they are well aware of the listener's desire to have a solid bottom end that is not overbearing at the same time. The average person does not have their speakers sitting 3-4 feet out into the room, they have the stereo and speakers up against or positioned close to a rear wall, not out in the middle of the room.

If a speaker is designed to have solid balanced bass against or close to a rear boundary wall, it is not likely to be designed so that flat measurements will be only valid, if I speaker is hanging in the middle of an infinite sphere.

It it was designed and measured that way and then the speaker was place on a floor in a home, then there would be a gain of +3dB, move the speaker next to close to a rear wall and then the speaker will gain another +3dB, for a total of a 6dB boost. Which, as has already been pointed out by @harby.

Almost all of the reinforcement that comes from a speaker placed near a room boundary, comes from bass frequencies, usually around 300Hz. and below.

So a flat measured speaker in whole space, would gain +6dB of bass "bloat" if positioned on a floor and against a wear wall.

As @Warren Jarrett stated;

So pulling speakers out into a room is not necessarily the best thing for many speakers. If they were intended to be used acoustically coupled to the floor and next to or close to a rear wall, then placing them on stands and pulling them out into a room will result in decreased bass, which would mean that the bass response is n longer flat.

It make all the sense in the world to me that AN speakers are designed to be placed in corners, they are average size speakers designed to be used in average sized rooms that would be found in a typical room. Typical rooms in houses have corners. Even if the room is much larger, depth wise, the corner placement creating an eighth space boundary would be still enough to project the bass strong enough to be enjoyable and solid withing a normal listening position.

Most speakers without rear ports can be placed close to walls and corners. Klipsch Cornwall's are a excellent example of such a speaker, hence the moniker "Corn + wall".

Rear ported speakers are more difficult to place, since many will exhibit bass bloat, if placed to close to a rear wall and are better balanced and less bloated bass, if pulled out into a room.

 

This makes a lot of (common) sense to me. I know this forum is mostly US citizens, who don't seem to have a problem with room size, but in Europe room size is more often an issue. I just wonder how many chances of getting the sound right an average user have. Or maybe most of the people don't care about it, or they can't tell the difference. However, given the importance I give music, it seems unfair to me that anyone would have to go through an odissey to enjoy music. I did struggle a lot to get a decent sound out of my system (mostly thanks to you guys) and, even though I always enjoy learning new things, I always somehow feel that I could have used that many hours better. Just a thought...

 

There is a reason that speakers that are manufactured in England and designed for use in English homes that typically have smaller rooms, are designed with the intention of being not a heavy on the bass as typical American speakers.

This is nothing new, BBC monitor type speakers have been around for quite a long time and are purposely designed for use in small rooms. So many of these English houses are built out of brick, so the rooms also tend to be more reflective and the speakers are designed to not be overly bright sounding.

I have three pairs of vintage Wharfedale speakers, W60's, W70's and W90's. These speakers are from the 1960's. They were specifically manufactured in America in a manufacturing facility the Wharfedale set up in New York, to manufacture speakers with more bass, that American's seem to have more of a preference to than in England.

Some of this may have to do with room size, where room sizes in America tend to be larger that in England, you need more bass energy to effectively pressurize a large size room.

 

Hail american speakers !

 

The OP's original question makes me feel old! Ah, to feel that rush of youthful wonder when discovering audio truths!

 

Hmm, I can "see" I need to upgrade my listening room accoutrements...

 

At the very lowest frequencies, it doesn't matter. The only thing about rear ported speakers is that air needs to be able to flow in and out of the port, a jet of air, so if the wall is close it makes turbulence degrading the sound. The wall needs to be several port diameters away.

 

I REALLY, REALLY like the SH.tv forums, for its threads like this one. Even though I probably write as though I already know everything about audio, actually I learn A LOT from the rest of you. Thank you.

Obviously, as a dealer for Audio Note speakers, I need to learn as much as I can about this thread's topic.

 

Raising a glass to your FINE explanation. Well done.

 

Many thank's!

 

Being a fine dealer an an apostle for AN, maybe you could bring this discussion up with their speaker engineer and quarry him as to what conditions their measurements are taken, "space" wise. Considering they have long ago come to the brilliant conclusion that in homes there are corners, why not design modest size cabinets that are efficient and dig deeply into bass?

Of course, the easiest way to accomplish this, is to place them in corner's that they may take advantage of eighth space.

It would be nice to have an actual explanation on how they design and measure their speaker line.

Where does whole space, half space, quarter space and eighth space measurements come into play?

Inquiring minds want to know (and get a better understanding!)?

I think this sort of thing, might be really interesting.

S&G