





SnapTS
A Real Time ThieleSmall Measurement System
For The Woofer Tester Pro And Speaker Tester Systems
Understanding Real Time Measurements And how This Effects Results
The Woofer Tester Pro and Speaker Tester 'Snap TS' mode can be a useful real time mode for evaluating
T/S parameters or making A/B tests against a known golden unit. Additionally, the high power testing capability
of the WTPro often reveals fabrication issues that occur at higher drive levels. As with any test,
setup and result interpretation is important because this can either hide or reveal certain phenomena. In any
case when accuracy, stability and repeatability are important the more robust sine test is always preferred .
The information in this document should be helpful in understanding these concerns.
SnapTS Linear and NonLinear Driver Effects to Consider
Q and Fs Ring Down Time
A damped oscillation will occur when an impulse force is applied to a mechanical resonator such as a
loudspeaker. The time that it takes for the resulting oscillation to decrease from an initial level
of Vo to Vf is T=ln(Vf/Vo)*Q/(pi*Fs) where Q is the mechanical loss. For a loudspeaker Qts is
typically understood to be the parallel combination of the mechanical and electrical losses,
Qts=QesQms. However, when the voice coil is not shorted or connected to a low output impedance
amplifier there is no electromotive drag from the motor so Qes is not applied and Q=Qms. This condition also
applies during the TS test because a constant current source is used and the
output impedance is high. The consequence of this is that the ring down time is now much
longer, and if this time is too long, it can affect the SnapTS real time measurements.
Sine wave testing is immune to this effect because the test signal is applied for a
much longer time, especially given how the search continually narrows. The bottom
line is that ring down time is a proportional to Qms/Fs. A combination
of both high Qms and low Fs being the worst case. If this occurs, use a larger FFT
by increasing the frame size.
Compression Effects  Why T/S Parameters Are Measured Using Small Signals
Several loudspeaker parameters are affected by temperature. Voice coil heating is
well known as this causes Re to change and can be directly entered or modified in many
of the popular Thiele Small modeling software. Note: The resistance of
copper is directly proportional to absolute temperature (degrees Kelvin).
A less obvious, but actually a very large error contributor are changes in suspension
stiffness due to temperature. This relationship exists because the mechanical
components that are used are often made from plastics and heat cured resins that continue
to have strong temperature dependencies even when cured. This effect goes beyond
the initial breakin effect that tends to be a more or less permanent change.
What is not well documented is that as mechanical energy is absorbed (into mechanical impedance Rms),
the suspension tends to warm and therefor soften, often quite dramatically.
When very small signals are used mechanical heating is far lower. For this reason
baseline models should be derived using the WT2 port. Note: When the WT2 port is
used in Sine mode, the extremely high precision provided is often enough to
measure this compression effect even though the drive levels are very small. The
Woofer Tester Pro's HiZP port is simply an extension of this curve, often out to Xmax
drive levels where the curve tends to turn around as the suspension (or motor) begins to bottom out.
Test Environment Noise Immunity
Though being able to measure drivers in a noisy environment can be solved by simply moving to
a less noisy environment, eventually measurement limitations are likely to occur. The
interesting thing about a sine wave is that because it is a monotonic all of the driving energy is
packed into one frequency. On the input side it is also now possible to build a matched
narrow band filter. The effect is the same as using a larger signal and a very large FFT
making truly 'small signal' conditions possible.
Motor and Suspension Non Linearity
Non linearity becomes increasingly important when testing at higher 'amplifier' drive
levels with the WTPro. In this case warming and stretching effects will cause
the mechanical bias point to shift, often beyond the more simple example of reversing
the leads and shifting the resonance. Clearly not only has the resonance point
changed, but so has the Q, BL and other parameters. If a real time mode is
used, eventually the impedance curve will even begin to distort. This is because
the resulting impedance curves are different for each extreme of the inward and outward
stroke. In this case an FFT real time mode is clearly not suitable while the sine
test is far more likely to succeed.
FFT Results Averaging
Ultimately it is the shape of the impedance curve that determines the driver
parameters. However, given that FFT results are often noisy or sparse, averaging
is often employed to produce a smooth curve. Time averaging of several
frames does not damage the data, but binning does. Binning uses the neighboring
bins as a group to produce a smooth curve. This can however damage the
shape of the curve and therefor effect Q (mainly by lowering the Fs peak).
Furthermore, if Fs does not align with the FFT bins, the actual peak is smeared into
the adjacent bins. These types of errors also increase as Fs decreases. The
testers averaging options include selecting the octave width, and the number of desired
outputs. Ultimately this determines the number of bins that are averaged together.
DC Accuracy
The ADC used in the tester does not convert DC signals. Therefor the DC bin of an
FFT holds no useful information. Like our sine test, the DC value must be
interpolated from the next two lowest data points. However, unlike the FFT that is
restricted to the bin frequencies, the sine mode can measure any frequency.
Comparing SnapTS and Sine Mode Tests
The time to complete the QFs and Vas portions of the T/S test sweeps are given below. In both cases,
additional setup time must also be considered when breaking in a driver (hours), or when the Vas portion of
the test calls for adding a test mass (10~20 seconds), mounting the driver in a test box (minutes),
measuring the suspension diameter (10 seconds), etc.
SnapTS Preferred Setup (results settle in 2~10 seconds)
 Select LoZP or HiZP (WTPro only)
 Select any Real Time signal. Use Impulse or Chirp for Low Frequency sensitivity
 Enable input averaging (if needed)
 Enable output averaging (if needed)
 Set sampling rate to 48000 Hz
 Set Frame Size from 8k64k depending on the driver Fs and Q
 Set low Frequency limit to 10, and high to 20k
 1/30th octave binning (or less)  Does not smear and lower the Zmax peak
 Set 512 or more output points for smoother curves
 Use dB/Ph filtering (not RI vectors)
 If needed move to a quiet test area
Sine Mode Setup (QFs completes in ~1 minute, Vas in ~20 seconds)
 Select LoZP or HiZP (WTPro only)
 Select Sine test signal mode
 Set sampling rate to 48000 Hz
 Set frame size to 4k
 Set buffers to 2
 Set averaging to 2
 Set Sweep Ratio to 1.3
 Set Min Ratio to 1.001 (0.1% frequency accuracy)
 Set low Frequency limit to 10, and high to 20k (or less for high L woofers)
 Make adjustments as needed for curve smoothness, range etc.
Circuit and Signal Processing Back Ground
Real Time (Fast) and Swept Sine Impedance Curves
Traditionally Ohms law, R=V/I, is first
learned with static DC voltages and
current. When AC circuits are
considered, energy storage devices like
capacitors and inductors cause the applied
voltage and current to be out of
phase. The ratio V/I ratio is still
used, but the V and I parts now must
consider phase in the calculation. In
this case 'ohms' are now referred to as
having 'reactive impedance', commonly
referred to Z (and P for phase). Since
circuits are often made from more than one
device, and capacitive and inductive energy
storage is frequency dependent, the
impedance curve relative to frequency is
often complex. Note: loudspeaker
mechanical parameters such as mass and
suspension stiffness store mechanical energy
and are transposed as capacitors and
inductors.
An impedance curve (a series of Z values at
particular frequencies) can be generated in
two ways. The first is to apply a
single frequency at each frequency of
interest, measure the resulting voltage and
current, and then compute the Z=V/I
impedance ratio. The second is to
apply a complex signal that contains all of
the frequencies of interest...all at one
time. An FFT is then used to convert
the resulting complex voltage and current
signals to frequency domain
equivalents. Since the amplitude and
phase of V and I are now in matching
frequency domain arrays, Z can be computed
for each FFT bin frequency. However,
unlike the single sine method, and though
this is a quicker method, a number of
factors need to be considered when this
technique is applied to a nonlinear device
such as a speaker.
FFT Frame Size and Resolution
Fourier Transform's are used to convert a
sequence of time domain samples into
frequency. The highest frequency that
can be defined (with aliasing) occurs when
exactly two samples occur in one sine
period. This frequency is known as the
Nyquist frequency or Nyquist rate, and is
defined as Fn=Fs/2 where Fs is the sampling
rate. The lowest frequency that can be
resolved occurs when one sine wave just fits
the time sequence. If for
example a 48000 Hz sampling rate is used to
capture a 32768 sample sequence, the elapsed
time would be t=32768/48000, or 0.683
seconds and this would result in a minimum
frequency of 1.464 Hz. Another
property of the FFT is that the minimum
frequency is also the spacing between each
adjacent frequency bin. In the above
case the available 'exact' frequencies would
be 1.46, 2.92, 4.39... If an
analog frequency falls between these
points the energy is divided into the
adjacent bins. Additional math is
needed to find the exact frequency and
magnitude. Note: FFT's are typically
written for powers of 2 sizes for
algorithmic reasons
(2,4,8,16,...16384,32768,... etc.)
Signal Density of Real Time Signals
Depending on what needs to be measured, several test signal options are available
for FFT real time mode testing. For example, the impulse signal favors low
frequency analysis while MLS is best used for high frequencies. In any case what
is important is that all of the frequencies of interest (each FFT bin) are
represented. Since these signals are obviously different in both time and in
frequency, the energy density or spectrum and the phase become important.
Mathematically this is all sorted out when the V/I ratio is calculated. What is
less obvious is that the time and spectral energy density and the nonlinear
characteristics of a driver can result in differences when one signal is used as
opposed to another. A good example would be the impulse mode (actually modified
triangle) where most of the energy is packed into a single edge. In addition, the
driver clearly moves inward or outward depending on the connection polarity.
As shown below this DC bias has generated two completely different impedance
curves. DC biases also occur, but to a lesser extent, in all of the other signal
modes. The only exception is the sine mode (and it even has a DC bias when it starts).
Drive Signals
The WT2 output is a true constant current
source, which is very different from a faux (not quite real) constant current source
that is created when a high value resistor is used inline with a voltage
amplifier. As an example the traditional method was to use an amplifier
with a 1k series resistor. In this case, the amplifier output was sufficiently
large to achieve a measurable signal level, and as long as the measured impedance was
not greater than say 100 ohms (10% of 1k) the error would be considered 'small
enough'. Note: Non amplified 'sound card' levels are considerably lower so its
not uncommon to use an even smaller series resistor value in the 100470 ohm range.
Note: The WTPro HiZP port is traditionally connected directly to an amplifier and
constant current is simulated by adjusting the amplifier output level using one of the
feedback modes. An alternative would be to not use a feedback mode and instead
add a large series resistor, just like the 'faux' constant current described
above. In this case the 0.5 ohm series resistor can also be increased to decrease
the 5A current sensing range to something considerably less (use 5 ohms for a 500mA range).
Conclusion
SnapTS is ideal for quick real time evaluation of TS parameters. However, given its sensitivity
to systematic errors, the modestly slower discrete tone 'sine mode' test is preferred when published or
critical data is important.




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