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Relocating Speaker Ports

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Anonymous
March 24, 2005 10:51:10 PM

Archived from groups: rec.audio.pro (More info?)

Hi Friends,

I have a pair of Tannoy PBM-8s which are right up against the wall
because of the size of my room. Can I improve on this situation
somewhat by moving the rear-firing ports to the front on the cabinets?
Assuming I use the same size and length port, any major downside,
soundwise, to doing this?

Regards,
Joe
Anonymous
March 24, 2005 10:51:11 PM

Archived from groups: rec.audio.pro (More info?)

It seems to me there is some rule of thumb on port placement that says
it should be 2 to 3 times its diameter away from any obstacle, like a
wall or brace. It would seem that there is not much room on the front
available that would meet this criteria. Maybe on a side, if the port
is not too long...

As to phase and delay issues, I think if you look at the length of bass
waveforms, its placement should be a fairly minor impact.

Then again, I'm not sure I'd want to cut up my monitors...

--Peter

Joe Kramer wrote:
> Hi Friends,
>
> I have a pair of Tannoy PBM-8s which are right up against the wall
> because of the size of my room. Can I improve on this situation
> somewhat by moving the rear-firing ports to the front on the cabinets?
> Assuming I use the same size and length port, any major downside,
> soundwise, to doing this?
>
> Regards,
> Joe
Anonymous
March 24, 2005 10:51:11 PM

Archived from groups: rec.audio.pro (More info?)

In article <OSE0e.2980$gI5.2203@newsread1.news.pas.earthlink.net> musetrap@earthlink.net writes:

> I have a pair of Tannoy PBM-8s which are right up against the wall
> because of the size of my room. Can I improve on this situation
> somewhat by moving the rear-firing ports to the front on the cabinets?

I suppose you could, but the major downside is being against the wall.
Having the rear ports partially blocked is probably a good thing in
this case.


--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
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Anonymous
March 24, 2005 11:14:19 PM

Archived from groups: rec.audio.pro (More info?)

"Peter Sylvester" <peters_no_spam_please@not_here.org> wrote in message
news:D 1vi8r$oaq$1@newslocal.mitre.org...
> As to phase and delay issues, I think if you look at the length of bass
> waveforms, its placement should be a fairly minor impact.

Only the lowest frequencies are produced at any volume through the ports in
the first place, so I agree that phase issues would minor at least, possibly
not even measureable. Frequencies that are coming through both the ports and
the woofer may not be masked as much coing from the rear, but I would expect
would be closer to being in phase if the port was moved to the front. I
think there would be audible differences between the two placements, but
whether it was better or worse would be subjective.


> Then again, I'm not sure I'd want to cut up my monitors...
It would make more sense to replace the Tannoys for something that fit the
location better.

Sean
Anonymous
March 24, 2005 11:51:30 PM

Archived from groups: rec.audio.pro (More info?)

Joe Kramer wrote:
> I have a pair of Tannoy PBM-8s which are right up against the wall
> because of the size of my room. Can I improve on this situation
> somewhat by moving the rear-firing ports to the front on the cabinets?

It won't sound exactly the same, because at some frequencies there will
be significant sound coming from both the cone and the port, and moving
the port to the front alters their relative timing (the rear port has
extra delay due to the distance 'twixt front and back) so they will have
a different phase relationship.

--
Anahata
anahata@treewind.co.uk -+- http://www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827
Anonymous
March 25, 2005 1:08:43 AM

Archived from groups: rec.audio.pro (More info?)

anahata wrote:
> Joe Kramer wrote:
>
>> I have a pair of Tannoy PBM-8s which are right up against the wall
>> because of the size of my room. Can I improve on this situation
>> somewhat by moving the rear-firing ports to the front on the cabinets?
>
>
> It won't sound exactly the same, because at some frequencies there will
> be significant sound coming from both the cone and the port, and moving
> the port to the front alters their relative timing (the rear port has
> extra delay due to the distance 'twixt front and back) so they will have
> a different phase relationship.
>

Thanks for your reply. I won't have a problem with them if they sound
different; I will have a problem if they sound bad though. As far as
the phase relationship goes, there are plenty of speakers similar to
PBM-8s out there but with front-firing ports. Is the design of those
speakers (in general) really all that different, aside from the port
location?

Regards,
Joe
Anonymous
March 25, 2005 1:08:44 AM

Archived from groups: rec.audio.pro (More info?)

In article <LTG0e.3053$gI5.527@newsread1.news.pas.earthlink.net> musetrap@earthlink.net writes:

> As far as
> the phase relationship goes, there are plenty of speakers similar to
> PBM-8s out there but with front-firing ports. Is the design of those
> speakers (in general) really all that different, aside from the port
> location?

Have you looked inside the box to see how the port is constructed? (I
haven't) Typically, a port in a small speaker isn't just a hole in the
panel, there's a tube that projects into the box and establishes the
proper phase of the port signal (at least within the frequency range
of interest). When moving the port to the front, if you also moved the
tube, I'd think that it would work pretty much the same other than you
didn't have the delay of the port sound associated with the depth of
the cabinet.

You could certainly try it. The worst that could happen is that it
sounds worse and looks ugly when you put it back the way it was.

--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
Anonymous
March 25, 2005 1:08:44 AM

Archived from groups: rec.audio.pro (More info?)

Mike Rivers wrote:

> musetrap@earthlink.net writes:
>
> > As far as
> > the phase relationship goes, there are plenty of speakers similar
to
> > PBM-8s out there but with front-firing ports. Is the design of
those
> > speakers (in general) really all that different, aside from the
port
> > location?
>
> Have you looked inside the box to see how the port is constructed? (I
> haven't) Typically, a port in a small speaker isn't just a hole in
the
> panel, there's a tube that projects into the box and establishes the
> proper phase of the port signal (at least within the frequency range
> of interest). When moving the port to the front, if you also moved
the
> tube, I'd think that it would work pretty much the same other than
you
> didn't have the delay of the port sound associated with the depth of
> the cabinet.
>
> You could certainly try it. The worst that could happen is that it
> sounds worse and looks ugly when you put it back the way it was.
>
> --
> I'm really Mike Rivers (mrivers@d-and-d.com)

The smaller the box the more critical the size
_and length_ of the port is to the low end response.
Change this and your bottom will go away.
If you can duplicate the size and length of the port
when you relocate it, you should be OK.

good luck
rd
Anonymous
March 25, 2005 1:50:01 AM

Archived from groups: rec.audio.pro (More info?)

Joe Kramer wrote:

> Thanks for your reply. I won't have a problem with them if they
> sound different; I will have a problem if they sound bad though.

The Tannoys would have been designed with that delay included in the
calculations for flattest forward frequency response. If you move the
port and keep everything else the same, I doubt if the frequency
response will be so flat. Just how bad that will be, I really don't
know. Maybe not very different. Given I think you said you're in a small
room and putting the speakers with their back to the wall, that's not
the optimum position unless they are designed to be mounted that way,
and with rear ports they probably aren't. Your room is probably having
more effect on the overall response than which way the ports are facing.

> As far as the phase relationship goes, there are plenty of speakers
> similar to PBM-8s out there but with front-firing ports. Is the
> design of those speakers (in general) really all that different

If there's any truth in my theory, then yes - the optimum tuning for the
port will be different. You'd have to do the whole calculations (driver
Thiele-Small parameters, box volume etc.) to work out the best reponse
and that still might not be as good - I suspect that a rear-firing port
enables more extended bass than forward-firing for a given box size but
don't ask me for mathematical proof!

--
Anahata
anahata@treewind.co.uk -+- http://www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827
Anonymous
March 25, 2005 1:50:02 AM

Archived from groups: rec.audio.pro (More info?)

On Thu, 24 Mar 2005 22:50:01 +0000, anahata <anahata@reply-to.address>
wrote:

>If there's any truth in my theory, then yes - the optimum tuning for the
>port will be different. You'd have to do the whole calculations (driver
>Thiele-Small parameters, box volume etc.) to work out the best reponse
>and that still might not be as good - I suspect that a rear-firing port
>enables more extended bass than forward-firing for a given box size but
>don't ask me for mathematical proof!

I doubt it will make any difference as there's no parameter for port
location in any of those models.

Kal
Anonymous
March 25, 2005 4:33:38 AM

Archived from groups: rec.audio.pro (More info?)

On 3/24/05 2:51 PM, in article
OSE0e.2980$gI5.2203@newsread1.news.pas.earthlink.net, "Joe Kramer"
<musetrap@earthlink.net> wrote:

> Hi Friends,
>
> I have a pair of Tannoy PBM-8s which are right up against the wall
> because of the size of my room. Can I improve on this situation
> somewhat by moving the rear-firing ports to the front on the cabinets?
> Assuming I use the same size and length port, any major downside,
> soundwise, to doing this?
>
> Regards,
> Joe


As Mike said, Getting them away from the wall is good. There IS the option
of stuffing the port with foam or cloth. It WILL change the lo end but a
simple broad lo end tone control boost might compensate.
March 25, 2005 11:35:16 AM

Archived from groups: rec.audio.pro (More info?)

In article <1wJ0e.62175$Q83.43110@bignews5.bellsouth.net>,
"Sean Conolly" <sjconolly_98@yaaho.com> wrote:

> "Peter Sylvester" <peters_no_spam_please@not_here.org> wrote in message
> news:D 1vi8r$oaq$1@newslocal.mitre.org...
> > As to phase and delay issues, I think if you look at the length of bass
> > waveforms, its placement should be a fairly minor impact.
>
> Only the lowest frequencies are produced at any volume through the ports in
> the first place, so I agree that phase issues would minor at least, possibly
> not even measureable. Frequencies that are coming through both the ports and
> the woofer may not be masked as much coing from the rear, but I would expect
> would be closer to being in phase if the port was moved to the front. I
> think there would be audible differences between the two placements, but
> whether it was better or worse would be subjective.
>
>

There are reasons some monitors are only ported to the rear, of which
the manufacturer has already thought out. IMO It's not a very good idea.
In a high-end 2way speaker system, the midbass driver is regularly
crossed over as high as 3k. Changing the port location *may* make a
difference as there is sound resonating from the port after all.

As far as phase, once again changing port location *may* make a
difference. Without knowing the exact specifications of the drivers much
less the xo, of which the manufacturer will probably not disclose, who's
to say what can happen.

And on top of all this, the OP may or may not even notice a difference
if he changes it so all this would be moot.

> > Then again, I'm not sure I'd want to cut up my monitors...
> It would make more sense to replace the Tannoys for something that fit the
> location better.
>
> Sean
>
>

hth,

--
Cyrus

*coughcasaucedoprodigynetcough*
Anonymous
March 25, 2005 12:44:02 PM

Archived from groups: rec.audio.pro (More info?)

OSE0e.2980$gI5.2203@newsread1.news.pas.earthlink.net, "Joe Kramer"
>
> I have a pair of Tannoy PBM-8s which are right up against the wall
> because of the size of my room. Can I improve on this situation
> somewhat by moving the rear-firing ports to the front on the cabinets?
> Assuming I use the same size and length port, any major downside,
> soundwise, to doing this?

Blocking the ports is the LEAST of your worries with the speaker stuffed
up there. Don't you notice the low end is kind of... well... flabby?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
March 26, 2005 12:34:04 AM

Archived from groups: rec.audio.pro (More info?)

Hi All,

Thanks for all the thoughtful replies.

Scott Dorsey wrote:

> Blocking the ports is the LEAST of your worries with the speaker stuffed
> up there. Don't you notice the low end is kind of... well... flabby?
> --scott

I suppose I'd notice the low end was flabby if I had much opportunity to
hear other other rooms with my speakers, or other speakers in my room.
As it is, I don't. In this very subjective situation, the bass I'm
getting isn't too bad, but yes, maybe a little reserved dynamically.

I've already rebuilt the crossover with better parts and filled the box
with polyfill, both of which moves were marked improvements. The only
stock part now is the rather cheap and flabby woofer, which I also
intend to replace some day. There are some 4" thick foam panels on the
wall behind the speakers, and if as Mike says, the ports being blocked
might be a good thing considering the small size of my room, this foam
is probably helping filter some low end that would otherwise be an even
bigger problem.

You've all helped me come to the conclusion that I better put away the
hole saw for now. Instead I'll spring for some better woofers and see
how that works out. Thanks again for all your help.

Regards,
Joe
!