Two Driver Sealed Box Acoustical
Measurement Techniques
To Calibrate or Not to Calibrate
The
alignment of a closed box system drivers and their crossover usually
consists of taking an acoustical measurement then changing a crossover
component or adding stuffing or bracing to the box. This is then repeated
until the systems response is flat. The calibration of the sound card or
the microphone may not be necessary for this tedious work as long as all
acoustic measurements are always done at the same drive level. Once the
relative response is flat then the system can be calibrated for a single
absolute response measurement.
Look
at the response of the sound card below. It is flat to within +1dB
from 30 to 19000Hz when using an FFT size of 8192. This is usually
adequate for initial crossover adjustment. To determine the response of
the sound card connect the line out to the line in and send an MLS signal
through a SoundIO module in Record/Play mode to a Spectrum Analyzer in
FHT/FFT mode. Using the SoundIO Repeat feature pre-stimulates the sound
card greatly increasing stability.
Look
at the measurement microphone response (green) and phase (yellow) curves
below. The response only deviates by 1dB at frequencies above 12kHz.
Unless there are large deviations in the tweeter response above 10KHz,
calibration is not necessary as most of the work on response adjustment
for two way systems is usually done in the crossover (1 to 4 kHz) region.
Obtaining the Response Curve for
Two-Way System Adjustment
For
most two way crossover adjustments only the combined far-field woofer and
tweeter response is required. It is obtained by placing the microphone at
tweeter level, 1 meter from the tweeter dome. It is usually valid down to
250Hz which is usually well below the crossover frequency. An MLS from a
Signal Generator module is sent to a SoundIO module in Record/Play mode.
This is followed by a Spectrum Analyzer in FHT mode to create a time
domain impulse response. An Oscilloscope module is used to gate the
resulting impulse to mask room reflections. It is gated at the point
where the first reflection arrives at the microphone, usually
4 to 5 mSec for a typical size room. The gated
impulse is sent to a Spectrum Analyzer module in FFT mode for generation
of the frequency response curve.
Two
drivers with the responses below were installed in a 44 liter sealed
enclosure. The crossover board was suspended outside the enclosure so that
components could be easily changed.
Initially
a 2nd order Linkwitz-Riley crossover was designed for this
system but the woofers response (green) at the 2100Hz crossover frequency
caused phase problems that left a 4dB dip in the combined response
(yellow) between 800 and 3000Hz. Here woofer (green), tweeter (blue) as
well as the combined far-field measurements were separately taken and
pasted into the Datalogger.
This
was replaced with second order Butterworth network, with a 3dB peak at
the crossover, in the hopes that the dip would be eliminated. A 4dB
artifact remained between 2 and 3 KHz. The purple trace is the response
with the tweeter polarity inverted.
A 2nd
order Chebyshev, with a 6dB peak at the crossover frequency, replaced the
Butterworth network. Although this did not remove the majority of the dip
it brought it more in line with the rest of the tweeters response
Once
the tweeter attenuation was adjusted the following far-field +2.85dB
response resulted.
Combining the Acoustical Response
Curves
After
the response in the crossover region is flattened the system is then
calibrated so that the system sensitivity is measured correctly. In order
to create a frequency response graph for the dual driver sealed box
system the two measurements must be combined.
First
the near-field woofer response is obtained. This is done by placing the
microphone at 0.25” from the woofers dust cap with the Signal Generator,
SoundIO, Spectrum Analyzer, Oscilloscope, Spectrum Analyzer setup from
above. Impulse gating is usually done at about 50 mSec to get a response
down to 20Hz.
The
near-field response is pasted into plot1 of a separate instance of a Datalogger
module. Note the very high SPL due to the close proximity of the
microphone.
The
combined far-field woofer and tweeter response is obtained (gating at 4 to 5 mSec) and pasted into plot 2 of the
datalogger.
The
level of the near-field response is brought into coincidence with that of
the far-field by reducing the gain of plot 1. Select Plot1 in the Sel:
Combo box of the Plot Adj: group and press the Gain: Dn button until the
levels of the two plots are equal at the proposed merge point. In this
case the near-field data is valid up to about 718Hz (fmax = 4311/
effective cone diameter) and the far-field data is valid down to 250Hz.
Both curves are reasonably flat between 200 and 300Hz so 250Hz might be a
good merge point.
Then
the near-field response is merged with the far-field response. Enter the
merge frequency in the Freq: edit control in the Mode group box and
select Merge in the Sel: combo box. Note the response dip below 500Hz due
to spreading loss caused by the enclosures front baffle. Also note that
the system sensitivity is that of the tweeters original free air graph.
Sonic beacon is a Canadian
organization founded in 1997. It is located in Pakenham Ontario which is near Ottawa, Canada. Ottawa is also home to
the National Research Council of Canada's anechoic chamber, a key
facility for acoustics testing. The chamber has been instrumental in the
development of Canadian loudspeakers, hearing aids and microphone arrays.
July
8, 2009:Sonic beacon Version 1.1.0.6 released. Data-Logger
can now save in .FRD and .ZMA file format. Data-Logger and Spectrum
Analyzer can perform .FRD and .ZMA clipboard copy transactions. Signal
generator can output signal complements on each sound card channel to
allow bridging in many consumer audio interfaces. Module status bar shows
current data type, FFT size and sample rate.
