Bass port facing wall - problem?

[Liquid Shadow]

My studio monitors (Alesis M1Active 620) have rear-facing bass ports that are about 5 inches from the wall. I have 12 panels of OC703 coming (ATS exact substitute actually, for $10 cheaper) and I'm going to put them in the corners. I know they are not the 4" FRKs but these will have to do for now.

My question is should I be worried about the rear-facing bass ports and do something about it? Like maybe put a panel on the wall behind the speaker? I know this may sound dumb because I have never seen other people do that.

Side question: the panels are 48x24, which is really too big for the corners in my room. I planning on cutting a couple of them length-wise, so they'll be 48x12. This will be way more corner-friendly for my room and I understand that the space between the panel and the corner will be much smaller, but I figured it's still better than nothing. You guys think this is a good plan?

[LeeYoo]

To my knowledge, placing a rear-ported speaker against the wall is effectively lengthening the port, which lowers the port frequency. That is, if the space is about the diameter of the port, or less.
I don't see a technical problem with 5" distance. What might be a problem is a vibrating wall.

A simple measurement will tell you how much lower the port frequency is.
If you have some sort of sound pressure meter.
Use a variable sine wave generator on 30-50 herz (or use software, or make a series of frequencies on your DAW), and find the frequency with the lowest output on the s.p. meter, close to the woofer cone.
This is the port frequency.
Do this with the box freestanding, and close to the wall.

Don't know about panels, but I think that they don't work on low frequencies (30-40 herz) anyway.

[SAC]

You can simply leave a space equal to or greater than the width of the port in order to avoid loading the port...in other words 4" from the wall or boundary.

This spacing will provide an area in the lateral directions equal to the port area, thus preventing additional back pressure and loading.

How you can calculate this (all units are inches):

the cross-sectional area of the port = Pi(r^2) = 50.2656
the min. area of the required spaced band = the area of the port = 50.2656
the circumference of the port forming the length (L) of the spaced band = 2pi(r) = 12.566
the area of the spaced band = L x W >= 50.2656 = 12.566(W) = 50.2656; W= 50.2656/12.566 =~4

[Liquid Shadow]

Originally Posted by LeeYoo

To my knowledge, placing a rear-ported speaker against the wall is effectively lengthening the port, which lowers the port frequency. That is, if the space is about the diameter of the port, or less. I don't see a technical problem with 5" distance. What might be a problem is a vibrating wall.
Don't know about panels, but I think that they don't work on low frequencies (30-40 herz) anyway.

You're right that the panels probably won't do anything to those low frequencies, I guess I should just leave them be?

[LeeYoo]

Hi sac,

The area of the port - Pi(r^2) = 50.2656
Do you mean the hole?
? in square inches?

Looks like about 1.5-2" dia port to me, that makes it less then 3.5 square inch.

[SAC]

Somhow in skimming the figure of 4" for the port stuck.
I have no idea what the actual port diameter is. And such positioning limitations are one reason that I am not a fan of rear and side facing components in a loudspeaker with a supposed controlled Q/dispersion.

But the numbers are insignificant - the math is still valid. So instead of a radius=2", substitute the actual radius of the port into the formulas.

But generally, whatever the port diameter, you simply keep the speaker that distance away from the boundary.

[Liquid Shadow]

Originally Posted by SAC

But generally, whatever the port diameter, you simply keep the speaker that distance away from the boundary.

Got it, thanks. Right now the distance between the port and the wall is definitely way bigger than the diameter of the port itself.

[LeeYoo]

Found a picture of the back on the net, and estimated the size from there.
Happy with the 4" minimum distance.

[tonio]

What about a rectangular port? Much the same I suppose?

T

[LeeYoo]

If the distance is more than the average port diameter, and the air can escape 4 ways, there should not be a big problem with port frequency change.

[Rod Gervais]

Folks,

I have a novel idea..... (don't have em often so I get excited when I do) why not read the manufacturer's set up instructions in the manual........ ??????

Rod

[LeeYoo]

Yeah!!


And then make a list on GS of the manufacturers you can trust.

[SAC]

Originally Posted by LeeYoo

And then make a list on GS of the manufacturers you can trust.

You mean of the manufacturers who actually provide valid full range polar response plots, and information regarding spatial loading response curves and possible emphasis settings to compensate for a variety of spatial loading scenarios?

Hmmmm...but that would come dangerously close to making sense.

And that wouldn't look nearly as nice as the embossed free field anechoic response plot that almost all of the manuals feature... and which corresponds to none of the intended uses...



On the other hand, the fact that so many make these units with ports or radiators firing to the back and or sides should provide an ample hint that such options were never intended, nor probably even considered.

[tonio]

Rod,

Brilliant idea!! unfortunately most mfg's do not have much info, other than an equalateral triangle set up.

I have Dynaudio BM6A's which are not cheap. Any suggestions as to what price point would a mfr even provide a full on instructions , not to mention polar plots etc?

[JohnTheRevelator]

JBL puts a lot of pertinent info into their middle line of monitors on up. Sad thing is that all these companies should have this info on hand as they should be making it to design and tweak the speaker into 'good enough for the money'.

I haven't seen as much info on any other monitors.

The sad state of audio,

Dan

[Rod Gervais]

Originally Posted by tonio

Rod,

Brilliant idea!! unfortunately most mfg's do not have much info, other than an equalateral triangle set up.

I have Dynaudio BM6A's which are not cheap. Any suggestions as to what price point would a mfr even provide a full on instructions , not to mention polar plots etc?

I would assume that the info is available upon request - but my response was for the OP - when I went on-line and looked at the manufacturer's documentation they were very specific as to how to handle single and corner boundaries with their speakers.........

Which makes me wonder why one would not just check the documentation.

Rod

[Rod Gervais]

Originally Posted by tonio

Rod,

Brilliant idea!! unfortunately most mfg's do not have much info, other than an equalateral triangle set up.

I have Dynaudio BM6A's which are not cheap. Any suggestions as to what price point would a mfr even provide a full on instructions , not to mention polar plots etc?

Tonio,

They have excellent documentation on those speakers (as regards setting them up - dealing with boundaries, etc.) - just for the heck of it I have requested further information than they provide on their website - I will let you know how they reply.........

Rod

[LeeYoo]

Two separate problems in these posts.

1. The de-tuning of the port when facing a wall. This was the original question.

2. The effect of placing a speakerbox (any speakerbox) against a wall, or in a corner. This should be textbook stuff.

[Liquid Shadow]

Of course I have read the manual, they have a bass density switch on the back. The problem is I CANNOT move the monitors, I was merely asking if it's a problem and if I can improve things with some OC703.

[Rod Gervais]

Originally Posted by Liquid Shadow

Of course I have read the manual, they have a bass density switch on the back. The problem is I CANNOT move the monitors, I was merely asking if it's a problem and if I can improve things with some OC703.

Asking us if it is a problem is difficult for us to answer, seeing as we are not there - and cannot listen to (or test) what is happening inside of the room.........

I take it that there is a problem - seeing as you wish to know if it can be solved with 703 - but you have not yet stated what the problem is, what extent of treatment there is (already) in the room or any other information about the room that would lead to an answer any more generalized than you have already received.

Rod

[Liquid Shadow]

Don't worry about it.

[foamboy]

Originally Posted by Rod Gervais

I would assume that the info is available upon request - but my response was for the OP - when I went on-line and looked at the manufacturer's documentation they were very specific as to how to handle single and corner boundaries with their speakers.........

Which makes me wonder why one would not just check the documentation.

Rod

Hey Rod. would you mind posting a link to the information that you found regarding these monitors? When I went to the Alesis site and clicked on the link for the manual I was directed to a page that wanted some personal info including email and such. Not a problem,but I just hate to get on another mailing list. Did you just go ahead and submit your personal info simply to get a pdf of the manual?

Thanks

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