Originally posted by Doug Osborne:
Boundry Effect?

... so the best fix is to not have a wall so close to the back of the speaker. ...

Originally posted by David Sykes:

Quote:

Originally posted by Doug Osborne:
Boundry Effect?

... so the best fix is to not have a wall so close to the back of the speaker. ...

Could anyone quantify "so close" in terms of a general rule/guideline for, say, ns10s and 1032s?

(granted, ears make the final determination)

Thanks
David

Originally posted by miroslav:
Won't dead=depth...or the illusion of...?

I have both front and rear walls partially dead, plus wood diffussers on the wall in forn of the monitors.

Distance between walls is approx. 14'

Seems to work quite well.

Originally posted by Bill Mueller:
It’s been a while since I taught this but I will give it a try.

Speakers that are placed near a wall, floor or ceiling surface radiate mid-low and low frequency energy from the rear that couples with and reflects off of the surfaces to both positively and negatively interfere with the direct sound emanating from the front of the speaker. One surface usually boosts frequencies at a quarter wavelength or lower by about 3db. Two surfaces boost up to 6db and three surfaces (a corner with a floor or ceiling surface) up to 9db. A 20hz wavelength is 56.5’ long. That means that if a speaker is within 14’ from any wall it will reinforce a 20hz signal. The closer it is, the higher the boosted frequency will be. However there will also be a dip at 40hz (1/2 wavelength) because the returning signal will be out of phase. These two relationships will fight with each other up the harmonic scale causing peaks and dips along the way. These anomalies cause the typical small room to vary by as much as 30db from one position to another position just a few inches away. These effects are also strong with low mid frequencies because most loudspeakers are still fairly omni-directional between 100hz and 500hz and don’t start beaming until they get a bit closer to their crossover frequency, usually 800-1000hz. At those frequencies the side spill from a monitor is more important than the rear spill. Early reflections from side spill fall within the Haas effect of 16-25ms delay causing not a flutter echo but a comb filtering that runs harmonically all the way through the signal to very high frequencies. These effects combine to make a perfectly good sounding monitor sound boomy and lifeless in a typical small room.

The perfect place to put a speaker is not next to a wall, but INSIDE the wall. Then, the wall becomes a waveguide to low frequencies, effectively extending the low frequency cutoff of the cabinet to below the driver’s resonant frequency, improving linearity greatly. IMHO, side surfaces should be damped down to at least 200hz to remove splash and the rear wall (behind the listener) should be far enough back that the returning signal will be more than 25ms delayed in time. Sound travels at 1130ft/sec so the minimum distance from the listening position to the back wall for early reflections to be safely outside of the Hass effect should be 14.124’. 1130/1000=1.13X25=28.25(round trip)/2 =14.125’ (distance from listener to back wall).

The back wall (again IMHO) should be diffuse enough to give a sense of “openness” to counteract the very necessarily large amount of full bandwidth absorption needed on the front and side walls and to eliminate any possibility of flutter echo with the ubiquitous glass window directly in the way in the most sensitive area of the listening field, the center. True bass trapping is also necessary because in most small rooms the walls become diaphragmatic at low frequencies and resonate wildly at their resonant frequency and it’s first few harmonics. This just adds to the chaos. Bass traps are best placed in room corners because the coincidence of the walls tend to filter out upper harmonic resonances leaving very strong low frequencies perfect for exciting columns of air in helmholtz type traps or membrane absorption in membrane type bass traps.

I hope this helped.

Best regards,

Bill

Originally posted by Bill Mueller:
It’s been a while since I taught this but I will give it a try.

Speakers that are placed near a wall, floor or ceiling surface radiate mid-low and low frequency energy from the rear that couples with and reflects off of the surfaces to both positively and negatively interfere with the direct sound emanating from the front of the speaker.

...

I hope this helped.

Best regards,

Bill

As I said before, the speaker/wall frequencies of interaction are dependent on the distance between them. The closer we move the speakers to the wall, the higher the null frequencies move and the more of the bass frequencies are uniformly reinforced. At 3 inches from the wall the negative interference is up in the 1000hz range. (I had a glass of wine so I hope you all don’t hold me to the exact frequency). This is now in the range where the speaker is not radiating any energy to the rear as Ethan properly suggests so the nulling is minimal. Therefore, the really important absorption should again be placed on the SIDE walls to remove the aforementioned splash and accompanying comb filtering.

Originally posted by Bill Mueller:
PoPstAr,

We are left with two problems however. First, the speaker is close to the wall, making leakage into the studio area a worse problem. Second, the speaker has an unnatural bass boost caused by the positive interference with the surfaces behind it. To address the first problem, we can stiffen the wall in the hope of attenuating some of the bass from going into our acoustic guitar mics. We can also monitor softer when tracking acoustic guitar. We can also place bass traps in the front corners of the room and attenuate some of the bass that way. We can also apply deep absorption and try to attenuate as much of the bass as possible that way. However as Ethan already said, it is very difficult to absorb bass with fiberglass. (I do believe that applying mid/high absorptive material to the front wall in a small room can remove flutter echo emanating from the rear wall and improve intelligibility in a room with parallel surfaces. I think Ethan may agree with this point.)

Addressing the second problem is actually easier depending on the speakers you use. Many speakers use a bass attenuation switch that is designed exactly for this situation. If you place your speakers close enough to the wall and use the low frequency attenuation switch as it is designed, you can end up with a very clean and clear monitor setup. Speakers like Yamaha NS10s are acoustic suspension speakers that are designed to be place on shelves (bookshelf speakers) using a wall immediately behind them to reinforce their bass. That’s one reason why they sound so damn thin perched on a meter bridge.

To me the real challenge is getting the most linear, flutter echo/phase cancellation free performance from our small room/small speakers combo, and knowing how they affect each other is key.

Best Regards,

Bill

Originally posted by Ted Nightshade:
12 ms = 12 foot sound path, so the monitors would have to be 6 feet from the wall behind the monitors, and you'd want the sound path bouncing off any other walls, floor, ceiling to be travelling at least 12 feet.

Bob Katz recommends 20 ms for the earliest reflections in a "reflection free zone".

Popstar, with the wall and the console both right up agains the monitors, perhaps you're getting coupling both ways? Too much of a good thing perhaps? Or too much of a not so good thing, I expect. There's comb filtering off the console anyway.

Originally posted by Bill Mueller:
PoPstAr,

A few points if I may.

As Ethan properly points out, the single surface bass boost that occurs when a monitor is right up against the wall is about 3db and LINEAR. Therefore it can be corrected either with built in speaker eq or an inserted shelving eq (not my favorite however). My Mackie 824 monitors have a great little switch just for this.

Originally posted by popstar:
[QUOTE]Originally posted by John Sayers:
[qb]guys - put a signal into your console and split it so one is direct and one goes through a delay unit. Now pan one signal hard left and the delayed hard right.(no delay at this point) Balance the speakers so you hear the resultant signal in the phantom center.

Now start adding delay to the delay side. It will still sound in the center UNTILL you reach around 18msec where suddenly the signal seems to split into two left/right signals.

try it;)

cheers
john

Bill,

I too have the Mackie 824 monitors.

I'm using them about 1.5' away from the wall and all the cut/boost settings a their neutral/factory default positions.

So...with yours up against the wall...when you mention the little switch on the back....are you referring to the Acoustic Space switch?
Are you setting it to A(quarter space -4dB) or B(half space -2dB)...I have mine currently on C (normal 0dB).

Also...do you boost/cut either the High or Low Frequency switches, or do you leave them on 0dB?

In your setup...how much is the passive radiator "obvious"...especially with the speakers up against the wall?
Oh…and exactly how close do you have them to the wall…1”…2”…3”…?

Thanks for your input.

--------------------
Miroslav