3 questions...

[the kid]

1. Do I understand correctly that it is more desirable audiophile -wise to have your cc be verticle (upright) instead of horizontal? And this pertains to the width of the projected sound field but not necessarily the sound quality?

2. Any way possible of estimating (guessing) where room gain occurs?

3. Would the length of 2 ports be half each of the total for 1 longer port?

[billfitzmaurice]

Re: 3 questions...

1. Do I understand correctly that it is more desirable audiophile -wise to have your cc be verticle (upright) instead of horizontal? And this pertains to the width of the projected sound field but not necessarily the sound quality?

Yes and yes.

2. Any way possible of estimating (guessing) where room gain occurs?

Starting where the room's longest dimension is equal to 1/2 wavelength, at rate of 12dB/octave.

3. Would the length of 2 ports be half each of the total for 1 longer port?

Usually just the opposite. It depends on both the total duct area and length.

[Pete Schumacher]

Re: 3 questions...

1. Do I understand correctly that it is more desirable audiophile -wise to have your cc be verticle (upright) instead of horizontal? And this pertains to the width of the projected sound field but not necessarily the sound quality?

2. Any way possible of estimating (guessing) where room gain occurs?

3. Would the length of 2 ports be half each of the total for 1 longer port?

1. MTM centers laying down suffer from lobing in the horizontal that can cause a loss of midrange output when the listener is off axis.

2. Room gain starts at the frequency where the largest room dimension defines the wavelength. It's not a hard and fast definition since rooms are rarely sealed up, with openings and such, that will reduce the effect.

3. If all the ports are the same diameter, then two will need to have double the length of a single port to keep the same tuning.

[the kid]

Re: 3 questions...

1. Thanks for the confirmation
2. I believe you. I just don't understand it yet
3. Drat!!!!

(thanks guys)

[benchtester]

Re: 3 questions...

3. Would the length of 2 ports be half each of the total for 1 longer port?

I'm not sure if you mean half the function or half the length. If you need shorter then multiple ports of smaller diameter makes sense. The goal is to tune to the right frequency and have enough port area that the air velocity isn't too fast and noisy.

The easiest way to calculate is to use programs like ajvented or winISD. They allow you to pick the number of ports and calculate the port length (diameter I think is also an input).

[Max_Andrews]

Re: 3 questions...

Multiple ports won't help with length because for the same tuning you need the same cross sectional area and length. So two ports, each half the area of a single, larger port, will still need to be the same length. What using multiple ports CAN offer you is increasing the resonant frequency significantly, to move it beyond the passband of the woofer and reduce any resulting midrange grunge you might get. This is more of an issue in bottom end woofers for a 3-way that are used up past 300hz than subs. A large port for a sub might have a 250hz resonance, which is already outside the passband. But the same resonance in a 3-way woofer used to 350hz would cause some midrange issues, so halving the port area and using two of them would get your resonance up to 500hz, which would be better as it's out of the passband and therefore will not be heavily excited.

Edit: actually, winisd doesn't show any resonance change when using multiple ports of the same length and area, so multiple ports does not help with resonance, therefore I can't see any benefit at all (unless you need a larger area that's non-standard, for example two 3" ports instead of one 4.5" port).

[mattk]

Re: 3 questions...

One way to handle a horizontal cc is to do a wmtw, like this one with 2XRS150-4 woofers, a Morel MDM-55 mid and a Vifa d26 for the high end. I had started this as a much smaller box to fit on top of a rear projection lcd tv, but the light engine started to go bad so I bought an lcd flat panel. Obviously on top wouldn't work, but having a cc underneath helped raise the tv to a better viewing height. Still, I didn't want it too high, so at 6.5" high, this is about as short as I could go with speakers that had at least some bass. The box is much bigger dimensions than the actual speaker enclosure so that the tv's base would be fully supported. Beats the heck out of the cheap horizontal mtm infinity that I had sitting in front of the tv before I finished this.

[Max_Andrews]

Re: 3 questions...

One way to handle a horizontal cc is to do a wmtw, like this one

Indeed, this is a nice solution because the midrange and tweeter are still vertically aligned giving you great off-axis performance, but keeping close to the footprint of a horizontal center channel. You also get better sound in general because you have a dedicated midrange (provided you employ it correctly). An even slimmer way to do it would be a quad of 4" woofers in a WWMTWW, with the "MT" section in the middle being a little 3" tang band wide-range driver with a dayton neo stuck right above it. Cross at 350 to the mid, 2700 to the tweet. Total box height about 5"

4x Hivi M4n woofers, a tang band W3-881SJ, and the ND20 would do well.

[the kid]

Re: 3 questions...

Thanks guys. ( I like Max's explanation ).

Matt - I assume those are ports in the front? the camera makes them appear to be just a hole. Good idea turning it into a tv stand.

The issue I have with my current cc is I can't seem to keep myself noticing the sound coming from 'it' rather than 'the front'. I am not sure that beaming is a correct description, but at least in my opinion the sound isn't quite in the right location. Could be the cc quality itself. I have been using an Optimus Prime xx I mean an Optimus Pro CS-5 # 40-4065 (1997). Best at the time. Got it re-furbed for $45. Thought that was great.

Currently I have it over the top of my projector screen (no option for bottom mount). But, I have it greatly angled down trying to keep that sound field at the screen and not directly at me or over me.

[mattk]

Re: 3 questions...

Yes, those are ports, each 1.5" diameter X 4" long. Without a flash they are just black holes. In fact it is dark enough in the room most of the time you can't even see the drivers, dark enough that my wife hasn't bugged me about putting a grill on yet, but I did mount neodymium grill magnets just in case.

[mattk]

Re: 3 questions...

If you have a sonically transparent projector screen, then the best solution would be a vertical mtm behind the screen with the tweeter in the center of the screen. Of course, those screens tend to be a bit pricey.

[dbe]

Re: 3 questions...

[QUOTE=the kid;1578715]2. Any way possible of estimating (guessing) where room gain occurs?

If you want to figure out room gain and the math doesn't freq (pun, here) you out, here is a link for you.

http://www.silcom.com/~aludwig/Physics/Main/Cavity_modes.html#Room_resonant_frequencies

This site is pretty good for fundamental understanding of room acoustics without being overly technical.

Here is the parent link to Al's site:

http://www.silcom.com/~aludwig/Physics/Main/Physics_of_sound.html#Physics_contents

There are also boundary effects that apply to driver to boundary (wall, ceiling, floor) distances which determine the relative smoothness or lack thereof of LF room response. These also play on the overall room response and room gain.

Dave

[the kid]

Re: 3 questions...

I considered the acoustic screens, but then budget reality set in and I couldn't do it.

Thanks for the links Dave.

[dbe]

Re: 3 questions...

I considered the acoustic screens, but then budget reality set in and I couldn't do it.

Thanks for the links Dave.

What kind of problem are you trying to solve?

Dave

[the kid]

Re: 3 questions...

What kind of problem are you trying to solve?

Dave

Some tonal quality, but mostly directional issues. Again, to me it seems the center sound stage (at least dialog-wise) is coming a little too much from 'up there' where the speaker is. I have a built in 5 band eq just for the cc that I can set with the receiver. It helps. But then I could lose some sound preference when trying to fix this.