[net.audio] table saws, KEF B110, transmission lines

sasaki@harvard.ARPA (Marty Sasaki) (08/07/85)

If things are set up properly, a table saw is much more accurate than
a hand saw.  I've built jigs and special cutoff boxes that allow
extreme accuracy and very quick cutting.  In any case, always use a
sharp blade on the saw, keep your fingers clear of the blade, and use
some kind of eye protection. Ear protection is also a good idea.

The KEF B110 has been mentioned several times recently as a pretty
good speaker.  I've heard from some folks that there are now better
units that cost less, have flatter response, etc. Does anyone have any
experience with possible replacement drivers?

I'm in the process of building a transmission line enclosure. The
classic wisdom is to make the length of the line equal to one fourth
the wavelength of the driver's resonant frequency. The line is then
stuffed with some kind of damping material. I understand the reasons
for the quarter wavelength length, and the use of damping material.
What I don't understand is why using both works. The length of the
line in all of the articles that I've seen is calculated by the speed
of sound in air, not the speed of sound in the damping material.
Intuition tells me that the length should be calculated by the speed
of sound in the damping material. Help!
-- 
----------------
  Marty Sasaki				net:   sasaki@harvard.{arpa,uucp}
  Havard University Science Center	phone: 617-495-1270
  One Oxford Street
  Cambridge, MA 02138

rdp@teddy.UUCP (08/08/85)

In article <291@harvard.ARPA> sasaki@harvard.ARPA (Marty Sasaki) writes:
>If things are set up properly, a table saw is much more accurate than
>a hand saw.  I've built jigs and special cutoff boxes that allow
>extreme accuracy and very quick cutting.  In any case, always use a
>sharp blade on the saw, keep your fingers clear of the blade, and use
>some kind of eye protection. Ear protection is also a good idea.
>
>The KEF B110 has been mentioned several times recently as a pretty
>good speaker.  I've heard from some folks that there are now better
>units that cost less, have flatter response, etc. Does anyone have any
>experience with possible replacement drivers?
>
>I'm in the process of building a transmission line enclosure. The
>classic wisdom is to make the length of the line equal to one fourth
>the wavelength of the driver's resonant frequency. The line is then
>stuffed with some kind of damping material. I understand the reasons
>for the quarter wavelength length, and the use of damping material.
>What I don't understand is why using both works. The length of the
>line in all of the articles that I've seen is calculated by the speed
>of sound in air, not the speed of sound in the damping material.
>Intuition tells me that the length should be calculated by the speed
>of sound in the damping material. Help!

One of the projects I spent quite a bit of time on was research of
transmission line bass enclosures. There turn out to be quite a few
myths about these, and the above writer seems to be a victim of them.

I believe that the hoopla about transmission lines is not in a small
part due to the relative success of I. M. Frieds (IMF) transmission
line enclosures of the early 70's. They did, in fact, attempt to utilize
the 1/4 wave phenomenon on the bass for some assist. However, one of the
things that I discovered was underdamping the line sufficiently to allow
for reinforcement caused a 1/2 wave cancellation an octave above the
1/4 wave point. Note all of the published curves for IMF and similar
TL's The IMF stufio, for example, had a slight peak (only 1 or 2 db or so)
at about 30-60 Hz, below which things died pretty rapidly, then they had
a depression (3 db or so) around 120 hz, followeed by the vague suggestion
of a peak again at about 180 Hz. 

What is happening here? Well, any attempt to use the 1/4 line resonance for
reinforcement causes enough energy to find it's way through to the end of the
line where it cancels the direct radiation from the woofer. This results in
a cancellation where the line is a full wavelength long.

The best lines that I was able to come up with were those that DID NOT
attempt to use the line characteristics to augment the extreme low end.
The superiority of transmission lines, it turns out, is not necessarily in
the deep bass, but in the mid bass region, say 100 to 300 Hz. 

What a transmission line buys you is a close approximation of a true infinite
baffle for these wavelengths. By forcing theses signals down a long absorbtive
path, the chances of response anomolies arising from internal cabinet
reflections is much reduced, resulting in exteremely accurate reproduction
in the midbass, presuming the driver is alright to begin with. 

This is ussually not achievable in normal cabinets because the absorbtive
characteristics of most materials decreases with decreasing frequency. Even
woool, which has been shown to have very good low frequency absorbtion, looses
it almost completely for real life thicknesses (say a foot or so) below 250 Hz.
The transmission line merely provides a far longer path, and, if properly done,
a non-reflective termination for the back wave of the woofer.

In designing such a system, I discovered that a properly done transmission
line provides relatively little effective reactive loading to the driver.
This is suggested by the fact that the system resonance characteristics do
not differ greatly from free air characteristics. So what is neede is a driver,
of impeachable quality, with a low resonance (whatever is desired) that has
a free air total Q of between .5 and .7. Most drivers are magnetically over
damped for this application, but one driver I did find suitable was the KEF
B200 SP1014, the small magnet variety. Some of IMF's later systems included
Peerless drivers (and the like), with too much damping, the result being that
many "tricks" had to be used to get any bass out of them, such as underdamping
the line (making it boom, essentially).

I would have to check my current listing of driver manufacturers to see what is
available. If there is any interest, I might be inclined to put together a 
source list of available high-quality drivers.

As an aside, the best that you can do to reduce velocity of sound with damping
is on the order of the square root of two, due to the change of operating
conditions from adiabatic to isothermal (or vice versa, I don't exactly recall)
due to the damping.

If this didn't make out into all of net-land, would someone forward it for me?

Dick Pierce