Null @ 20hz. Needs EQ or bad DIY?

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.

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.

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.

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?

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

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.

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.

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?

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

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 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

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

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.

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.

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

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.

I'd guess that's your room and not the speaker; a close mike measurement right at the driver would tell.