rear port vs front port

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I was going to use a rectangular port in the back of the speaker at the bottom. Instead of a round port in the front about 12 cm away from the bottom. They will have the same cross-section area and length. What are the draw backs? I can make a rectular port in the same place in the front 12 cm away from the ground if it sounds better or near the ground or anywhere.

This is the port I want to modify http://www.eton-gmbh.de/dlstream.php?FileId=71&PHPSESSID=585e438f5baea1ce3b05651eab49ad77
 
I like this question. Some people hate front ports yet it shouldn't really matter. I suppose there is potential to have mid frequencies audible through a front port but I've not had that problem. I'm also understanding that having a LF source near the floor will help so a low port is good.
 
A badly positioned / dimensioned port will exhibit pipe mode
resonances that simply shouldn't be there, minimal or badly
placed internal damping can contribute to the effect.

A bad port is better facing backwards.

A ports position has no influence at its bass resonant frequency,
except for a speaker designed to be up against a wall, where
postioning in the base of the speaker or the front is needed to
prevent port blocking.

:) sreten.
 
Mice! bah thats funny, reminds me of a story I read on the web about a mouse who had camped out inside sum1 PC, made a nice little home for itself. All the wires mouse arranged to make a nest thing, I think it also got other stuff in there too to make it more coasy problem is the mouse got stuck and died :( Bad smell.
 
I had some KEF R104/3 once. These are coupled cavity loading for the twin bass units, so the vent on the front is tuned to around 65Hz. This means that the vent was around 4" diameter and only 2 or 3" deep. I found that my cat would crawl inside and lay on the cone of the internal upward firing bass driver!!
That is until I realised that she was doing this. Then I waited for the moment I found her in there, and then I turned on some music.
She never went in again. I should say that I did not play the music loud. I would never be that cruel. Just loud enough to surprise her.

Andrew

p.s. on a technical note...rear firing vents, depending on the tuning frequency will have a slightly different response than a front firing vent because of the added delay, but this won't make a huge difference in practice. It will however couple very differently into the room resonances and this can make a significant difference to the in-room response.
 
horns???

I have a similar question (please don't accuse of stealing the thread), but I'm curious what the difference is when the mouth is on the front or back of horns. For example the Fostex recommended enclosure for the FE103 has a mouth in front, but the Buschhorn MK2 and Ed Schilling's horn have them in the rear. Wouldn't the output from the front horns be out of phase from the driver? It seems like the rear is the logical choice for these type of designs. I could be completely wrong as my knowledge of acoustics is limited.
 
All of the LF acoustic output is from the woofer driver at frequencies well above the vent tuning frequency of the box, that is, the F(B). At lower frequencies, specifically a narrow band of frequencies centered on the box tuning frequency F(B), all of the acoustic output is from the vent. Phase is not really an issue.
 
sreten said:
A badly positioned / dimensioned port will exhibit pipe mode
resonances that simply shouldn't be there, minimal or badly
placed internal damping can contribute to the effect.

A bad port is better facing backwards.

Okay I'm leaning toward the original port design (round 70 mm diameter by 110 mm and 12 cm from the ground). I couldn't find a 70 mm diameter pipe only a 3" or 76 mm, that's why I wanted to change the design. Also I think I will not like the sound of the port and keep it stuffed most of the time and a stuffed port in the front might look bad. Maybe I should just build it sealed. As you can tell I'm pretty bad at making decisions :D

Timn8ter said:

But mice will

Mice get in everything. I made some speakers with a rectangular port in the back near the ground and found a piece of pizza crust on top of the polyfil stuffing when I took the bottom woofer off. Also they ate or chewed a piece of rubber or foam above my air intake manifold in my car.
 
Timn8ter said:
All of the LF acoustic output is from the woofer driver at frequencies well above the vent tuning frequency of the box, that is, the F(B). At lower frequencies, specifically a narrow band of frequencies centered on the box tuning frequency F(B), all of the acoustic output is from the vent. Phase is not really an issue.

Oh yes, phase *is* an issue. But you are still right in that placement is not very important. It is the reverse phase of the port below Fb that makes the drop 24 dB/oct (compared to the closed box' 12 dB/oct). The contributions from the port and driver largely cancel. Above Fb there is a range where the port and driver are of similar magnitude and phase and of course the phase of two sources is important also here.

But:
There will not be much added phase difference due to port placement anyway due to the long wavelength in the bass. At 30 Hz the wavelength is ~10 m and any dimension for sane sizes of boxes will be smaller than this. The placement may have *some* effect but only to a degree corresponding to the wavelength/distance ratio.

Actually there was an article the other year in JAES that examined the effects of different placement of a port on a spherical cabinet.
As I recall it they found differences, but they were small.
 
If you're building a floorstander, it might be worth having the port away from the bottom. A port near floor level will excite the floor to ceiling room mode.

In our small room at the Bristol Show the bass varied tremendously depending on the height at which you listened. It was fine for those sitting down but very boomy for anyone standing up.

It might be worth bearing in mind, depending on your room.
 
Just to defy everyones logic on traditionally placed ports in reflex cabinets, I’ve been designing Octet floor standing versions for years....from massive to small...Motto... Low frequencies tend to <cling to the periphery>........With tuned port in base of enclosure offers a far better response distribution and equalisation. This cabinet sounds just as good away from the corner. The obvious advantages of far less internal reflections within cabinet gives better sound than others when not placed in corner. We should all know what happens when large LF speakers are put in room corners, the +3dB or more per side creates an overpowering LF response..some systems need it.

The only serious disadvantage is the time and dedication taken in making such octagonal shaped cabinets. The design requires much less internal bracing than normal rectangular types and using modern day portable angle circular saws with angular adjustments and a good workbench makes ideal.

Photo shows post construt stage, commence glueing Hartmann fiber egg boxes to sides. If one is going to use a 1st order crossover filter, (as I do) then some absorbent in the lower midrange is inevitable. Also a HP filter in the power amp below fb is necessary.

With a small vol .....as in photo...3.88Cuft/100lit....enables two 10“ bass units in same cabinet to provide a much more fuller sound than a single 12“ incher.... the bass can be felt and heard. This is how it should be.
With a Q of 1.2, and box res of 33Hz, the bass is not boomy but those who have heard it say it was <full >. The relationship with the tuned port and the room floor offers a longer path, so a shorter cabinet port with a big port area (15cm dia tube) can be used with low windage and distortion noises. The advantage using a base-tuned port on loose flooring or in flats is to shake the vermin and people from the building. In this example the port criteria i,e 2:1 (length shouldn’t exceed two width‘s).
Using Thiele/Small software doesn’t quite tell the whole story......in most parts yes.....the tuned port for floor loading must be made 20% longer to start with and trim it til the equal double hump appears on test equip. You will get boom if you don’t bring the lower peak up. This is a classic example of two smaller drive units out -performing a single driver, i.e single 12“ or 15" driver unit.
Some argue that a Q of 1.2 is to high, contributing to boom and tiring, but this isn’t the case. If you want to feel the lowest boom of the carnival drum, then your stomach will feel it from this system ...If you want purest classical, at a Q of 0.65 then bung up the port. Leaving it reflex on classical, it brings the cello’s and other low-end-stuff a bit up-front.
Anyone else done sim construction ????

Question: Anyone know why the larger low frequency instruments are placed back stage ?


VIVA 2 driver reflex.

rich
 

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I don't know if I should start a new thread for this?

But here goes.

I've desided to build a ML TL design using Martin King's Mathcad TL simulations. I want to post the simulated response, but I haven't figured how to save the mathcad FR as a jpg file, so I can post it here (it has a little 2-3 db dip at 110 Hz, might be a problem). Anyone know how?

Also I still have the problem of front or rear port? I haven't seen any ML TL's with a rear port which is what I'd like to use. I'm assuming that this port might behave as a regular ported box, but the driver/port interactions are much more wild (for lack of a better word, looking at the FR graphs) than a regular driver/port.

Also, this is somewhat insignificant, unless I made a mistake, but the Eton 8-800 driver seems not to work well at all with a open end TL enclosure and I'm assuming it's the specs of the driver.
 
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