A Phase Linear Transmission
Section 3: Transmission Line
Figure 3.1a. shows the response of the empty transmission line together with
the driver near field response. We can see the first null very clearly around 140
Hz. The fundamental resonance is then about 70 Hz, corresponding to a physical line
length of around 1.2 m. The hump at 30 Hz stems from the driver Fs.
Gradually, stuffing was added to the line and the driver and port output was measured.
From the measurements, the following observations were made :
- The LF augmentation
is present from 30 Hz (driver Fs) up to 120 Hz. If we compare this with
the phase plots, we see that this happens in the region where the phase difference
for the driver and port is in the -90 to +90 degree area (Figure 3.1c). The driver has the least
amount of phase shift.
- Adding stuffing, straightens
out the amplitude and phase while reducing the amplitude of the first null. The -90
degree difference frequency shifts sligthly downwards in frequency.
- For low stuffing densities,
both the driver NF response and the port response changes as stuffing is added.
- Above 150 g, the driver
NF response stays the same (Figure 3.5)
- The system phase sum
changes up to around 200 g, above this the phase does not straighten out any more
- Stuffing more than
200 g, port output is dampened (Figure 3.4b), reducing the summed output
without improving the phase response to any significant degree (Figure 3.6).
- The optimum stuffing
could be defined as getting the most gain possible from the port with as straight
as possible phase response. In our case, this is found to be for a stuffing weight
of 200 g.
- More stuffing (250-300
g) might be added if the LF response is too much for the listening room in question.