Consequence of 'small' port size in large mid-bass reflex enclosure.

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SteveRB

Hello,

I have re-built an Altec Valencia 846B. I now have the 15" 416-8z woofer in the 6.6 cuft (185 litre) enclosure with the horn on top of the box. I have been re-tuning the enclosure and have found that the 'best' response curve and impedance plots have come with moderately low stuffing and what seems to be a very small port: 4" diameter x .75" deep -- basically a 4" hole in the front baffle (much like the classic Altec utility cabinets). The system is tuned to around 35Hz.

The goal of this project was never to get full deep bass. Currently everything sounds punchy and clear.

I have been reading about general port sizing. And most everything I find states that 4" is too small stating port noise as the major issue.

What are the consequences of a small port for such a large woofer?

Impedance plot below.


SteveRB

Additional info:

two-way set up, crossed at 1200Hz. This is a music only system.

JohnR

I've not measured these effects : http://www.subwoofer-builder.com/port-compression.htm

From a quick sim using some data I found on the web for the woofer, port velocity does get quite high but at low frequencies (< 40 Hz) and it's pretty high SPL.

FullRangeMan

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What are the consequences of a small port for such a large woofer?
Small port, small bass, bass depend more on the box stuffing type and quantity used.


JohnR

What are the consequences of a small port for such a large woofer?
Small port, small bass, bass depend more on the box stuffing type and quantity used.

The response of a ported cabinet is determined by its volume, port length and area, and woofer parameters. The stuffing will have some effect but it's secondary.
« Last Edit: 21 Apr 2017, 12:17 pm by JohnR »

SteveRB

So, to summarize the articles above. The tuning of the system is reliant on the port area, port length and enclosure volume (stuffing and dampening effectively cheating the volume somewhat). However, air velocity and turbulence inside the port effect the compliance of a woofer's suspension. Based on those physical constraints, it is best to have a port area of a set minimum radius:

A bass reflex speaker should be matched in size with its port. Here are some recommendations on minimum vent size compared to speaker size :

1″ vent => 4″ speaker
2″ vent => 4″ or 5″ speaker (6″ max)
3″ vent => 6″ speaker (8″ max)
4″ vent => 8″ or 10″ speaker (12″ max)
6″ vent => 12″ or 15″ speaker


The above recommendations being calculated through some heavy math (not shown).

So even though I am getting decent plots and frequency sweeps with a short 4" opening, I am likely shorting the system due to the physical restraints and pressure put on the woofers suspension. Meaning... I need to double my port area and tune them again to length. 

Also, flaring both ends of the port should help at the extremes (both Hz and SPL) reduce turbulence further.

nickd

I tend to think the Altec engineers knew more about box tuning than the new computer models would imply. The factory tuning seems to match the woofers output very well. We are just used to modern over damped "audiophile" designs now. A bit of DSP room correction and classic Altec bass is a force to recon with. I have yet to hear a modern speaker that I prefer in that area.

JohnR

Hi Nickd, I guess that explains the response curve I got... the port looked too short.

Steve, bear in mind those are "typical" recommendations - the velocity depends on the tuning frequency i.e. a sub tuned to 18 Hz will be a very different thing to your woofers. And, it also depends on SPL. Interested to know how you go.

SteveRB

I tend to think the Altec engineers knew more about box tuning than the new computer models would imply. The factory tuning seems to match the woofers output very well. We are just used to modern over damped "audiophile" designs now. A bit of DSP room correction and classic Altec bass is a force to recon with. I have yet to hear a modern speaker that I prefer in that area.

I certainly want to keep the Altec sound as part of this design. The 416 is used in cabs from 6 to 10 cuft. The Model 19 is a great example of shallow port in huge box. I'm dealing with 6.5 cuft and want to get the most out of it. I think a lot of their smaller utility cabs are tuned high for the music of the time. Thinking that two 4" ports tuned to 30Hz-35Hz will get me where I want to be.

This is a great document.

http://www.lansingheritage.org/html/altec/plans/1974-enc-manual.htm


JRace

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FRM is right.
http://audiojudgement.com/bass-reflex-speaker-design/
What?

Your link contradicts FRM

Quote
But I am going to mention that (for a given speaker) the tuned frequency of the box (fB) will depend on 3 factors :
Length of the port
Radius of the port
Volume of the box (VB)

Quote
Damping material. Like with sealed enclosures, damping material can be used to reduce panel resonances and standing waves. Unlike the sealed enclosure, you don’t stuff the box with damping material, because you will obstruct the port. Just place 1-2″ absorbent material on the the walls of the enclosure (on one of each opposite sides or all of the walls).

Like JohnR says damping is secondary.

FullRangeMan

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The response of a ported cabinet is determined by its volume, port length and area, and woofer parameters. The stuffing will have some effect but it's secondary.
Correct, a woofer wont do midrange freq as a midrange driver.

roscoe65

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Correct, a woofer wont do midrange freq as a midrange driver.

Tell that to an Altec 414A.

lokie

Quote
Correct, a woofer wont do midrange freq as a midrange driver.

Uh? Tell that to Tannoy, Shindo, Altec, Western Electric and.....

In my DIY 2 way, I am putting on various size back panels to experiment w port sizes. The differences aren't much. Even compared to a sealed cabinet. In fact, I have been experimenting w one speaker sealed and the other (see picture) w a port. I can barley hear the difference and the imaging is rock solid.

In contrast, taking out the internal dampening (3/4" wool covering all the walls) and replacing w bracing was a huge improvement. Now, I only have dampening on the back panel. Still tinkering here.







.






JLM

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Some estimate that some very poorly braced cabinets (especially large unsupported open baffles) can produce more resonant spls than woofer.  But your bracing could be greatly improved by using window framing type design.  (Run bracing in a box shape lining the inside of the cabinet walls at intervals equal to the the cabinet width/depth and perhaps adding cross bracing or a sheet of plywood with a large hole cut out of it at midpoints of the box shapes.)

Agree with nickd on the overly constipated "audiophile" sound of many modern designs.

Agree with JohnR on the effectiveness of lining the inside of the cabinet but do keep in mind that half of the driver's sound radiates inside the cabinet and that the driver cone is the most sonically transparent material of the speaker, so some of that inside sound will come out via the cone in delayed manner, making for a smearing of the intended sound.  This is one of the advantages of using tapered pipes or transmission line cabinets that can redirect the sound away versus trying to absorb it with a thin layer of stuffing.

FullRangeMan

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Tell that to an Altec 414A.
Uh? Tell that to Tannoy, Shindo, Altec, Western Electric and.....

In my DIY 2 way, I am putting on various size back panels to experiment w port sizes. The differences aren't much. Even compared to a sealed cabinet. In fact, I have been experimenting w one speaker sealed and the other (see picture) w a port. I can barley hear the difference and the imaging is rock solid.

In contrast, taking out the internal dampening (3/4" wool covering all the walls) and replacing w bracing was a huge improvement. Now, I only have dampening on the back panel. Still tinkering here.
Why you guys dont quote the John post?
Many cone midrange drivers do good quality bass, but I suspect a woofer dont will do a nice 2kHz tone imo.

roscoe65

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Why you guys dont quote the John post?
Many cone midrange drivers do good quality bass, but I suspect a woofer dont will do a nice 2kHz tone imo.

I'll say it plainly:  you are flat-out wrong.  The Altec 414A will run clean 3-4khz.  In fact, you can run a 414A with no crossover and bring a tweeter in at 3kz - 7khz with a first order crossover.

There are a handful of exceptional drivers, but they may be out pf production or quite expensive now.

FullRangeMan

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I'll say it plainly:  you are flat-out wrong.  The Altec 414A will run clean 3-4khz.  In fact, you can run a 414A with no crossover and bring a tweeter in at 3kz - 7khz with a first order crossover.

There are a handful of exceptional drivers, but they may be out pf production or quite expensive now.
I dont know this 414 but it seems possible if its a vintage driver, these modern drivers sounds like midranges to me not woofers, in last years usually woofers dont go 1kHz with decent sound quality, unless they are quite small as 6 or 8''.

JohnR

I think I should have said "The low-frequency response of a ported cabinet is determined by its volume, port length and area, and woofer parameters."

Midrange response is a different matter. Current "pro style" drivers often go out to several kHz (on axis) e.g. here is Faital Pro 15PR400 (15" woofer, modeled -3dB at 40 Hz, 118 dB midband SPL without exceeding Xmax from 35 Hz up):



OTOH "hifi style" drivers generally don't.

FullRangeMan

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Thanks for inform, Faital are somewhat Ferraris of speakers, so far I have not seen it in regular pro audio makers as Beyma.