"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:

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.

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

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.

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.



.... Phil

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...

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

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 10:22 PM, Peter Chant wrote:
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.

Cheap adjustable hole saw has arrived. It it works it will pay for
itself in saved hassle in one use. However, if baffle is a min of a
foot wide one wavelength between front and back will be at over 400Hz,
so roll off will be quite high. I wonder if I can put the speakers in a
scrap piece and temporarily fix a larger sheet so I don't have to drill
my nice unsullied sheet of ply!

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?

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


Have done that. Two bass / mids mounted with tweeter on a piece of
kitchen counter top. Mid-woofers in series but with 2x6.8uF cap across
one. Somehow it sounds a bit 'boxy'. Mid range seems accentuated. I'll
have to wire the drivers in parallel and see what happens. I wonder if
this is an effect of the baffle arrangement or a fundamental limitation
of the drivers.

Peter Chant wrote:


Have done that. Two bass / mids mounted with tweeter on a piece of
kitchen counter top. Mid-woofers in series but with 2x6.8uF cap across
one.

** Wow, so he took my suggestion seriously after all.


Somehow it sounds a bit 'boxy'.


** LOL - an open plain baffle can sound a bit "woody" if it is made from ply or pine.

Mid range seems accentuated.

** Gee - a lack of low bass will do that ...

I'll
have to wire the drivers in parallel and see what happens.

** It will be 6dB louder and have even more midrange.

I wonder if
this is an effect of the baffle arrangement or a fundamental limitation
of the drivers.


** Trying to balance and passively equalise a home brew *two way* speaker by ear alone is a near impossible task - I know cos I have tried many times.

Since you cannot adjust the mid frequency level independently, you have to be damn lucky with your choice of drivers. However, with a three way design, options for tweaking the response really open up.

A couple of years back, I picked up a pair of old AR2axs for A$100. These are a three way design with a really primitive x-over and cheaply made drivers that had deteriorated to the point of useless. The money got me two, nice looking cabinets of about 45 litres volume.

I stripped them both out and started again with three Peerless drivers chosen to have similar dB/watts and in sizes to fit the existing baffle cut outs. The chosen woofer's response was modelled in the same volume box using WinISD and it was near perfect.

So, I purchased a 10 inch woofer, 4 inch midrange and 1 inch soft dome with a big mounting plate. The new mid got its own 2 litre enclosure, made from a thick cardboard port tube running back to front- which also helped to stiffen the front baffle. The original AR mids were "sealed back" types, just like old fashioned paper cone tweeters - yuck.

Much effort and time was spent on the passive x-over which was externally connected to the box until I felt no further improvement was possible. Testing was done first with sine waves and then 1/3 octave pink noise using a Rode calibrated SPL meter. Further testing was done with an AKG CK2 condenser mic and 4 cycle sine wave bursts observed on a scope - a very informative technique for revealing colouration and one which works well inside a room.

X-over frequencies ended up at 450Hz and 2.2kHz with 3dB of attenuation for mid and tweeter. I bought a 4 ohm tweeter to make sure it would have enough level to match the woofer cos you sure as heck do not what to have to attenuate that.

The tweeter's HP filter is 18dB/octave while the others are all 12dB/octave.. The x-over uses three air cored inductors and five 250VAC rated poly caps plus a couple of big bi-polars for the woofer. The mid enclosure was stuffed tight with damping material and sine wave burst testing showed there were virtually no internal echoes emerging.

The new speakers are use mainly for watching DTV and DVD movies - so had to sound natural on speech and they do.

FYI: My previous speakers were Quad ESL57s which I had used for over 20 years - they needed major restoration work done by an expert, so I sold them to a guy who was willing to finance getting that done.


..... Phil

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

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.