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Measurements of Cover and Cap Compression Driver Loading

Posted by bp on June 7, 2001 at 05:18:16

I got curious about the actual effects of the back cover and loading cap and decided to do some measurements. I also thought that the frequency response might be of some general interest. These measurements are by no means the last word in accuracy but seem to be fairly consistent. These are the measurements performed on an Altec 802-8T driver mounted on a damped 511B horn. The measurements were taken at a distance of 39" on axis with a Radio Shack sound level meter mounted on a tripod. The source tones were the warble tones on Styke's Basslist Test CD. The Radio Shack meter was uncalibrated and stock with no corrections made to the values to compensate for the known inaccuracy of this meter at high frequencies.

  
  
 Effect of Back Loading on Altec 802-8T Compression Driver on 511B horn 
          
        Sound Level (dB)* 
   
 Freq.   Cover ON Cover ON Cover OFF Cover ON 
 (Hz)   +40uF       +Loading Cap 
 ---------------------------------------------------------- 
   201  56  57  56  56 
   254  64  67  70  63 
   320  76  80  79  75 
   403  81  84  83  79 
   507  80  81  80  76 
   640  82  83  83  78 
   806  86  87  87  81 
   1150  86  85  86  82 
   1280  87  87  88  84 
   1612  85  84  84  82 
   2029  80  81  81  82 
   2560  83  83  83  85 
   3226  84  84  84  85 
   4064  84  84  84  85 
   5120  85  85  85  86 
   6450  86  86  86  87 
   8124  84  84  84  84 
   10240  84  84  84  84 
   12902  82  81  81  81 
   16256  66  66  66  66 
   20480  60  59  59  58 
   
 *The 2nd column showing the sound pressure levels with the cover on are  
 measurements with a 40uF 500Hz crossover capacitor in series with the  
 driver. A 1/4" felt pad was used for the measurements in columns 2 and  
 3 with the cover on and no loading cap. The cover was unscrewed for  
 column 4 and left hanging as far away from the driver as the leads  
 would allow. For column 5, the black bakelite loading cap was assembled  
 onto the diaphragm and the cover replaced. 
   
 Here is a correction chart that was posted previously: 
   
   200 _Start_______________________________________ Flat 
   250 _____________________________________________ Flat 
   315 _____________________________________________ Flat 
   400 _____________________________________________ Flat 
   500 _____________________________________________ Flat 
   630 _____________________________________________ Flat 
   800 _____________________________________________ Flat 
   1kH______________________________________________ Flat 
   1.25_____________________________________________ Flat 
   1.6 _____________________________________________1600HZ = .1DB 
   2 _______________________________________________2000HZ =.2DB 
   2.5 _Start______________________________________ 2500HZ =.3DB 
   3.15_____________________________________________3150HZ =.5DB 
   4 _________________________________________________4KHZ =.8DB 
   5 _________________________________________________5KHZ =1.3DB 
   6.3 _____________________________________________6.3KHZ =2DB 
   8 _________________________________________________8KHZ =3DB 
   10 _______________________________________________10KHZ =4.4DB 
   12.5____________________________________________12.5KHZ =6.2DB 
   16 _______________________________________________16KHZ =8.5DB 
   20 _______________________________________________20KHZ =11.2DB 
  
  
  

It appears that there is no measureable effect of leaving the back cover off. This dosn't rule out that small reflections beyond the precision of the measurements will influence the sound.

However, the loading cap does have a very large measureable effect on the lower midrange from 200Hz-1600Hz. The sound pressure in the lower midrange is decreased by 4-6dB. The high frequencies seem to be relatively unperturbed. With the correction factors, the response begins to roll off somewhere between 13 and 16 KHz and is down about 10dB at 16kHz.

The drivers in the 511B horns have a fairly good low end response down to around 400Hz where it begins to roll off. There also appears to be a peak from around 800Hz to 1200Hz.

This post is made possible by the generous support of people like you and our sponsors:

Sonic Craft

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