2 < Page > 4
Adding the additional impedance to the RS225 in a vented design result in more significant changes.
Once again the red curve is the modeled response with 0.1 ohm Rg. The cyan curve is with the total
series resistance increased to 1 ohm. Note that the ripple is now nearly 1 dB and 0.5 dB less sensitive
than with the ideal model. The red curve is the result of increasing the box volume from 63 liters to 85
liters, a 35% increase, and reducing the tuning frequency from 28.7 Hz to 25.7 Hz. Happily, this also
results in about a 4 Hz extension in f3.
Other observations: The change in voice coil impedance will also negatively impact the expected
response of the LP and zobel circuits, increasing the crossover frequency, reducing the effectiveness
of the zobel, and may result in audible peaking around the crossover frequency.
The tweeter, in most home applications, will be relatively unaffected by thermal issues, due both to the
relatively low power applied in its passband, and the use of ferrofluid, aluminum formers, etc.. However,
should increased voice coil impedance occur, it will lower the crossover frequency of the HP, and
increase the peak in the summed response at the crossover frequency. The tweeter may also end up
sounding bright at higher SPL, due to the loss of sensitivity of the mid and woofer. This may partially
explain why some prefer the tweeters slightly rolled off from flat.
Amplifiers with relatively high impedance outputs of more than an ohm are going to exacerbate all the
negative issues mentioned above. In addition, drivers other than woofers may be affected by the
reduction in damping.
The addition of series resistance will result in a lower f3, which in some cases will offset the required
increase in enclosure size.
-And their consequences on system Q