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Author Topic: Max SPL on a sub  (Read 10923 times)

Mark Wilkinson

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Re: Max SPL on a sub
« Reply #30 on: June 01, 2017, 07:34:21 am »

I would agree with you.  But how often do you see spec measurements with HP and LP filters in place?  That would throw the numbers off (and not in a good way.

I do agree that this would give a better idea of the loudness in the intended range.  But would still "favor" the freq that are higher in level.

Hi Ivan, What difference does it make how often do we see measurements with HP and LP filters in place?

The question is whether or not a useful, valid, single SPL number exists.
And IMHO the answer is YES.

My guess is someday, an innovative company with sufficient balls and integrity will start using it, or something similar.
Maybe lead/push other stand-up companies to follow suit...
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Mark Wilkinson

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Re: Max SPL on a sub
« Reply #31 on: June 01, 2017, 07:51:46 am »

First of all, if you actually want to measure the peak, you MUST have a meter that can do so.

Most audio (pros even) do not want to spend that much money on a meter.

Without the meter, it does not matter what procedure you have.

I use an NTI XL2 meter.  There are others that are less expensive.

Even some good apps.  But the problem with the apps is that they are limited by the max SPL the mic or program can handle.

And that is well well below the 144dB of the NTI.

Hi again Ivan,  I think the great majority on these forums realize how tenuous peak readings are.
Personally, they have no meaning for me, nor really do A and C weightings.

And I'm with Helge,...
It's almost like always bringing up peaking measurements as an example of what's wrong with measurements in general,
or consistently talking about prevalent misleading industry practices.........
instead of offering solutions.......
becomes part of the problem itself...no offense meant :)
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Josh Ricci

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Re: Max SPL on a sub
« Reply #32 on: June 01, 2017, 03:26:11 pm »

FWIW, a real world usage measurement I've come to like, is Smaart's LEQ, using pink band-passed with the actual HP and LP filters I intend to use, in place.  IMO, this gives an average that is a decent proxy for "a one number meaningful spec".

And taken one step further, I measure LEQ with whatever eq I intend to use to smooth response.
I mean, that's where the sub is going to live, with HP, LP, and eq.....what can it do there?

Using a good RMS voltmeter with averaging, vs the LEQ readings, ....then power handling and even a sensitivity specs make sense to me.

Again FWIW.  I know how much experience you have, please advise if you see anything wrong with this methodology.

One issue with using averaging is that technically dB should be converted to Pa (Pascals) averaged and then converted back to dB. This method always favors speakers with a less uniform response in a direct comparison of a single SPL number. This type of method does not reveal if the subwoofer is producing the output primarily at 70-80Hz and is 20dB down at 30Hz versus another unit that has a much more uniform amount of headroom 30-80Hz for example.

Mark Wilkinson

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Re: Max SPL on a sub
« Reply #33 on: June 01, 2017, 03:54:42 pm »

One issue with using averaging is that technically dB should be converted to Pa (Pascals) averaged and then converted back to dB. This method always favors speakers with a less uniform response in a direct comparison of a single SPL number. This type of method does not reveal if the subwoofer is producing the output primarily at 70-80Hz and is 20dB down at 30Hz versus another unit that has a much more uniform amount of headroom 30-80Hz for example.

Thanks Josh,  any idea how Smaart does the LEQ calc ?
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Josh Ricci

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Re: Max SPL on a sub
« Reply #34 on: June 01, 2017, 04:52:28 pm »

I have my own opinions that I've developed over years of testing, on how to fairly spec a speaker or subwoofer. For the sake of the discussion, I thought this might be an entertaining experiment...I've uploaded some measurements from a subwoofer I designed. What I'd like to do is have anyone interested look at them and give their opinion on what the "spec sheet" should say. Just assume that these are internal measurements from design and development and that you are in charge of putting the spec sheet together for marketing purposes. I already have my own that I would personally use, if this was a commercially available device that was for sale, which it is not. I will list a brief description of each measurement and the conditions. The equipment is high quality and calibrated. I've been doing this for about 10 years now. If there are any questions just ask.

The system is a large subwoofer intended to be used between 25-100Hz.

First off we have an impedance measurement. What nominal impedance should this cab be rated at?






Note for all further measurements shown here.
1.)These measurements are taken in a large, flat, outdoor area. (Half space)
2.)The cab is placed horizontally on the ground.
3.) The microphone (EW's M30) is placed on the ground at a specified distance from the cab.
4.) There is no processing at all in these measurements. No high or low pass filters and no EQ.
5.) The measurements are taken with a very slowly ascending sine wave sweep for higher noise rejection and better resolution.
6.) There is no smoothing applied to the measurements.

Let's look at a voltage sensitivity measurements.
There are 4 measurements on the chart corresponding to microphone distances of 1m, 2m, 4m and 10m from the cab. The voltage used was 14.1 volts measured at the speakon terminal to the cab for each measurement. The measurements are then normalized relative to each other back down to a level corresponding to 1.41 volts at 1m. This being a large sub with multiple radiating surfaces there is some variation in the measurements depending on the mic distance. 10m is more accurate IMO.
How would you rate the voltage sensitivity of this cab? Is 1.41 volts correct or should a higher voltage of 2 or 2.83 volts be used resulting in a 3-6dB higher sensitivity rating?


Let's look at the raw frequency response.
What would you rate the frequency response of this cabinet as?



Now let's look at determining the maximum output of this active subwoofer. I'll present 2 measurement types that are on opposite ends of the spectrum.

Here's the first one. This is measured at a distance of 4 meters ground plane and normalized to 2 meters ground plane (Approximating the SPL of 1m full-space). The high pass filter is engaged for this measurement. It is a 23Hz 12dB/octave BW filter.
The signal used is an ascending sine wave sweep covering 2Hz to 120Hz. It has a duration of 23 seconds.
The amplifier is at the point of limiting and / or clipping throughout the entire measurement. This is basically "wide open" worst case scenario type stuff. Basically this tests accelerated long term heat build up in the voice coils, issues with driver over excursion or bottoming and finds "problem" frequencies. This is considered to be the output limits with long term, low crest factor signals.
Based on this what should be the " Long term" maximum output rating? Should this be rated as is as 1m full-space, or pick up the free 6dB by rating it at 1m half-space?




The short term output is quantified by a different type of measurement. I use 1/3rd octave, Hann windowed, 6.5 cycle long burst signals. This signal type was popularized by Don Keele years ago and has since been incorporated into speaker, subwoofer and amplifier measurements. It is available in Clio, REW and Igor Pro softwares and possibly others. Typically distortion thresholds would also be used to determine the maximum useful output of the speaker. I prefer to test to the distortion limit and also beyond for the sake of completeness and to determine the speakers absolute limits if passive and whether it is protected well enough not to blow if active. Measurements are taken at each 1/3rd octave interval. The 2 graphs below are the same data presented different ways. The measurements are referenced and normalized to a distance of 2 meters ground plane, though the measurements may actually be made at 4 to 8 m for larger cabs such as this.(Again the SPL reported here corresponds to 1m full-space SPL.) Also the SPL is not truly a "peak" measurement it is an "rms" type of reporting. It is directly comparable to the sine wave measurements.

Whew...Ok, with that out of the way what would you rate the short term or burst SPL of this subwoofer as? Do you add the free 6dB by making it a 1m half space number? Do you add 3dB to make it a peak reading? Also note that in the top graph this subwoofer is the center blue line only! The other 2 are historic min and max.

« Last Edit: June 01, 2017, 05:01:45 pm by Josh Ricci »
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Josh Ricci

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Re: Max SPL on a sub
« Reply #35 on: June 01, 2017, 04:55:29 pm »

Well that turned out to be a much bigger post than I thought.



Thanks Josh,  any idea how Smaart does the LEQ calc ?

I have no idea. I've actually never used Smaart. Maybe it has more information in the user notes of help file?

Chris Grimshaw

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Re: Max SPL on a sub
« Reply #36 on: June 02, 2017, 03:59:55 am »


First off we have an impedance measurement. What nominal impedance should this cab be rated at?


In the interest of playing it safe, I'd actually call that a 2ohm cab. A more "nominal" value would be 3ohm, but when it comes to very low impedances (and choosing amplifiers to match), I think it's best to be conservative.

- I think so long as the sensitivity measurement is clear (ie, 2.83v @2ohm or whatever), then anything sensible will be fine. If you're looking at buying something like this, you should be able to convert as needed.

- I'd call it 30Hz-150Hz.

- The most common usage for subwoofers is placed on the ground, so I'd probably standardise for that, with the note that 6dB would be lost when flown high up.

- Rating the maximum SPL is tricky since the frequency response is sloped. I'd probably rate maximum continuous and burst at the top and bottom of the passband (noted above).

Chris
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Ivan Beaver

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Re: Max SPL on a sub
« Reply #37 on: June 02, 2017, 07:30:20 am »

Well that turned out to be a much bigger post than I thought.



I have no idea. I've actually never used Smaart. Maybe it has more information in the user notes of help file?
ANd the bottom line is that you can get different "numbers" from the same loudspeaker-depending on what you are looking for/at.

When the only spec is "max XXdB SPL", that REALLY doesn't tell very much.

And without any supporting data, it kind of "says", we don't want people to know where the numbers come from.  At least to me.

Manufacturers used to give lots of data-but it is becoming less and less these days.

Yet, measurement systems are very affordable-as compared to years past.
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Ivan Beaver
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Josh Ricci

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Re: Max SPL on a sub
« Reply #38 on: June 02, 2017, 05:23:51 pm »

In the interest of playing it safe, I'd actually call that a 2ohm cab. A more "nominal" value would be 3ohm, but when it comes to very low impedances (and choosing amplifiers to match), I think it's best to be conservative.

- I think so long as the sensitivity measurement is clear (ie, 2.83v @2ohm or whatever), then anything sensible will be fine. If you're looking at buying something like this, you should be able to convert as needed.

- I'd call it 30Hz-150Hz.

- The most common usage for subwoofers is placed on the ground, so I'd probably standardise for that, with the note that 6dB would be lost when flown high up.

- Rating the maximum SPL is tricky since the frequency response is sloped. I'd probably rate maximum continuous and burst at the top and bottom of the passband (noted above).

Chris

Both you and Ivan make points I agree with and both of your posts, in combination with my own post of the measurements of a cab, illustrate the problem. There is no way to express the maximum SPL of speakers, in a way that would be meaningfully comparable, with only a single data point. The same could be said of voltage sensitivity.

Let's say that the pro industry started to move towards standardized ratings of the maximum SPL of a subwoofer. They still want a single data point for the consumers. A 50Hz test frequency is settled on as that's right in the wheelhouse of any subwoofer. You have to settle on a test signal too. Let's say Mr. Keele's 6.5 cycle shaped burst. You also have to agree on the measurement conditions say outdoor ground-plane and the reporting method. Say 1 or 2m half space, or whatever. A measured result is required.

Fast forward a few years and the market starts to see the new max spl @50Hz spec factoring into subwoofer sales. What happens logically? The manufacturers start to design their subs to make the most sound possible at 50Hz as it could result in increased sales. Of course they may be shortchanging the deep bass extension and headroom or other aspects of the device to do that. It's still only a single data point.

I don't think it would be that difficult to make a fairly simple industry spec for subwoofers. I'd like to see this.

1.) Graph of measured impedance for passive cabs covering 1/2 octave below and 1/2 octave above the speakers rated bandwidth of use.
2.) Graph of measured voltage sensitivity of the cab with voltage applied and other required measurement data (ground plane or flown? Mic distance, smoothing used, etc.)
3.) Graph of frequency response for active cabs. (Voltage sensitivity takes care of passive cabs.) Again the measurement specifics need to be noted.
3.) Adopt something similar to CEA-2010 to rate the maximum output. Use the 6.5 cycle shaped bursts at 1/3rd octave intervals. Use the distortion thresholds for a pass/fail result. The consumer home audio industry is far ahead in this regard in my opinion. At least they have some sort of sensible protocol for it that has gathered traction. Personally I would throw out the averaging all together and a few other things from their standard. I'd like to see the subwoofers be tested over their full bandwidth in 1/3rd octave intervals. If your sub is rated down to 20Hz (Never mind that it is well below tuning and 20dB down by then) it needs to have the maximum SPL reported at that frequency. That would prevent overzealous claims of extension that isn't useful in the real world. With active cabs this will quickly show that the flat response shape dialed in with heavy EQ at lower volume levels, does not hold up in the least as the output demands are increased. With passive cabs it would be best to use a voltage source which is limited to the maximum recommendation by the manufacturer. A format similar to mine I posted earlier makes a lot of sense.

In my opinion the 3 measurement graphs listed above would do about 90% of it. A bit of text on a spec sheet isn't really the way to do it.

Ivan,
Danley already has 2 of the 3...

Ivan Beaver

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Re: Max SPL on a sub
« Reply #39 on: June 02, 2017, 07:16:46 pm »

Josh, I am sure you know this, but the problem with providing information that nobody else does, is that some people will try to use this information to "judge" the performance-without actually knowing or understanding that data.

Let's look at distortion.  Many people would like to see this.

But in what way?  A percentage of the output?  A specific SPL at a specific distance?  Is it for a specific freq? The maximum at some freq?  An average across the range?  If an average-what is the range?

So let's say that a manufacturer comes up with a "single number" (like amplifiers do).

Let's say it is 2%.

If the level is pretty high, that is very low for a loudspeaker.

But yet some people would see it as extremely high-as compared to amplifiers or other electronics.

So they would claim that the particular model has really high distortion.

But what about a speaker that they like?   Do they know what the distortion is-and was it measured the same way?

Unless you have a USEABLE reference to compare, you have no idea if the numbers being presented are good or not.

YES-it get pretty complicated pretty quick.
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Ivan Beaver
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