Dayton Audio DATS Dayton Audio Test System - Manual (Page 2)

In contrast to the small-signal parameters, the large-signal parameters are intended to characterize the
driver’s performance limitations as it becomes non-linear. Large-signal parameters are usually best
left to the manufacturer to measure, as they may require disassembly or destructive testing. The large-
signal parameters include:
P
E(MAX)
The thermally limited input power
X
MAX
The maximum linear excursion
X
MECH
The mechanical excursion limit
VD The maximum displacement volume
There seems to be a bit of confusion in some DIY speaker circles about what signal level is
appropriate for measuring small-signal parameters, so let’s look at this question more closely. In his
groundbreaking paper titled “Loudspeakers in Vented Boxes,” Neville Thiele
1
discusses the test signal
level and states:
“The value is not of great importance, but a standard test figure is 1 volt.”
In his excellent book titled “Testing Loudspeakers,” Joe D’Appolito
3
writes:
“The T/S parameters are “small signal” parameters. It is important …to
keep drive levels as low as your instrumentation will allow while still
providing reliable results.”
So there is no reason to measure Thiele/Small parameters at any particular level, as long as the
measurement is not contaminated with noise and is well below the large-signal threshold. DATS
allows the user to set the level of the test signal sweep anywhere in the range from +5 dBu (2 volts
peak) to -10 dBu (0.35 V peak) within the software. This constitutes the small-signal range for the
vast majority of speakers, but note that some micro-speakers could be pushed into the non-linear
region at DATS maximum output level.
The objection is heard occasionally that f
S
can change with drive level, so it would seem that the
parameters should be measured at higher power levels. It is easy to demonstrate—in the case of well-
designed transducers that are operating normally (i.e., not damaged)—that the f
S
of a driver does not
change significantly over a very wide range of operation. In fact, a shifting in f
S
at high drive levels
would indicate the onset of nonlinearity and would not constitute a valid small-signal measurement.
At the other extreme, a shift in f
S
at low drive levels is an indication that a driver has mechanical
obstructions such as debris in the magnetic gap. This behavior is the basis for the DATS rub and buzz
test. For example, Figure 2 shows the impedance of a transducer (Dayton Audio RS-100) measured at
eight different drive levels over a 70 dB range in 10 dB steps. At the lowest drive levels, the
resonance vanishes into the noise but the shape (Q) and center frequency (f
S
) remain unchanged over
the 70 dB range of measurement signal level. This is typical behavior for a good driver.