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PostPosted: Sat Sep 03, 2011 12:52 am 
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The closer you get to a wall the more the bass will build up. I've seen a lot of designers vary the baffle step compensation in their designs based on their speaker placement, but I've never seen equations to predict this behavior. These designs that I've seen were not for studio monitoring so it wasn't a critical listening.

If you had to go this way you could minimize the problems with a lot of absorption behind the speakers. Heck, it might even work in an LEDE room, it wouldn't be my choice though.

If I remember correctly (which I probably don't) you should be at least 3 feet from the nearest walls.

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PostPosted: Sat Sep 03, 2011 1:00 am 
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If I remember correctly (which I probably don't) you should be at least 3 feet from the nearest walls.

I think the ITU recommendation is 1.5 m, (but my recollection might not be any better than yours! :) ).

I'll see if I can find that.

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PostPosted: Sat Sep 03, 2011 2:31 am 
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Hmmmm... not finding much. Genelec recommends 1.1 m minimum, but also add that the main factor is the cut-off frequency of the specific speaker you are using. Makes sense: They give this comment:

Quote:
... position the speakers far enough from the wall to move the first order interference dip below the lower cut-off frequency of the speaker. To move the dip down to 30 Hz, the distance needed is 2.8 meters. This would not be possible in most control rooms simply because of lack of space.


So I guess the best answer is "it depends on your speaker". If you have a really small thing with no useful bass, then you can put it anywhere, but if your speaker has decent bass response, then the minimum distance to the wall depends on the lowest frequency that the speaker can reproduce.


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PostPosted: Sat Sep 03, 2011 8:30 am 
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hmmm ...

So HOW does this all fly when the starting 'IDEAL' point of 38% from the front wall
is to be considered ???? :|

Keeping in mind that it's only a start reference point ... ok ... but ...

for ex: Some of us have small control rooms [12x16]. So ... any thoughts on the preferred
way to balance this all out ??

Thanks


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PostPosted: Sat Sep 03, 2011 9:14 am 
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So HOW does this all fly when the starting 'IDEAL' point of 38% from the front wall
If you are not soffit mounting, and can't get that kind of distance from the walls that you need, them go with Method Number 3: get the speakers as close as you can to the front wall, which forces the issues up into the higher end of the spectrum, where it hopefully won't be too noticeable. IE, put them right up tight against the front wall, or really close.

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PostPosted: Sat Sep 03, 2011 10:00 am 
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Hi Stuart !

Thanks for working this thread .... and the other flush-mount thread. Very educational.

Regarding monitor positioning [not soffitted] ... should you have a moment to re-check
my build thread [UAN- COntrol room]
viewtopic.php?f=1&t=14794&start=390

and see that I do have monitors [these OLD beat-up ones] quite close to the wall.

Directly behind the monitors is my plain wall, while between, I have 2 - 2'x4'x4" panels, along
with chunkies in the corners.

I'm doing listening test ... but the concern is, The inner monitor edge is about 1" in FRONT
of the panel plane ... do you see any issues with that. [I probably should be posting in my
thread ... don't mean to deflect nor hijack].

Thanks!


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PostPosted: Sat Sep 03, 2011 3:45 pm 
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I would pull the speakers out away from the wall and angle them in so you don't have to move back the mix position. You don't have to have the equilateral triangle with the speakers and your mix position. You can move them out and angle them at 45 degrees and not have to change your mix position.

Your mixes against the wall will probably be bass light because of the boost they get from the wall and you will compensate.

edit:clarified the last statement and spelling of compensate :oops:

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Last edited by Eric Best on Sun Sep 04, 2011 7:01 am, edited 1 time in total.

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PostPosted: Sun Sep 04, 2011 5:35 am 
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RJ, you might find this interesting, from Genelec:

http://www.genelec.com/learning-center/ ... cellation/

Their recommendations on how to place speakers relative to the front wall:

1) Flush mount (soffit mount)
2) Right against the wall.
3) Very far from the wall.
4) Make the wall extremely absorbent.

They give their reasoning for each case (very logical).


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PostPosted: Sun Sep 04, 2011 7:02 am 
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Very important last sentence in case 2

"Additionally, the low frequency boost should be compensated for when the loudspeaker is mounted close to the wall (+6 dB)."

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PostPosted: Sun Sep 04, 2011 8:06 am 
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Eric Best wrote:
Very important last sentence in case 2

"Additionally, the low frequency boost should be compensated for when the loudspeaker is mounted close to the wall (+6 dB)."


Hi Eric,

Very useful/educational material you've posted. Thank-you.

I was particularly interested in the passive EQ circuit too.

At the moment, I have an old pair of ALTEC studio monitors in the room for listening & testing,
but these old boyz are NOT quite in the sonic shape they USE to be :| I probably need
to replace the drivers, and most likely, rebuild the x-overs. I've heard that caps can 'drift'
after some 50 years :shock:

My next plan is to audition some new monitors ... looking at several lines like the Focals,
Opals, BF, etc... [would like to stay within a $2-3k [US] price range if possible].

Again ... thanks for your posts!!


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PostPosted: Sun Sep 04, 2011 2:02 pm 
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Which Altecs do you have?

If the surrounds are OK and the cone looks fine you shouldn't have too much problem.

Replacing the capacitors would probably help.

There is a company http://www.greatplainsaudio.com/ that specializes in Altec speakers. I have some 902 drivers on 511B horns that I bought replacement diaphragms for the drivers. They were very helpful and might be able to replace or refurbish your drivers, if they are monitors that you know well and like.

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PostPosted: Wed Sep 21, 2011 2:33 am 
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Eric Best wrote:
Very important last sentence in case 2

"Additionally, the low frequency boost should be compensated for when the loudspeaker is mounted close to the wall (+6 dB)."


Just a data point for anyone following this thread. I'm going to flush mount my ATC 100 ASL speakers. The tech guy at ATC told me I could expect a "real world" LFB of about +4 dB. So, at least in this case, there's a slight difference between the projected boost and the mfr. tested boost. I'm sure this varies with the speaker under consideration and its crossover. The ATCs have no provision for switchable response, so I plan to test after the room is done and build a software filter in my Metric Halo D/As for the compensation.

Craig

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PostPosted: Wed Sep 21, 2011 3:07 am 
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Quote:
The tech guy at ATC told me I could expect a "real world" LFB of about +4 dB. So, at least in this case, there's a slight difference between the projected boost and the mfr. tested boost. I'm sure this varies with the speaker under consideration and its crossover.
That's a surprising comment, coming from a speaker manufacturer.

The base boost doesn't really depend on any aspect of the speaker itself: it depends on the simple fact of soffit mounting. This is basic physics. Compare two identical speakers, one radiating freely into full space, and one radiating into half space. There will ALWAYS be a boost of 6dB for the one radiating into half space, since that half of the power that was going "backwards" is now forced to go "forwards". Doubling the power radiated forwards is an increase of 6 dB, regardless of the speaker, manufacturer, crossover, planet or galaxy in which this is done. A 100% increase in radiated power is a 6 dB boost, no doubt about it.

In fact, the boost could theoretically be GREATER than 6 dB if the speaker also happens to be getting a boost from other walls, or the floor, or the ceiling. For example, if you were to mount the speaker right in the "tri-corner" between two walls and the ceiling, you'd get a major boost. The only way that the boost could be LESS than 6 dB is if the original location of the speaker was such that it wasn't radiating into full space to start with. But if that were the case, then once again the boost would be identical for all speakers used in that location, regardless of brand, power, model or crossover. The soffit boost is independent of the speaker, amplifier, crossover, or anything else. It is an effect created by the soffit itself, and always gives a 6dB increase, as compared to the same speaker radiating into full space.

The only difference that the size/shape of the soffit makes, is that it changes the frequency at which the boost starts, and thus the frequency at which the roll-off should start. But it does not change the 6dB peak boost: that is a function of just soffit mounting, itself, nothing else.

Maybe he was talking about interaction with the rest of the room? In other words, a 4dB boost from soffit mounting AS COMPARED TO just having the speaker in a small room, close to walls? That would make sense, but if that's the case he explained it wrong: The roll-off you need is still 6 dB as compared to full-space, but only 4dB as compared to being in a small room in proximity to walls.

Quote:
The ATCs have no provision for switchable response, so I plan to test after the room is done and build a software filter in my Metric Halo D/As for the compensation.
Software EQ may or may not work well: It would have to be phase-linear for optimum performance.

Testing the room response is always a good idea, but you'd have to be very careful to differentiate between the modal response of the room, and the bass boost from the soffit. For any small room, it is possible to confuse the two issues, unless you use a good analysis program, such as REW. And of course, you cannot compensate for modal response issues by using EQ, since that is a time-domain problem, but you MUST compensate for the soffit bass boost, since that is purely a frequency and power issue.

The circuit that Eric mentions is pretty simple and easy to make, and should do the job well.


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PostPosted: Wed Aug 22, 2012 9:09 am 
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Based on the very well explained Genelec flush mounting techniques, and the techniques mentioned on this forum, i was wandering if i can make the outer wall from concrete blocks, and fill the inner space with absorbant material, and also place an opening (like a speaker port) near the bottom of this concrete wall, so that the air can enter and by convection cool a bit the speakers. In this case, do i get any significant advantage on using concrete blocks instead of wood or mda? should i calculate this port based somehow on the speaker size? thanks in advance guys!!!

Cheers from Argentina!!

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PostPosted: Sun Feb 17, 2013 10:38 am 
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Ooops! Just realized that I never responded to this! Oh well, hopefully "six months late" is better than "never"!

Quote:
i was wandering if i can make the outer wall from concrete blocks, and fill the inner space with absorbant material,
Are you talking about the soffit walls, or the room walls? In both cases, the answer is "yes".

Quote:
and also place an opening (like a speaker port) near the bottom of this concrete wall, so that the air can enter and by convection cool a bit the speakers.
OK, so it the soffit wall you are talking about. Yes, you can do that, and in fact you MUST do that: your speakers will need some cooling, even if they are passive, and if they are active the probably need a lot of cooling.

Quote:
In this case, do i get any significant advantage on using concrete blocks instead of wood or mda?
Both will work, provided that the MDF is very thick. The front baffle of the soffit hast to be very rigid and massive to work properly. Light-weight thin MDF would not be good, but thick, heavy MDF would be fine.

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