Tricky mic position create random response mesure

[B&KMan]

Hi everybody,

After small controverse in regards of position mic for evaluation calibration SPL, I perform for members, a small experience of danger of mic in near field for coherent response and amplitude value.

I put my microphone in near field : exactly same space mic-driver of all 3 mesures but just keep mesure in arroud top edge of driver center of driver and low edge of driver...

Ok if you look pict the change position is really small maybe 2 inch for all 3 position in vertical and no in deep and no in lateral...

[B&KMan]

In according of picts the result of 3 mesures is catastrophic random response in frequences and amplitude...

conclusion at this closeup all positions is too critical for good evaluation and reproductible datas. each 1/16 inch back, angle, down or side affect strongly mesure... do not put your SLM or mic too close... inside of 2 feets the close-up is too critical and push a complexe procedure for extraction of fairs stable datas.

I hope this help to better calibration..






Jean.

[B&KMan]

So At frq 16K and at exactly same power output, the difference value is more than 12DB depend just 1 inch vertical position!! at 1/8 inch higher position; differente curve and power response....

So if you put your big slm into near field for evaluate SPL add the reflextion of instrument + the type of mic, plus if you load in weight "C" or "Lin"...

Indianas jones jungles sound heres baby....

If you consider any this type of response for calibrate high or med or Low Frq, what your chance to drop notch right on the good value ??


No surprise many sceptical is claim : hey my ear is the best (youafafafaf).



Jean.

[Zilch]

Oh, Jean, Jean...

I BELIEVE Ian was measuring INTEGRATED near field SPL, not FR, and he advised using a tripod such that the microphone could be precisely positioned at the desired location for most reliable determinations.

In any case, as he subsequently reiterated for clarity, the measurement and adjustment is RELATIVE, NOT absolute.

[I'm thinking OTHER things, too, but I'll just stifle them for now....]

[johnaec]

I think a much more meaningful test would be at the standard 1 meter distance, which allows for normal wave development and propagation, especially with a slot type of radiator. Up so close, you're probably dealing with speaker "quantum mechanics" effects. And standard 0, 15, 30 and 45 degree offsets would seem to relate much closer to what you're actually hearing and listening for.

John

[B&KMan]

Oh, Jean, Jean...

I BELIEVE Ian was measuring INTEGRATED near field SPL, not FR, and he advised using a tripod such that the microphone could be precisely positioned at the desired location for most reliable determinations.

In any case, as he subsequently reiterated for clarity, the measurement and adjustment is RELATIVE, NOT absolute.

[I'm thinking OTHER things, too, but I'll just stifle them for now....]

hey hey Zilch... talk your all your point of view with no reserve ( )


for spl with sinewave:

mmmmm what is interest to keep relative SLP of sinewave of 16K is any 1/16 inch change on couple Db the SPL ??? any mesure in interesting if reproductible... this is a basis of any datas...

the interest to expose the many differents fr response in DB and expose the yoyo SPL if you choose generate one frq or other...

choose any fr and check the difference in reagrds at same fr but just little difference keeping...

Look the Global spl line in white and the total of thi difference is gange more than 4 db in pink noise !! (this is result of mesure exposed )...

so the measuring INTEGRATED near field SPL in this method is very very tricky...

So maybe I have completely in field but I do not understand what is interest record arbitral value with very tricky reproduction value for the same driver and for other...

so the result is
not conform to power spectral energy of driver at all fr and at specific fr
and not according to environnemental area listening
Low reproductible value

So open my eyes and explain what good aspect is possible in this set-up

This is my 2+2 cents point....

[Ian Mackenzie]

Okay,

I am now Back, the Daleks nearly got me..lucky for my sonic screwdriver I escaped!

Zilch is correct.

My reference is in effect to the design of the system with respect to relative Spl pressure response. The drivers respond to a specific spl based on sensitivity and the voltage drive applied via the filter sections. Therefore for each driver to operate at a matched spl of acoustic output from flat amplitude the voltage drive to each driver must be adjusted via the L PADs.

In order to understand this it is important to realise we are refering to net spl of each driver at a specifc descrete point. The overall flatness and integration is a function of the filter slopes, phase response and location of the drivers on the baffle. JBL have already done this design work so our JOB is quite simply to adjust the voltage drive.

After some trial measurements about 18 months ago I realised using FFT, MLS and Pulsed Analysers lacked the instringic precison for performing a measurement of net SPL based on differential of the voltage drive.

So I pulled out my trusty Tandy meter and mounted it on my tripod and fed a sine wave via a PC contolled function generator to the amplifier. By placing the meter a specific distance from the baffle in front of each driver I was able to measure with great precision the net SPL via fine adjustment of the L pads.

I used frequencies of 600, 5000 and 150000 for the midrange, horn and slot respectively.

I started with the slot and positioned the meter directly on axis about 2 inches from the baffle and wound down the pad on the horn fully and adjusted the level with the Slot Pad fully open to +4 db on the meter. I then carefully adjusted the L pad till I got 0 db, after repeating this several times I marked the postion on the foil Cal.

I then place a masking tape over the Slot mouth and repeated the same test with the horn with the meter again on axis with a reference of +4db (and wound down the midrange L pad fully) and then adjusted the horn L pad to 0 db.

The same technique was applied to the mid come using several measurements to assure precison of the +3 attenuated back 0 db level.


The location of the meter should be directly on axis but exact location is not overly critical. The reason is simple in the that we are only concerned with the net relative spl after adjustment from fully open L pad back to a desired point for 0 dba. As JBL advise this on their foil cals in Dba and the voltage drives from the crossover filters are already pre determined it is fairly straight forward to adjust the exact levels of each driver.

I think it is reasonable to assume that JBL with all their vast resources would have found the 0 dba reference point.

In fact I have found using this technique the only reliable and repeatable method of adjusting the relative balance of the drivers on a 4 way system.

Mid and far field measurements using even gated analysers do not offer the same precision for this kind of measurement.

When I then played the system it has portrays the smoothest and most remarkable coherent sound and the imaging is amazing.

I find the need for further adjustment completely un necessary

Needless to say my tongue in cheek remark (since edited) in the crossover modification thread is probably apt. It's simply a case of interpreting the available information and applying it with some simple techniques to solve a problem.

But if we come back here in few years time we will probably find people scratching their heads with the same problem.

The Doctor