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Re: Comparison of systems [message #28196 is a reply to message #28194]

Wed, 18 October 2006 17:36

Wayne Parham
Messages: 18676
Registered: January 2001

Illuminati (33rd Degree)

Doubling impedance cuts current in half, which is equivalent to a 3dB reduction in power. However, impedance of loudspeakers is complex, in that it isn't a flat value like a resistor. What we did last year was to use the minimum impedance to calculate the voltage level that would be needed to produce the desired power level. So we tested last year using fixed power levels rather than fixed voltage levels. The 100 watt power level at 10 meters corresponds exactly to the 1W/1M SPL level. Last year we also measured response at one fixed voltage level, 28.3v, which then gave the 2.83v/1M SPL value since we measured at 10 meters distance.

2005 Prosound Shootout Test Results

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Power / Voltage / Decibel Conversion Charts [message #28197 is a reply to message #28174]

Wed, 18 October 2006 19:57

Wayne Parham
Messages: 18676
Registered: January 2001

Illuminati (33rd Degree)

The measurement datasets are all referenced to 2.0v levels. So the conversion chart below is useful for calculating SPL in terms of power and comparing with other levels. The chart is referenced to 1 watt.

Voltage   Load Impedance  Power       dB
========================================
1.0v          16Ω        62.5mW    -12dB
1.0v          12Ω          83mW  -10.8dB
1.0v           8Ω         125mW     -9dB
1.0v           4Ω         0.25W     -6dB
1.0v           2Ω          0.5W     -3dB
2.0v          16Ω         0.25W     -6dB
2.0v          12Ω         0.33W   -4.8dB
2.0v           8Ω          0.5W     -3dB
2.0v           4Ω           1W       0dB
2.0v           2Ω           2W      +3dB
2.83v         16Ω          0.5W     -3dB
2.83v         12Ω         0.67W   -1.7dB
2.83v          8Ω           1W       0dB
2.83v          4Ω           2W      +3dB
2.83v          2Ω           4W      +6dB
4.0v          16Ω           1W       0dB
4.0v          12Ω         1.3W   +1.25dB
4.0v           8Ω           2W      +3dB
4.0v           4Ω           4W      +6dB
4.0v           2Ω           8W      +9dB
========================================

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Distortion Measurements [message #28200 is a reply to message #28174]

Fri, 20 October 2006 13:28

Wayne Parham
Messages: 18676
Registered: January 2001

Illuminati (33rd Degree)

Many thanks to David Lee for making the Praxis measurements at this year's Prosound Shootout. He did a great job. We were able to gather valuable data on several subwoofers, and can compare response of each of them, seeing how they all perform outdoors, with no room interactions or boundary reinforcement.

We didn't perform distortion measurements at the 2006 Prosound Shootout, but we did in 2005. We measured response and distortion (THD+N) at 28.3v, 100 watts, 200 watts, 400 watts, 800 watts, and so on up to 2400 watts or the maximum allowed by the owner of the cabinet.

Prosound Shooutout 2005 - RESULTS

You can easily read the LMS charts from the 2005 Prosound Shootout. The amplitude response (SPL) is shown as a blue line, with the distortion shown as a violet line. To know the amount of distortion present, simply subtract the distortion from the SPL to read a decibel value, -35dB, for example.

Then to convert to a percentage, use the following conversion chart:

-60dB = 0.1%
-55dB = 0.2%
-50dB = 0.3%
-45dB = 0.5%
-40dB = 1%
-35dB = 2%
-30dB = 3%
-25dB = 5%
-20dB = 10%
-15dB = 15%
-10dB = 30%
-5dB = 55%

If the distortion level falls below the line shown in the noise floor measurement, then distortion is unknown. One can only know it is below the noise floor. The noise floor was 65-70dB up to 50Hz and fell to 50-55dB above 50Hz. Typical amplitude response ranged from 105dB to 120dB, so the average dynamic range was approximately 40dB. That means distortion below about 1% was too low to measure in that setup. To get more resolution, we could have moved the microphone closer but since we were primarily concerned with high power levels, 1% resolution was sufficient.

I wish we had done distortion measurements this year too, so we could obtain that figure for all the systems tested. I'd also like to have measurements of the LABhorn and some other subwoofers that weren't represented at either the 2005 or 2006 events. Hopefully we will have samples of these other subwoofer systems next year. The only charts I've seen of the LABhorn were done indoors, and I'm not sure what power levels were used. This is significant for both response and distortion datasets. It would be nice to have all of the systems compared using the same measurement system and test plan, and performed in the same environment, preferably outdoors in a wide open space like we have had at the Prosound Shootout. So we'll continue to gather information about more systems as time goes by at subsequent Prosound Shootouts, and we'll archive the data here.

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Re: JBL SRX728S [message #28201 is a reply to message #28188]

Fri, 20 October 2006 21:47

Pascal Pincosy
Messages: 1
Registered: May 2009

Esquire

JBL's specs state that the sensitivity of this box is 98dB. The measurements here are showing a max sensitivity of 104dB. Any ideas?

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Re: JBL SRX728S [message #28202 is a reply to message #28201]

Sat, 21 October 2006 00:38

Wayne Parham
Messages: 18676
Registered: January 2001

Illuminati (33rd Degree)

The response curve published by JBL looks very close to the one we made at the Prosound Shootout. But JBL doesn't include response above 200Hz, probably because the intended use is below 100Hz. You'll notice that response is rough above 200Hz and the speaker shouldn't be used that high. Peak output is about 106dB at 500Hz, and there's a peak to 104dB at 220Hz. But there's also a 10dB dip at 200Hz and a 20dB dip at 450Hz.

I'm very pleased with the quality of measurement data we've obtained at the last two Prosound Shootouts. I don't know of any other similar events that have been done outdoors with equipment like this, so I'm glad we've taken the opportunity to take these measurements. I'm looking forward to obtaining more measurements of other systems as time goes by.

JBL SRX728S Specification Sheet

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Re: JBL SRX728S [message #28206 is a reply to message #28201]

Wed, 01 November 2006 14:36

Tom Danley
Messages: 16
Registered: May 2009

Chancellor

Hi Wayne

Pascal may have caught something worth checking out, perhaps there is a mic cal issue or something.
Compare the measured response on the JBL web site for that sub. Your measurement is consistently higher.
Compare your 100W 10M measured response for the 12 pi from last year VS this year, it is also higher.
This may be from something as simple as a mic cal issue (needing a new reference Voltage setting).

Tom

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Re: JBL SRX728S [message #28209 is a reply to message #28206]

Wed, 01 November 2006 21:09

Wayne Parham
Messages: 18676
Registered: January 2001

Illuminati (33rd Degree)

Did you pay attention to the reference level? We measured at 10 meters with the drive signal normalized to 2v. The response we measured is very close to that shown by JBL, within 1dB.

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Re: JBL SRX728S [message #28212 is a reply to message #28209]

Thu, 02 November 2006 06:55

Tom Danley
Messages: 16
Registered: May 2009

Chancellor

Hi Wayne

If you were going to normalize the curves, why not do them so that they are comparable to the ones last year, that is to reflect a 1W 1M equivalent as opposed to something one has to re-calculated for each curve.

Also, if you check those boxes here on the comparison page, and account for the 28.3V drive into a 4 Ohm load, these are still too high in comparison to last year and JBL's published data.

http://www.audioroundtable.com/ProSpeakers/messages/373.html

Tom

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Re: JBL SRX728S [message #28214 is a reply to message #28212]

Thu, 02 November 2006 14:44

Wayne Parham
Messages: 18676
Registered: January 2001

Illuminati (33rd Degree)

Each of those charts was done with a 2v signal. Since the distance was 10 meters, add 20dB for the 1m value. Add another 3dB if you want to normalize to 2.83v. The JBL SRX728S response chart shows about 98dB/2v/1M at 100Hz, 97dB at 70Hz and a smidge under 96dB at 50Hz. Peak output is about 106dB at 500Hz, and there's a peak to 104dB at 220Hz. There's a 10dB dip at 200Hz and a 20dB dip at 450Hz.

I think what may be causing some confusion is the way Praxis works. The data captured hasn't been post-processed or anything like that. There is a sense line that reads amplifier voltage and presents the data normalized to a reference level. In each of the datasets captured at this year's Prosound Shootout, the reference level was 2v. This is comparable to last year's 28.3v measurement, but offset by 23dB. There is also a decibel/voltage/power conversion chart posted so that people can compare SPL between systems having different impedance.

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