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post #2131 of 4701 Old 02-13-2017, 12:40 PM
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Seems like a couple tabs would be much cheaper
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post #2132 of 4701 Old 02-13-2017, 12:59 PM
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Does our forum have a sensory deprivation flotation tank talk section?
You have to go to noavs.com for that.
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post #2133 of 4701 Old 02-13-2017, 02:01 PM
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Looks like I'm gonna be playing "catch that buzz" as well. Was playing around with my 708i's today and as soon as the amp powers on, I hear the buzz so it has nothing to do with the processor.
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post #2134 of 4701 Old 02-13-2017, 02:32 PM
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A few thoughts / comments:

When I talked to Charles Sprinkle at JBL (who was instrumental in designing the 708s and M2s), he said not to really worry about toe in. Slightly angled toward MLP is probably fine.

I know most of us want to move on from the "should subs go down below 20 hz" discussion, but I thought I would put this capper on it (it's a capper at least for me):

After all the back and forth discussion here, I wanted to make sure my thinking and associated comments were accurate. I've mentioned a few times that I've mixed over half a dozen movies, but as admitted, they were independent features. Since my personal experience is somewhat limited, I picked up the phone last week and called Formosa Group out in L.A. and talked to one of their lead sound mixers - one who has done ATMOS mixes for several major titles we all would be familiar with. He stated that it is standard industry practice to filter out everything below 20 hz, and that even the main mix stage probably doesn't go any lower than that. He also stated that having energy below 20 hz in the recording messes with metering and is just considered a nuisance. When I mentioned the term "slop," he said "I never heard it called that before, but that's a perfect term for it." Was happy to have my suppositions confirmed

Also, I took advantage of the time to talk about reference levels and he confirmed my statements that 78 - 79 db is good for a smaller room, and that he even calibrates one of the large mix stages at Formosa at 83 db, and that stage is about the size of a smaller commercial theater.

Hopefully this helps brings some perspective; I do not intend this to start up another debate

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post #2135 of 4701 Old 02-13-2017, 03:32 PM
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The point of the discussion was the roll off of the JBL subs, not that they don't go under 20 hertz, it's that they don't really go down to 20 hz. Negative 6 dB is pretty drastic, but to each their own.

Everyone else spun out on all the <20 material.
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post #2136 of 4701 Old 02-13-2017, 03:50 PM
 
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Quote:
Originally Posted by John Schuermann View Post
After all the back and forth discussion here, I wanted to make sure my thinking and associated comments were accurate. I've mentioned a few times that I've mixed over half a dozen movies, but as admitted, they were independent features. Since my personal experience is somewhat limited, I picked up the phone last week and called Formosa Group out in L.A. and talked to one of their lead sound mixers - one who has done ATMOS mixes for several major titles we all would be familiar with. He stated that it is standard industry practice to filter out everything below 20 hz, and that even the main mix stage probably doesn't go any lower than that. He also stated that having energy below 20 hz in the recording messes with metering and is just considered a nuisance. When I mentioned the term "slop," he said "I never heard it called that before, but that's a perfect term for it." Was happy to have my suppositions confirmed
Exactly what I've been saying all along. Thank you for calling him.

Also, by filtering out the slop it allows more amp power and cone excursion capabilities, both in commercial cinemas and our home systems, to get louder, cleaner, and stronger 20Hz to 30Hz content, the part they actually intended, which does matter.
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post #2137 of 4701 Old 02-13-2017, 05:22 PM
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Originally Posted by Jsin_N View Post
The point of the discussion was the roll off of the JBL subs, not that they don't go under 20 hertz, it's that they don't really go down to 20 hz. Negative 6 dB is pretty drastic, but to each their own.

Everyone else spun out on all the <20 material.
How do they rate their subs frequency responses? If they're rated full space with no boundaries, there's a good chance they are flat to 20hz in a typical room situation.

The SUB18 is at -10db at 20hz which I agree, is pretty drastic. If they could make it 3db down at 20hz, that would be huge. But again, I'm not sure how that's rated. Maybe measuring the sub the ways other subs are rated (assuming they're different) would show different specs. I would like to see data-bass test a SUB18 so we can see how it compares to the others.
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post #2138 of 4701 Old 02-14-2017, 12:27 AM
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Quote:
Originally Posted by John Schuermann View Post
A few thoughts / comments:

When I talked to Charles Sprinkle at JBL (who was instrumental in designing the 708s and M2s), he said not to really worry about toe in. Slightly angled toward MLP is probably fine.

I know most of us want to move on from the "should subs go down below 20 hz" discussion, but I thought I would put this capper on it (it's a capper at least for me):

After all the back and forth discussion here, I wanted to make sure my thinking and associated comments were accurate. I've mentioned a few times that I've mixed over half a dozen movies, but as admitted, they were independent features. Since my personal experience is somewhat limited, I picked up the phone last week and called Formosa Group out in L.A. and talked to one of their lead sound mixers - one who has done ATMOS mixes for several major titles we all would be familiar with. He stated that it is standard industry practice to filter out everything below 20 hz, and that even the main mix stage probably doesn't go any lower than that. He also stated that having energy below 20 hz in the recording messes with metering and is just considered a nuisance. When I mentioned the term "slop," he said "I never heard it called that before, but that's a perfect term for it." Was happy to have my suppositions confirmed

Also, I took advantage of the time to talk about reference levels and he confirmed my statements that 78 - 79 db is good for a smaller room, and that he even calibrates one of the large mix stages at Formosa at 83 db, and that stage is about the size of a smaller commercial theater.

Hopefully this helps brings some perspective; I do not intend this to start up another debate
From the data that's available on Data Bass, I would say that low frequency filtering and/or reduction is *common* industry practice but not *standard* industry practice.

The data at DB is not representative, but I if I had to guess, I'd say about 1 in 3 BD releases don't get filtered at all. This includes many big name blockbusters and other films that review very well for sound quality.

The part about filtering at 20 Hz is somewhat inaccurate. If a movie is filtered, it's more commonly done at 30 Hz. Many films are only shelved down several dB rather than using an actual high pass. We've also seen filters that kick in at 40 Hz or even higher (like the recent "Star Trek: Beyond") and some that don't cut until below 10 Hz. Clearly there is no industry-wide standard practice.

It also deserves mention that most filters alter the phase response of the sound in the pass-band, especially in and around the cut frequency. Furthermore, essentially all playback systems exhibit phase shift of their own which is usually greatest at the cut frequency for their own bandwidth. The cascading of these phase shifts, when they occur in both the production chain and the playback chain, increases the likelihood that these phase shifts will cause audible ringing. So if the goal were to reproduce bass down to 20 Hz with utmost fidelity, it is prudent to use filters that don't cut until somewhat below 20 Hz.

A similar situation exists at 20 kHz. The 44.1 kHz sampling rate was chosen for CD because the nyquist frequency (the maximum representable frequency) of 22.05 kHz allows some breathing room for filters to cut to zero. The filters must cut to zero (or very close to it) because any content that exceeds the nyquist frequency causes nasty aliasing distortion. Unfortunately in practice, the 44.1 kHz sampling rate required very steep analog filters in order to preserve all the content below 20 kHz. Optimal filters were impractical without great expense, and realizable filters had to compromise between the flatness of the pass band (below 20 kHz) and the phase shift involved. Both issues had audible consequences. The issue was largely solved by oversampling.

Unlike the case at 20 kHz, however, aliasing is not a problem on the low end. Such filters are only necessary to protect a band-limited monitoring or playback system or to remove actual slop from recordings such as dialog using cut frequencies *well below* those that are intended to be reproduced accurately. Also, the audibility of sound drops off very fast once you get above 15 kHz or so. Very few people can actually hear 20 kHz. But, almost everyone can hear to 20 Hz and below. As such, we can expect that any ringing induced by excess filtering will be more audible at the bottom end than it is at 20 kHz.

IMHO, it's better to "first do no harm" and avoid filters in the production chain, allowing the playback system to implement whatever filtering it needs to do to optimize headroom and/or protect itself. Almost all competent playback systems do this already.

On the subject of reference levels, the advice given is reasonable, but I'd also add that the cinema standards are seriously flawed because they are based on measurement of continuous sounds that include the room too much. A big issue with choosing playback level is the impact that tonal balance has on apparent loudness. For example, adding 1 dB to the upper mids or treble can sometimes make content sound several dB louder.

Because cinemas and dub stages calibrate tonal balance using continuous signals, there is poor consistency in tonal balance between dub stages. This means that there is poor consistency in tonal balance and loudness in the actual content. Which in turn means that you won't ever find a single calibration SPL or EQ configuration that will sound good with every movie you watch.

As I understand it, the situation has improved lately only because mixers are becoming wiser to the fact that every dub stage sounds different. They may be limiting the use of EQ altogether (since much of this presumably happens at earlier production stages on smaller room systems anyway), and may be making an effort to mix to sound consistent with other content on that dub stage rather than going with what actually sounds best.

Most cinema stuff these days sounds OK on home systems that otherwise sound good for music, but back in the 90s especially a lot of stuff was mixed very hot in the treble to overcome the X-curve, and the result is very uncomfortable to listen to on typical home systems without an X-curve in the playback chain. Too bad few people have the ability to engage such a filter.
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post #2139 of 4701 Old 02-14-2017, 05:37 AM
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Hi all. In my upcoming room, the optimal positions for the subwoofers (theoretical) is exactly where my M2's are positioned.
I found the picture below from another setup, which inspired me regarding how to place the subwoofers as they should and have the M2s where they should.

Do you see any issues by having the M2s upside down like that?
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post #2140 of 4701 Old 02-14-2017, 09:13 AM
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Nope.
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post #2141 of 4701 Old 02-14-2017, 10:08 AM
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Looks like I'm gonna be playing "catch that buzz" as well. Was playing around with my 708i's today and as soon as the amp powers on, I hear the buzz so it has nothing to do with the processor.
I'll be playing that game all weekend long in KC @Gooddoc , you still comin?

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Originally Posted by John Schuermann View Post

He stated that it is standard industry practice to filter out everything below 20 hz, and that even the main mix stage probably doesn't go any lower than that. He also stated that having energy below 20 hz in the recording messes with metering and is just considered a nuisance. When I mentioned the term "slop," he said "I never heard it called that before, but that's a perfect term for it." Was happy to have my suppositions confirmed
I certainly respect his statement, but I am certainly not buying that it is "Industry Standard"

All you have to do is look at the PVA's of say the last 20 action blockbusters that have been released to realize there really is NO standard here. We got some like that Star Trek beyond that is beyond abysmal with its rolloff:

https://www.avsforum.com/forum/113-su...l#post47581481

and then we have ones like hacksaw ridge:

https://www.avsforum.com/forum/113-su...l#post50643049

Just a bit of disparity I'd say.

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post #2142 of 4701 Old 02-14-2017, 10:30 AM
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I'll be playing that game all weekend long in KC @Gooddoc , you still comin?
Well, I have a non-refundable plane ticket and hotel reservations...and just came down with a pretty severe head cold/sinus infection. If it doesn't clear up I won't be able to fly. Only time is going to tell.
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post #2143 of 4701 Old 02-14-2017, 10:57 AM
 
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Do you see any issues by having the M2s upside down like that?
It depends on the speaker, the designer, the room, the seated distance, and the frequency range it addresses.

A speaker manufacturer has no control about where you mount a speaker in a room so the distance the woofer is to the left wall, the right wall, and the back wall will often vary between any two setups (and will greatly alter the sound) however the designer does have control of the exact distance to one important room boundary: the floor. If you greatly alter that distance by flipping the speaker upside down, keeping the tweeter at the same height, the woofer's direct signal path distance won't change very much however the major woofer reflection off the floor bounce travel path will increase significantly and when that combines with the direct signal path the constructive/destructive interference patterns can often cause an objectionable dip in the frequency response. This is often called the "Allison Dip" or "Allison Effect" named after my recently departed friend, Roy Allison, who spent a great deal of time analyzing this issue and he published papers on it [Roy F. Allison, The Influence of Room Boundaries on Loudspeaker Power Output, JAES, June 1974] . Here is an example of the problem it may cause with my crude stick figure drawing. First the correct way and then the inverted speaker way:


Now the inverted way pulling the woofer far from the floor but maintaining the proper ear height for the tweeter:


Sorry for the crude image and this is just a simplification of the issue so it is easy to understand. In truth the room has zillions of later (secondary, etc.) reflections so it all gets scrambled, but this issue with the strongest (loudest) first reflection off the floor is real, audible, and can be measured.
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post #2144 of 4701 Old 02-14-2017, 11:27 AM
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Originally Posted by m. zillch View Post
It depends on the speaker, the designer, the room, the seated distance, and the frequency range it addresses.

....

Sorry for the crude image and this is just a simplification of the issue so it is easy to understand. In truth the room has zillions of reflections so it all gets scrambled, but this issue is real, audible, and can be measured.
But what about the ceiling reflection then? Wouldn't that be the same issue if the speaker is placed as intended?

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post #2145 of 4701 Old 02-14-2017, 11:32 AM
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Well, I have a non-refundable plane ticket and hotel reservations...and just came down with a pretty severe head cold/sinus infection. If it doesn't clear up I won't be able to fly. Only time is going to tell.
If it's the same thing I am still getting over, you'll have a down day and half and then you'll come out of it pretty well. Lingering cough that just doesn't seem to want to go away, but that's about it. Hope you get to feelin better.

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post #2146 of 4701 Old 02-14-2017, 11:34 AM
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But what about the ceiling reflection then? Wouldn't that be the same issue if the speaker is placed as intended?
YES! Here's all your reflection points, duh


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post #2147 of 4701 Old 02-14-2017, 11:52 AM
 
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But what about the ceiling reflection then? Wouldn't that be the same issue if the speaker is placed as intended?
Since a tower speaker's tweeter usually isn't exactly half way up your wall, it is below that, at say seated ear height, when you invert the speaker and put the woofer far way from the floor, but maintain the exact same tweeter height, you aren't suddenly remounting the woofer the same distance to the ceiling as it was from the floor.

Ceiling reflection are important though, yes.

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post #2148 of 4701 Old 02-14-2017, 11:59 AM
 
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This exact same dip in the frequency response is also a problem if you mount your speakers at a bad distance to the back wall. The exact same problem happens here too: the arrival time to the ear of the back wall reflected sound will almost perfectly cancel out that of the direct sound path (because they are out of phase), at a specific frequency, hence a dip at that frequency:

http://arqen.com/acoustics-101/speak...-interference/

How to avoid the bass suck out is discussed in the link. [Although I haven't read the page in any great detail, I'm just supplying the first googled thing I found which discusses this known issue.]

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post #2149 of 4701 Old 02-14-2017, 12:21 PM
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This exact same dip in the frequency response is also a problem if you mount your speakers at a bad distance to the back wall. The exact same problem happens here too: the arrival time to the ear of the back wall reflected sound will almost perfectly cancel out that of the direct sound path (because they are out of phase), at a specific frequency, hence a dip at that frequency:

http://arqen.com/acoustics-101/speak...-interference/

How to avoid the bass suck out is discussed in the link. [Although I haven't read the page in any great detail, I'm just supplying the first googled thing I found which discusses this known issue.]
That is the EXACT problem I have in my room, it's at the same frequency as well. Unfortunately my placement is limited. I can't pull the speakers out more from the back wall or I'll block the door to the room.
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post #2150 of 4701 Old 02-14-2017, 12:26 PM
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That is the EXACT problem I have in my room, it's at the same frequency as well. Unfortunately my placement is limited. I can't pull the speakers out more from the back wall or I'll block the door to the room.
Let me guess - you have the speakers ~27 inches from a boundary? The is actually the exact reason I want to turn my speakers upside down. I can move the subs closer to the side wall and raise the dip above 80hz.

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post #2151 of 4701 Old 02-14-2017, 12:36 PM
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Let me guess - you have the speakers ~27 inches from a boundary? The is actually the exact reason I want to turn my speakers upside down. I can move the subs closer to the side wall and raise the dip above 80hz.
Very close, they're about ~24 inches or so from a boundary. I'd put them closer to the wall but I need to get behind them to turn them on (powered)
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post #2152 of 4701 Old 02-14-2017, 12:45 PM
 
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Yes, I can see that in some instances flipping is actually beneficial. Controlling the exact right distance to all of the room's six boundaries can be a nightmare. There are computer programs which can help with that but the distance of the woofer to the floor is sometimes very specifically selected by the designer and messing that up can have consequences.

I goofed up my stick figure diagram, by the way, and have replaced them with slightly better versions. I forgot that the angle of incidence must match the angle of reflection. My Photoshop skills suck but at least that part is now a tad better and the difference in the reflected signal path's length is even easier to see now. Check it out.

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post #2153 of 4701 Old 02-14-2017, 01:38 PM
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That is the EXACT problem I have in my room, it's at the same frequency as well. Unfortunately my placement is limited. I can't pull the speakers out more from the back wall or I'll block the door to the room.
Do you have broadband absorbers on the front wall? That can help a lot with SBIR.
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post #2154 of 4701 Old 02-14-2017, 01:51 PM
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Does any one use the JBL 708P for surround sound and how do they sound?

Also, can you plug JBL 708P 7 Series 8-inch Bi-amplified Master Reference Studio Monitor into a regular pre/pro using XLR? I have a Classe SSP-800.
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post #2155 of 4701 Old 02-14-2017, 02:11 PM
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Do you have broadband absorbers on the front wall? That can help a lot with SBIR.
I have these in the front corners. They're GIK alpha bass traps. I know they don't extend to thensure ceiling, I'll have to put that on my to do list.
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Does any one use the JBL 708P for surround sound and how do they sound?

Also, can you plug JBL 708P 7 Series 8-inch Bi-amplified Master Reference Studio Monitor into a regular pre/pro using XLR? I have a Classe SSP-800.
Yes, plenty of people use the 708i's as surrounds (myself included soon) and they do wonderfully. The 708ps will be just as good. And yes, you can plug them right into your pre/pro via XLR.
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post #2156 of 4701 Old 02-14-2017, 08:40 PM
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when you have SBIR you can correct it with absorption but the absorption needs to be able to handle the frequency you need. Many 2" absorption panels will not go work at 125hz, 4" ones typically can handle that freuency. Make sure you look at the specs of the panel you will be using. When you move the speaker closer to the wall the cancellation dip will happen at a higher frequency so that can be one solution for SBIR with absorption panels.
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post #2157 of 4701 Old 02-14-2017, 08:48 PM
 
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^ Also to add to that many speaker manufacturers are expecting you to get a dose of boundary reinforcement but if you dissipate it by using tremendously thick absorptive panels which nullify the bounced energy [interestinlg as heat, I believe] you may find your speaker sounds a bit thin/tinny because the bass isn't augmented by that boundary, at the proper distance, as was intended by the maker.

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post #2158 of 4701 Old 02-15-2017, 02:24 AM
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Originally Posted by m. zillch View Post
It depends on the speaker, the designer, the room, the seated distance, and the frequency range it addresses.

A speaker manufacturer has no control about where you mount a speaker in a room so the distance the woofer is to the left wall, the right wall, and the back wall will often vary between any two setups (and will greatly alter the sound) however the designer does have control of the exact distance to one important room boundary: the floor. If you greatly alter that distance by flipping the speaker upside down, keeping the tweeter at the same height, the woofer's direct signal path distance won't change very much however the major woofer reflection off the floor bounce travel path will increase significantly and when that combines with the direct signal path the constructive/destructive interference patterns can often cause an objectionable dip in the frequency response. This is often called the &quot;Allison Dip&quot; or &quot;Allison Effect&quot; named after my recently departed friend, Roy Allison, who spent a great deal of time analyzing this issue and he published papers on it [Roy F. Allison, The Influence of Room Boundaries on Loudspeaker Power Output, JAES, June 1974] . Here is an example of the problem it may cause with my crude stick figure drawing. First the correct way and then the inverted speaker way:


Now the inverted way pulling the woofer far from the floor but maintaining the proper ear height for the tweeter:


Sorry for the crude image and this is just a simplification of the issue so it is easy to understand. In truth the room has zillions of later (secondary, etc.) reflections so it all gets scrambled, but this issue with the strongest (loudest) first reflection off the floor is real, audible, and can be measured.
Were you really friends with Allison? That's pretty cool if so. When I was 6, my Dad invested in a stereo system included Allison speakers. I think they were model 7s. They sounded good. I can't say for sure how much those speakers inspired my interest in audio later in life, but they definitely had a significant influence on me early in my life.

I do want to say a few clarifying things here. First of all, the Allison paper concerns power response which is not the same thing as what you are illustrating in your picture. Power response depends on the proximity of the driver to one or more boundaries, so this is something the designer can more or less control by varying the height from the floor, provided that the speakers are always placed far enough away from all the other boundaries. However, your picture shows something more akin to on-axis frequency response measured in the presence of the floor. This response will vary substantially depending, not just on the vertical distance between the woofer and floor, but also on the distance between the listener and woofer.

An important question that arises is whether flat power response or flat on-axis frequency response is more important for low frequencies (i.e., under 500 Hz) in the presence of a boundary. I lean toward on-axis frequency response being more important but with power response playing a bigger role than for higher frequencies. I'm curious as to Harman's opinion on the matter, if anyone can point me to any relevant research.

With that said, the M2 measurements suggest that the anechoic on-axis response is flat all the way down to 40 Hz. This means that JBL did not implement *any* compensation in the design for fluctuations in either power response or on-axis SPL that arise from placement on a floor. If Harman regarded flat power response as being more important than flat on-axis frequency response, then I believe we would see compensation of some kind in their measurements. Instead, they measure completely flat in the anechoic chamber.

On the other hand, if on-axis frequency response is regarded to be the more important factor, then this response will depend on the listener's distance (and head height, but that's pretty consistent). If JBL implemented any compensation, it would only work for listeners at a fixed distance. Instead, optimal results would require custom in-room EQ. But there is another issue.

If indeed on-axis frequency response is the more important factor, then in all likelihood, the relative tonal balance of the upper bass and low mids is very inconsistent among a wide variety of music masters. Pretty much everyone environment will have boost below 100 Hz from the floor, but there will be a big dip somewhere above there centered at a frequency whose half-wave length is the path length difference between the direct and reflected sound. Below that dip, wherever it lies, is where the response will start to rise. Above that dip, the response exhibits a comb filter ripple with peaks and dips alternating at multiples of the frequency first dip. However, at some point, the direct sound becomes distinguishable from the reflected sound so the ripple may not be audible like the initial dip and the bass boost below it may be.

As an example, I ran some numbers with the M2 to calculate where the on-axis SPL dip will reside, depending on listener distance and assuming listener ear height of 40". I also assumed an M2 woofer height of 30", which is a guess being that I can't find a drawing:

10 feet: dip at 370 Hz
15 feet: dip at 530 Hz
20 feet: dip at 700 Hz
25 feet: dip at 865 Hz

I calculated out to 25 feet because the M2 specs give a max SPL figure for 8 meters. Note that for 20--25 feet, the crossover has effects on the level and phase of the reflected sound, and this may suppress the appearance of a dip in the response at these distances.

As another example, consider a small speaker driver situated at ear level and 1 meter away, as might occur in a near-field monitoring situation. The distance difference is quite high when sitting so close, and the dip is all the way down at 240 Hz or so.

And just so no one gets confused, you won't necessarily see dips at these frequencies in your measured frequency response because your frequency response gets modified a lot by later arriving reflections. However, the reflections that cause the dips documented above arrive so early that they can't be distinguished from the direct sound. This means that it is likely that the brain will hear these dips even if the measured frequency response doesn’t show them or shows dips elsewhere.

To answer the original question about placing the M2s upside down, I think that should be perfectly fine. And a setup in any configuration can be further improved with custom in-room EQ for the low frequencies.
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post #2159 of 4701 Old 02-15-2017, 12:36 PM
 
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Originally Posted by awediophile View Post
Were you really friends with Allison? That's pretty cool if so. When I was 6, my Dad invested in a stereo system included Allison speakers. I think they were model 7s. They sounded good. I can't say for sure how much those speakers inspired my interest in audio later in life, but they definitely had a significant influence on me early in my life..
Interesting. That means I was selling audio professionally when you were 6.

Yes, my family was friends with Roy. [If I recall correctly the last time I saw him was when my family attended a classical music performance with him at Jordan Hall, decades ago]. He was one of the most important speaker designers to ever live but he was a kind, honest, modest, and humble gentleman, not one to boast or pat himself on the back, at all, ever, and that sort of personality unfortunately often doesn't exceed in commercial success. He was however quite gifted in his engineering talents. For instance, Stereo Review magazine, now called Sound and Vision, once said of one of his designs, the IC-20, that they were the most accurate they had ever measured to date!

Your Dad's small, but floor standing model 7 would be an example where the designer, Roy, cleverly took advantage of, and precise control of, not one but rather two of the room boundary reinforcements: By placing the woofer perpendicularly to the front edge so its acoustical center was very close to the rear and instructing the customer to place the speaker against the back wall for optimal performance, Roy now knew exactly the distance the woofer would be both from the floor surface and the rear wall so he could design the output to take advantage of that position in the room.



Quote:
the Allison paper concerns power response which is not the same thing as what you are illustrating in your picture.
Actually, the power response is the summed acoustical power measured in all directions, in an anechoic environment, whereas Roy's paper addresses the much more important issue of what happens to this in a reverberant field, i.e. a living room, rather than a meaningless anechoic chamber none of us live in. I don't believe there is an exact name for this other than "typical, reverberant field (i.e. living room) acoustical power response measured at the ear in the far field". People, including Roy, may abbreviate this down to just "power response", for example, even though that isn't 100% technically accurate, simply for the sake of brevity and to keep the wording, including in titles and abstracts, concise.

My stick figure drawing helps explain the nature of the Allison effect in a simple two dimensional model that is easy to understand for people unfamiliar with the concept and as I already explained this is both a simplified explanation and all the numerous later reflections (although much quieter) plus other room boundary reflections besides just the floor have important roles as well:
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Originally Posted by m. zillch View Post
Sorry for the crude image and this is just a simplification of the issue so it is easy to understand. In truth the room has zillions of later (secondary, etc.) reflections so it all gets scrambled, but this issue with the strongest (loudest) first reflection off the floor is real, audible, and can be measured.
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And a setup in any configuration can be further improved with custom in-room EQ for the low frequencies.
Actually, as the link I provided earlier mentioned, attempting to eradicate a problematic room boundary dip due to an unavoidable placement issue you can't correct is sometimes futile: as you raise the response electrically at the dip frequency by say +6 dB you simultaneously also raise the room boundary reflected signal's cancellation wave by the exact same amount, +6dB. The net result is that the dip you hear remains, but you've just wasted available amplifier power.

As the link put it: "Unfortunately, you can’t correct for SBIR using EQ. If you apply a correction filter to try to boost the signal at a cancellation frequency, you will also boost the refection that’s causing the interference!"

The best solutions are to design the speaker to be as immune to this as much as is possible (and for the most room boundaries as you can) and to reposition its distance to the room boundary causing the canceling wave. EQ may help though with the other reflections, i.e. by raising the sound at that point where there is no contradicting (phase cancelling) wave to worry about.

None of this may actually matter to the person asking if inverting the M2s matters, I'm simply explaining that any speaker's sound can potentially change, sometimes audibly, when the woofer's distance to the room boundaries changes. I'm sorry if I didn't make that clear before.

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post #2160 of 4701 Old 02-15-2017, 02:54 PM
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Quote:
Originally Posted by m. zillch View Post
Actually, the power response is the summed acoustical power measured in all directions, in an anechoic environment, whereas Roy's paper addresses the much more important issue of what happens to this in a reverberant field, i.e. a living room, rather than a meaningless anechoic chamber none of us live in. I don't believe there is an exact name for this other than "typical, reverberant field (i.e. living room) acoustical power response measured at the ear in the far field". People, including Roy, may abbreviate this down to just "power response", for example, even though that isn't 100% technically accurate, simply for the sake of brevity and to keep the wording, including in titles and abstracts, concise.
I read the paper you posted top to bottom last night. Yes, power response is the sum of acoustical power through the entire sphere (or hemisphere or quarter sphere, depending on boundary conditions). However, the same measurement that's done in an anechoic chamber can be done in the presence of boundaries, and the result changes. That's exactly what Allison did in that paper.

Furthermore, he only considered the presence of one or a few of boundaries near the speaker. His measurements were not in an enclosed room. That's not a big deal for his purposes because boundaries that are farther away than one wavelength or so from the speaker have little effect on its power response at a particular frequency. Those boundaries do still contribute reflections and increase the average/smoothed SPL in steady-state frequency response measurements that use continuous signals.

The Allison dip, as articulated in that paper is a dip in the power response that arises due to the presence of boundaries. Power response is essentially the same as the actual acoustic output of the speaker vs. frequency. It can be stated in units of watts for a particular electrical voltage input. And in the presence of nearby boundaries, the total acoustic output of the speaker changes as compared to anechoic conditions.

The difference in meaning appears slight, but it's very important. One of Allison's primary goals in his speaker designs was flat power response, and this study reflects the fact that power response changes in proximity to boundaries. However, Harman's later research emphasized that flat anechoic on-axis response is more important than flat power response.

Quote:
Originally Posted by m. zillch View Post
My stick figure drawing helps explain the nature of the Allison effect in a simple two dimensional model that is easy to understand for people unfamiliar with the concept and as I already explained this is both a simplified explanation and all the numerous later reflections (although much quieter) plus other room boundary reflections besides just the floor have important roles as well:
A more correct interpretation of the Allison effect, based on your drawing, would be to imagine making a whole bunch of drawings, each depicting a microphone placed a different position in the room but all being the same distance from the driver. The distance chosen must be far enough away for near-field effects. Then you take the RMS average of pressure across all of those locations, which when reported as a level in dB tells you roughly the total amount of output from the speaker at each frequency.

Quote:
Originally Posted by m. zillch View Post

Actually, as the link I provided earlier mentioned, attempting to eradicate a problematic room boundary dip due to an unavoidable placement issue you can't correct is sometimes futile: as you raise the response electrically at the dip frequency by say +6 dB you simultaneously also raise the room boundary reflected signal's cancellation wave by the exact same amount, +6dB. The net result is that the dip you hear remains, but you've just wasted available amplifier power.

As the link put it: "Unfortunately, you can’t correct for SBIR using EQ. If you apply a correction filter to try to boost the signal at a cancellation frequency, you will also boost the refection that’s causing the interference!"
Yes the EQ will boost the level of the reflection, but the boost will still be there in the combined response. It's simple math. A 6 dB increase is the same as multiplying pressure by 2. So if in the absence of EQ, the direct sound is A and the reflection is B, the response is A+B. Note that A and B are quantities with phase that may sum constructively or destructively. With EQ, the direct sound is 2*A, the reflected sound is 2*B, and the response is (2*A + 2*B) = 2*(A + B) or a 6 dB increase. It doesn't matter whether the combination of constructive or destructive, the end result will still be boosted by 6 dB.

There may be other problems with boosting by 6 dB (or whatever). Doing so will increase both acoustic output and amp demand by 4X at the particular frequency. The strong boost in acoustic output may lead to coloration from uneven response in other early reflections and/or reverb/decay response. The big question is how much to weight combined on-axis response vs. power response when deciding how much EQ to apply. I don't there is a clear answer on this issue.

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Originally Posted by m. zillch View Post
The best solutions are to design the speaker to be as immune to this as much as is possible (and for the most room boundaries as you can) and to reposition its distance to the room boundary causing the canceling wave. EQ may help though with the other reflections, i.e. by raising the sound at that point where there is no contradicting (phase cancelling) wave to worry about.

None of this may actually matter to the person asking if inverting the M2s matters, I'm simply explaining that any speaker's sound can potentially change, sometimes audibly, when the woofer's distance to the room boundaries changes.
But again, you can't design a speaker to be immune to what your picture illustrates. Well, technically you *can*, but you end up with something like a line array in which the floor bounce can be canceled entirely, regardless of listening distance. Allison's approach aimed to compensate for changes in power response and not changes in on-axis frequency response.

I think as far as the original question is concerned, I would recommend consultation with JBL/Harman. I'm actually curious as to what their response on the matter would be and what technical reasoning they would offer to support their recommendation. My guess is that they would recommend custom in-room EQ adjustments, just like I suggested. The big question though is exactly what those adjustments should be for optimal subjective performance. I'm not sure Harman even has a clear answer to that question.
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