STONES SOUND STUDIO    
       
            Loudspeaker System & Driver Measurements
                                                   by

                                               Russell  Storey                        Draft 10 _610               


 Stones Sound Studio, Speakers and Drivers (transducers) are measured using an array of  test 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-amplifier.

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  are ambient 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  the 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  temperature under power
The driver ( transducer ) magnet , voice coil , pole piece  temperatures are measured  
by Fluke thermal bead  direct contact  and  Laser  Infrared  digital temperature meters when doing
 
IEC  Power or RMS Power   Measurements
(See below )
unless other wise stated.

 

 T/S Thiele & small parameters  New Driver  measurement pre conditioning  mechanical  compliance

                       

Movie >   Running in the  speakers at Stones Sound Studio   Laboratory

The Peerless  830883   6.5''   speakers   are  wired in  series /parallel  to an amplifier  and  driven by a    20 Hz  sine wave  for  around 8 hours   before  any measurements
are carried out .  Running in the drivers  helps free up the new  surround , spider and lead out wires  and also  reduces the resonance   frequency  and enables  changes  in
 driver  mechanical complianceto that of a "real world working speaker " .Running in the drivers  insures  more accurate   frequency    Impedance  and  '' Thiele/Small"
 electromechanical parameters that define the specified low frequency performance of a loudspeaker driver  result in more accurate modelling  of  the  crossover and
enclosure design .Running in the drivers in  also  aids   listening evaluations and  improves the  speakers  bass  mid  depth and deta

 

Subwoofer , Woofer , bass /mid  drivers are normally  hooked up to an amplifier with a sine wave  source  between 20 to 30Hz.  The  signal  level is  adjusted till  the cone
excursion is  around 70% of the drivers maximum   X max  ( mm)  -/+ specification   . The  driver is   run in  for 6 to 8 hours .
Will   loosen  the  mechanical stiffness  of the new  speaker , spider , tinsel and cone  surround compliance 
 

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

   Stones Sound Studio , Laboratory  test equipment 

Fig 1  Example of  soft dome Tweeter  THD -Distortion Test

 

Stones Sound Studio , Laboratory  test equipment