If a subwoofer's frequency response is 28Hz on the low end, what does it really get to?

If a subwoofer says it goes down to 28Hz, does is it really go down to that level?

Does that deteriorate over time, if the driver is in good shape?

Originally Posted by BeeMan458 

Our eighteen year old ported (with radiator) sub was spec'd at 28Hz and pretty much, that was all it was good for and then it dropped like a stone. It would play lower but the more energy that was applied, the flappier the woofer got. Don't recall flappy and subwoofer being used together being a good thing.

Recently, I replaced the driver and radiator in both boxes and currently, we're going through a fifty hour break in period to loosen the new drivers up a bit. Once the break-in period is over, I'll dial things in using a sound meter, DSPeaker, Anti-Mode, 8033S II and Audyssey, MultEQ XT. From there I'll be able to do some measured room sweeps with REW and compare the differences.

Here's a graph of the old dialed in woofers being used at the same time. The sweep parameters were set to: 15Hz - 200Hz. Both subs are spec'd for 28Hz - 120Hz. The dip on the left is expected for a ported sub using a radiator. Don't know why but that's what I read.

If the new drivers and radiators don't correct the null on the right, I'll either have to accept it or hope placing an acoustic panel in the one location I have behind the null position will correct the problem. Currently, all speakers are crossed over at 60Hz, so the rest of the speakers will help with filling in the hole.

Hope the above helps with your question.

Graph of our 28Hz to 120Hz subwoofers:



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Originally Posted by likemovies 

If a subwoofer says it goes down to 28Hz, does is it really go down to that level? Does that deteriorate over time, if the driver is in good shape?

Originally Posted by likemovies 

edit: is that low end all marketing?

Originally Posted by BeeMan458 

LFE, THX Reference, 0.1 channel information, 20Hz - 120Hz. (nuff said)

Originally Posted by BeeMan458 

LFE, THX Reference, 0.1 channel information, 20Hz - 120Hz. (nuff said)

Originally Posted by sputter1 

Lots of ~20hz content. Nuff said.

Originally Posted by BeeMan458 


And the above has what to do with an ~90Hz null and content encoded above 80Hz in the LFE channel? ...

Originally Posted by sputter1 

That's your response? lol nvm then.

Originally Posted by likemovies 

If a subwoofer says it goes down to 28Hz, does is it really go down to that level?

Originally Posted by BeeMan458 

LFE, THX Reference, 0.1 channel information, 20Hz - 120Hz. (nuff said)

Originally Posted by craig john 

To be more exact, with DD and DTS, the LFE channel is "specified" to be 3 Hz to 120 Hz. With the lossless codec's, Dolby TrueHD and DTS MA, the LFE channel is a full range channel specified as 3 Hz to 20 kHz.

Still, there is very little content in the LFE channel above 80 Hz in any of those codec's. Recording engineers just don't put that content there.



Craig

Originally Posted by J_Palmer_Cass 

Most of the time there is plenty of content recorded on the LFE channel that goes up to 120 Hz.

Originally Posted by craig john 

Not according to Rodger Dressler. He's so convinced of this that he sets his LPF of LFE at 80 Hz.

Craig

Originally Posted by craig john 
Not according to Rodger Dressler. He's so convinced of this that he sets his LPF of LFE at 80 Hz.

Dolby Digital's LFE channel carries additional bass information from 120 Hz on down. This is not a roll-off but a digital brick wall (i.e., no 121 Hz info), so the content is usually rolled off by the sound engineer starting around 80 Hz for a smoother blend.

Originally Posted by BeeMan458 

I distinctly recall stating THX reference (standards), not DD or DTS with any mention of CODEC's.

If the boss (THX) says up to 120Hz, what do I care and it sounds like those who LPF their subs at 80Hz are being ego bound (stubborn) as opposed to being flexible as 80Hz or 120Hz, what does it matter if the LFE channel (the subwoofer) is set to 120Hz? Sounds more like Rodger is digging in his heels to support his "personal" thesis as opposed to trying to get along with THX reference standards.

As to the null, I know it's the subs interacting with the room's acoustics, so stop going there. Some have reported that simply changing subs in the same location has cured their null so there's a bit more to the whole, sub, room, interaction thingy than some are willing to deal with. In the meantime, once the new drivers are "BROKEN IN," I'll do some room sweeps, dial things in and have my wife stand around with a big thick love seat pillow (yes, furniture seating cushions do have absorptive properties) and see what REW has to report back.

Some like to discourage and say don't bother going there because I know everything. Others like to encourage and say, here are the facts as I understand them so what the heck, it's all good; have a good time with your journey.

(And for the record, I'm not trying to reinvent the wheel. I'm busy learning about the wheel being used.)

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Originally Posted by bfreedma 

I think I'll side with Craig and Rodger on this one.

Originally Posted by pokekevin 

So if my XO for my fronts are set to 110hz it would be wise to just leave the LPF at 120hz on the sub right?

Originally Posted by pokekevin 

So if my XO for my fronts are set to 110hz it would be wise to just leave the LPF at 120hz on the sub right?

Originally Posted by pokekevin 

Not trying to be argumentative here but why would you guys want to set the LPF on the sub to 80hz? I know you are saying that there is barely any >80hz content in the lfe channel but what if there is on certain mixes? Is there a benefit to setting the lpf to 80hz? It's like saying there is some sub 20hz content in mixes but since there's not a lot than all those crazy people trying to get flat to 10hz are...well just crazy lol. Might as well just set an hpf at 20hz?

Like I said, not trying to be argumentative. Just want to see what is the pros of setting the lpf to 80hz

Originally Posted by BeeMan458 

I distinctly recall stating THX reference (standards), not DD or DTS with any mention of CODEC's.

If the boss (THX) says up to 120Hz,...

Originally Posted by BeeMan458 

...what do I care and it sounds like those who LPF their subs at 80Hz are being ego bound (stubborn) as opposed to being flexible as 80Hz or 120Hz, what does it matter if the LFE channel (the subwoofer) is set to 120Hz? Sounds more like Rodger is digging in his heels to support his "personal" thesis as opposed to trying to get along with THX reference standards.

"Helped Dolby launch its first Dolby Surround technology, and every one since, including Pro Logic, Surround EX, PLII, PLIIx, Dolby Digital, DD+, Meridian Lossless, TrueHD."

I think many have a gross misconception as to the real difference between a 120Hz vs. 80Hz low pass. Personally I always have found I prefer an 80Hz setting. This isn't the difference of having a low pass or not having one. This is just setting the filter lower. If instead we look at the difference between a 120Hz vs. 80Hz low pass, we see it is in fact a shelf filter. Assuming a 4th order filter, moving to 80Hz has no real effect below 40Hz, reduces the 80Hz level by ~4dB, 100Hz by ~7dB, 120Hz by ~9dB and ultimately reaches -14dB somewhere above 200Hz. If the low pass filter is 2nd order, cut those differences in half.

If you have your subwoofers rising to say 3-6dB above the main speakers, the lower crossover point would have the 80-100Hz range pretty much in line with a flat calibration and a smooth rise below there, which many like the sound of.

Originally Posted by beezar 

Ok, back to the original question:

I know lots of subs say the low end +/- 3dB anechoic (eg: 28Hz +/- 3 dB). I thought, however, that the room can enhance the lower end so that the extension can go flat significantly below the specified low end. Thus lots of people with sealed subs claim extension flat into the teens despite the sub having a likely above 20Hz low end. All of this depends upon the room and where the sub is placed in the room, of course. Correct me if I'm wrong.

Typically, I'm more clear (read verbose) with regard to PVG, like I did in this excerpted post;

"Pressure Vessel Gain (PVG), or room gain, is the scenario whereby the longest dimension of the room can no longer support full propagation of the waveform. At this point, the acoustic propagation transitions to acoustic pressurization. A typical myth is a small cabin cannot support the lowest frequencies.... nothing could be further from the truth. The manner in which the sound is reproduced into the space changes from a normal cyclic propagation, to pressurization because the wavelengths are too big for the space. The frequency at which this occurs is approximately the point whereby half the wavelength of a given frequency is equal to the rooms longest dimension. So, a 20 hz frequency has a wavelength 56.5 feet. So half of that, 28.25 feet, is the point of transition. Any frequency below that point pressurizes the room, and any frequency above that point propagates freely. So in this room that's approximately 28 feet in the longest dimension, from 20 hz downward, the room gives back acoustically. This is room gain, cabin gain, or more specifically PVG...Pressure Vessel Gain.

At this frequency, the results are a gain in acoustic pressures in the room that grows as the frequency decreases. This acoustic support reciprocity, is theoretically 12db per octave. The percentage of the 12 db/octave gain one achieves, entirely depends on the integrity of the boundary walls and surfaces. If it was the theoretical concrete bunker, a full 12db/octave boost would occur. Typically, somewhere between 6-10 db octave could result. Also, in addition to the walls and surfaces flexing, other aspects may affect the point at which room gain begins. Furniture, cabinets etc, anything that consumes a certain measure of cubic feet, may slightly alter the transition frequency merely because the items take up space.

This acoustic pressurization, room gain, is the proverbial free lunch. It is essentially headroom that's thrown back into the system. And unlike horn subs, the distortions and non-linearities are not magnified. An IB sub system is a sealed alignment. Sealed alignments roll off second order. Room gain also is second order. So one can see how integrating a sealed alignment may offer substantial benefit when attempting to integrate the system to the room via time and frequency equalization. The -3db point of the IB, may typically be deeper than the transition point where room gain begins. Properly adjusted, this would result in substantial headroom added back in for significant capability for the big LFE effects."

Originally Posted by BeeMan458 

First things first, there was no mention of LPF'g the LFE channel as the THX reference standard is set, my understanding, to a max of 120Hz.

Originally Posted by BeeMan458 

The only thing I'm doing with the LPF on the sub, is making sure it mirrors the THX standard. Anybody is welcome to characterize what I'm doing in in fashion they wish but I'm going with my descriptors.

Originally Posted by BeeMan458 

As to other standards running up to 20kHz, considering the frequency range or capability of a subwoofer driver, isn't it a safe bet that any standard far exceeding a driver's capability, is being grossly ridiculous?

Originally Posted by pokekevin 

Hmmm...makes me wish I could set my XO to 80hz so I could set the LPF to 80hz... Audyssey keeps setting it to 110 for my room. Blah