Just as a thought, have you considered trying one or both
speakers (series or parallel) on an open baffle to see what
they sound like? If you are not wanting bass then a piece of
wood maybe a foot or so square might be a starting point?

I once tried it with a drawer from an old dresser which had
a thicker than might be expected floor panel. Worked
surprisingly well.



--
Woody

harrogate three at ntlworld dot com

Peter Chant wrote:
On 12/09/2014 12:22 PM, Phil Allison wrote:

Probably making a vented enclosure. Is it better put the drivers in a
single enclosure or to make essentially two separate enclosures side by
side? Can't really think of any pros and cons except to do the sums and
see if the box sizes are realistic.

** Long as the 4 inch drivers are closely similar, using a common
enclosure and one port is the way to go.


Two Audax AT100M0 drivers. Found data sheets on-line last night.
http://www.audax.com/archives/AT100M...gue%201994.pdf

I love the internet, that info would have been harder to get hold of
when I bought the drivers. Don't speak French so ringing them up might
not have gone well.

The Bose 802 ( pro audio version of the 901 ) used 8 x 4 inch drivers
and two ports - not that it is any shining example of design.

Seriously, one large port IS better than two smaller ones - less losses
and wind noise ie chuffing.

Less surface area of the port compared to its volume / area - makes
sense. Suppose front / back is unimportant. Back is probably neater if
I have one port only.


If you have the basic TS parameters for the drivers ( Vas,Fs and Qs) and
pop them into WinISD Beta you can easily see how altering the box volume
and port frequency changes things.

Don't run windows if that is a windows program. Probably could run it
under wine. There seem to be plenty of on-line calculators. I have a
few 'cookbook' type design books as well, so if I have the parameters I
could also try them.

Given that these two tiny 4in paper drivers are a lot smaller than the
twin 6.5in polypropylene ones in my mains and that the centre is running
speech not full range I wonder if I ought to try something else? I
wonder what happens if build some sealed enclosures which are slightly
larger than normal to give a Q of around 1 so there is no rise in the
bass before they roll off. Would that avoid boominess that I might get
with ported boxes or if I used a Q of 0.7?

http://www.diyaudioandvideo.com/Calculator/Box/

I've worked this out* for a _single_ driver.
(* got someone else's website to do it for me)

Sealed Box Q=1

Vas = 4.41 lts
fs = 64 Hz
Qts = 0.48
Qtc = 1
Dimensions
Vb = 0.05 ft3 = 1.32 lts
= L x W x H
f3 = 104.82 Hz
fb = 133.33 Hz

That seems rather

With the more normal Q of 0.707 I get:
Sealed Box Q=0.707

Vas = 4.41 lts
fs = 64 Hz
Qts = 0.48
Qtc = 0.707
Dimensions
Vb = 0.13 ft3 = 3.77 lts
= L x W x H
f3 = 94.28 Hz
fb = 94.27 Hz

Vb: Speaker Box Internal Volume
f3: 3dB Cutoff Frequency
fb: Enclosure Resonant Frequency

Annoyingly I can't find my books.

Website recommends a ported enclosure - perhaps I should follow that advice:
http://www.diyaudioandvideo.com/Calc.../SealedPorted/


Ported. Guess cone dia at 3.5in.
(can't be bothered to find a rule right now)
Ported Box

Vas = 4.41 lts
fs = 64 Hz
Qts = 0.48
D = 3.5 in
Dimensions
Vb = 0.28 ft3 = 7.83 lts
= L x W x H
f3 = 49.72 Hz
fb = 53.57 Hz
Dv = 1 in = 2.5 cm
Lv = 1.85 in = 4.53 cm


Or I could just read the datasheet which gives various box volumes and
port sizes underneath a chart showing the various frequency responses...
Volume anywhere between 1.7 and 10lt! I suppose the
manufacturerintended this to be used in ported boxes.

This is of course for a single driver. So I do need to do the sums for
a twin driver enclosure - unless I simply make two single driver boxes
side by side.

Assuming doubling Vas is all I need to do then running that through the
last website gives:
Ported Box

Vas = 8.82 lts
fs = 64 Hz
Qts = 0.48
D = 3.5 in
Dimensions
Vb = 0.55 ft3 = 15.65 lts
= L x W x H
f3 = 49.72 Hz
fb = 53.57 Hz
Dv = 1 in = 2.5 cm
Lv = 0.53 in = 1.26 cm

Hmm. Double the size but I do save 1/2p making a shorter port...

You could try a little series resistance on the woofers to push up the Qts
and be more suitable for closed box?

This probably will not help you, but here was my quest. 15 years ago ?
http://www.pitt.edu/~szekeres/center.htm

Greg

"Woody" wrote:
"Peter Chant" wrote in message
...
Chaps,

Just wondering if anyone has any experience. Given I have
some days off
I might do this.

Have a couple of 4" bass units and a tweeter kicking
around the garage
and the desire that _anything_ must be better as a centre
speaker than a
horrid injection molded plastic speaker that came with the
(oldish TV).


Probably making a vented enclosure. Is it better put the
drivers in a
single enclosure or to make essentially two separate
enclosures side by
side? Can't really think of any pros and cons except to
do the sums and
see if the box sizes are realistic.




Are you trying to achieve bass or just better/louder audio?

If you are wanting bass (and TV sound is as good - some say
better - than FM radio) then you might want to consider
using signal level filtering and then driving both speakers
with one amp in one enclosure. If you are wanting to achieve
better audio quality in stereo the two enclosures physically
separated and driven is a better choice.

Remember that ultimately it is all about moving air and up
to around 120Hz (some say 150Hz) there is little
directionality so a common so-called sub-bass unit is a good
option in this situation as each driver will only have to do
half the work.

One thing not to overlook: how will adjust volume levels? If
your TV has the option of feeding audio out at line level
that is volume controlled then you don't have a problem -
although you say it is an older TV so this is unlikely as it
is only a feature that has come in over the last maybe five
years or so with the larger takeup of home cinema. If this
is the case then you have to use the TV headphone output
with all its attendant noise and distortion which begs the
question of whether it is worth doing in the first place.

Assuming you have a LCD/Plasma TV even if it is old, have
you considered fitting deflectors under the (usually)
downward facing speakers to direct the sound to you viewing
position? You'd be surprised how much difference such a
small change can make.


Ha, I taped cardboard under my tv to reflect. It works.

Greg

Peter Chant wrote:


** If you put the two woofers in series, that will make the impedance 16 ohms and increase effective the amplifier power rating - then put say a 20uF cap across one of them. The impedance will drop towards 8ohms above 1 kHz and the no cap driver gets 6dB more level - just what you need to maintain flat response.

Interesting thought. Does that not kill the electrical damping from the
amp, as at resonance each speaker sees mainly the impedance of the other
speaker

** Nonsense.

The two drivers do not act independently so no such scenario can exist.


Just found my copy of the Loudspeaker Design Cookbook. Perhaps I ought
to re-read part of it.


** Read it all again if you ****ing like.

Won't mean a thing to a narcissistic, know nothing nutter like you.




.... Phil

On 12/10/2014 07:46 AM, gregz wrote:

You could try a little series resistance on the woofers to push up the Qts
and be more suitable for closed box?


Interesting. This might actually reduce the influence of the amp and
cables. I have one book which I cannot find which was obsessed with
measuring the amp impedance and matching the speaker to the amp.

This probably will not help you, but here was my quest. 15 years ago ?
http://www.pitt.edu/~szekeres/center.htm

Not far from what I have in mind. Thanks. Might go push-pull as there
seems to be pros and the cons are that they are a little ugly.

On 12/10/2014 06:15 AM, Woody wrote:
Just as a thought, have you considered trying one or both
speakers (series or parallel) on an open baffle to see what
they sound like? If you are not wanting bass then a piece of
wood maybe a foot or so square might be a starting point?


No. If I do that I'll have to put a foot on the bottom. Perhaps
sides... Actually if I put a foot on the bottom then the speakers are
near the screen and effectively the TV is part of the baffle. Apart
from making sure it does not fall off the TV there are few reasons not
to try this.

I once tried it with a drawer from an old dresser which had
a thicker than might be expected floor panel. Worked
surprisingly well.

On 12/10/2014 12:44 AM, Dave Plowman (News) wrote:
In article ,
Peter Chant wrote:
Likely MTM - unless there is a brighter idea. Got a large CRT TV at
present. Will likely look at size suitable to stand a likely LCD on it
for the day when the CRT breaks. Of course acoustic parameters are a
prime driver for box size as well.

You might find it a problem sighting a speaker with a powerful magnet
close to a CRT set.

The bass drivers are sheilded.

Peter Chant wrote:
On 12/10/2014 07:46 AM, gregz wrote:

You could try a little series resistance on the woofers to push up the Qts
and be more suitable for closed box?


Interesting. This might actually reduce the influence of the amp and
cables. I have one book which I cannot find which was obsessed with
measuring the amp impedance and matching the speaker to the amp.

This probably will not help you, but here was my quest. 15 years ago ?
http://www.pitt.edu/~szekeres/center.htm

Not far from what I have in mind. Thanks. Might go push-pull as there
seems to be pros and the cons are that they are a little ugly.

You can probably find programs that you can add series resistance and find
output results. I used an old dos program. A little resistance will not
change efficiency much but will change low end results.

Greg

"Peter Chant" wrote in message
...
On 12/10/2014 07:46 AM, gregz wrote:

You could try a little series resistance on the woofers
to push up the Qts
and be more suitable for closed box?


Interesting. This might actually reduce the influence of
the amp and
cables. I have one book which I cannot find which was
obsessed with
measuring the amp impedance and matching the speaker to
the amp.


I can understand that approach with a valve amp where there
is an output transformer involved, but matching with a
transistor amp? Eh? The main factor with a transistor amp is
having a power supply that could deliver the (often short
term) currents needed. JLH proved it could be done easily by
having a regulated PSU. In the '80s I built a dual mono
power MOSFET amp with regulated supplies a la JLH and tests
showed it would do 110W into 8R and 220W into 4R - which is
what it is all about. [I would still be using it today save
it has developed a dc offset problem on one channel and I
have never had the time to sit down and find out why.] I had
a pair of the Bailey designed transmission line speakers at
the time (rather like the Imhoff TLS80] and that could
rattle windows at 10 paces with that amp!

A few decades ago there was an obsession with getting the
amp output impedance as low as possible to increase the
damping factor, until someone - could have been JLH or Doug
Self or someone like that - proved that it is easy to over
damp a circuit. Indeed the lower the amp output impedance
becomes the more effect the resistance of the
interconnecting cable has which was why I believe there was
a move to direct amping. I believe it was only with
increasingly powerful computer modelling that it was
discovered that much could be achieved by better driver,
cabinet, and particularly crossover design and at the same
time more could be made of signal level filtration so bi or
tri-amping became popular and to an extent survives today
(although very little in the UK from what I read.) As a
result of the work on crossovers, in my very limited
experience you will often find a small series resistor on
the output of the bass section of a crossover.

Now I know this will be contentious, but from what I heard
years ago the Motional Feedback speaker marketed by Philips
did far more to achieve purity of (bass) sound than anything
I've ever heard. I remember going to one of the hi-fi shows
in Harrogate probably in the '70s. I walked into the
ballroom at the Old Swan Hotel (of Agatha Christie fame) as
I could hear what I thought was a brass band playing -
possibly Grimethorpe - so you can imagine my surprise when
all I saw was two MFB loudspeakers. Staggered was not the
word. The only reason that I can think they never took off
was (a) the price which compared with the price of some
so-called hi-fi kit these days would now be seen as cheap
and (b) because it was done by Philips who were not
perceived to have hi-fi capability. I often wonder what
would have become of the technique if the design had been
done by someone like an early Linn?



--
Woody

harrogate three at ntlworld dot com

In article , Woody
wrote:
"Peter Chant" wrote in message
...
On 12/10/2014 07:46 AM, gregz wrote:

You could try a little series resistance on the woofers to push up
the Qts and be more suitable for closed box?


Interesting. This might actually reduce the influence of the amp and
cables. I have one book which I cannot find which was obsessed with
measuring the amp impedance and matching the speaker to the amp.


I can understand that approach with a valve amp where there is an
output transformer involved, but matching with a transistor amp? Eh?

I'm not clear what "matching" means above. An engineer would probably
assume "arrange for the two impedances to be equal". However it may have
been used more vaguely, akin to "suit one another" in some other way.

The normal view is that the required output pressure-time pattern is
defined by the *voltage*-time pattern the amp asserts at the terminal of a
speaker system. Then its the job of the ampliier to supply what current the
speaker demands for that voltage to be applied.

The complications are of course, thing like the difference between speaker
units and complete speaker systems and effects of items like crossover
newtworks, etc, and how they are taken into account.

"Matching" in the "make impedances the same" respect seems odd given how
much the impedance of a typical speaker will vary with frequency. Also
likely to vary with other factors. I guess this is why people adopted the
presumption for "the voltage defines the waveform and you then supply the
demanded current".



Now I know this will be contentious, but from what I heard years ago
the Motional Feedback speaker marketed by Philips did far more to
achieve purity of (bass) sound than anything I've ever heard. I
remember going to one of the hi-fi shows in Harrogate probably in the
'70s. I walked into the ballroom at the Old Swan Hotel (of Agatha
Christie fame) as I could hear what I thought was a brass band playing
- possibly Grimethorpe - so you can imagine my surprise when all I saw
was two MFB loudspeakers. Staggered was not the word. The only reason
that I can think they never took off was (a) the price which compared
with the price of some so-called hi-fi kit these days would now be seen
as cheap and (b) because it was done by Philips who were not perceived
to have hi-fi capability. I often wonder what would have become of the
technique if the design had been done by someone like an early Linn?

A snag which may have held back MFB is the need to buy the speakers and amp
as a package deal. This means losing some degree of choice, then and in the
future. And how many makers make good speakers *and* good amplifiers?

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html