What is to be gained by using a larger box volume? In some speakers, I
estimate the internal box volume of 2-3 cu feet for 12" drivers, some even
less. Some designs call for 4 cu ft. or even more. In ported boxes I see the
port can be shorter for a given frequency and the efficiency may be higher,
what else can I expect?

I plan to build a pair of speakers. My design allows for 4 cu ft, but can
make the box shallow or deeper as necessary. I need to select the drivers
and get a book to help with calculations. What driver parameter values
should I take into consideration when designing a larger box? Should I be
using PA drivers (they tend to get costly)? This may help me narrow down the
drivers to choose from.

I plan on tuning the port in the range of 45-50 Hz. Hopefully I can get
extention down to 35Hz. The goal is strong bass in the 40-80 Hz range.

Thanks for your advice. -S

"SimonLW" wrote in message


What is to be gained by using a larger box volume?

Other things being equal, a larger box either has deeper bass or higher
efficiency.

In some speakers, I estimate the internal box volume of 2-3
cu feet for 12" drivers, some even less. Some designs
call for 4 cu ft. or even more. In ported boxes I see the
port can be shorter for a given frequency and the
efficiency may be higher, what else can I expect?

See above.

I plan to build a pair of speakers. My design allows for
4 cu ft, but can make the box shallow or deeper as
necessary. I need to select the drivers and get a book to
help with calculations.

The magic words are "Thiel Small Parameters". The Thiel Small parameters
for a drive can help you determine the ideal box for your application.
Online and freeware calculators and tutorials abound.

What driver parameter values
should I take into consideration when designing a larger
box? Should I be using PA drivers (they tend to get
costly)? This may help me narrow down the drivers to
choose from.

So-called PA drivers, if of high quality tend towards far higher efficiency,
loudness capacity, but only moderate bass response. JBL, B&C, Beyma and
some of the higher end Eminence drivers fit into this category.

I plan on tuning the port in the range of 45-50 Hz.
Hopefully I can get extention down to 35Hz. The goal is
strong bass in the 40-80 Hz range.

The magic words are "Thiel Small Parameters".

When it comes to bass, the important parameters are diaphragm size, Xmax,
and to a lesser degree, efficiency.

In article , "SimonLW" wrote:
What is to be gained by using a larger box volume? In some speakers, I
estimate the internal box volume of 2-3 cu feet for 12" drivers, some even
less. Some designs call for 4 cu ft. or even more. In ported boxes I see the
port can be shorter for a given frequency and the efficiency may be higher,
what else can I expect?

You get different alignments using different volumes. An efficient driver
normally requires a bigger box. The lower the resonance
the driver has, the bigger the box.

I plan to build a pair of speakers. My design allows for 4 cu ft, but can
make the box shallow or deeper as necessary. I need to select the drivers
and get a book to help with calculations. What driver parameter values
should I take into consideration when designing a larger box? Should I be
using PA drivers (they tend to get costly)? This may help me narrow down the
drivers to choose from.

Well getting a book is a step in the right direction. A computer program
is invaluable. Start plugging in drivers and values, and you will see
what comes up with different numbers.

I plan on tuning the port in the range of 45-50 Hz. Hopefully I can get
extention down to 35Hz. The goal is strong bass in the 40-80 Hz range.

Thanks for your advice. -S

Whats your application?

greg

SimonLW wrote:
What is to be gained by using a larger box volume? In some
speakers, I estimate the internal box volume of 2-3 cu feet for
12" drivers, some even less. Some designs call for 4 cu ft. or
even more. In ported boxes I see the port can be shorter for a
given frequency and the efficiency may be higher, what else can
I expect?

Well, unfortunately, you can expect that if you build speakers
based on your assumptions above, they will not work as you
expect them.

A lot of misconceptions abound over the role of enclosure volume
and ports and such. Many of them are based on a fundamental
misunderstanding of the interaction of several very important
speaker system parameters: enclosure volume, efficiency and
cutoff frequency. This relationship is stated in a fairly
well-known equation:

n0 = kn Vb F3^3

where

n0 = reference efficiency
kn = efficiency "constant"
Vb = enclosure volume
F3 = low frequency cutoff

Now, one thing that the equation, as it is stated, suggests, and
one thing that many people assume (as you did), is that the
dependent variable is efficiency, n0, while all the others are
independent variable. Thus, it seems to make intuitive sense
that if you increase the volume Vb and maintain the same F3 and
kn, then efficiency goes up. In other words, put the driver in a
bigger box and get more efficiency.

Unfortunately, it doesn't work that way. For any given driver,
n0 is fixed, and changing any of the other variables will NOT
change it. What will happen is simply changing the volume for a
given driver will change BOTH F3 and kn. kn is where the key to
this seeming conundrum lies: it's value is dependent upon a
number of system parameters, and those system parameters CHANGE
when you change, for example, the enclosure volume.

Also, it seems intuitively obvious that putting a port in a
speaker or changing the port dimensions will change efficiency,
and this is wrong as well: doing so also changes F3 and kn such
that n0, for a given driver, remains constant.

What the relation shows is how these four parameters in a SYSTEM
(driver and enclosure) are interelated, and tell you what's
possible, not what is. They are most useful when they are
rearranged. In the form above, what it tells you is what's the
efficiency you could expect from a given volume, cutoff
frequency and system alignment. Now, you have to go find a
driver of that efficiency (and other parameters) to meet that
goal.

In another form:

Vb = n0/(kn F3^3)

this tells you that for an efficiency of n0, an alignment that
results in some kn and a needed cutoff frequency, what's the
smallest volume that could possibly be used to achieve it.

Another variation:

kn = n0/(Vb F3^3)

will tell you what system alignment is needed to achieve a given
efficiency, boc size and cutoff frequency.

Finally:

F3 = cuberoot(n0/(kn Vb))

tells you what the cutoff frequency would be for a given box
size, alignment and efficiency).

I plan to build a pair of speakers. My design allows for 4 cu
ft, but can make the box shallow or deeper as necessary. I need
to select the drivers and get a book to help with calculations.
What driver parameter values should I take into consideration
when designing a larger box?

All relevant parameters: WHat sort of system response do you
want and what sort of enclosure configuration (sealed of what
Qt?, reflex with what alignment parameters? etc.)? WHat
efficiency do you require? You need to know effectively the
complete Thiele-Small parameter set for the drivers to even see
if they are suitable for the enclosures.

Should I be using PA drivers (they tend to get costly)? This
may help me narrow down the drivers to choose from.

Why, what properties of PA drivers do you think have advantages
for your application? In general, PA drivers are not well suited
for subwoofer usage: they tend to have light cones to maximize
efficiency for a given motor structure, stiff suspensions to
lower mechanical damage potential, but more importantly to make
them usable in poorly designed, often open-back enclosure.

I plan on tuning the port in the range of 45-50 Hz.

Bad idea. Deciding where you;re going to tune the port without
having any idea of the properties of the system or the driver
you're going to use is about the worst way to design a reflex.

The optimum way is to decide what you need for efficiency and
cutoff frequency and what you have for enclosure volume. Or
conversely, constrain cutoff frequency and alignment, you
determine what efficiency you need. From the values for the
alignment type, cabinet volume and efficiency, it then tells you
what the required electromechanical parameters of the driver
need to be. Then you go out and see if you can find a driver
that has those parameters. Likely you won't find anything on the
first try, so you'll need to go back and change your
assumptions by changing your system, parameters and see what
pops out for new driver specs. And so on, iteratively, until you
end up wioth something that's reasonable.

Conversely, if you have drivers in mind, you have to know their
parameters, and from that your system efficiency is set. Now,
from the parametersm you'll derive cabinet volume and tuning
frequency (if appropriate).

Hopefully I
can get extention down to 35Hz. The goal is strong bass in the
40-80 Hz range.

From a tehcnical standpoint, your requirements don't mean much,
and you're trying to get a technical answer to your question.
What do you mean "strong bass? A prominent rise in output
response over that reagion? Good flat response? High power
handling? And what does "extension down to 35 Hz" mean? Is that
the F3 response? Flat to 35, rolling off under that? Giving
different answers to these questions means sometimes very
different designs.

"SimonLW" wrote in message
...
What is to be gained by using a larger box volume? In some speakers, I
estimate the internal box volume of 2-3 cu feet for 12" drivers, some even
less. Some designs call for 4 cu ft. or even more. In ported boxes I see
the port can be shorter for a given frequency and the efficiency may be
higher, what else can I expect?

I plan to build a pair of speakers. My design allows for 4 cu ft, but can
make the box shallow or deeper as necessary. I need to select the drivers
and get a book to help with calculations. What driver parameter values
should I take into consideration when designing a larger box? Should I be
using PA drivers (they tend to get costly)? This may help me narrow down
the drivers to choose from.

I plan on tuning the port in the range of 45-50 Hz. Hopefully I can get
extention down to 35Hz. The goal is strong bass in the 40-80 Hz range.

Thanks for your advice. -S

Thanks for all the information. I found a pair of drivers (not PA) that had
good efficiency (95db 1w/1m). The manufacturer recommends they are used in a
vented enclosure. using TS parameters and formulas I found in a book, the
box volume should have been 2.8 cu. ft. Next I built a test box that I could
vary the volume buy stacking solid objects in it. I connected my volt meter
to another speaker across the room and tested different port lengths and
different box volumes by playing sign wave tones from my function generator.
This setup was not for testing frequency response because this would vary
depending on the room and response and the speaker the meter is connected
to. It would give an indication of the performance of the different volumes
and port tube lengths at a given frequency. After the tests, I went with a
volume of 4 cu. ft. and a port length of around 2.5 inches. I'm getting
strong response down to 40Hz and still fairly strong to 30Hz. After
listening tests, the bass sounds smooth across the range. One problem I've
noticed is turntable rumble in the MP3s I encoded from my turntable. The
previous speaker peaked then rolled of quickly below 50Hz so I never heard
that.

I'm still tinkering with the midrange and teeters. I'm not quite happy with
the sound yet.
-S

If you got a woofer with an efficiency of 95dB @ 1W @ 1M, good luck finding
a mid range and a tweeter to match! You might want to look at horns.


"SimonLW" wrote in message
...
"SimonLW" wrote in message
...
What is to be gained by using a larger box volume? In some speakers, I
estimate the internal box volume of 2-3 cu feet for 12" drivers, some
even
less. Some designs call for 4 cu ft. or even more. In ported boxes I see
the port can be shorter for a given frequency and the efficiency may be
higher, what else can I expect?

I plan to build a pair of speakers. My design allows for 4 cu ft, but
can
make the box shallow or deeper as necessary. I need to select the
drivers
and get a book to help with calculations. What driver parameter values
should I take into consideration when designing a larger box? Should I
be
using PA drivers (they tend to get costly)? This may help me narrow down
the drivers to choose from.

I plan on tuning the port in the range of 45-50 Hz. Hopefully I can get
extention down to 35Hz. The goal is strong bass in the 40-80 Hz range.

Thanks for your advice. -S

Thanks for all the information. I found a pair of drivers (not PA) that
had
good efficiency (95db 1w/1m). The manufacturer recommends they are used in
a
vented enclosure. using TS parameters and formulas I found in a book, the
box volume should have been 2.8 cu. ft. Next I built a test box that I
could
vary the volume buy stacking solid objects in it. I connected my volt
meter
to another speaker across the room and tested different port lengths and
different box volumes by playing sign wave tones from my function
generator.
This setup was not for testing frequency response because this would vary
depending on the room and response and the speaker the meter is connected
to. It would give an indication of the performance of the different
volumes
and port tube lengths at a given frequency. After the tests, I went with a
volume of 4 cu. ft. and a port length of around 2.5 inches. I'm getting
strong response down to 40Hz and still fairly strong to 30Hz. After
listening tests, the bass sounds smooth across the range. One problem I've
noticed is turntable rumble in the MP3s I encoded from my turntable. The
previous speaker peaked then rolled of quickly below 50Hz so I never heard
that.

I'm still tinkering with the midrange and teeters. I'm not quite happy
with
the sound yet.
-S

"James Lehman" wrote in message
...
If you got a woofer with an efficiency of 95dB @ 1W @ 1M, good luck
finding
a mid range and a tweeter to match! You might want to look at horns.


I got a midrange horn and a horn tweeter. The midrange horn is way too hot,
so I'm tinkering with it now. I mellowed it down with a 10 ohm series
resistor and adjusting the crossover capacitor accordingly. Sounds better
for PA use, but I'm going to add a switch to mellow them more for home use.
-S

An L-Pad is made just for this purpose. It has two variable resisters ganged
together on the same shaft. One of these resisters is in series with the
driver to reduce the power and the other is in parallel to compensate for
the increase in resistance of the other. The idea is that 8 ohms is
maintained.


"SimonLW" wrote in message
...
"James Lehman" wrote in message
...
If you got a woofer with an efficiency of 95dB @ 1W @ 1M, good luck
finding
a mid range and a tweeter to match! You might want to look at horns.


I got a midrange horn and a horn tweeter. The midrange horn is way too
hot,
so I'm tinkering with it now. I mellowed it down with a 10 ohm series
resistor and adjusting the crossover capacitor accordingly. Sounds better
for PA use, but I'm going to add a switch to mellow them more for home
use.
-S