Front vs rear port?

[KenM10759]

PMC does some very good things with transmission line & front ports. I'm dying to hear the newer Twenty5 series.

 

[TLS Guy]

PMC does some very good things with transmission line & front ports. I'm dying to hear the newer Twenty5 series.

The physics of a port in a TL like PMC offer is totally different from a ported tuned box. The TL port if the line has a 3:1 taper will be one third of the area of the high pressure closed end. It is the opening of a specialized Gedeckt pipe and at the port there is an node of pressure and an antinode of displacement. The area is large, and there is never any chuffing, and since the pipe is folded and critically damped there is no midrange radiation. The whole thing is totally different from the Helmholz resonators under discussion here. The TL in its correct iteration, not mass loaded, is a stopped pipe, of the Gedeckt family.

 

[Bucknekked]

It is the opening of a specialized Gedeckt pipe and at the port there is an node of pressure and an antinode of displacement. The whole thing is totally different from the Helmholz resonators

TLS Guy
Dang ! Gedeckt pipes, anti-nodes and Helmholz resonators. All in one paragraph.
If I knew what any of those 3 things were, I'd be impressed as hell with myself.
As it is, I tip my hat to you for understanding what they are AND being able to use them in a sentence.

I have the Salk Songtowers which I believe are modified TL cabinets with a circular rear port. So they may be in your subject area. I'm happy with them and I've never heard the ports make any noise.

 

[3db]

My first foray into HT was a total Technics packaage which included a pair of SB-T200s which had both a front and back port as shown;

 

[TLS Guy]

TLS Guy
Dang ! Gedeckt pipes, anti-nodes and Helmholz resonators. All in one paragraph.
If I knew what any of those 3 things were, I'd be impressed as hell with myself.
As it is, I tip my hat to you for understanding what they are AND being able to use them in a sentence.

I have the Salk Songtowers which I believe are modified TL cabinets with a circular rear port. So they may be in your subject area. I'm happy with them and I've never heard the ports make any noise.

Yes, you have a hybrid speaker. You have an abbreviated pipe terminating in a tuned cavity which is a Helmholz resonator. This was popularized by Martin J. King. You end up with a smaller enclosure and do extend F3 downwards a bit compared to a traditional reverse tapered TL

So you have variations on this theme, and there are a lot.

However this type of design behaves much more like a ported resonant reflex speaker than a closed pipe. The volume of the enclosure has a lot more impact on tuning than length. There are two peaks of impedance just like any other ported enclosure. The roll off below F3 is 24 db per octave, just like a reflex enclosure. So this is a resonant design.

This is the impedance curve of a mass loaded TL (Red Line).

A pure Gedeckt TL looks like this.

And this.

This is the impedance curve of the first line.

This the second line.

The first TL I showed you is the smaller line of my dual TLs, the second my center speaker.

This is the impedance curve of the larger line in the dual TL speakers.

Note in each case there is only one tuning peak of impedance.

The peaks of impedance of the dual lines are 48 Hz and 27 Hz, about half an octave apart.

The two combine to give this.

Since response is extending out to 20 Hz you can't see that roll off is second order 12db per octave.

However you can see it on the center line.

You can see that F3 is 44 Hz, and at an octave lower at 22 Hz response has only dropped 12 db.

In addition once the pipe is sufficiently damped to get one tuning peak and not two, then the speaker is aperiodiodically damped and non resonant.

Since we are talking ports.

This is the port of the shorter line.

The larger lines have a port on each side at the top.

The object is to get a clean but solid powerful bass without any hangover, that is highly articulate.

 

[Bucknekked]

Yes, you have a hybrid speaker. You have an abbreviated pipe terminating in a tuned cavity which is a Helmholz resonator.
The object is to get a clean but solid powerful bass without any hangover, that is highly articulate.

Mark
As usual, you have the most detailed and annotated replies that just continue to amaze.
What I got from that is I am the proud owner of a Helmholz resonator hybrid speaker that's clean, articulate and doesn't suffer from hangovers. That's a good match for me since I havn't had a hangover since I got out of the Air Force 40 years ago. And I try to be articulate.

Now, I have to use Helmholz regulator in a sentence 3 times or I will forget the word. I shall practice on other unsuspecting members of the forum.

 

[killdozzer]

Mark
As usual, you have the most detailed and annotated replies that just continue to amaze.
What I got from that is I am the proud owner of a Helmholz resonator hybrid speaker that's clean, articulate and doesn't suffer from hangovers. That's a good match for me since I havn't had a hangover since I got out of the Air Force 40 years ago. And I try to be articulate.

Now, I have to use Helmholz regulator in a sentence 3 times or I will forget the word. I shall practice on other unsuspecting members of the forum.

Hurry up!! You already called it a regulator.

 

[Bucknekked]

Hurry up!! You already called it a regulator.

at least I aint pretending to know what it is. I'm just trying to get it in a sentence before I forget what a Chemowitz farfinator is. A mind is a terrible thing to waste

 

[PNWer]

What are the pros and cons of either option?

I get that there is little or no difference if a subwoofer is down-firing, front firing, or side firing, but do the frequencies of the typical speaker port not matter that it is firing into the back wall?

I am planning to order a pair of Philharmonic BMR's and want to make sure I choose wisely!

Also, might as well throw in the option of plugging the port (a sub will likely be in play).

I own 3 Philharmonic BMR, one with front port.
From over 10 feet away I do not hear any port noise from front port (even at concert levels), nor do I hear any sonic difference with the rear ported pair.

As I already want the Salk cabinets, additional $50 upcharge for front port was not significant.

Philharmonic speakers come with speaker terminals sticking out the rear (about 5/8")