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Thats great, we have to show consistency and that is why you should start measuring at 75db. That is what everyone else is holding to at REW forum, unless there is a house curve or for measuring for compression.
Why 75dB? I think it's more important to start somewhere where compression becomes a legitimate concern. If the driver is moderately efficient (mid 80's), compression isn't of concern until you're at least there. So, to me, 75dB testing is moot.

"While 75db is the default for REW it is hardly a standard per se. There has been a lot of emphasis over the years on 75db being a standard of some sort but it is really just a reference guideline for level setting speakers. "

Right, so stick with it! If you started at 75db then you could go up in 10 db increments and have this reapeatable for all of your measurements. Would this not make sense from a sciencetific/reference standard.

Looking at your graphs of the 5 woofers that you measured there are no target lines. If you are not measuring at 75db you then need to state the target level for the measurenent taken. IE: on the graph for the Emotiva where would you place the target level? And in the graph for the SVS is the target level 116db? If so you missed it by 20db!
I have a very fundamental issue with the talk around compression testing here. What's catching me off guard here is people saying to test compression by increasing the output to achieve 10dB steps. This is flawed. Compression shouldn't be solely focused on the FR. Compression is a measurement of input vs output. It is not a measurement of response as you increase the output itself. For example, if you increase the input voltage at a speaker's terminals then you should get that same relational value in dB output by the DUT. Anything less is due to the effect of compression.

Yes, you are looking to see how the response changes but what's the point if a driver is so inefficient at higher output that you're having to feed it twice as much as you should for the same output?

Consider this: What happens if you're increasing SPL by 10dB but your voltage ratio from your previous input voltage of 4v to your new voltage input, which should be 12.6 volts, is actually 14.6v? All you've done is increase the output but you've failed to acknowledge the 2v loss in your test, which equates to about 14watts or 6dB! Yikes! So, yea, the curve at the reference frequency increased by 10dB, but it's not illustrating the fact that you just had to make up more than 6dB by turning the amp gain up higher and higher.

This is what compression testing should tell you. Again, voltage in vs SPL out vs what should be there. Then you get the FR curve, but most importantly, you get to see how efficiently the driver is able to use the power provided to it.

Hope that makes sense. Maybe my assumption on how this kind of testing is being performed is wrong so please correct me if so.

- Erin


Below I've attached a picture of compression testing I did on a Seas w18nx driver. This is the 20-110hz band. As long as the lines are stacked on top of each other, what you're seeing is what is expected; no loss in output vs voltage input. Where the lines deviate, is an indication of how much output (in dB) is lost with the input vs the initial voltage vs frequency. As you go higher in frequency in the chart, you can see a loss of about 0.4dB from 1v to 8v input.






This is the same thing but from 400hz to 6khz. You can see a loss of about 0.8dB at 3800hz. Likely due to inductance issues (yet to verify).

 

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Discussion Starter #22
Thats great, we have to show consistency and that is why you should start measuring at 75db. That is what everyone else is holding to at REW forum, unless there is a house curve or for measuring for compression.
Bob, I understand what your preference would be however; I have clearly outlined what the methodology is and how I will go about taking the measurements. If I were to cater to everyone's preference I would never get anything done. The purpose here is to compare my measurements to the measurements posted, published, etc by the manufacturer. 75db is not a standard. I do not know how else to explain that. Additionally this is not REW forum discussion, The Sub Zone is it's own forum and the fact that I happen to use REW is irrelevant to this discussion outside of the defined methodology. The consistency comes from the measurements listed and outlined below taken directly from the methodology.

Frequency Response (FR)
Frequency response is measured utilizing a 512K (11.9s) sine sweep from 0 Hz to 200 Hz via REW (Room Equalization Wizard). The microphone is placed on the ground at a 2 meter distance measured from the center of the subwoofer driver. In the event a particular subwoofer requires a deviation from this it will be noted within that particular set of measurements. For example, if a ported/vented sub with multiple drivers requires different mic placement in order to capture accurate results, it will be spelled out within that particular subwoofer test.

Long Term Output
Maximum long-term output is measured using a 1M (23.8 second) sine sweep from 0 Hz to 200 Hz. The first sweep is taken after level matching 50Hz at 90db. The level is then raised by 5 db for each successive sweep until the output level is clearly compressed. Compression measurements are taken at the same time as the max output. The compression graph reflects the relative compression to the original 90 db sine sweep.

Right, so stick with it! If you started at 75db then you could go up in 10 db increments and have this reapeatable for all of your measurements. Would this not make sense from a sciencetific/reference standard.
You have just made my point. I could start using 75db, but it is irrelevant to the tests that I perform. Saying it repeatedly does not make it true. You are more than welcome to start your own forum and start all of your measurements at 75db or whatever number you would like. Starting at 75db and moving it up 10db per sweep is no more scientific than starting at 90db and raising it by 5db as I have outlined in the methodology.

Looking at your graphs of the 5 woofers that you measured there are no target lines. If you are not measuring at 75db you then need to state the target level for the measurenent taken. IE: on the graph for the Emotiva where would you place the target level? And in the graph for the SVS is the target level 116db? If so you missed it by 20db!
Uhm.. What? I did state the target. 'From 0 Hz to 200 Hz starting at 90db and raising it 5db until the signal compressed. Please let me know what part of this is unclear to you.

Of course it would, you are describing what you hear in your room, we need to see the actual in room freq resp to correlate what is measured with what is heard.
One has absolutely nothing to do with the other. You cannot correlate a subjective opinion to a sine sweep. The best you could hope for is to correlate it to a given part of a song or film that I happen to capture that I also commented on. Subjective, emotional responses to audio are not measurable.
 

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Discussion Starter #25
Why 75dB? I think it's more important to start somewhere where compression becomes a legitimate concern. If the driver is moderately efficient (mid 80's), compression isn't of concern until you're at least there. So, to me, 75dB testing is moot.



I have a very fundamental issue with the talk around compression testing here. What's catching me off guard here is people saying to test compression by increasing the output to achieve 10dB steps. This is flawed. Compression shouldn't be solely focused on the FR. Compression is a measurement of input vs output. It is not a measurement of response as you increase the output itself. For example, if you increase the input voltage at a speaker's terminals then you should get that same relational value in dB output by the DUT. Anything less is due to the effect of compression.

Yes, you are looking to see how the response changes but what's the point if a driver is so inefficient at higher output that you're having to feed it twice as much as you should for the same output?

Consider this: What happens if you're increasing SPL by 10dB but your voltage ratio from your previous input voltage of 4v to your new voltage input, which should be 12.6 volts, is actually 14.6v? All you've done is increase the output but you've failed to acknowledge the 2v loss in your test, which equates to about 14watts or 6dB! Yikes! So, yea, the curve at the reference frequency increased by 10dB, but it's not illustrating the fact that you just had to make up more than 6dB by turning the amp gain up higher and higher.

This is what compression testing should tell you. Again, voltage in vs SPL out vs what should be there. Then you get the FR curve, but most importantly, you get to see how efficiently the driver is able to use the power provided to it.

Hope that makes sense. Maybe my assumption on how this kind of testing is being performed is wrong so please correct me if so.

- Erin


Below I've attached a picture of compression testing I did on a Seas w18nx driver. This is the 20-110hz band. As long as the lines are stacked on top of each other, what you're seeing is what is expected; no loss in output vs voltage input. Where the lines deviate, is an indication of how much output (in dB) is lost with the input vs the initial voltage vs frequency. As you go higher in frequency in the chart, you can see a loss of about 0.4dB from 1v to 8v input.






This is the same thing but from 400hz to 6khz. You can see a loss of about 0.8dB at 3800hz. Likely due to inductance issues (yet to verify).

Wow Erin, that is a great example! Thank you for contributing your knowledge to the conversation!
 

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Hi Dale, I will give you that what you are measuring, the reason why and your methodology is different form mine. As this is your forum, so be it.

Though, I have no idea why you would bother measuring speakers like this, but thats OK, I am a little behind the obvious learning curve. Does it not matter most what the speaker is doing in your room, rather than out in your driveway? But that is your methodology, and I will respect it.

But when you measure your system as you listen to it in your room, and give SUBJECTIVE reports to how it sounds then I am sure you will go by the standard REW specifications.

You have not answered my question as to which SPL meter you use, .... so please do.

Dale said, "Uhm.. What? I did state the target. 'From 0 Hz to 200 Hz starting at 90db and raising it 5db until the signal compressed. Please let me know what part of this is unclear to you.

Uhm....Yes that is correct, you have to state a target level as you are the one measuring. Otherwise I can infer any level I want to on your graph if you don't state what is the actual level. So you say that in all of your graphs, the first measurement is 90db going up in 5db increments, is that correct? Then looking at the following graphs:
the Axiom your initial reading of 90db is 4db too high.
the Chase is so wrong I dont know what to say
the Emotiva is 5db to high
the power soud is about correct is about correct but we need to see it overall freq response since it is clearly not a "subwoofer"
the svs graph clearly shows that you did not correlate 90db with you intended target.

Dale said, "One has absolutely nothing to do with the other. You cannot correlate a subjective opinion to a sine sweep. The best you could hope for is to correlate it to a given part of a song or film that I happen to capture that I also commented on. Subjective, emotional responses to audio are not measurable."

I say, really? Then why are you measuring? If measured freq response has no correlation with what you are hearing, why bother measuring?


Bob
"Make more progress, less excuses"
 

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Though, I have no idea why you would bother measuring speakers like this, but thats OK, I am a little behind the obvious learning curve. Does it not matter most what the speaker is doing in your room, rather than out in your driveway? But that is your methodology, and I will respect it.
I can answer this one...

Take a look at our Subwoofer Tests - Archived forum.

That forum has more views than any other forum on our site with as few threads as it has. 10's of thousands of views for most of the threads... most likely over a million views total. Obviously there are quite a few viewers who appreciate this type of testing. We are indeed going one step farther with subjective reviews on many of these we will be testing. We will try to do them all if we can. However, the purpose of our testing has been thoroughly explained.
 

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Discussion Starter #28
Hi Dale, I will give you that what you are measuring, the reason why and your methodology is different form mine. As this is your forum, so be it.

Though, I have no idea why you would bother measuring speakers like this, but thats OK, I am a little behind the obvious learning curve. Does it not matter most what the speaker is doing in your room, rather than out in your driveway? But that is your methodology, and I will respect it.

But when you measure your system as you listen to it in your room, and give SUBJECTIVE reports to how it sounds then I am sure you will go by the standard REW specifications.

You have not answered my question as to which SPL meter you use, .... so please do.

Dale said, "Uhm.. What? I did state the target. 'From 0 Hz to 200 Hz starting at 90db and raising it 5db until the signal compressed. Please let me know what part of this is unclear to you.

Uhm....Yes that is correct, you have to state a target level as you are the one measuring. Otherwise I can infer any level I want to on your graph if you don't state what is the actual level. So you say that in all of your graphs, the first measurement is 90db going up in 5db increments, is that correct? Then looking at the following graphs:
the Axiom your initial reading of 90db is 4db too high.
the Chase is so wrong I dont know what to say
the Emotiva is 5db to high
the power soud is about correct is about correct but we need to see it overall freq response since it is clearly not a "subwoofer"
the svs graph clearly shows that you did not correlate 90db with you intended target.

Dale said, "One has absolutely nothing to do with the other. You cannot correlate a subjective opinion to a sine sweep. The best you could hope for is to correlate it to a given part of a song or film that I happen to capture that I also commented on. Subjective, emotional responses to audio are not measurable."

I say, really? Then why are you measuring? If measured freq response has no correlation with what you are hearing, why bother measuring?


Bob
"Make more progress, less excuses"
I keep forgetting the SPL, it is a Galaxy CM140. For measuring I am using the IBF.

the first measurement is 90db going up in 5db increments, is that correct?
No, what I stated was that the volume is level set at to 90db AT 50Hz and then I will begin the sweeps and raise the volume by 5db.

the Chase is so wrong I dont know what to say
If you would actually read the words instead of just looking at the pictures you would have seen at the top of the Chase test that I stated:

"Before we start I would like to say that the tests that I did back in February on the Chase VS-18.1 were performed before any methodology was written for The Sub Zone and as a result the measurements for max output were based on maintaining <10% THD. This is different than other tests that have been performed on this beast utilizing the CEA 2010 standard."

I say, really? Then why are you measuring?
To show an estimated room gain benefit.

Bob, I am done trying to help you grasp this as it seems you only really want someone to argue with.
 

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Discussion Starter #29
I guess no spl capability will be tested since that would be a very long power run; even at low current it will require a decent gauge cord.

Cool idea
Hey Glen, sorry I missed this one. I have been working out the SPL thing. I actually think it will be possible but probably will not be added until next Spring.
 

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Dale,
I really do implore you to consider what I wrote above regarding the fundamental purpose of compression testing. Raising the output by 5dB doesn't really tell us anything about compression other than how the curve changes. That's fine... but that's only one (small) part of compression. We need a way to gauge input voltage for each step vs output SPL. You can do this pretty easily with online calculators. Just figure what each voltage input will be, in steps, and that should tell you what your output should be. Make a table with 6-7 stepped voltages, then the theoretical (what you should get, if compression were not inherent in design), and the real result. Then you'll see how much output in SPL is lost due to inefficiency at your reference frequcney of 50hz and you can still provide your FR curves so we can see how the overall curve is affected. It's not quite as pretty as the examples I gave above, but it'll suffice. Once I get set up, I will be providing the compression testing the same way as I did above. Flying in the face of apparent standard REW convention. lol.


Playing devil's advocate: As far as subjective vs objective, I actually agree. I'm more of a 'let the data talk' kind of guy. I stay away from subjective thoughts regarding sound; I freely talk subjectively on my likes/dislikes with a product interface, usability, etc. I do agree that when a subjective review is given, objective data should be provided to help the both the reviewer and the viewers (possibly) understand why your subjective analysis came out the way it did. Subwoofers are especially prone to placement and as we know placement will dictate response at the seated position. A simple 6 point spatial average of frequency response would suffice in objective/subjective correlation.


Just my $.02....

- Erin
 

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Discussion Starter #31
Dale,
I really do implore you to consider what I wrote above regarding the fundamental purpose of compression testing. Raising the output by 5dB doesn't really tell us anything about compression other than how the curve changes. That's fine... but that's only one (small) part of compression. We need a way to gauge input voltage for each step vs output SPL. You can do this pretty easily with online calculators. Just figure what each voltage input will be, in steps, and that should tell you what your output should be. Make a table with 6-7 stepped voltages, then the theoretical (what you should get, if compression were not inherent in design), and the real result. Then you'll see how much output in SPL is lost due to inefficiency at your reference frequcney of 50hz and you can still provide your FR curves so we can see how the overall curve is affected. It's not quite as pretty as the examples I gave above, but it'll suffice. Once I get set up, I will be providing the compression testing the same way as I did above. Flying in the face of apparent standard REW convention. lol.


Playing devil's advocate: As far as subjective vs objective, I actually agree. I'm more of a 'let the data talk' kind of guy. I stay away from subjective thoughts regarding sound; I freely talk subjectively on my likes/dislikes with a product interface, usability, etc. I do agree that when a subjective review is given, objective data should be provided to help the both the reviewer and the viewers (possibly) understand why your subjective analysis came out the way it did. Subwoofers are especially prone to placement and as we know placement will dictate response at the seated position. A simple 6 point spatial average of frequency response would suffice in objective/subjective correlation.


Just my $.02....

- Erin
Thank you Erin and I would like to discuss this in more detail when you have time for me to call. I understand all about letting the data talk however; there are other elements outside of hard numbers that need to be told as well. I do plan to do in room measurements, moving forward but am not yet committed to a methodology around it.

Thanks for the input, I sincerely appreciate it.
 

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no worries. shoot me a PM and let me know when is best to call you and when/if I get a chance tonight I'll give you a shout. :)
 

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I take back what I said about doing a spatial average for the subwoofer subjective analysis. If you're only concerned about the low end response, there's no reason to do a spatial average here... not unless the listening room is haunted by ghosts. the response is not going to vary here within a few inch window. If you're measuring multiple seats, it's a different story.

anyway, one simple FR capture with a LONG window (enough to allow fine resolution of resonance/modes) and a good CSD (waterfall) should help with your subjective to objective correlation if you choose to go this route. Personally, I'd say do the subjective, post the data of the driver in the 'ideal' situation as your baseline and then post the listening position response.
 

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Dale,

I am in catch-up mode here, have read through the thread, and clearly have soooooooo much to learn about subs and sub testing. What an amazing resource you are providing.

Two questions, and beg pardon if they were covered and I missed them, or if they totally miss the mark somehow:

1. It would seem that there should be a direct relationship between the compression measurements and harmonic distortion, i.e. as the measurement level reaches onset of compression, harmonic distortion would also be going through the roof. Does this appear to be the case? If so, is there any value in correlating the two as a measurements sanity check of some sort? If there is no such correlation, what am I missing?

2. As I see it, your measurements are half-space measurements, due to obvious physical limitations. How does that compare to typical manufacturer's measurement methods and specifications for subs? It is clearly a restatement of the obvious that whole space versus half space versus quarter space versus corner loading would have a big effect on frequency response measurements for a sub. Is there any industry-standard for how manufacturers approach this, and what is your feeling for how it affects your ability to correlate your measurements with manufacturers specs and measurements?

Thanks again, appreciate your work and feedback.
 

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Discussion Starter #35
1. It would seem that there should be a direct relationship between the compression measurements and harmonic distortion, i.e. as the measurement level reaches onset of compression, harmonic distortion would also be going through the roof. Does this appear to be the case? If so, is there any value in correlating the two as a measurements sanity check of some sort? If there is no such correlation, what am I missing?
Thanks for the questions. The relationship between the compression measurement and THD measurements are based on the THD measurements being taken at the same time. Compression is not necessarily indicative of high distortion. Take the SVS SB-13 Ultra, which is an awesome sub by the way, if you look at the Max output before compression you can see that the signal falls on itself at around 80Hz but the THD at that point was only 2%. So compression does not necessarily indicate higher distortion is present.

2. As I see it, your measurements are half-space measurements, due to obvious physical limitations. How does that compare to typical manufacturer's measurement methods and specifications for subs? It is clearly a restatement of the obvious that whole space versus half space versus quarter space versus corner loading would have a big effect on frequency response measurements for a sub. Is there any industry-standard for how manufacturers approach this, and what is your feeling for how it affects your ability to correlate your measurements with manufacturers specs and measurements?
Normally most manufacturers will only post their response but not the actual measurement graph. It has been my experience thus far that those that do not post graphs usually post their frequency response then a typical in-room response. I am starting to get in the habit of taking an in room measurement to add to the tests or review.
 

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1. It would seem that there should be a direct relationship between the compression measurements and harmonic distortion, i.e. as the measurement level reaches onset of compression, harmonic distortion would also be going through the roof. Does this appear to be the case? If so, is there any value in correlating the two as a measurements sanity check of some sort? If there is no such correlation, what am I missing?
To add to Dales comments...

You can almost look at compression as "when does it stop getting louder". The deeper the bass goes the more it requires from the amp and driver, that's why you'll most often see the compression occur on the lower notes first. But that "collapse" in the linear response curve doesn't always mean distortion.

A lot of budget subs don't have effective limiters, so they'll just keep getting louder until you either can't handle the dreadful sound from all the distortion or they self destruct. But the better subs will use a DSP (Digital Sound Processor) to 'tailor' the frequency and compression response. Using Dales example of the SB-13 Ultra it appears the SVS engineers decided to start limiting the volume/output before the driver produces distortion. That essentially means it won't get any louder at certain frequencies, but it won't sound like garbage either. Makes the parts last longer too, which for me is always preferable.
 
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