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Old 6th February 2011, 12:26 AM   #361
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Quote:
Originally Posted by jbell View Post
AND.... the real game changer here.... TH's are directional.... I've measured up to 12db front to back. Which explains why I LOVE the 28v10m test.....

Makes a BR vs TH comparison, like bringing a knife to a gun fight.
Jim,

I appreciate the work you have done and shared.

Like you, I like to make sawdust, then see if results match predictions.

Sorry you seem defensive about a detail that could easily be confirmed with a simple a/b test.

Sonic memory is rather fleeting, I prefer to see actual side by side tests done rather than believe simulations. The simulations only show a 3 dB difference however, while guns and knives are more like 10 dB different :^).

Directionality does increase on axis sensitivity. However, for a horn to start to impart directionality, the mouth size needs to be about 1/4 wavelength wide. That size would happen in multiples, but a single horn the size of the SS15 won’t have much directionality below 140 Hz.
Directionality follows the inverse square law, it makes no difference the distance the measurement is taken at.

It is easy to see this effect when looking at polar charts of any horn, once the mouth size is less than a wavelength, the pattern starts widening.
If you have measured greater directionality of a single SS15 at low frequencies, please direct me to where I could see them.

Art
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Old 6th February 2011, 12:54 AM   #362
jbell is offline jbell  United States
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art:

you missed olivers point with his horn resp.... (it was what I was getting at with my 'show me the hornresp comment from earlier.')

A 220liter reflex with a 3015lf doesn't tune well to do an A/B vs the SS15. You either tune it lower than SS15, or you make the box smaller.

an A/B won't work.


On the directivity, what happens (in general) when you put 2 sources close to each other?

Last edited by jbell; 6th February 2011 at 01:06 AM.
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Old 6th February 2011, 02:19 AM   #363
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Quote:
Originally Posted by jbell View Post
art:

you missed olivers point with his horn resp.... (it was what I was getting at with my 'show me the hornresp comment from earlier.')

A 220liter reflex with a 3015lf doesn't tune well to do an A/B vs the SS15. You either tune it lower than SS15, or you make the box smaller.

an A/B won't work.


On the directivity, what happens (in general) when you put 2 sources close to each other?
Frankly, I really don't care exactly how the low end of the 3015LF in a box compares to the SS15, even a sealed box test would show the sensitivity of the driver between around 120 -200 Hz (it should be about 99 dB, as the specs indicate), an A/B would determine whether your dB meter is correct, or reads a couple dB high as it appears.

Directivity depends on the size of the sources, but two stacked sources will reduce the vertical coverage while maintaining the horizontal coverage.

Since the ground plane reflects the vertical height, a vertical sub stack is acoustically twice as high as its physical height, imparting greater directivity than one would think based on physical height alone.

This point is often missed by proponents of “center cluster” subs, by the time they are put in a long line across the front of a stage, the horizontal coverage is reduced, so side coverage is lacking, while the subs are still omni in the vertical dimension, putting out nearly as much energy behind as in front.

Last edited by weltersys; 6th February 2011 at 02:39 AM.
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Old 6th February 2011, 02:40 AM   #364
jbell is offline jbell  United States
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Quote:
Originally Posted by weltersys View Post
Directivity depends on the size of the sources, but two stacked sources will reduce the vertical coverage while maintaining the horizontal coverage.
very good.

how many sources of sound are there in a tapped horn?
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Old 6th February 2011, 04:23 PM   #365
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Default 2 Sources

I believe JBell is referring to these sources:

1. sound from the back of the loudspeaker driver
2. sound from the front of the loudspeaker driver that has travelled through the enclosure.

I can certainly say that it appears to work for me.

Regards, Ben
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Old 6th February 2011, 04:35 PM   #366
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Default But how does Behringer measure it?

Quote:
Originally Posted by jbell View Post
Here's a behringer 2031p... with well published specs. It's 89db 1w1m 4ohm. So... 2v should get something close to 89db.

well I'm within 2db of their published spec... on the cheap-o spl meter using the EXACT same setup as I did with the ss15.

now it's cold... I'm not dragging this stuff back out in the cold again.


I'm sure I am not the first to think of this, but I don't think anyone has mentioned it:

if Behringer tests in an anechoic chamber, and you test on the ground... the 2 dB difference to the specs could be accounted for by this?

Regards, Ben
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Old 6th February 2011, 07:22 PM   #367
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Originally Posted by jbell View Post
very good.
how many sources of sound are there in a tapped horn?
The amount of sound sources would be the amount of drivers contained in the tapped horn, though at low frequencies closely spaced drivers are virtually the same as a single larger driver.

The output of a tapped horn is a single acoustical source comprising the summed response of the speaker cone front, rear, and internally reflected radiation.
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Old 6th February 2011, 09:13 PM   #368
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Originally Posted by salzburgsoundsystem View Post
I'm sure I am not the first to think of this, but I don't think anyone has mentioned it:

if Behringer tests in an anechoic chamber, and you test on the ground... the 2 dB difference to the specs could be accounted for by this?

Regards, Ben
Testing in an anechoic chamber would imply a free space measurement, which would be up to six dB less than a half space (ground plane) measurement at low frequencies.

For frequencies in wavelength less than the center of cone to baffle edge, the baffle creates a half space, so measuring high frequencies in half or quarter space shows less increase than would be predicted by pure physics, which use a single point source radiating 360 degrees as the model, which never happens in the real world.

Having no response curve to look at, my speculation would be Behringer may have merged a ground plane measurement with a free space measurement, this makes the upper portion of the graph look smoother, and shows more LF, commensurate with the speaker placed next to a wall as is common in many small room situations.

If the speaker actually had a flat response to 60 Hz or so 89 dB in free space, it would measure around 95 dB in half space. A ported speaker the size of a B2031P would not be that sensitive that low, hence my previous assessment.

Manufacturers use various amounts of smoothing (or averaging), more smoothing can reduce +/- 6 dB peaks to +/- 3 dB (the usual range) or less.

Since a dB meter responds to the loudest frequency peak, it will always read higher than the average level of a speaker, unless the speaker happens to be perfectly flat response.

Since Jim did not do a frequency response test on the speaker, he only determined that the loudest point in the speaker’s response was 91 dB as read by his SPL meter, which unfortunately tells little about the speaker, or accuracy of the meter.

In the chart below, the same small tapped horn (15” x15” x26.5”) with a 10” speaker are compared, the blue trace is using an Eminence Kappa Pro 10”, a +/- 3 dB response from 53 to 160 Hz.

The orange trace is wimpy magnet 10” speaker from an abandoned Fisher stereo cabinet .

Using a dB meter and pink noise band limited to around 160 Hz, the Fisher speaker would read within 1 dB of the Kappa Pro, even though it is 10 dB down (half as loud) for most of the sub range !

Measuring can be fun, interpreting the results even more so.
Having studied the squiggly lines since around 1977, it becomes easier to interpret results, but without knowing precise details of the test procedure and equipment used, results can be very ambiguous.
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Old 7th February 2011, 12:38 AM   #369
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Quote:
Originally Posted by jbell View Post
A 220liter reflex with a 3015lf doesn't tune well to do an A/B vs the SS15. You either tune it lower than SS15, or you make the box smaller.
...or you design it with an expanding vent. This should boost the passband response a bit, if done right.

I've included an example below. This is an 8" design I'm looking at, where an expanding vent is used to counteract that inductance rolloff and flatten the passband. A dB or so is added, and even more can be added if I'm willing to go with a larger box size.
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Old 7th February 2011, 06:08 PM   #370
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Quote:
Originally Posted by jbell
A 220liter reflex with a 3015lf doesn't tune well to do an A/B vs the SS15. You either tune it lower than SS15, or you make the box smaller.

Quote:
Originally Posted by Brian Steele View Post
...or you design it with an expanding vent. This should boost the passband response a bit, if done right.

I've included an example below. This is an 8" design I'm looking at, where an expanding vent is used to counteract that inductance rolloff and flatten the passband. A dB or so is added, and even more can be added if I'm willing to go with a larger box size.
Regardless of the contribution of the vent, any ported or sealed alignment (excluding bandpass ) of a given speaker will be within a dB or so in the region from 120-200 Hz.

The question of actual sensitivity of a specific tapped horn compared to ported cabinet will remain until an A/B test using the same speaker is performed.

If the actual measured SS15 level from 45-120 using a 3015LF were to exceed the front loaded SPL of the same speaker at 120-200 at the same drive voltage, that would be reasonable proof the SS15 would exceed a normally damped ported alignment.

Art
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