Help understanding room gain

[garyrc]

John,

Wow, I don't know.

Could it be as simple as the 92 dB sensitivity of the subs being so far below that of the La Scalas that you would have to pump many times the power into them for them to seem equal to the La Scalas dynamically, during those famous movie surges? I know you probably have the subs and all other speakers set up for equal SPL from the listening position with test noise signals, but movies really push the envelope with dynamic bass SPL. La Scalas are rated by various writers anywhere from 98 dB to 105 dB, probably dependent on the "space" they are in -- I think 1/8 th produced the 105). With everything fairly near a corner (since you say the 92 dB includes some room gain) there is a 13 dB maximum difference, or ~~~ 20X the power needed??? If your subs are "clip proof," as some powered subs are, maybe they are just plateauing away when they run out of amp power. 50 watts into a La Scala (perfectly plausible according to PWK's chart, with 63 watts producing 115 dB in 3,000 cu. ft.) might conceivably require 1,000 watts into the subs to match ..... I doubt it but it's conceivable.

The other annoyance is that some movie DVDs -- unlike the sound stripes of the movies themselves --- have compressed dynamic range. Who knows why? Most brand new movies are O.K. --- maybe they know the youth audience wants big bass, and full dynamics. Maybe the people who assemble the master from original visual, music and sound elements (when this is done) don't check a print of the best format original to see how the filmmakers mastered it, especially when transferring older, highly dynamic films, but they need to check the new ones as well. Examples of DVDs with compressed dynamic range, compared to the movies themselves, in well equipped San Francisco theaters are:

Amadeus

The Last Waltz and almost any other Rock movie, until recently.

Phantom of the Opera (the musical version)

ET

Alien -- the "thing" that does you-know-what was so loud in the theater that I had fingernail marks in my arm (from my future wife) for days after. I'm surprised it didn't peel the paint off the theater walls ...

Superman

Ben-Hur (If you set it so the music is loud enough, the dialog is much too loud ... if you set it so the dialog is at a plausible conversational level, even some of it is lacking in dynamics ... the two times Mesala shouts are much too soft --- and the big bass effects are way below the theater levels, unless you ride the volume control)

And almost any musical that was recorded and presented in magnetic stereo.

[JohnA]

John,

...

With everything fairly near a corner (since you say the 92 dB includes some room gain) there is a 13 dB maximum difference, or ~~~ 20X the power needed??? If your subs are "clip proof," as some powered subs are, maybe they are just plateauing away when they run out of amp power. 50 watts into a La Scala (perfectly plausible according to PWK's chart, with 63 watts producing 115 dB in 3,000 cu. ft.) might conceivably require 1,000 watts into the subs to match ..... I doubt it but it's conceivable.

The other annoyance is that some movie DVDs -- unlike the sound stripes of the movies themselves --- have compressed dynamic range. Who knows why? Most brand new movies are O.K. --- maybe they know the youth audience wants big bass, and full dynamics. Maybe the people who assemble the master from original visual, music and sound elements (when this is done) don't check a print of the best format original to see how the filmmakers mastered it, especially when transferring older, highly dynamic films, but they need to check the new ones as well.

Most VMPS subs are unpowered. Mine are not powered. Both are driven by an Acurus A250 (soon to be 2) capable of delivering 430 watts RMS continuous (480 at 1k) and 700 watts RMS dynamic power to each. So, I could easily have 10 dB more amplifier heading to the subs as my La Scalas and Heresies actually get. Adding a second A250 will give each sub 800 watts peak and I'll configure the power so only the power amps pull off of a single 20A breaker.

Due mostly to the shape of the room, I don't notice nodes when I walk around, other than the obvious bass increase very near walls.

I have discussed compression with my audio guru I bought most of my gear from. I have suspected it, as does he, but you are the first to confirm compression on DVDs. My thought was that it is done to keep from blowing up HTIB systems. I sure hope not! I've put too much effort into my room and system to put up with bad source material.

The calculation of whether I need another pair of subs and the second A250 driving them entirely revolves around the actual output of the ones I have while placed in a corner. While I have the 2nd amp already, adding another pair of subs raises the cost another $2k, meaning I need to apply the correct engineering to the problem.

[Islander]

Some AV receivers have a compression setting hidden in a submenu somewhere. You might want to check that in case its default position adds compression. That's a separate setting from the Night mode, which also adds compression.

[JohnA]

I don't have a reciever; don't believe in 'em.

I'll recheck my DVD players, but the first thing I do is remove all compression for anything.

[colterphoto1]

Great looking layout there John, very spacious and everything in it's place.

Have you tried picking one or the other corners and placing your subs side by side (or stacking if applicable) ? That gives another 5-6 db increase.

I see the relative inefficiency of those subs as your major hurdle, that and the power handling capacity- they just can't keep up with the LaScalas. Have you considered changing to the Klipsch THX sub system?

Michael

[JohnA]

Yes, I have. For about $2500 to $3k I can add a KPT-884 and a power amp. It's alittle less to add 2 more VMPS.

It's time to do some testing to see if I can determine the actual 1 watt output in my room.

[Andy W]

Here's a visualization of 1/2, 1/4, 1/8th space. http://www.trueaudio.com/st_spcs1.htm

full space = 4pi steradians
half space = 2pi
quarter space = pi
eighth space = pi/2

For each additional halving of the space from full space to eighth space the SPL increases the 6dB (theoretical). In the real world you never quite get 6dB because the boudaries are never perfectly rigid (some energy is lost by sound going into and through the boundary) and due to the fact that a subwoofer is not a point source and can't be placed exactly in the plane or corner of the boundary.

The reason you get +6dB instead of +3dB is due to the squared term in the inverse square law of power radiation.

Room gain is partly due to the boundary effects and partly due to the "cabin gain" effect seen in cars. The transition from boundary gain to cabin gain depends on the dimensions of the room. When the wavelengths exceed the dimensions of the room, cabin gain begins at 12dB/octave below that frequency. Essentially the entire room is being pressurized by the subwoofer. The longer the wavelength, the greater the pressurization effect. In a car, with small dimensions and sealed subs, the cabin gain begins at frequencies of ~100Hz. Which is why car subs can get away with very small boxes - the 12dB/octave rolloff in the sub is compensated for by the cabin gain, giving a "flat" frequency response down to very low frequencies. The same goes for Klipsch's in-ear headphones... except the effect starts much higher in frequency.

The same thing happens in a living room, but usually doesn't take effect until ~30Hz... and the losses (leaks) present in the room (where it transitions to other rooms in the house) put a damper on the "cabin gain" effect. Cars leak too... especially when you roll down the windows.

Boundary gain can have effect up into the midrange, if the speakers are close enough to the wall (in-wall speaker take the most effect). At all frequencies the distance to the walls in front of and behind the speakers or subwoofer determine where you will have peaks and dips in the frequency response.

[JohnA]

O.K. So if I assume I get no "room" gain, because my room is so lossy, connected to other rooms and has an acoustic tile ceiling, the 92 dB rating of each sub is 86 dB. Let's assume that would be in 1/2 space, as there is no other practical way to test something this big. But, corner placement raised that. So, if I add 4 dB per boundary instead of 6, I get 86 + 8 = 94 dB. 700 watts peak is +28.5 dBw, so 94 + 28 = 122 dB. Add +6 for another sub and amp = 128 dB peak at one meter!

Repeat the above with 430 watts continuous, 26.3 dBw, and I get 126 dB continuous at 1 meter.

My La Scalas are colocated with the subs, and they are not capable of 126 dB at one meter.

[garyrc]

So, why do I have so much trouble getting theater level bass effects?

It's still a mystery, isn't it? Here is some grasping at straws.

• Just in case the DVDs or Blu-rays you have trouble with are dynamically compressed or have the deep bass shaved off, or both, can you list a few that seemed to have distinctly less bass than you experienced in the commercial movie theater? Then others can weigh in on whether they are getting less bass at home than in the theater with those particular movies.

• I don't remember what the standard duration (of the peaks) is supposed to be when measuring amplifier peak power or dynamic power ... is it 10 milliseconds (1/100 th of a second) ?? .... but for what it's worth, aren't deep bass peaks relatively long in duration ("fat") compared to the leading edge of higher pitched peaks (say, the leading edges of cymbal crashes)? I think there are often some rather long lasting bass peaks in modern movie soundtracks, which might place a great demand on the peak power of amplifiers. Is it possible that an amplifier capable of "peak power," of 700 watts, as measured in milliseconds, might fail to produce that power for longer "fat" bass peaks? I would estimate some big booming peaks in movies to be as long as 1/2 second, with loud rumbles between these peaks, instead of the usual, deeper troughs found between peaks in other kinds of music. How do your subwoofers do when playing the shorter bass peaks on orchestral CDs or Lps? Examples of what I'm thinking of are the ~~~ 1/8 to 1/16 second leading edges (only) of the huge bass drum & timpani booms in the Riccardo Muti version of The Firebird (EMI CDC 7 47099 2), or the fff timpani/bass drum on the Crystal Clear Recordings direct to disc version of Fanfare for the Common Man (on a disc called something like Sonic Seasonings) -- this one also has big peaks on the Tam-Tam, that have leading edges more in the lower midrange, for comparison. The timpani/bass drum beats should flap your pant legs in the wind, unless you are too far away (in my room it happens at 15 feet). A wooden chair (kitchen or Captain's chair) should seem to move, buckle, or go out of square for a moment, and a wood floor will move. If your system can reproduce the short bass peaks in Classical, Romanist, or early 20th century orchestral music, but not deliver the long bass peaks in movies, maybe the difference is peak duration.

• For the record, in the THX Sound System Program Instruction Manual Architect's and Engineer's Edition (copyright 1987 -- this is for dubbing stages, and by implication, commercial theaters), they report measuring "infrequent, but recurrent, peak levels of 108 dBC, fast (10 ms time constant) maximum." For Return of the Jedi, they did pick up 110 dB peaks, specifically at 63 Hz. About an octave lower (31 Hz) the peak was "only" 106 dB.

• I'm assuming that the walls right near the subs are very rigid, right?

• If you haven't tried it already, I wonder what a Room EQ Wizard analysis, with the microphone at the various listening positions (including head height) would show (http://www.hometheatershack.com/roomeq/) .... maybe you have a dip that room walking doesn't make obvious.

• I wonder if your favorite nearby theaters have jacked up or put a bump in the bass? Someone at JBL once admitted to that (in the '70s or '80s).

• In the days before theater subs, studios would sometimes instruct projectionists to ride the volume a bit, or turn it up at a certain part of the movie (I know they asked for it to be turned up at a specific time part way into the movie for the 70 mm version of Paint Your Wagon). The DVD transferrers might be clueless in regard to these instructions. An old trick they might not have given up was to have an automated way to raise the volume for certain sequences (the 70 mm versions of Oklahoma! and Around the World in 80 Days [1956], and even in one of the several Fantasound systems for Disney's Fantasia in 1940!).

[gaspr]

John, have a look at the attached white paper written by Floyd Toole

LoudspeakersandRoomsPt3.pdf

[gaspr]

...and another

Loudspeakers%20and%20Rooms%20-%20Working%20Together.pdf

[JohnA]

While thinking about my problem, I decided to reverse the polarity of the subwoofers.

I got more "punch" from the subs that way. No curves, yet, but I like it, initially. Loreena McKinnett's "Live at tha Al Hambra" sounded much more like she did at the Tennessee Theater in Knoxville a couple of years ago.

Maybe that was all I needed. I did check the polarity of the subs before I did it and a positive voltage caused both active drivers to move forward.

[Islander]

While thinking about my problem, I decided to reverse the polarity of the subwoofers.

I got more "punch" from the subs that way. No curves, yet, but I like it, initially.

"More punch" from subs usually means "more correct" settings, positioning, etc. As I understand it, changing the subs' polarity fine-tunes their time-arrival at the listening position to match up with the other speakers, so more punch means they're now in phase with your other speakers, therefore the subs are working with them and not against them.

[SWL]

I decided to reverse the polarity of the subwoofers.

I got more "punch" from the subs that way.

What are your subs crossover settings set at?

[DrWho]

John, have a look at the attached white paper written by Floyd Toole

Interesting, I've not read that one before (speakers and rooms pt 3)....very interesting once you start digging into it.

[gaspr]

Hey Doc...if you have not read his book yet, I highly recommend it. "Sound Reproduction Loudspeakers and Rooms" This guy really knows his stuff.

[DrWho]

I own it...just haven't read it yet [] ....I actually forgot about it until you brought it up just now...

[gaspr]

When you do get around to reading it, I would love to hear your review...

[DrWho]

As an item of discussion, I'd be curious what you think about the
transient nature of a wave through the room versus the steady state
analysis that describes room modes....

As a very simple analogy,
look at a wavefront that starts with the subwoofer at the front of the
room with a listener sitting in the middle of the room. Ignoring all
the other reflections except the front and rear walls, the first sound
heard at the listening position is the direct sound from the subwoofer.
As time goes on, the wavefront moves through the listening position and
then eventually reflects off the back wall. This reflection then
arrives at the listening position with a different phase, so the
pressure amplitude at the listening position is the product of that
interaction. If it's in phase, it adds...outta phase and it subtracts.
The waves continue to move through the listening position with a
constant phase angle difference, but when the direct sound stops,
there's still a long period where the reflected sound continues to move
through the listening position.

Let's say your subwoofer played a
single cycle of a frequency whose wavelength was 4x longer than the
distance of the listening position to the rear wall. For the first half
cycle, you would get a normal amplitude at the listening position and
then for the next half cycle you would have no sound, and then for
another half cycle you would have sound again at the normal amplitude.
Your single wave now went from a simple sine-wave to a half-wave
rectified sine-wave that lasted half a cycle longer than intended.

If
you let the subwoofer play more than one cycle (so play the same tone
for a long period of time), then the first half cycle would be normal
followed by a long period of silence (the steady state standing wave)
and then at the very end you would have an extra half cycle.

For
what it's worth, you can clearly see this behavior in a water wave
table as it takes some time for the standing wave to establish.

I
am very curious what kind of audible impact the transition into and out
of the steady state has on the music, and what happens to the transient
behavior when you move the speaker/listener around and EQ in the manner
that Toole is referring to in his article. It is the first and last
half cycle in my analogy that makes me very hesitant to just accept
that room modes are minimum phase....I can agree that they behave like
minimum phase, but I think there is a compromise there...

I'd be curious what Toole's thoughts would be on something like this:

http://www.avsforum.com/avs-vb/showthread.php?t=837744

where basically an active means is being employed to cancel out the reflections...

I've attached a German article that is linked to in that one thread that discusses the topic further.

tmt2002.pdf

[ZAKO]

Long periods of time??? I dont think so,