Russell Storey Draft 1 _709
Stones Sound Studio,
Speakers and Drivers (transducers) are measured
using an array of equipment in conjunction with
the LMS 4 Loudspeaker Analyser which is calibrated in true SPL db
(Sound
pressure level
).
Test Equipment Calibration:
Our Test
& measurement Equipment
is run for around 2 hours prior to
calibration to warm up the electronics and stabilize
the measurement
circuits .
The Equipment is then individually "calibrated " to
minimise
measuring errors before measurements can be taken.
Taking into account errors in measurements
due to speaker cable and microphone leads etc
Measurement of test lead impedance , microphone db
loss , test speaker cable , cable high and low frequency
insertion losses must be subtracted from the actual
measurement to reduce measurement errors
and provide thus provide accurate measurement data of the
device under test ( speaker or transducer )
Microphone and Pre amp Calibration
First a Brüel
& Kjær
laboratory reference microphone
pressure calibrator @ 94 SPL db is used to calibrate
the microphones
and LMS 4 Speaker Analyser and
pre-amplifer.
SPL db definition re
-Wikipedia
Sound pressure level
(SPL) or sound
level Lp
is a
logarithmic measure of the
rms sound
pressure of a sound relative to a reference value.
It is measured in
decibels (dB) above a standard reference level
The commonly used reference sound pressure in air is pref
= 20
µPa (rms)
which is usually considered the
threshold of human hearing (roughly the sound of a
mosquito
flying 3 m away).
Ambient temperature
The Environment ambient temperature is
measured and noted before any measurements are
made.
For frequency response ,measurements
a constant voltage amplitude sine wave sweep set to 2.83V
rms across the speaker terminals or driver
The 2.83V rms across the speaker terminals or driver is
used as a reference point unless otherwise
specified
.
With Artisan Speakers measurements the microphone is set at a distance of
1 to 2.7 Mtrs "depending on the required measurements "
.
The Microphone
The Microphone is centred between ,at ,and ,or around the tweeter /bass
mid axis on the front baffle depending on the speakers driver /tweeter
configuration be it, MT ,MTM,etc its set different .
Up to 40
different readings are taken over a period 2Hrs as the drivers warm up ,then
the
readings are computer averaged
over a the specific measurement frequency range to
compute the SPL db sensitivity.
Manufacturer Specifications
, Speaker ( transducer ) Sensitivity and graphs errors
SPL db ( pressure level)
measurements of any transducer (speaker)
can vary buy as much as -/+ 3db to -/+ 6db
in the
ambient temperature range from 0 to 45 degs C.
Other
factors (variables) change the transducer
measurement parameters and will affect the SPL db
readings.
Some of the measurement variables to
consider areambient temperature, humidity ,
barometric pressure
( height above sea level ), transducer motor (magnet) system temperature,
voice coil temperature and
compliance of the cone and
spider materials heating losses in components in the
crossover are all taken into account.
Why, ? All this if you interested read more below
Measurement Accuracy and
Measurement errors:
Speaker SPL
db sensitivity ( speaker pressure level) varies depending on the
driver type cone size ( diameter ) materials measurement
location
( room ,car park, shed , hall , anechoic chamber etc ) and
also the measurement type application calibration and
measurement bandwidth
accuracy of equipment and
most importantly the measurement environmental temperature
Speaker
Systems
sensitivity in
(SPL )db is usually measured over the 10Hz to 50KHz range (
depending on the type and application of speaker)
A low distortion constant voltage sine wave of 2.83 V rms ( ref
1KHz) is applied across the speaker terminals with the LMS
loudspeaker Analyzer.
Measurement Bandwidths
Single Drivers( transducers )
Subwoofers 10 to 500Hz,
Woofer 10 to 3KHz ,
Mid /bass 10 to 10 KHz ,
Mid 150 to 12Khz ,
Tweeter
400 to 100 KHz
Ambient temperature
Ambient temperature is the most important factor
when measuring any transducer or loudspeaker system this
is set
normally at 22deg C on average unless other
wise stated.
Speaker Impedance (T/S
Thiele & small parameters)
Measuring transducer or Speaker
enclosure Impedance or (T/S
Thiele & small parameters) specification is like measuring
a
“sponge with a pair of metal callipers "Every time you take a measurement you will get a
different reading as device , test equipments,
|ambient and internal temperature change
" which measurement is 100% correct ? "the answer is none " the
measurements are only a guide to work from when
designing speaker systems .
At Sound Studio
Studio we have a laboratory with an array of reference
calibrated test equipment to
enable consistent reliable and accurate
measurements for our
customers
and clients " see
Stones Sound Studio , Laboratory
test equipment below
In an ideal world you need to measure several transducer or
speaker samples and then take an average of the readings
however this is not
always practical
Speaker and Driver (transducer ) Power rating
Speaker and Driver (transducer ) Power rating is a very
complex subject and suffers from a lot of debate and arguments world wide
.
Speaker Power rating , rms power handling specifications and
method of testing etc vary from manufacturer to manufacturer and
Engineering Standard
Societies USA and Europe like AES , IEC , ALMA , etc
so unfortunately there are several world standards to chose from
Power Density
Speaker and
Driver (transducer )
Power density
measurements using band limited pseudo random pink/white
noise or multiple modulated sine wave
sweeps can provide a simulated
" average music power" or "Normal speech and music" which
provides a more meaning specification thank a fixed sine wave
or short length pulses as there is a lot more heat and power
energy generated in the driver motor and voice coil , or speaker system
For more details of IEC Power Measurements see : power test definition-peerless loudspeaker dk
Fig 1 Example of soft dome Tweeter THD -Distortion Test
Stones Sound Studio , Laboratory test equipment
