front porting vs rear porting

[steveksux]

Now lets get a bit more realistic. Some people are in better shape than others, some are more naturally athletic etc..... so they all reach the wall at a different time. That's Group delay

Sounds like the same thing as phase alignment? Different term for same thing, or 2 similar but different things?

The port's going to put out a delayed signal compared to front wave since its a longer path, and the constant time shift from the longer path results in various phase shifts to different freqs since each freq has its own wavelength. So many ms delay might be 1/2 wave at 40 hz (causing cancellation) but would be a full wavelength at 80Hz and reinforce the front wave.

Randy

 

[metron]

When you hear bad group delay at a live show its about the worst thing. I saw a band one time at a large outdoor ampitheater (red rocks) and we were way in the back hanging out. The bass notes were getting to us about a full second after everything else and it was awful.

 

[Petebass]

When you hear bad group delay at a live show its about the worst thing. I saw a band one time at a large outdoor ampitheater (red rocks) and we were way in the back hanging out. The bass notes were getting to us about a full second after everything else and it was awful.

Be careful not to confuse Group delay with plain old bad acoustics. What you experienced was Mid and high frequencies beaming forwards while the low frequencies bounced around the ampitheatre for while before they hit your ears.

I guess if we pretened the ampitheatre is a giant speaker cabinet, we get an idea of how group delay happens.

As an example, lets say a cab is tuned to say 40Hz. The frequencies at and around 40Hz bounce around the cab looking for the port(s) and leave a few milliseconds later than the other frequencies. So obviously if the cab produces a lot of 40Hz, then all this activity might create a bit of mud in your sound. If you speaker is a typical 15" PA and has already started rolling off at 40Hz, then there's no relationship between the group delay and the frequency response, so the sound will be percieved as tighter.

So you're probably thinking "well just tune every cab really low so the group delay happens at a frequency that's lower than what we'll be reproducing". That doesn't work. It destroys the cabs frequency response. It'll rob so much bottom end that it'll be useless for bass guitar. Hence begins the cab designer's balancing act of tradeoffs.

 

[metron]

i thought it was group delay because that venue is known for having wonderful sound. thanks for the info though!

i have always wanted to ask... is that goony goo goo as in delirious?

 

[Petebass]

i have always wanted to ask... is that goony goo goo as in delirious?

Yep! We're showing our age

 

[fdeck]

since port placement has nothing to do with a "tighter' sound... what is the main factor in tight bass response? i know sealed cabs have tighter response but what gives a ported cab tighter response?

Based on my DIY speaker building experience, I think the main factor in tight response is limiting the lowest frequencies. Small sealed cabs do this automatically. With ported cabs, you have to avoid getting too greedy for low end response. Doing either of these things also seems to correlate with reduced group delay.

One reason to avoid rear ports is that they are not protected by the grille, and as a result, things can fall in such as childrens' toys, small cats, and so forth. Also, as others have pointed out, it limits your choices as to speaker placement.

 

[Petebass]

.......as others have pointed out, it limits your choices as to speaker placement.

A little bit. Basically don't obstruct the port. So don't put the cab right up against a back wall.

The generally accepted rule is to keep as much from the distance from the wall as the diameter of the port. i.e. 3inch port, then keep the cab 3" from the wall.

I challenge this though. Why? Because to me, cabs with larger ports have less air pressure (wind) passing through and therefore blow the air to a lesser overall distance. I've proven this accidentally with a DIY cab I built recently that had 2 x 2" ports originally (front ported). I was using small ports trying to save space and make the cab as small as possible. It backfired because I could feel the wind from the ports hitting my face from 4 feet away. In other words, if this was a rear ported cab, I would have had to put it at least 4 feet from a wall, contradicting the above generally accepted statement that 2 inches would have been enough.

I redesigned the cab with a 4" port at the same tuning frequency. To do this I had to port it at the rear because there just wasn't enough room for a 4" port on the front baffle. Now I can't feel any wind unless I put my hand within 1 or 2 inches of the port. This is much better. I can this rear ported cab 2" from the back wall and not have any dramas. to me that's pretty much right up against a back wall.

Oh and for the record. Moving the port from front to rear produced no noticable change to the midrange.

 

[metron]

Yep! We're showing our age

I have to say that when it came out I wasnt allowed to see it. "If you dont like it..."

 

[fdeck]

I can this rear ported cab 2" from the back wall and not have any dramas. to me that's pretty much right up against a back wall.

Thinking about it a bit more, I agree. The reason is that the effective port area in the air space between the cab and the wall is large compared to the port itself.

Hmmm... maybe I won't be so concerned about rear-porting any more.