About The Sub Zone: Testing Methodology and Environment - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #31 of 37 Old 11-03-12, 05:07 PM Thread Starter
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Re: About The Sub Zone: Testing Methodology and Environment

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Erin H wrote: View Post
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.

Regards,
Dale Rasco



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post #32 of 37 Old 11-03-12, 05:08 PM
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Re: About The Sub Zone: Testing Methodology and Environment

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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post #33 of 37 Old 11-03-12, 05:25 PM
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Re: About The Sub Zone: Testing Methodology and Environment

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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post #34 of 37 Old 02-02-13, 05:09 AM
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Re: About The Sub Zone: Testing Methodology and Environment

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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post #35 of 37 Old 02-02-13, 07:11 AM Thread Starter
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Re: About The Sub Zone: Testing Methodology and Environment

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AudiocRaver wrote: View Post
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.

Quote:
AudiocRaver wrote: View Post
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.

Regards,
Dale Rasco



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post #36 of 37 Old 02-02-13, 12:49 PM
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Re: About The Sub Zone: Testing Methodology and Environment

Quote:
AudiocRaver wrote: View Post
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.

-Jim

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post #37 of 37 Old 02-02-13, 01:14 PM
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Re: About The Sub Zone: Testing Methodology and Environment

Thanks for the detail. I was wondering if there might be an actual compressor/limiter function built into the signal processing somewhere to protect the driver. Shows how much I still have to learn about subs.
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