Null @ 20hz. Needs EQ or bad DIY? - AVS Forum
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post #1 of 28 Old 04-02-2012, 03:56 PM - Thread Starter
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So after building my first sub and running some sweeps in REW I have noticed a huge null around the 20hz range. I do not have any measuring equipment but the null is pretty defined. Measuring equpiment is something I am working on as well as getting a MiniDSP. The sub sounds great but when I get around 20hz the whole room gets quite then picks up around 18hz and gets the room shaking at 14.5.

My build is a Dayton RSS390HF in a 4.3 ft^3 cube powered by a Crown XLS 1000 with an Onkyo RC260. I have run Audyssey. My question is is it possible I have a leak in the sub that would cause this or does it just need some good EQ action. I understand the effects of different room placement, and I have tried moving the sub around but I get the same effect no matter where it is. I see the sub moving but the room is almost dead silent. No vibrations... nothing, I hear the woofer moving but no bass. I used a ton of silicone to make sure everything was sealed and used the lighter trick to check for seals. However when checking around the mounting screws I cannot tell if there is a leak or it is just the woofer making the flame move.

Any thoughts?
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post #2 of 28 Old 04-02-2012, 04:02 PM
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Not sure if this applies to the XLS, but the Crown xti1000 measured only 90 watts @ 20hz 4ohm...

http://www.avsforum.com/avs-vb/showt...6#post13869426

You could also try measuring from a different location to see if the null is still present.


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post #3 of 28 Old 04-02-2012, 04:17 PM - Thread Starter
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Quote:
Originally Posted by Jay1 View Post

Not sure if this applies to the XLS, but the Crown xti1000 measured only 90 watts @ 20hz 4ohm...

http://www.avsforum.com/avs-vb/showt...6#post13869426

You could also try measuring from a different location to see if the null is still present.

I have tried moving the sub around and the same issues. I'm a little ignorant when it comes to these things. From what I read if I am understanding correctly, that the amp produces less wattage @ 20hz than other frequencies? If thats the case that seems pretty lame and I feel ripped off dropping $300 bucks on the wrong amp.
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post #4 of 28 Old 04-02-2012, 04:17 PM
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Nulls are most often placement related, especially if you are dealing with only one sub. Definitely experiment with placement before spending money on eq. Mind you, eq may still be necessary and also beneficial, but if you're getting it only to address the null, try the simple (and free) things first.
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post #5 of 28 Old 04-02-2012, 04:30 PM - Thread Starter
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Originally Posted by wth718 View Post

Nulls are most often placement related, especially if you are dealing with only one sub. Definitely experiment with placement before spending money on eq. Mind you, eq may still be necessary and also beneficial, but if you're getting it only to address the null, try the simple (and free) things first.

Again, I have tried multiple locations fromt he middle of the room to every corner and every wall with the same results. I am also limited on placement of the sub. So if it's not because of my handy work and not due to the amp, I have no problem investing some money in an EQ to get that flat response at the LP.
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post #6 of 28 Old 04-02-2012, 05:08 PM
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I believe Ricci had the same problem but even lower in freq., and it was either his floor or one of his walls resonating and acting as an absorber.

A room would have to have a dimension of 27 ft to have an acoustic mode or null at 20 Hz.

Noah
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post #7 of 28 Old 04-02-2012, 05:08 PM
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Quote:
Originally Posted by djdcm0722 View Post

Again, I have tried multiple locations fromt he middle of the room to every corner and every wall with the same results. I am also limited on placement of the sub. So if it's not because of my handy work and not due to the amp, I have no problem investing some money in an EQ to get that flat response at the LP.

See my graphs under Sublimator Duo build. It's could be the low fs of 18 which is a good thing. How much of a dip are we talking? Is the box damped with polyfill?


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post #8 of 28 Old 04-02-2012, 05:14 PM
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Here's some reading material. http://www.aes.org/tmpFiles/elib/20120402/13680.pdf


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post #9 of 28 Old 04-02-2012, 05:55 PM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

I believe Ricci had the same problem but even lower in freq., and it was either his floor or one of his walls resonating and acting as an absorber.

A room would have to have a dimension of 27 ft to have an acoustic mode or null at 20 Hz.

The room is 32x13x8.5. It is also an L shape. The dimensions are the long part of the L with my sub at the top and my LP about a third of the way.

Quote:
Originally Posted by coolrda View Post

See my graphs under Sublimator Duo build. It's could be the low fs of 18 which is a good thing. How much of a dip are we talking? Is the box damped with polyfill?

I don't have exact measurements but it is extremely noticeable. I plan on getting a mic here soon so I can take level readings. Also, no polyfill. I thought about stuffing it and I was going to ask about that. I don't feel like having polyfill shipped to my location so I think I will buy some cheap pillows, which should have the same effect correct?
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post #10 of 28 Old 04-02-2012, 05:58 PM - Thread Starter
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Quote:
Originally Posted by kgveteran View Post

Here's some reading material. http://www.aes.org/tmpFiles/elib/20120402/13680.pdf

Thanks for that. I saw a lot of equations hopefully I can wrap my head around it. I flipped through it real quick and looks like there is some good info. I will read it tomorrow - 3am here, time for sleep
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post #11 of 28 Old 04-02-2012, 06:00 PM
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Try taking the subwoofer outside with an extension cord, if you don't get 20hz out there then it ain't the room.

If you can get lower frequencies and the cone is moving at 20hz and you're not getting sound, then it IS the room.
Could be a hallway or door or a number of things causing cancellation.

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post #12 of 28 Old 04-02-2012, 06:10 PM
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If it's an acoustic mode from room dimensions, there should be peaks as well as nulls, which should be evident when you walk around the room with 20 Hz playing.

Noah
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post #13 of 28 Old 04-03-2012, 07:10 AM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

If it's an acoustic mode from room dimensions, there should be peaks as well as nulls, which should be evident when you walk around the room with 20 Hz playing.

So after some extensive moving of furniture and walking around its come down to the room which is disappointing. The sub has to stay where it is. So hopefully the EQ will help bring up the 20hz even if slightly. I'll be moving soon so hopefully the next house will have better acoustics.

Thanks everyone for your help. Happy it wasn't poor box construction.
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post #14 of 28 Old 04-03-2012, 08:53 AM
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Just be careful when trying to boost a null....I believe it's generally considered a bad idea. Can you try some bass traps in your room instead?
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post #15 of 28 Old 04-03-2012, 09:07 AM
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I would not do anything or make any conclusions until you can actually measure everything. Otherwise it will just be guessing - assuming going on.

James
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post #16 of 28 Old 04-03-2012, 09:36 AM - Thread Starter
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I would not do anything or make any conclusions until you can actually measure everything. Otherwise it will just be guessing - assuming going on.

James

True. I hope to be getting my equipment soon.
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post #17 of 28 Old 04-03-2012, 12:28 PM
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Quote:
Originally Posted by djdcm0722 View Post

So after some extensive moving of furniture and walking around its come down to the room which is disappointing.

Not sure what that means; did the output vary a lot as you walked around or not?

If not, check your walls/ceiling/floor for a lot of motion at that freq' there's some chance that something could be done about it.

Noah
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post #18 of 28 Old 04-03-2012, 12:38 PM
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"I believe Ricci had the same problem but even lower in freq., and it was either his floor or one of his walls resonating and acting as an absorber."

this is probably what is happening.

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post #19 of 28 Old 04-03-2012, 02:01 PM
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Quote:
Originally Posted by djdcm0722 View Post

So after some extensive moving of furniture and walking around its come down to the room which is disappointing. ...

Quote:
Originally Posted by djdcm0722 View Post

The room is 32x13x8.5. It is also an L shape. The dimensions are the long part of the L with my sub at the top and my LP about a third of the way....

Quote:
Originally Posted by LTD02 View Post

"I believe Ricci had the same problem but even lower in freq., and it was either his floor or one of his walls resonating and acting as an absorber."

this is probably what is happening.

Think about this last bit for a moment... something sucking out 20Hz energy, but not at 18Hz... could be done electronically, but I don't buy a physical system with that sharp a tuning.

Think also about the conventional wisdom of treating rooms, especially for bass mode suppression - we add absorbers to a room to make bass louder when wave interference is the cause of the dip. Conversely, 20Hz may be fine, he's really hearing a strong peak at 18Hz (1/2 wave fits the room).

You see, I have a very lossy room, of comparable dimension, and no suckouts. I wondered why for years, then learned a little about absorbers and started to see all the bass traps: 15' of sliding glass doors, paneling installed on standoffs, as well as 1 connecting wall, no upper floor and open basement below.

To the OP: what is room wall construction? Is it above or below grade? What kind of furnishings or acoustic treatments?

Have fun,
Frank
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post #20 of 28 Old 04-03-2012, 03:19 PM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

Not sure what that means; did the output vary a lot as you walked around or not?

If not, check your walls/ceiling/floor for a lot of motion at that freq' there's some chance that something could be done about it.

I noticed the 20hz pick up a lot by moving 5 feet to the rear of the LP, but then 14hz wasn't as loud.

Quote:
Originally Posted by fbov View Post

To the OP: what is room wall construction? Is it above or below grade? What kind of furnishings or acoustic treatments?


The walls are similar to drywall but a little more gritty (sheet rock maybe?). What do you mean by above or below grade? Not really any acoustic treatments. Tile floor, sectional couch, one rug in between the couch and TV, curtains on all glass doors, one wall completely bare. When I get a chance I will post a pic of the layout of the house and maybe some pictures. I would love to get some acoustic panels (pending WAF), but I move about once a year.
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post #21 of 28 Old 04-04-2012, 12:57 PM
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...What do you mean by above or below grade? ...

Above ground or below ground, as many HT are in the basement, and cinderblock walls reflect bass like nobody's business. From your description, you have a very live room that will benefit from some treatment, be they acoustic absrober wall hangings, or just a padded carpet.

Let's also be careful about "hearing" vibrations at extremenly low frequecies. I have no illusions about hearing the 16Hz pedal organ note at the start of "Also Sprach Zarathustra" (happened upon by chance), but I definitely could sense it and knew the song from the single note. But I did not hear a 16Hz tone... (yes, big sealed sub used gently so little HD).

If what you're hearing is a harmonic of the subsonic fundamental, you'll get the wrong node locations, the wrong spatial pattern. This is one time a $50 Radio Shack SPL meter is worth it's weight in gold.

Have fun,
Frank
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post #22 of 28 Old 04-05-2012, 12:15 PM - Thread Starter
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Above ground or below ground, as many HT are in the basement, and cinderblock walls reflect bass like nobody's business. From your description, you have a very live room that will benefit from some treatment, be they acoustic absrober wall hangings, or just a padded carpet.

Let's also be careful about "hearing" vibrations at extremenly low frequecies. I have no illusions about hearing the 16Hz pedal organ note at the start of "Also Sprach Zarathustra" (happened upon by chance), but I definitely could sense it and knew the song from the single note. But I did not hear a 16Hz tone... (yes, big sealed sub used gently so little HD).

If what you're hearing is a harmonic of the subsonic fundamental, you'll get the wrong node locations, the wrong spatial pattern. This is one time a $50 Radio Shack SPL meter is worth it's weight in gold.

Have fun,
Frank

It's above grade. Been a little busy so I haven't been able to post any pics. I would like to get a RS spl meter but (1) I havent been able to find it on their website and (2) I live in Germany so no Radio Shack available. As I said I have been busy but I keep meaning to make a post in lilmikes mic thread to see if anyone has any spares they are willing to sell.
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post #23 of 28 Old 04-05-2012, 03:29 PM
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Quote:
Originally Posted by fbov View Post

If what you're hearing is a harmonic of the subsonic fundamental, you'll get the wrong node locations, the wrong spatial pattern. This is one time a $50 Radio Shack SPL meter is worth it's weight in gold.

How?

The RS meter only reads the integrated SPL for all freq.

Something like OmniMic would be informative, using the SPL?Spectrum that shows total SPL as well as a graph of SPL vs. freq.

Noah
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post #24 of 28 Old 04-07-2012, 07:00 PM
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This null exists in my HF 15's as well. This is a well know issue and the reason
it happens is a severe impedance spike that lowers the power delivered at that
frequency. I's actually at 19hz not 20hz. Pretty much every woofer has an impedance
spike somewhere.
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post #25 of 28 Old 04-08-2012, 03:22 AM
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Quote:
Originally Posted by LTD02 View Post

"I believe Ricci had the same problem but even lower in freq., and it was either his floor or one of his walls resonating and acting as an absorber."

this is probably what is happening.

Yup, have the same thing in my front row, room is too small to have a mode that low, but the ceiling above moves about 1/8" at 17Hz at around 100dB at the main listening row. You can only do so much sometimes. But let's face it, just 7 years ago getting a 'flat' freq response on a sub was simply a matter of luck, and there were very little EQ or measurement options for either subs or mains/center/surrounds. Things like REW, Audyssey, YPAO and MCACC have made the home listening experience MUCH better. How did we EVER think we got good sound with simply an SPL meter and tape measure to set delays and levels on a receiver that had only one crossover freq selectable (Large or Small)?? By chance.

We live in a golden era of HT, where we can get more ULF extension than in any theater, at the expense of getting the midbass right due to room size. Systems being made by DIYers now are capable of running 6-10dB BELOW power and excursion limitations, for a truly clean sound. How many times have you been to a cinema and heard the mains being pushed too hard?

I have a very cheap and compromised system, but I have enough resolution and detail to uncover poor mic'ing jobs in films (lots of the voices in TF:ROTF are clipped). Would have never picked that up before on the old system....

OK, way too OT now....but seriously, if your only serious null is at 20Hz, you have done well. It would suck to have that null at 50Hz and take away the visceral 'punch' effect....

JSS
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post #26 of 28 Old 04-08-2012, 11:55 PM
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Yes, and except for high Q designs, there's no peak and certainly no dip at that freq.

The impedance peak is from driver resonance and results in higher efficiency at that freq; if the VC allowed the same power in as at other freq there would be a large response peak.

Quote:
Originally Posted by mcsoul View Post

This null exists in my HF 15's as well. This is a well know issue and the reason it happens is a severe impedance spike that lowers the power delivered at that frequency. I's actually at 19hz not 20hz. Pretty much every woofer has an impedance spike somewhere.


Noah
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post #27 of 28 Old 04-09-2012, 09:23 AM
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Quote:
Originally Posted by noah katz View Post

Yes, and except for high Q designs, there's no peak and certainly no dip at that freq.

The impedance peak is from driver resonance and results in higher efficiency at that freq; if the VC allowed the same power in as at other freq there would be a large response peak.

I'm not sure the sensitivity totally makes up for the impedance. It looks like about a 25 ohm spike, which could amount to 6 db of attenuation. It seemed very real when I was listening to test tones in my HT. 19hz dropped off, 18hz came back and went on to 16hz where I stopped. I did not have a db meter at the time. I should re-test, but I hate running test tones because i fear damage to the subwoofer.
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post #28 of 28 Old 04-09-2012, 10:58 AM
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I'd guess that's your room and not the speaker; a close mike measurement right at the driver would tell.

Noah
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