11
12. Acoustic set-up procedure
Set up Overview
There are three areas that are covered by this setup routine: notch/boost
filters, low pass filter, and phase compensation filter. The notch and boost
filters in the Mezzo 9 subwoofer are designed to cancel out the peaks and
dips in frequency response that are caused by room acoustics. The low
pass filter rolls off the high frequencies and provides integration with the
full range speakers. The phase compensation filter helps the subwoofer
and the full range speaker sum together correctly. The following setup
routine helps you to measure the frequency response of the subwoofer in
your room, and then apply the correct filters to give you the most accurate
response.
The idea is to measure the frequency response of the subwoofer in your
room at your actual listening position and then apply any required
equalisation to make it as flat as possible. Then adjust the crossover
slope and phase as necessary to join the frequency response of the
subwoofer to that of you main speakers as smoothly as possible.
Fully read through the How it works sections for Notch/Boost, Low Pass
Filter and Phase Compensation below before starting to make any
changes or attempting the Acoustic Setup procedure. These give an
overview of the setup process, why it is necessary and what it can
achieve.
Set-up CD
The setup CD contains 17 tracks of test signals. Track 1 is wide band
noise that allows you to set your subwoofer to a reference level. Tracks
2-16 are narrow band modulated tones that allow you to measure the
level of the subwoofer in small frequency steps ranging from 30Hz to
100Hz and create a frequency response graph. The 17th track is a special
test tone for setting up the phase filter. Also on the CD is a PC application
that helps you set the Notch/Boos Filter.
The CD is used with the "Acoustic setup procedure". See later section.
How It Works
Notch/Boost
A typical subwoofer response in a room may look something like Figure 1
below.
As you can see in this example there is a peak at 80Hz (B) and a slight
deficiency at 35Hz (A).
To correct these issues, a notch filter would be applied at 80Hz and a
boost at 35Hz. After these corrections have been made, the net result
would hopefully be a response much closer to that shown in figure 2
below:
Low Pass
Next we need to join the subwoofers response to that of the main
speakers.
The low pass filter adjustment sets the crossover point and slope of the
filter that prevents high frequencies going to the Sub and this joins its
response to that of the main speakers.
The frequency of this point can be moved up or down for best fit for your
loudspeakers and its slope adjusted to best match the natural rolloff of the
low frequency response of the main speakers.
If the Low Pass Filter is set too high the Subwoofer and main speaker
responses can overlap too much and create an unwanted hump in the
overall response.
Conversely, if the Low Pass Filter is set too low (see diagram 1) the
Subwoofer and main speakers responses might not join properly and
create an unwanted dip in the overall response.
Frequency (Hz)
Level
(dB)
Figure 1
Figure 2
10 100
A
B
1K 10K
Frequency (Hz)
10 100
A
B
1K 10K
Level
(dB)
Level
(dB)
10 100 1K 10K
Level
(dB)
=
10 100 1K 10K
Level
(dB)
10 100 1K 10K
Level
(dB)
=
10 100 1K 10K
Level
(dB)
Variable cross
over point
Sub Full range
12dB/Oct low pass slope
Large overlap
24dB/Oct low pass slope
Large gap
Diagram 1
