Help understanding room gain

[JohnA]

O.K. I'm aware that adding a surface near a loudspeaker increases output by 3 dB. Half-space is +3, quarter-space is +6, 1/8th space is +9, correct?

Room gain is said to be +6 dB. I assume that is from the effect of 2 walls being close by. So, if I move that same speaker, rated at xx dB/w/m including room gain, to the corner I'll only get 3 additional dB of gain?

I think I'm seeing this is a major difference in the rating systems between home and pro audio speaker systems. The pro stuf is always rated in half-space.

[DU73]

i always thought it was when one adds an extra room to an existing house []

in all seriousness I can not help as this is far to complex for my understanding.. hopefully someone else can chime in

[pauln]

Seems the half, quarter, and eigth space gains are generated in the near field by local boundaries but propogated into the far field.

The room gain is the contribution from the proximity in the far field space of the walls, floor and ceiling boundaries which reflect and "recover" a portion of the sound that would be lost in free space.

If this is all true, then the room gain would be variable from room to room,

but for a particular room it would be fairly constant - somewhat independent of the speaker placement,

in the sense that the gain would be about the same, not the actual level which would vary with speaker placement. So some of room gain is placement based.

So I think you are correct that moving the speaker from a back wall to a corner would add 3dB (with two speakers it would add 6dB), and probably boost the room gain a little, too.

I'm sure it is much more complicated because the room gain would be frequency dependent with respect to the absorbtion/reflectivity of the room,

as is the placement of the speaker where the corner will add more low than high.

[Don Richard]

Quarter space, half space, etc. involves boundary reinforcement and radiation angles. Room gain is dependent on the size of the room and its reflective and absorptive characteristics.

[garyrc]

So the question might be "compared to what?" Wasn't there an old Jazz song with that refrain? Les McCann? In the '70s, when most of my friends backed speakers up to the wall, but did not put them in a corner, PWK said something like put a speaker -- any speaker -- on the floor, in a corner, and it will be like making a 25 watt amplifier into a 100 watt amplifier. That would be +6 dB. I don't know what he was comparing it to, but the article was about direct/reflecting speakers like the Bose 901 or the JBL Aquarius series, so he might have been comparing the SPL in the corner to that of a speaker out in the middle of a room.....?

[sfogg]

" Room gain is said to be +6 dB. "

Room gain is basically a second order (12dB/octave) increase in bass from the point it begins at. Where it starts kicking in totally depends upon the dimensions of the room. Basically it starts at the half wavelength of your rooms largest dimension. So in a 17' room you will start seeing room gain at about 32hz. That is assuming a well sealed room, the more lossy/leaky your room the less boost you will see in the bass.

Shawn

[JohnA]

My room is sort of hatchet shaped. A hall enters one corner that is about 18 feet long.

I'm actually trying to calculate the power and number of subwoofers I need to reproduce the low bass effects I hear in movie theaters. I found out my subs are rated with 6 dB of room gain (half-space?). I don't know what configuration is used to measure La Scalas, but I suspect it is 1/2 space since they are basically a pro speaker. If I'm correct, I need to double my power and double my cone area (2 more subs) or quadruple the power into the subs I have. That would be almost 3x the power rating of the subs.

That calculus involves spending as much as $3000, so I'm not about to guess.

[Islander]

Have you experimented with different sub positions? Sometimes the location that looks logical is not at all the best. Generally, between the main speakers should be best, but not in every room. Sometimes you'll get an annoying bass peak or dip that can be reduced or eliminated by moving the sub(s) as little as a foot. Also, your subs will deliver a lot more power to you if they're pointing at you. They could actually be cancelling each other with the positioning shown in your room diagram.

Have you experimented with the sub levels? I'm using a single 10" 500 watt sub with a pair of bi-amped JubScalas with 1000 watts available to each speaker and it keeps up just fine with its level control in the 10 o'clock position and the receiver's LFE output set at -6dB. During certain movies, the bass effects can make the room feel like it's moving, or like the sofa is about to move. I've never had that sensation in a cinema.

Also, the La Scalas start to roll off their bass output as high as 90Hz. I get the flattest measured bass response with the sub crossed over at 150Hz and all the speakers set at Large and the LFE set at Both. I tried setting the speakers at Small, but with nothing below 150Hz getting to them, the bass was noticeably less full. Using Both added 8 square feet of bass horn (2 LS bass bins @ 4 sq ft each) and it sounds better to me that way.

I listen to more music than movies, so my reference is the sound of live instruments. With the settings I'm using, bass instruments, even 5-string bass guitars, sound realistic and neither under- or over-emphasized.

My room is 18' x 19' and it opens out to other rooms, so its effective size is more like 18' x 30'. With your room and equipment, I would think that you've got plenty of hardware and power to drive it already. It could be that your existing system is not operating at its optimum.

Also, you will only get room-moving effects at high volume. If the vocals are down to whisper levels, the bass effects will be lower than they should.

John, I know you've been on this forum for a long time, but don't be offended when I mention all this basic stuff. When I was a mechanic and a vehicle wouldn't start, I would first ask if there was gas in the tank and was the transmission in Neutral (motorcycles)? You always check the basic stuff first, before looking for a more complex solution to the problem.

[sfogg]

" My room is sort of hatchet shaped. A hall enters one corner that is about 18 feet long. "

Is that hallway sealed off? If not you won't see very much room gain there since that hallway is extending the longest dimension of your room.

What room gain is where the reflections from your walls are becoming more and more in phase with the sound still coming out of your subwoofer(s). The sound couples and you get an increase in SPL throughout the room. Basically the area where you entire room is turning into a mode. You won't get say 6dB increase in output for 10-80hz. What one could get is say 0dB from 80-40hz, then a second order increase from there down. In other words a curve that would give you +12dB at 20hz, +24dB at 10hz...etc. But that would be perfect room gain which won't happen if you are leaking bass. You have to overlay the room gains curve over the response curve of your speakers. So for example if your speakers are ported (or horn loaded) they roll off fourth order. So if they happened to start rolling off where the room gain kicks in (40 hz in this example) instead of your acoustic response being down 24dB at 20hz you would only be down 12dB. Instead of being down 48dB at 10hz you would be down 24dB.

The real trick with room gain is to make your subs response be the inverse response of room gain. In other words have them rolling off second order at the same place as your room gain kicks in. If you do this you can keep your subs flatter longer without trading additional bandwidth for output as one would if they tried to EQ the subs flatter.

Since room gain is second order you need a sub that rolls off second order... a sealed design. My rooms longest dimension is 17' so room gain should be kicking in at about 32hz. The room has no windows, has double drywall walls and ceiling all screwed and glued on its own interior studs and a concrete floor. The door is all sealed up. It is a solid room that leaks bass much less the most rooms. My L/R subs are 6 cubic foot sealed cabinets each with 2 JBL Sub1500s in them. The f3 of these are about 33hz with a !0.5. So in other words they naturally are rolling off to be down 3dB at 33hz and follow a typical second order rolloff. At 16hz in free space they would be down 12dB. However in room the room gain keeps them flat to at least 10hz. Possibly lower but the measurement equipment I used stops at 10hz.

My LFE sub is ported (passive radiators) with a natural f3 of 14hz. It rolls off fourth order though so on the really deep stuff my impression is that the sealed subs actually go deeper in room even though their designed f3 is about 33hz. The room gain matches the rolloff of the sealed woofers, in only cuts the rolloff of the ported woofer in half.

"I need to reproduce the low bass effects I hear in movie theaters."

That is not a good goal. There is far more bass effects on DVD/BluRays then most movie theaters can really handle. Good home theater isn't about trying to match a movie theater, but to exceed it.

What is the target SPL you want to be able to hit? If you listen at THX Reference level the LFE channel can hit 115dB at the listening position.

Shawn

[JohnA]

" My room is sort of hatchet shaped. A hall enters one corner that is about 18 feet long. "

Is that hallway sealed off? If not you won't see very much room gain there since that hallway is extending the longest dimension of your room.


"I need to reproduce the low bass effects I hear in movie theaters."

That is not a good goal. There is far more bass effects on DVD/BluRays then most movie theaters can really handle. Good home theater isn't about trying to match a movie theater, but to exceed it.

What is the target SPL you want to be able to hit? If you listen at THX Reference level the LFE channel can hit 115dB at the listening position.

Shawn

No, the hall is not sealed.

I tested the system once and the subs were -4 at 16 Hz. I don't think I have a performance issue other than output. I have some real doubt about being able to hit 115 dB at my listening position, 3.4 meters away. It does a great job reproducing music, but has never had the thump a Klipsch powered theater has. It will exceed the local Carmike theater with JBLs.

I can get pretty reasonable gunshots by turning the subs up 10 to 12 dB, but that is well above calibration levels at my seat.

[garyrc]

Do Klipsch systems used in commercial movie theaters use horn loaded subwoofers? In the "best" klipsch equipped theaters, what subs are used?

As I've said before, strictly IMO:

• To get my movie bass to match the levels in the best theaters, I have to set the sub to a higher level than I would ever use for music listening. When playing music CDs or hybrids, there is often a "room rumble" that is annoying and causes me to turn down the sub, or turn it off. The sub (Klipsch RSW15) is not nearly as "clean" in the bass as our Klipshorns, which don't have as deep a reach.

• The best bass response in today's best theaters seems to excel only below about 40 Hz, and seems to go down to well below 20 Hz.

• The best 70 mm presentations of the late 50's and the '60s, in the best theaters (often those originally set up by Mike Todd), virtually all with horn loaded bass, seemed to have more dynamic bass -- above about 40 Hz -- than modern THX or other theaters. Even though they had very attenuated output below about 40 Hz, conccrete floors would shake, and quite a wind would hit the audience during heavy bass passages (example, the thunderstorm and earthquake sound in the 70 mm prints of Ben-Hur).

• When THX started, their literature contained a justification for moving away from completely horn loaded bass, on the grounds that horns had higher THD, and were thus not preferred. They did not seem to measure Frequency Modulation distortion, which PWK always thought was more irritating, and kept bass from sounding as clean.

[Don Richard]

So the question might be "compared to what?" Wasn't there an old Jazz song with that refrain? Les McCann? In the '70s, when most of my friends backed speakers up to the wall, but did not put them in a corner, PWK said something like put a speaker -- any speaker -- on the floor, in a corner, and it will be like making a 25 watt amplifier into a 100 watt amplifier. That would be +6 dB. I don't know what he was comparing it to, but the article was about direct/reflecting speakers like the Bose 901 or the JBL Aquarius series, so he might have been comparing the SPL in the corner to that of a speaker out in the middle of a room.....?

PWK tested the BoSe 901s with a spectrum analyzer and reported the results in The Dope From Hope in the 1970s. He found that the 901s performed best when turned backwards from the recommended position and placed in a corner. At the time the wife had a pair of 901s so I tried that and found that he was correct. In fact I did not need to use the EQ box that came with the speakers. Still sounded lousy, though. Better but lousy.

[dBspl]

O.K. I'm aware that adding a surface near a loudspeaker increases output by 3 dB. Half-space is +3, quarter-space is +6, 1/8th space is +9, correct?

Adding a boundary theoretically will increase the Sound Pressure Level (SPL) by 6 dB. The Sound Power Level (PWL) will only be increased 3 dB.

Of course this would have to be a perfect boundary and present an ideal mirror image to the loudspeaker. This is usually only realizable at low frequencies (long wavelengths), but in half space (and under the proper measurement conditions) it works fairly well throughout the bandwidth of the loudspeaker.

In 1/8th space, the SPL can increase as much as 18 dB compared to free field!

dBspl

[JohnA]

So, why do I have so much trouble getting theater level bass effects with 2 subs rated at 92 dB/w/m placed in the corners and driven with 700 watts peak, each? The 92 dB includes +6 dB of "room gain". The room, shown above, is 322 sq ft with 8' ceilings.

[Islander]

So, why do I have so much trouble getting theater level bass effects with 2 subs rated at 92 dB/w/m placed in the corners and driven with 700 watts peak, each?

Try pointing the subs at the listening position. Bass is less directional, not non-directional, as some people think.

[JohnA]

They are. You can see my configuration in the link in my sig.

I'm going to try moving the subs closer to the TV.

[Islander]

I guess I was misreading your diagram. It looks like the two subs are directly facing each other.

[JohnA]

When I rebuilt the room I bought new, lowboy cabinets in cherry and put my La Scalas on top.

[Islander]

Have you tried walking around the room while music is playing, to see where the bass peaks and nulls are? Could it be that your sofa is in the middle of a bass null region? With my old setup, there was a really strong but small (2-3 feet across) bass peak region near one of the chairs and it was funny to hear the bass get so much louder when I passed through it. It wasn't where anyone would sit, so I didn't worry about it.

[ClaudeJ1]

They are. You can see my configuration in the link in my sig.

I'm going to try moving the subs closer to the TV.

I have VMPS large woofers also. Mine are firing backwards, into the corners, and with the POLARITY reversed at the speaker cable connection (red to black). They shake everything.They created a suckout until I reversed the connections.........vefified by $3,000 worth of borrowed test gear.