High Frequency Driver Far Field Response Measurement
Here we measure the acoustical frequency response of a
common double chamber 1” dome tweeter unit. This unit has a voice coil
resistance of 6 ohms and has a nominal power rating of 100 Watts. Its
resonant frequency is 750 Hz making it a good candidate for a two-way
system. The driver is flush mounted in a box with front panel dimensions
of 11" x 25". The room is semi-reverberant and measures 6.5m(l)
x 5.5m(w) x 2.4m(h).
In the procedure below we will do the following:
- Modify the "32768_MLS_Impedance_Measurement.process"
to measure frequency response.
- Wire the measurement
circuit for level and frequency calibration.
- Set the level of the
sound card and optional amplifier to 2.83V.
- Calibrate the sound card
and optional amplifier input level and frequency response.
- Run the process and
window out floor and ceiling reflections so that we can observe the
relative SPL vs. frequency response of the driver in the spectrum
- Apply a microphone
calibration curve so we can view the absolute SPL vs. frequency response
of the driver in the spectrum analyzer.
- Observe the absolute SPL
vs. frequency response as we gradually move the microphone closer to
- Present the SPL vs.
frequency response vs. microphone distance from the DUT in the
Loudspeakers are typically specified in terms of dB
SPL for 2.83 volts peak input. The sound pressure level is measured
on-axis in anechoic conditions at a distance of 1 metre from the
loudspeaker. 2.83 Volts corresponds to the voltage across a standard 8 ohm
speaker driven at 1Watt. The current required to supply 1 watt to 8 ohms
is 0.353 Amps.
Although +12V DC is available on all computer PCI
backplane connectors, the analog output stage of a typical AC97 audio
interface is powered from +5VDC. At best these -10du (40-300 ohm) single
ended line outputs can produce a full scale output voltage of about
1.25Vrms which falls short of the 2.0Vrms (0.7071 x 2.83Vpeak) required
to produce 1 Watt across 8 ohms. Better quality sound cards may have +12V
powered, swappable, in common 8-Pin Dip packages (as below) op-amps in a
balanced configuration on the line outputs but these at best can deliver
8V into a 600 ohm load (13mA ~100mW).
Figure 1 Eight Pin DIP Op-Amp Packaging
The table below shows the maximum output voltage of two
motherboard based audio codecs into 4 load impedances.
Load Impedance (ohms)
Output Level ALC888S Codec (Vpeak)
Output Level AD1885 Codec with LF353 Amp (Vpeak)
Infinity – Open Circuit
Table 1: Maximum Sound Card Output
Voltages vs. Load Impedances
In addition to the line level outputs, some older sound
cards such as the sound blaster AWE 32 had speaker outputs that could
easily drive 2 Watts into an 8 ohm load. If you have such an output this
would be the one to use for frequency response measurements.
The four pin USB 2.0 port found on most computers can
output 500mA at +5V or 2.5 Watts. Most USB powered audio interfaces only
have balanced international studio line level outputs (+4dBu or 2.19Vpeak
nominal). These 300 ohm outputs can produce about +10dBu or 3.49Vpeak
maximum but are not suitable for driving 8 ohm loudspeakers. Their
headphone outputs are only good for about 100mW into 32 ohms. Although
eight pin +12V and +24V powered USB and six or nine pin Fire-wire audio
interfaces could easily meet the above standard these types of audio
interfaces are usually equipped with the same +4dBu outputs as the USB
There are three things you can do to compensate for low
audio interface output levels.
- Purchase a 1 Watt or greater stereo power
amplifier such as the Dayton DTA1 (2x10 Watts -> 4 Ohms @ 0.1%
THD @ 1KHz), Pyle PCA1 (2x3 Watts-> 4 Ohms @ 1.0% THD @ 1KHz) or
PTA2 (2x8 Watts -> 4 Ohms @ 1.0% THD @ 1KHz), the Sonic Impact
TA2024 (2x6 Watts -> 4 Ohms @ 0.1% THD ) or the Nady XA300 (2x120
Watt RMS -> 8 ohms @ 1.0% THD @ 1KHz) ranging in price from
$39.00USD (29EUR) to $100.00USD (74EUR).
- If you already have a consumer HiFi receiver or
amplifier you can purchase a 3.5mm TRS to ¼” RCA male adapter cable
(as below) and connect the sound cards line-out to the equipments
Aux-In input and drive the DUT with the amplifier.
Figure 2: 3.5mm
TRS to ¼” RCA Male Adapter Cable
- You only need a single amplified channel to
measure loudspeaker on axis frequency response. The +6V split
supply powered, +15dB gain class AB amplifier on the left below can
be inexpensively constructed and can deliver 4 Watts to a 4 ohm load
at less than 0.1% THD at 1 KHz. The +12V single supply amplifier on
the right can do the same but is not recommended due to the large
output decoupling capacitor required to isolate the DC from the
loudspeaker. At 20Hz this 2200uF capacitor would have an impedance
of 3.6 ohms. This has the effect of reducing the voltage at the
loudspeaker terminals as frequencies are lowered so that 1 Watt is
no longer dissipated across the voice coil. Although the IC’s in the
circuits below have thermal shut down a 6C/Watt heat sink is
Figure 3: Class AB Amplifiers Capable of 1 Watt
Just connect the amplifier in
series with the audio interface outputs and adjust the mixer levels to produce
an output of exactly 2.83Vpeak (2.00VRMS) into the loudspeaker.
If you are measuring impedance or relative responses
(such as you would when you are adjusting cross-over components) you do
not need to calibrate levels. If you are measuring absolute responses
like we are here you should calibrate levels as you probably would like
to refer your measurement to some manufacturers data sheet. In order to
calibrate a sound card line input you need to compare it to a reference
standard. An inexpensive way to do this is to purchase a pocket
multimeter such as the 4 1/2 digit EXTECH DM110. It costs about $34.99
USD or 27.41 EUR. Its AC RMS voltage accuracy from 40-400Hz on its 4.000V
range is +1.0% + 10 digits. This translates to about 0.05V. More accurate
6 1/2 digit meters such as the Agilent 34401A with VAC accuracies of
better than +0.06% are available. You must ensure that your meter
measures AC voltage at the frequency (~200Hz) that sonic beacon outputs
during level calibration. For instance, during voltage calibration for a
sample rate of 96000S/S at an FFT Size of 1024 a 187.5Hz waveform is
We will modify "32768_MLS_Impedance_Measurement.process"
to perform the frequency response measurement. This process ships with
the release version of this product. When we are done it will consist of
six modules. The first is the signal generator, which generates an 8192
length MLS stimulus to excite the DUT. Second is the SoundIO module,
which plays the stimulus and records the response of the driver. Third is
the Arbitrary Filter, which will invoke our microphone compensation
curve. Fourth is a Spectrum Analyzer, which will perform an FHT on the
MLS time domain data in order to convert it to an impulse response. Fifth
is the Oscilloscope module, which will allow us to window the impulse
response in order to remove room reflections. Finally another Spectrum
Analyzer, which performs an FFT on the impulse and allows us to view
amplitude vs. frequency and phase vs. frequency graphs.
Install the driver in the desired enclosure or baffle. Bypass any driver
crossover unit. Place the enclosure on a stand about 1 meter from the
Open “C:\Users\Public\Documents\Sonic Beacon\Sonic
Beacon\32768_MLS_Impedance_Measurement.process” from the applications
File…Open… menu. Press OK if the “No Compatible
Calibration File Present” message box appears.
Select Options…Process from the applications menu. The Process
Select dialog will open. Highlight Oscilloscope in the Module
List box. Highlight Arbitrary Filter in the Available
Modules list box. Press the Insert button. Highlight Oscilloscope
in the Module List box. Highlight Spectrum Analyzer in the Available
Modules list box. Press the Insert button again. The Process
Select dialog should appear as in Figure 2.
Figure 4: Driver Measurement Process Select Dialog
Press the Ok button in the Process Select dialog. The modified
process will open as in Figure 3. Press OK when the “No
Compatible Calibration File Present” message box appears.
Figure 5: Driver Measurement Process
Open the FFT Options dialog from the applications Options…FFT…
menu and change the FFT Size to 8192. Press OK in
the FFT Options dialog box. Press OK when the “No
Compatible Calibration File Present” message box appears.
Select 4 from the SoundIO Repeat Sequence: combo box.
Select the Step: 4 Spectrum Analyzer module. Select FHT
from the Type combo box and check the Apply Freq. Cal. checkbox
in the Options group box.
Figure 6: Step 4: Spectrum Analyzer
Select the Step: 6 Spectrum Analyzer module. Select Log100
from the Xaxis Sel: combo box. Select dBRel from the Yaxis
Sel: combo box. Select 5dB/div scale from the Yaxis Sel:
combo box. Enter 1000 into the Ref1: Edit control and press
the enter key. Select FFT from the Type combo box and check the Apply
Freq. Cal. checkbox in the Options group box.
Figure 7: Step 6: Spectrum Analyzer
Wire the circuit for calibration as shown in Figure 1. Use short, low
resistance or shielded wiring. Note that external amplifiers should be in
the calibration loop. These devices will produce more output swing than is
tolerated by the sound cards line inputs so keep levels low.
Figure 8: Driver Measurement Process
You need to adjust the level of your selected sound card recording path.
If you are running Windows 7 or Vista right click the sound icon on the
windows task bar and select Recording devices from the popup menu
that appears. Double click the selected sound card in the Sound dialog
box Recording tab. Select the Levels tab and adjust the
slider to its one-quarter setting. Press the OK button. If you are
running XP or below; select the Levels tab Press the Open Mixer
button the SoundIO modules Options group. Select Options…
Properties… Choose your sound card from the Mixer Device and
press the Recording radio button in the Adjust Volume for
group. Press the OK button. Deselect all Record Control
mixer paths except the Line In. Adjust the Line In mixer
slider to its one-quarter setting and equalize its balance slider.
You need to adjust the output level of your selected sound cards playback
path. If you are running Windows 7 or Vista select the Playback
tab of the Sound dialog box and double click the selected sound card.
Select the Levels tab and adjust the Line In to its 25% setting.
Otherwise if you are using XP or lower select Options… Properties…
Press the Playback radio button in the Adjust Volume for
group. Press the OK button. Mute all Playback mixer gain
settings except the Volume Control and the Wave Out.
Equalize the Volume Control and the Wave Out mixer balance
sliders. Adjust the Volume Control and the Wave mixer sliders
to their one-quarter settings.
You now need to calibrate the sound card if you do not already have a
valid calibration file loaded for the input and output devices selected
in the SoundIO module. Press the Calibration button in the SoundIO
module. The Calibration dialog box will open.
Select Input from the Calibration Type Select combo box and
press the Run button to output the applications AC internal
reference to Line-Out. A sine wave of the closest harmonic of 200Hz
possible with the processes SoundIO module sample rate and FFT size will
be directed to Line-out. Plug in one end of the TRS cable into
Line-Out (green) and measure the signal amplitude with an Oscilloscope or
AC voltmeter. Then connect the other end of the TRS cable to the Line-in
input (blue) and enter the measured amplitude in peak (oscilloscope) or
RMS (AC voltmeter) volts in the Reference Level Edit controls. Select
either Vpeak or Vrms in the Reference Levels Combo Boxes
depending on the measurement instrument used and press the Stop
Button to update the gain that is applied to the all the processes
modules that display amplitudes.The dialog boxes Reference Level edit,
Sound Card Input and Input Level static text controls will contain the
user entered reference or measured Line-out level (must not be 1.0), the
selected input of the processes SoundIO module and the mixer gain setting
of the input channel (0-65535) respectively as below.
Figure 9: Level Calibration Group Box
Controls after Pressing Stop Button
Select Auto from the Calibration Type Select: combo box in
the Calibration Status group box. Select MLS from the Signal
Type combo box in the Frequency Calibration group box. Press
the Run button and wait for the Frequency Calibration Complete
status message to appear. If a “No data in record buffer” message
box appears Press the Open Mixer button and increase the
applications output level slider. The SoundIO modules input device level
may also be increased by right clicking on the Windows Task Bar Sound
icon and selecting the Recording and Levels and adjusting
the devices slider but Levels Calibration will have to be repeated.
Frequency calibration may be restarted by pressing the Run button. If
successful, the calibration dialog should look as in Figure 3. The
Calibration Progress bar may not update in certain versions of Windows.
Figure 10: SoundIO Module Calibration
Controls After Calibration, Input Level Settings May Vary Depending On Sound Card
Press the Save button to save the calibration file to disk. Enter
a file name when the Save Calibration File dialog appears. Press
the OK button in the calibration dialog and select Yes when
the “Calibration Parameter Has Changed. Save To Process File?”
message box appears. Do not change the Recording Control Line-In
gain settings after this point or re-calibration will be required.
Changes to the Playback mixer gain settings may be made and will
not affect signal level measurements.
Now rewire the circuit as shown in Figure 9. Place the driver enclosure
on a stand about 1 meter from the floor. Place the mike about 1 meter away
so that it is in line with the dust cap of the driver.
Figure 11: Driver Measurement Process
Press the Run button. Four bursts of MLS sequence will be sent to
the driver. Now observe the resulting trace in the oscilloscope. You
should see a single large aberration at the beginning of the trace
followed by a group of aberrations about 3 mSec later. This trace
represents the impulse response of the driver and the room. The first
aberration is the driver’s impulse response and the later aberrations are
room reflections. They will distort the response curve and must be
excluded from the measurement. Place the mouse at the beginning of
the oscilloscope trace and press the left button. Now sweep the
mouse to the first room reflection and release the left mouse
button. Only the trace highlighted in inverse video will be included
in the measurement. See Figure 10. The distance (d2) for any height (h)
and microphone placement (d1) may be calculated as follows.
The time to first
reflection can be calculated as follows.
Figure 12: Windowing the Drivers Impulse
Now press Run again and observe the spectrum analyzers channel 1
trace as in Figure 11. This is relative response of the driver with
respect to a 1KHz reference frequency.
Figure 13: Drivers Relative SPL
Now we will enter microphone sensitivity in order to make true sound
pressure level measurements. In the second spectrum analyzer module
select dBVSPL from the YAxis: Sel: combo box. Find the
microphone sensitivity in its manufacturers data sheet. It is usually
expressed in terms of output voltage (mV) per Pascal (Pa). One pascal is
equal to 94dB SPL. The spectrum analyzer dBVSPL scale is referenced to
94dB SPL. If the microphone sensitivity is expressed in some other terms,
conversion is necessary (see the conversion factors below). Our
microphone sensitivity is 0.031V/Pa. Now enter 0.031 into the Spectrum
Analyzers YAxis: VRef: edit box and press the enter key.
Now press the Run button and observe channel one of the second
spectrum analyzer. Figure 12 is the absolute sound pressure level of the
driver at 1 meter.
= 20uN/m2 = 20uPa = 0.2nBar = 200udyne/cm2 = 2.9015
X 10-9 lb/in2 = hearing threshold:
SPL = 0.1 N/m2 = 0.1 Pa = 1uBar = 1dyne/cm2 = 1.454
X 10-5 lb/in2 = average factory noise
SPL = 1N/m2 = 1Pa = 10uBar = 10dyne/cm2 = 1.454 X
10-4 lb/in2 = air compressed riveter
SPL =101,330 N/m2 = 101,330Pa = 1Bar =10133 X 10+6 dyne/cm2
= 14.693lb/in2 = 1atmosphere.
Figure 14: Drivers Absolute SPL
Now we introduce the microphone frequency and phase correction data to
the process. If your microphone does not come with an ASCII format
correction file you may enter one manually in the Arbitrary Filter edit
control using the manufacturers supplied frequency and phase response
curves. See “To enter an arbitrary response envelope manually via the
keyboard” in the arbitrary filter section of the user manual. Our
microphone comes with an ASCII file so we simply open the file in the
arbitrary filter module. The arbitrary filters edit control can translate
almost any correction file that has the form shown in Figure 13.
Figure 15: Required Correction File
A note of caution;
most correction files come with the actual response curve of the
microphone element. In this case the response curve must be inverted. You
must press the Invert button in the arbitrary filter dialog bar
for these files to give the correct response output. The arbitrary filter
multiplies the input signal amplitude response by its amplitude response
and adds its phase response to the input signals phase response.
Highlight the arbitrary filter module and press the Edit button on
the dialog bar. The edit control should open on the right side of the
module. Press the Open button on the dialog bar. Select the All
Files(*.*) option from the Files of type combo box. Select the
correction file from the Open dialog and press the OK
button. Press the Update button on the dialog bar. If a Frequency
Element out of Response Range message appears, go to the given line
number and place a semicolon in front of it. This will cause the line not
to be parsed. You may also delete it altogether. This application has a
maximum response range of 0 Hz to 22050 Hz. Now look up the microphone
manufacturers frequency response curve. If the plot in the graph window
is equivalent to the curve press the Invert button. Now press the Save
button and enter a file name in the Save As dialog File Name
edit control. Press the Save button.
Now we can measure the response verses the microphone distance from the
driver. Each time the distance is halved the response should increase by
6dB. We will create a new process consisting of only the DataLogger
module to illustrate this point. Press the New button from the
applications File menu. Open the FFT Options dialog from
the applications Options…FFT… menu and change the FFT Size
to 8192 and Sample Rate to 44100. Press OK in the FFT
Options dialog. From the Options menu select Process...
The Process Select dialog box will open. From the Available
Modules list box select DataLogger and press the Insert
button. The DataLogger will appear in the Modules List box.
Press the OK button. When the DataLogger opens change the Xaxis
scale selection to F1Log100. Change the YAxis1 scale
selection to dbVSPL. Enter the microphone sensitivity in
the YAxis1 Ref: edit control. Change the YAxis2 scale
selection to None. Press the Title button and enter a plot title
in the Title edit control.
Figure 16: Data Logger Settings
We begin by taking a measurement at 1 meter. Highlight the “8192_MLS_Response_Measurement.process”
and press the Run button. When the process is done sweep out a
time window in oscilloscope module as shown in the Table 2 below.
Distance (meters) d1
Bounce Distance (meters) d2
Time Window Sweep (mSec)
Table 2: Loudspeaker to Microphone Distance
Verses Floor Bounce and Impulse Windowing Time
Press the Run button again. Now highlight the oscilloscope
module and press the left mouse button. Select Sel Ch1 from
the popup menu that appears. Press the left mouse button again and
select Copy from the popup menu. Now highlight the DataLogger
process and select Plot1 from the Plot Adj: Sel: combo box.
Press the left mouse button and select Paste from the popup
menu. Repeat for the remaining microphone distances each time sweeping
out the new oscilloscope time window and incrementing the DataLogger plot
number. You should end up with four plots in the DataLogger each
separated by about 6 dB as shown Figure 15.
Figure 17: Driver Absolute SPL
Response Verses Mic Distance