Re: Why Do TEF Systems read 3-5 dB high ?
Lest anyone thinks I am picking on DSL, JTR, or any specific company using TEF, that simply is not the case.
I have put together another example of the disparity that seems to occur with TEF systems below, with information taken from the B&C spec sheet for the BC18SW125.
Some background information is included for those not familiar with bass reflex (BR) and tapped horn (TH) designs, which are used in the graphs below.
BR uses a port tuned to the cabinet internal volume, the output of the port combines with the speaker output to bring up the low end response of the speaker.
If the speaker is tuned for flat response, the output down low will be no greater than the upper response the speaker has in a sealed box.
By making the port/cabinet tuning frequency (Fb) higher than optimum for a flat response, it is possible to create a bump in the response above the 3 dB down point (F3), the upper response of the speaker will fall back to the normal level it has in a sealed box of similar size.
The Lab 2x12” ported design in the lower left graph does this, a nominal 92 dB speaker (two 89 dB speakers +3 dB) puts out 94 dB at the bump. Details of the 2x12" design are in the DIY section of this site.
As can be seen in the upper left chart, the 95 dB sensitive BC18SW125 speaker rolls off at about 12 dB per octave in a sealed box. In a “classic” ported box, shown in the simulation in the lower left, response falls at about 24 dB per octave below Fb.
The simulation does not accurately show what happens to the upper response of the speaker, the port creates a “pipe” resonance, internal reflections create peaks and dips.
To get a low F3 requires a low Fb, if you want the speaker to have relatively flat response, the box has to be big. The upper right graph of a B&C BR design shows the falling response of a fairly big box tuned quite low.
Something “funny” has happened to the speaker’s output in that upper right TEF graph, the output should not be any higher than 95 dB at 100 Hz, but it shows about 98 dB, THREE DB MORE THAN IT SHOULD BE.
Now, a very little information on TH, Tom Danley has a full TH white paper on the DSL website for those that want to know more than the very basics. The TH more effectively uses the front and back radiation of the speaker than a BR, the rear side of the driver enters the horn at a location (tap) at a distance far enough away from the throat, where the front of the driver is driving the horn.
The TH that I designed and built, called the Keystone, is about 6 dB louder than a BR cabinet using the same driver, with a similar F3.
Two of those BR cabinets would still be almost 3 dB down from the Keystone TH and take up more truck space.
The lower left graphs show several different cabinet designs and speakers tested at the same outdoor location, same drive level. All except the TH with extender used a BW 24 filter at 25 and 125 Hz.
0 dB on the lower left graph should be about 91 dB, if the published charts of B&C, Eminence, and JBL are correct.
The Lab 2x12” (FB 36 Hz) is nominally 3 ohms, the rest are all nominally 4 ohms.
The BC 18BR38 (FB 38 Hz) is of similar size to the 18 SW 115 simulation, it appears quite close to the 94 dB that sim shows.
The Keystone TH (TH18S no plate) is 45 x 26.5 x 22.5, double the size (volume) of the Lab 2x12”, a bit bigger than a DSL TH-118, a bit smaller than a JBL SRX728.
The JBL SRX728 is rated at 98 dB sensitivity, on the published chart it reaches around that above 60 Hz.
The Keystone TH is almost exactly the same sensitivity as the 728 from 45-80 Hz, louder above and below (other than below 32 Hz).
The Keystone, using the same B&C speaker as the TH-118, being larger, and having a slightly higher LF corner, should theoretically be louder than a TH-118, according to Hoffman's Iron Law.
Given Tom Danley’s design genius, a difference of 3 dB between the two tapped horns (mine being the lesser) seems possible.
The side by side manufacturer’s graphs (I dropped the DSL graph by 3 dB to account for voltage difference) of the TH-118 and the SRX728 show the TH-118 to be around 7.5 dB louder over most of the sub range.
It simply does not seem plausible that two tapped horns of similar size , frequency response, using the same speakers would be 7.5 dB different in level.
Art Welter
