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Good speakers with tiny subwoofers, and why that's backwards.

Chromatischism

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I was inspired by another thread to start this.

I've seen so many systems which have fairly decent speakers, even large towers paired up with tiny little subwoofers, like they were an afterthought. The little stepstool of a sub looks adorable, but that thing is not going to add much to the system. If anything, it will seriously hold it back, especially if your system is also used for movies.

My subs can run circles around my speakers (120db vs 100db) and it's not a problem – you can't have too much sub – because:
  1. Movie LFE is run up to +10 dB louder than speaker channels by design
  2. Redirected bass from all speaker channels just adds more work for the subwoofer to do. Combined with the LFE track, this can push your sub(s) +20dB above your speakers in some scenes, especially if you run them a bit boosted anyway. If you have a surround system where lots of bass is being redirected to one small sub, you are likely running into compression at an 85 dB playback level which negatively affects sound quality and dynamics.
I always warn people to avoid the mistake of going too small. It will be nice to not have buyer's remorse and later sell at a loss, have to ship, etc.

In summary folks, don't match subs to your mains - subs should be the most powerful speaker in the system!

My 2 pesos, from experience. Started with a 10" Velodyne.
 
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wwenze

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+3dB is two times the power
+10dB is 10 times the power
+20dB is 100 times the power

How many speakers would you need to redirect to hit +20dB
 
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Chromatischism

Chromatischism

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+3dB is two times the power
+10dB is 10 times the power
+20dB is 100 times the power

How many speakers would you need to redirect to hit +20dB
Read carefully: the +20 guesstimate includes the extra +10 base from the LFE channel and the effect of a bass boost since most people don't run their subs flat.

Now, it isn't going to happen with all movies, but there are some scenes where bass is pushed to all channels. Play the opening scene in Bladerunner 2049 on a 5.1 or higher system and you'll see what I mean about subwoofer compression. Headroom becomes important even if running at a typical 85 dB level from your speakers, which is -20 on the master volume of a THX calibrated AVR. At that point a powerful 15" sub, typically ported, becomes the minimum to recreate the track faithfully since you'll need 105 dB at 20 Hz, because of signals from bass managed channels and a sub boost.
 
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ernestcarl

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@Chromatischism

I believe I know which bass thread you're referring to...

Now, it isn't going to happen with all movies, but there are some scenes where bass is pushed to all channels. Play the opening scene in Bladerunner 2049 on a 5.1 or higher system and you'll see what I mean about subwoofer compression. Headroom becomes important even if running at a typical 85 dB level from your speakers, which is -10 on the master volume of a THX calibrated AVR.


Well, I do reckon people who have smaller subs do not listen at those levels. I don't even think most people who own multichannel systems with decent-sized subs come close to listening at cinema level SPLs... I myself typically listen somewhere around mid 70 dB for movies -- also, 'loudness' DSP compensation keeps things sounding relatively balanced regardless of volume level. If the film is played back with volume leveling set properly, there's not even a remote possibility for the sub to compress. But I am speaking about my own small listening room setup here where levels have been set with the speakers' SPL limitation in mind. Others with much larger rooms and a smaller sub(s) and/or those who aren't particularly this methodical (i.e. OCD) in setting things up might get into trouble sooner, I imagine.

Fortunately, I've been able to use the gigantic lower bass room mode peaks along the length of my listening couch to my personal advantage :p

1629960883336.png

*I think very few people realize how much boost that +10dB alone in the LFE channel actually amounts to so I've pasted the above from my own measured setup as an example.

Soundmixer and audio2920 in that thread said they preferred to run their (living room) main monitors without bass management, which I found somewhat interesting. But since my cheapo monitors do not play down to 20Hz, I thought maybe I should just try running all of mine full-range with their bass cloned to the sub channel so everything's widely overlapping. The shared LFE and bass managed subwoofer channel are both low-passed at 180Hz using linear phase crossovers (sub's mid-point front of the room and hidden so no chance of localizing). Then I carefully applied additional IIR & FIR correction so phases and magnitude were all in line and balanced along the whole length of zone of interaction -- BTW, I measured the entire couch for this prior any filtering to each channel -- and then only later processing this large set of averaged raw measurements as the basis for any filter modification. ANYWAY, to cut things short, it turned out much better than I expected so I'm keeping it. :D


there are some scenes where bass is pushed to all channels. Play the opening scene in Bladerunner 2049 on a 5.1 or higher system and you'll see what I mean about subwoofer compression.


Huh. Played the first 10 minutes or so of Bladerunner 2049 just to see what happens:

1629958818282.png


1629959572965.png


At the levels I've calibrated the fronts and surrounds, the channels do not compress at 0dBFS. The single sealed 12" sub that I use does, however, start to compress right around -3.4 dB -- but this is primarily below 20Hz (infrasonic).

In the end, I do still find myself disabling volume leveling from time to time when watching action-heavy blockbuster films just to add some extra oomph of volume here in my somewhat SPL limited (non-THX calibrated system), but even then, I still cannot seem to recall of having even hit close to the maximum peak level of my meter.
 
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Chromatischism

Chromatischism

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Huh. Played the first 10 minutes or so of Bladerunner 2049 just to see what happens:

index.php


index.php


At the levels I've calibrated the fronts and surrounds, the channels do not compress at 0dBFS. The single sealed 12" sub that I use does, however, start to compress right around -3.4 dB -- but this is primarily below 20Hz (infrasonic).
Was this with bass management on or off?
 
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Chromatischism

Chromatischism

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During the opening scene of that movie I measured peak levels of 120-125 dB at 30 Hz at -10 on the AVR. My subs are only boosted about 6 dB, but there is of course room gain adding some efficiency. Crossovers at 80 Hz. Lesser subs would have been maxed out before then. Therefore, the subs you put into the system define your experience.
 

wwenze

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Read carefully: the +20 guesstimate includes the extra +10 base from the LFE channel.

Now, it isn't going to happen with all movies, but there are some scenes where bass is pushed to all channels. Play the opening scene in Bladerunner 2049 on a 5.1 or higher system and you'll see what I mean about subwoofer compression. Headroom becomes important even if running at a typical 85 dB level from your speakers, which is -10 on the master volume of a THX calibrated AVR. At that point a powerful 15" sub, typically ported, becomes the minimum to recreate the track faithfully since you'll need 95 dB at 20 Hz, not including signals from bass managed channels which would take you to 100-105 dB asked of the subwoofer(s). And, that's if you don't add any boost to the subs, which most people do!

Correct. You have +10dB base from the LFE channel. Which is equivalent to 10 surround speakers.

You need to add 90 more surround speakers to reach +20dB.

10x base for the sub + 10 surround speakers worth = 20x = +13dB.

Point is, whatever is causing your sub to compress, is clearly not due to simple arithmetics of speaker power summing/redirection as suggested by the text highlighted in blue.
 

abdo123

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btw I really think you're projecting what you think is a good choice onto other people. The reason why you need beefier subs is BECAUSE you have smaller speakers.

let me demonstrate, the latest movie I watched was the The Witcher Nightmare of the Wolf, a very action packed movie. Looking at the audio we see that the center channel has an average RMS of -28 dB while the LFE channel has an average RMS of -40 dB (the LFE is probably boosted by 10 dB later on).

1629968749805.png


Ofcoruse the center channel has low frequency content, a lot of it.

1629968818320.png


So for a 5.1 setup, if All of your speakers are capable of Full range output. the subwoofer doesn't do much anymore. In fact most people who do 5.1 full range setups only purchase a subwoofer so they wouldn't lose the content in the LFE channel (perhaps some AVRs can upmix the LFE to other channels?)

So basically the more speakers you have that are not full range, the more bigger / beefier subwoofers you need. Otherwise there is barely any real content in the LFE Channel.
 
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Yorkshire Mouth

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btw I really think you're projecting what you think is a good choice onto other people. The reason why you need beefier subs is BECAUSE you have smaller speakers.

let me demonstrate, the latest movie I watched was the The Witcher Nightmare of the Wolf, a very action packed movie. Looking at the audio we see that the center channel has an average RMS of -28 dB while the LFE channel has an average RMS of -40 dB (the LFE is probably boosted by 10 dB later on).

View attachment 149657

Ofcoruse the center channel has low frequency content, a lot of it.

View attachment 149658

So for a 5.1 setup, if All of your speakers are capable of Full range output. the subwoofer doesn't do much anymore. In face most people who do 5.1 full range setups only purchase a subwoofer so they wouldn't lose the content in the LFE channel (perhaps some AVRs can upmix the LFE to other channels?)

So basically the more speakers you have that are not full range, the more bigger / beefier subwoofers you need. Otherwise there is barely any real content in the LFE Channel.

I have a link somewhere, which I can’t currently find. But the long and short of it is that Dolby and THX, forthe purposes of 5.1/7.1 surround systems, consider full range to be something like 100-150hz to 20khz to be ‘full range’, with subs picking up anything below that (+ anything above that if the 5.0 don’t go that low).
 

KMO

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There is this other thread here where it's pointed out that the bass management is rather overdoing the redirection, by simply summing the signals, causing excess power if fully in-phase. (Unless I'm misunderstanding something.)

So 7 channels of bass can end up 7x amplitude in the SW signal, so 49x power, +17dB. When it should arguably should be 7x power, so +8.5dB, or maybe somewhere in-between the two.

Anyway, given that amplitude summing, yes, I believe 7.1 could demand +20dB from the subwoofer. But it probably shouldn't.
 

abdo123

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consider full range to be something like 100-150hz to 20khz to be ‘full range.

Are you implying that AVRs will direct low frequency signals from 'Full range' speakers to subwoofers?

I hope someone else chimes in and confirms this, as It seems really counter intuitive.
 

Yorkshire Mouth

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Are you implying that AVRs will direct low frequency signals from 'Full range' speakers to subwoofers?

I hope someone else chimes in and confirms this, as It seems really counter intuitive.

Can be set for either.

But I believe in THX mode, the answer is yes.

EDIT: Here you go:

“THX requires an 80Hz crossover point in the setup menu to ensure proper bass management in every THX Certified AVR. This setting confirms that the subwoofer handles low frequencies, freeing up the loudspeakers to handle mids and highs.”

https://www.thx.com/product/thx-certified-av-receivers/
 

wwenze

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There is this other thread here where it's pointed out that the bass management is rather overdoing the redirection, by simply summing the signals, causing excess power if fully in-phase. (Unless I'm misunderstanding something.)

So 7 channels of bass can end up 7x amplitude in the SW signal, so 49x power, +17dB. When it should arguably should be 7x power, so +8.5dB, or maybe somewhere in-between the two.

Anyway, given that amplitude summing, yes, I believe 7.1 could demand +20dB from the subwoofer. But it probably shouldn't.

Interesting. I am being conflicted by two different methods of calculation. However one of the method which appears more correct gives me the gut feeling that it has to be wrong somewhere.

And that would be the amplitude one. Because 6 speakers = 6x the amplitude = 36x / 36 units of power. Yet the 7th speaker = add one more amplitude = this signal itself is responsible for an additional 13 units of power.

However in terms of implementation, I wouldn't be surprised if this is indeed being done.

This does lead to a problem however. This would mean that if we direct bass from satellites to sub, the frequency response will go all haywire because bass will be 49x while treble remain at 7x.

However again, isn't this what avr's eq is for. So the end result remains equal amplitude across frequencies. But this leads to problems again when some of the speakers are not playing. Hmm......

I'm sure there has to be some step I am missing out. Apart from power summing of course.
 

KMO

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In practice there rarely is bass in all channels, so I don't think the discrepancy hits that hard that often. Usually main channel bass is only in L+R or C. At least in studio mixes. Not sure about live multichannel stuff.
 

wwenze

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What about 2.1 speakers where <150Hz are usually XO-ed and simplistically summed? That happens often.

Mmm... I have 2.0 capable of 40Hz in-room. I also have a standalone stereo sub capable of 40Hz in-room. I have a 2.1 (edited because I discovered I am lazy). And I have a UMIK1. Let's do this.
 

KMO

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What about 2.1 speakers where <150Hz are usually XO-ed and simplistically summed? That happens often.

Mmm... I have 2.0 capable of 40Hz in-room. I also have a standalone stereo sub capable of 40Hz in-room. I have a 2.1 (edited because I discovered I am lazy). And I have a UMIK1. Let's do this.

Yes. That discrepancy should absolutely be visible in REW, if you compare bass-managed L, R and L+R. L+R should show +3dB for the subwoofer compared to 1 channel.

You can actually see the same effect doing trace arithmetic to add a pair of time-aligned bass-managed L and R traces in REW itself. Because both sweeps are going to be fully coherent in the subwoofer region, they'll fully sum in-phase, and you'll see the same +3dB for bass there in the "A+B" trace.

So arguably, the bass-management system summing isn't wrong.

It's just that the L and R channels now have fully coincident sub-bass drivers that do sum coherently. It's a perfectly valid crossover for each channel, but you've gone and stacked every channel's sub-bass driver together, rather than spreading them around the room, which boosts it.

I don't think there is an obviously better easy answer here. Only thing I can think would work would be to do matrix-style signal processing on the bass to effectively reduce the stacking effect.

Just as Dolby Pro Logic extracts the mono signal, then plays it at +3dB in the centre speaker, you could have the bass management try to isolate in-phase signals spread across L+R or L+C+R or L+C+R+S, to find a coherent multichannel mono signal. Then play that mono signal at the appropriate power-summed level (+3dB, +5dB, +7dB...), then add in any remaining non-mono signals via normal summing. No idea if that's remotely sensible. I can imagine getting different sorts of inconsistent results...

(Edit: also note that those power-summed numbers +3dB etc are probably too low, because the bass from 2 speakers isn't totally incoherent - see audio2920's real-life test numbers over in that other thread...)
 
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Chromatischism

Chromatischism

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Correct. You have +10dB base from the LFE channel. Which is equivalent to 10 surround speakers.

You need to add 90 more surround speakers to reach +20dB.

10x base for the sub + 10 surround speakers worth = 20x = +13dB.

Point is, whatever is causing your sub to compress, is clearly not due to simple arithmetics of speaker power summing/redirection as suggested by the text highlighted in blue.
I tidied up that response to better make the point since typing it on the go on the phone. Also, you excluded an important part of this.

It's simple. You need subs capable of +10 dB over your speaker capability due to the way the LFE channel works. That is base level functionality.

Then, you should have another +10 dB in reserve for the combined effect of redirected bass and a sub bass boost that most people apply.

That means if I plan on listening at 85 dB, which is -20 on an AVR calibrated to 0 = Reference, plan on having subwoofer(s) capable of 105 dB at 20 Hz to get the full experience without maxing them out. At 75 dB or -30, I would plan on having subs capable of 95 dB at 20 Hz. In that scenario it is possible to get there with a good sealed sub.
 
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