Why doesn't my room sound better than it does? - Page 3 - AVS Forum
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post #61 of 65 Old 05-06-2013, 11:25 AM - Thread Starter
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Originally Posted by fbov View Post

Try one thing before you trash the tweeter - hook it up in the opposite polarity. A reverse-polarity tweeter is not uncommon, and when designing, reversing from what you've designed is a diagnostic - a sharp deep dip at the XO frequency is a sign of a good XO. In this case, perhaps not the best XO, but with reverse polarity nonetheless!

Have fun,
Frank

I tried reversing the tweeter's wires this past weekend and unfortunately it did not make a difference. The good news - and what made me think to respond to this - is I just got a shipping confirmation for the replacement tweeter. Last week, I was told it would be end of July.
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post #62 of 65 Old 05-06-2013, 02:36 PM
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I'm seeing ~8dB range at T=0 from 90Hz down to the teens. Previous data was more like a 20dB range, so big difference in response!

Really hard to compare resonances due to scale change. Looks like the 10-20Hz resonance(s) is now -30dB rather than -25dB?

I agree that the first plot has a peak at 60Hz, but the resonance is still a little higher than that in both plots. Again, scale makes first one hard to be accurate, but I think I'm seeing the same behavior at 65Hz and 48Hz(?) in both plots.

The same data can be displayed as a "spectrogram" in REW. Pick your SPL range properly and it's easy to see certain threasholds. I talk about my room in this thread.


The sub-20Hz data looks a lot like your room... but without the other resonances (and junk above 125Hz). I'm attributing the sub-20Hz to adjacent room resonances, since the wavelength's too long to fit the room. the 60Hz peak doesn't align with the room, so perhaps I have some hum intruding as well.

In terms of mitigation, your 60-ish Hz ridge looks like the perfect match to a Helmholtz resonator, but it has to be tuned to the right frequency. I've been playing with some concepts based on bass reflex tuning, but with an adjustable port to fine tune the resonance. To work well, this should be a high Q device centered on the ridge frequency.


Sadly, I suspect one would have to build several before finding out all the nuances... so I'm thinking variable port length (fine tune) and diameter (coarse tune). This shows a 50Hz device.

Have fun,
Frank

PS Glad to hear the tweeter's in transit!
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post #63 of 65 Old 05-07-2013, 08:34 AM - Thread Starter
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Originally Posted by fbov View Post


I agree that the first plot has a peak at 60Hz, but the resonance is still a little higher than that in both plots. Again, scale makes first one hard to be accurate, but I think I'm seeing the same behavior at 65Hz and 48Hz(?) in both plots.

The same data can be displayed as a "spectrogram" in REW. Pick your SPL range properly and it's easy to see certain threasholds. I talk about my room in this thread.

In terms of mitigation, your 60-ish Hz ridge looks like the perfect match to a Helmholtz resonator, but it has to be tuned to the right frequency. I've been playing with some concepts based on bass reflex tuning, but with an adjustable port to fine tune the resonance. To work well, this should be a high Q device centered on the ridge frequency.

Sadly, I suspect one would have to build several before finding out all the nuances... so I'm thinking variable port length (fine tune) and diameter (coarse tune). This shows a 50Hz device.

Have fun,
Frank

PS Glad to hear the tweeter's in transit!

I generated some spectograms when measuring... just didn't post any yesterday. It shows the ridge is closer to 64-65hz so you have a good eye. I assume that means it isn't just an electrical hum.



I'm not familiar with Helmholtz resonators. Just did a quick google search and looks like some differing opinions on them and their effectiveness. Also seems like it would be a challenge to tune. I'll probably try some bass traps at some point and see what impact that has. So far, I've only taken measurements from the MLP so will try different mic positions too to see if this 65hz ridge is always present.
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post #64 of 65 Old 05-07-2013, 12:39 PM
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Now that I think about it, 60Hz hum usually shows at 120Hz, wher eyou have a strong T=0 peak. A leg like this one is a strong resonance. However, your spectrograph would benefit from a wider SPL range. 89-49 only covers 40dB, and below 100Hz, you're starting level is about 80dB, so you've displayed a -30dB decay. Starting louder won't hurt either, but I recommend at least a 50dB range on the plot if room noise floor supports it.

What you've read about Helmholtz resonators is true, and that's what makes them well suited to your needs. I used formulas from Everest. Page 220 has an example similar to your case.

The idea is that a narrow resonance responds best to a tuned trap, while general low bass issues (which you may have truncated) respond to a less-tuned, broader-band device. For these purposes, please understand that you don't want a true, broadband absorber as you have no need for absorption above ~100Hz that I can see.

I based the chart on a 50Hz design center, so I could estimate the performance difference achievable from minor design changes. As you can see, port length is a fine adjust; it's port diameter that drives the tuning frequency once the box volume is set. I'd put the port in front of a sub, to drive a resonance, and do a frequency sweep with a mic in the port, and no absorptive material in the box. Maximum output should be tuning frequency. Once tuned, I'd stuff it with fill to reduce re-raditaion. Then I'd place it where the desired resonance was strong.

And to be fair to you, this is all planning; I've built and tested nothing along these lines. .

HAve fun,
Frank
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post #65 of 65 Old 05-07-2013, 09:41 PM
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This is what hum looks like:


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