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

I can understand why rear-ported speakers would need to be (ideally) a few feet from the back wall but why are front-ported and even closed cabinet speakers asked by manufacturers to be away from the wall as well?

 

Adjacent walls increase efficiency and reinforce bass response. You achieve +3dB in gain, the closer you are to a wall. It's called the boundary effect. So, + 3dB from the floor, a position near a wall, another 3dB, and in a corner, another 3dB. All total, that's +9dB in room gain. Which is also +9dB in bass gain. Some speakers could use that extra reinforcement. But, if you already have speakers with good bass response, then you're usually only adding bass boom.

Now, my Acoustic Research speakers were designed to use room gain for flat response. My AR90's need to be placed against the back wall. The room gain was taken into account in the original design, and is used to provide flat response without boom. Roy Allison also designed speakers, that accounted for room gain, as well. There are others, but not as many as there should be.

 

But how does that factor in when it's the back wall that I'm referring to? A closed enclosure shouldn't be affected by the *back* wall, correct? The floor and whether it's close to a side wall adding more volume to some frequencies is good info to have. Just wondering what how pulling the speaker out from the back wall is recommended and supposedly affects the sound.

That is assuming the side wall and floor are of course at the same exact angle/position.

 

Diffraction off of the cabinet will cause some of the sound energy to hit the wall behind the speaker. This sound is the reflected toward your listening position with a shift in phase that can interfere with the sound that is directly coming from the speakers.

 

Floor, back wall, side wall; it doesn't matter. In acoustics, boundaries are boundaries. Boundaries reinforce. That causes peaks in response. And the lower the frequency, the longer the wave length. The longer the wave length, the easier it is to be reflected by boundaries. The more reflection there is, the more chance of reflected sound interfering with the primary sound too. Room modes come into play as well, creating havoc.

The room is king. No way around it. But, it can be improved, with proper positioning, good room treatment, and even DSP. It's a tough balance.

 

Thanks guys. That makes sense. Would I be correct in my assumption that front-ported and closed enclosures are less of a problem in that sense than rear-ported speakers?

 

I don't think the location of the port makes that much difference as far as wall placement. Sound is radiating primarily from the drivers, not the port.

The thing about speakers that are ported or vented in any location is that they typically work to extend bass response downward by about 1/2 octave, compared to a sealed box of the same volume. There's often a little bit of a midbass hump, too. And below the tuning point for the port, there's a steep rolloff- the bass response becomes less "controlled", but the steep rolloff keeps it from being a noticeable problem. But when bass response anomalies like a midbass hump and distortions below the tuning resonance are backed up against a wall, they get reinforced, and become more audible.

All of the speakers I know of with a volume over 1 cubic foot that are considered to be compatible with placement against the wall are sealed boxes- AR, Boston Acoustics, Allison. There are some ported minispeakers that are said to work well against a wall, like Wharfedale Diamonds, but I don't know of any larger ported or vented speakers that like wall placement.

 

By my experience rear ports only gives you better control of how the speaker interact with the rear wall.

 

Well, a rear ported speaker can help mask any potential for port noise. A front port might make port noise more audible, and noticeable. There's no-free-lunch.

 

I use a pair of slot front-ported Ascend Acoustics Sierra Horizons as my main left and right front speakers. They sit on shelves in a bookcase only a few inches from the back of the case and sound wonderful.

 

Strat,

Here is a little bit about boundries that might make it a bit clearer to you by example.

Sound and light both radiate out in all directions. Think of a candle burning, with the flame in the center of the sphere, light is emerging from all directions of the surrounding sphere.

Every thing here is simply the physics of the geometry of a sphere.

Now, if we slice the sphere across ir equator, we now have separated the sphere into two equal halves.

Sort of the same, like the ground and the hemisphere of the sky above. This is referred to a a boundary, which now creates a "half space"

If we then take a vertical plane, where we are viewing the flat part of the plane and slice it through the top part of the hemisphere, all the way to the "floor" of the hemisphere, we have now divided the sphere into equal quarters. Therefore it is now referred to as "quarter space", which is like a large room where you speaker is centered along a long rear wall.

Cut the sphere again (for one speaker, by running a plane vertically through, but at a 90° angle to the "rear wall" and we create a "corner" and this is known as an "eighth space" which is how it is "seen" by that individual speaker.

If we were to suspend a speaker inside the center of a giant sphere and then cut that sphere into two equal hemispheres (half space) the speaker would experience a gain of +3dB's.

Since, from the viewpoint of a loudspeaker and the real world, where speakers don't hand from giant sphere's, speaker measurements are taken with the speaker already sitting on the ground, already in "half space". When you read a speaker's sensitivity specification, it is given that "as measured in "half space" and usually from a distance of 1-meter.

Although, with legacy Klipsch and Altec Lansing speakers, half space measurement is taken at a distance of 4' (for reasons unbeknownst to me).

As illustrated here.



Here is an illustration of listening to a speaker in half space.



It wasn't really relevant to my example, but in "researching" my information, I happened to wander across it and thought it might greatly improve the overall presentation of my post, so I found a way to make it relevant.

S&G

 

Always good to keep it relevant.

 

Proximity to sidewalls and/or backwall is fine, if that is how the speakers were designed to be located, like Klipch-horns, and Audio Note speakers. But most speakers were designed for optimum frequency response linearity in an anechoic chamber, which acoustically has no walls at all. So for these speakers, farther away from any wall simulates better the environment that they were designed to sound best in.

 

I skipped right over your technical explanation, to your second illustration, and found her to be EXTREMELY relevant. Thank you for your research.

I am going to buy a blond wig that matches your example, and continue your research with my wife tonight.

 

How do these speaker designers settle on an room dimension/placement strategy to tune the speaker to?

 

Well, in the two examples I mentioned, overall room dimensions are not critical because the near-field effect of the tight corner location is overwhelmingly more significant than the rest of the room.

Placement strategy is simple. The speakers are designed to be as tight as possible into room corners. But the owner is welcome to adjust ("tune"), inch-by-inch, away from the wall and/or corner to his personal taste, synergy with his equipment, and specific characteristics of the room.

 

So a sort of zero inch placement is optimal? That makes sense. Would the designs have a minimum distance between?

 

Warren: Hey honey, I thought we might try this tonight.

Warren’s wife: Okay, but after you’ve finished putting that wig on could you please turn down the volume on the stereo, it’s giving me a headache.

 

Would room treatment noise-absorption panels placed right behind the speakers on the back wall help alleviate the issue?

My room isn't that wide so speakers are only a bit away from the back wall out of necessity.

 

Beat me to it.

 

The quarter-space emphasis that comes from near-wall placement mainly refers to the bass frequencies, as it is their long wavelength that allows the reflection to basically become part of the same radiated wave as the speaker, instead of being a separately-hear reverberation.

These same frequencies, however, are the least-affected by treatments; to manage bass, you really need huge bass traps that are much more cumbersome than just using tone control on your amplification.

The low frequencies are often what a listener wants to emphasize when they don't have a subwoofer with crossover (removing low frequencies from the main speaker) and so the bass boost is often enjoyable until it becomes unnatural (although wall placement effects can extend into the upper, boomier bass, this can be somewhat tuned by the exact distance to the wall).

Some powered monitor speakers have compensation for a near-wall (or on desk/floor) placement:

 

Well, actually, I planned to ask her to put on the long blonde wig, while I dim the lights and change the music to Metallica. But maybe you're right; for a change maybe she'd like to see me in a wig, instead my usual full leather riding suit.

 

Zero inch with respect the backwall, and experiment with inches from the side walls. Then the distance between speakers is set, not to be changed. The only variable left, for good balance of wide soundstage and firm center image, is the listening chair's position.

 

This is my definetely experience. Rear-ported speakers equals to boomy sound, closed or front ported equals to no boomy sound. At least in my listening room (5x5 meters). In both scenario they had to be placed about half a meter from the wall.

 

As other members have commented correctly, the port is more a function of the speaker design, and less to do with room boundary. Likewise, the port location has little to do with room boundary (except very close to the wall on a rear port)

The sealed acoustic suspension speaker isn't much different in its bass pattern. Even though the driver may be forward facing, the lower bass frequencies wrap around the cabinet in a full spherical pattern. The spherical omni-directional pattern for the typical home hifi speaker will be at about 160 Hz and lower. (tending to be spherical up to 300 Hz) The mids and hi freqs will be directional, since of course as one moves to the side or back of the speaker, the midrange and hi frequency sound intensity drops off, but bass doesn't.

It is this low frequency energy surrounding the speaker that bounces back from any boundary, floor, wall, ceiling. The distances from these boundaries determine which frequencies will be reinforced, vs which ones are cancelled (reflection out of phase) and some that will be neither reinforced nor cancelled. (reflective energy still there which can introduce harmonic distortion, sometimes "ringing or booming" .. a complex phenomenon and for another discussion) The cancellations have more affect on the total response in the room, since these nulls will be very deep. Reinforcement has a very audible effect, (typically +2dB to +9dB) but cancellation much more effect. (can be as deep as apprx -30 dB)

Let's say we have a sealed speaker or ported speaker 12 inches off the floor. If against the back wall all of the bass frequencies will be reinforced by about 3dB... and the floor by another 2dB or so. If the speakers are moved 18 inches from the back wall, upper bass room response will tend to reduce; but the lower bass unaffected... because its wavelength is long and still being reinforced in phase with the woofer. Move the speakers well away from the back wall, then the upper bass may become unaffected by that boundary and be neutral, but the mid bass room resp would be more reduced. The lowest bass would be somewhat reduced as well.

The distance from a boundary correlates to the wavelengths of sound that you want to reinforce, or cancel.