Your hypothesis is close.
Acoustically a large room and a small room are nothing alike at LFs, so one would suspect that this acoustical difference results in the large perceptual difference. Acoustically a large room is diffuse at LFs while small rooms are modaly discrete. The diffuse nature of the LFs along with the much larger reverb times of large rooms causes this perceptual difference. After many "paper" studies of the situation (including a PhD thesis) I arrived at the best "approximate" solution and that is multiple subs. This yields much more diffuse LF signal along with a much more uniform response.
But the lack of LF reverberation in small rooms is a problem because we sense sound level by amplitude as well as signal duration. Hence for a given sound level the longer the signal reverberates the greater its perceived level. I did some experiments with adding a random LF reverb signal to the LF sources and this was an improvement, but I didn't follow through on that aspect sufficiently to draw any clear conclusions as to how this should be done.
Camplo,
You don't mention weighting in your 80-90dB range at home, 90dB at 30Hz would only read 50dB on "A" scale as your SLM was set.
A pair of sealed 18" require about 46mm (1.75") excursion peak to peak to produce 122dB at 30Hz at one meter.
As Earl mentioned, the longer low frequency reverb times in a theater make a perceptual difference, but are you also reaching low frequency peak levels of 122.3 dB as your SLM indicated during the Black Adam movie when measured at at home?
Art
I see no reason why I should, especially after several bad experiences with commercial sealed speaker systems. That's not to say sealed can't work.
It seems a BR tuned to ~20hz... is hard to beat. Performance is identical to a TL with a matching exit CSA and volume, and the build complexity is much less. Maybe placing the excursion null ~20hz is a good idea. In the end, I see lessening of Radiation mass size, near tuning. My two 18"s facing forward, produce more bass than in the slot. Just like a compression driver, lowering the csa of exit lowers lf sensitivity.
If chasing direct energy, a front loaded horn or some other vented design with a flair that matched or exceeded the size of a big woofer, by the time we reach the mouth, would be desirable.... so sealed woofers win per square foot, is what I reckon.
When designing a horn, 4th order bandpass, are there rules about compression ratio? Like no more than 2 to 1? Now that I see that pressure increases 2nd order distortion, I wonder the outside limit to pressure. I would expect pressure to reduce excursion, and that is what we see.
Where are the subwoofers using small woofers with very small throats and expanding lines tuned to 20hz? If limiting the mouth to about 800-1300cm2 how small of an enclosure could one make I wonder.
I feel like I am using this term incorrectly. The Radiation mass, in the omni part of a sources spectrum. is going to be very round. The radiation mass also changes with signal level I believe.
I think I mean to say Radiating area...instead of mass.
I am wondering how literal are you using the word perceptual, and thank you for catching the weighting issue. Luckily the true peak meter had no weighting. I can't imagine that you guys are saying I could "feel" the bass because the room was big. What I've been thinking is that the only thing I could possibly be feel is pressure. For no reason that I can figure, we don't have numerous tools that display true peak vs frequency. Where exactly those 120db peaks landed in frequency is unknown. Is it likely that an Imax theater is set to be able to play 120db or higher, down to 20hz, with low distortion (low excursion). My one subwoofer is well past Xmax but not quite to Xlim by 46mm
Diffuse....I would think that there is a strong direct and indirect energy component here....and in the LF Diffuse room, Direct energy is higher (sitting front Row, speakers at the screens plane) vs what happens in the LF modal room (Me sitting 1 meter away from my Speakers). Increasing Direct energy yields a much more diffuse perceived signal along with a much more uniform response. The difference between a more LF Diffuse room vs increasing Direct Energy is the where the modes are....So though the multi sub in the small room will Yield a more "diffuse" LF signal it will not match the reverb component of the actual LF Diffuse performing room. The reverb amount of the small room will lessen but where the reverb takes place, ie the modes, will not change.
If true, is there merit in my desire to increase Radiation area in pretty much, as much of the spectrum as I can... I claimed that I could hear the difference in direct vs room, from my horn vs my 15" shrug I think Increasing Radiation area increases Direct energy, isn't that true?
From 1 meter do you think you could tell the difference in Direct/Room at 200hz from 15" vs a 4".....I tried it once already but I don't remember the results lol.
Where is the fundamental mode in the average domestic room. Where is the Fundamental mode in the Imax Theater
Interested in your guys responses.
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Hi Ro,
i think sealed is ultimately the best way to achieve bottom end......if properly implemented.....which almost never occurs.
Because proper implementation imo, does not use any EQ/DSP such as a Linkwitz transform or a combination or shelving/para EQs, etc.
If a sealed box's very low end is extended by increased excursion via EQ, the EQ invariably negates the sealed's natural low-order rolloff.
Transfers/achieved acoustic high-pass, head towards the same as vented, so might as well go vented.....
I think proper sealed implementation relies solely on increased displacement, .......either drivers that are larger and/or with more xmax.....or simply more boxes. Whichever route taken should obey the rule of needing 4x the displacement for each octave decrease.
This is the ticket i think... to have the sealed displacement needed for the lowest frequency and highest SPL desired......... with little to no boost used..
To me, you are misusing the concept of "direct energy." The concept in acoustics is direct field versus the reverberation field. At HFs we can sense the first arrivals of an impulsive sound before the reverberation starts to take place, but at LFs this is not possible. It takes many cycles of the sound for the ear to identify it. At LFs this time is greater than the onset time of the reverberation. Hence, one cannot detect a direct field contribution at LFs, it's all reverberation.
In my room the first mode is about 20 Hz. In an IMax theater it is about 2 Hz or less.
What exactly is wrong with LF EQ of the subs? I don't get it. If the excursion capability is there what's the problem?
And you think that these things are audible at the VLFs that we are talking about?
I must then again make sure that we are acknowledging what I first described
This whole time, discussing the theater experience vs home
I am not talking anything about Ears and Hearing.
I am speaking only on the tactile experience. (: of, relating to, or being the sense of touch)
I could feel the bass, physically affecting by body through the air....
With this being said, can we now, say, Direct Energy? Please lol!!! It could also be some high peaks at LF that I can't or haven't hit yet, in particular with low distortion that the Imax can.....I guess that part is just as realistically possible as the Direct energy concept. As I've displayed in the past....True Peak energy within mid and highs are generally neutral within what Peak bass levels are. So that would mean the 123db peaks I monitored with my phone suggest that the bass peaks where at ~123db...... I am fine with that outcome as well...I think thats what @weltersys was suggesting as well...
Pressure is pressure....regardless if its coming straight from the driver or bounding off a wall....
Still, Increasing Radiation Area.....Does this affect how far the Direct field extends from the driver??? Regardless of perception. You said that it takes many cycles for the the ear to identify it. What about my body?
20-30 Hz
One test using tones from 1-30 Hz at amplitude levels from 125-144 dB reported voice modulation, and abdominal and chest vibration.30-50 Hz
One series of testing conducted with tones from 31-50 Hz at 90-100 dB output levels compared how people perceived their sensations versus actual vibration levels of different areas of their body by hooking up accelerometers to the head, abdomen, and chest of their test subjects. It was found that although the head itself was not measured to vibrate as much as the abdomen and chest, head vibrations were perceived as being stronger, likely due to auditory structures within the head. Chest vibrations were measured and subjectively felt to be stronger than abdominal vibrations, and 50 Hz frequencies were more effective at causing vibrations and vibratory sensations than lower frequency sound at the same output level. Other tests conducted at much more powerful levels past 140 dB reported respiratory rhythm changes, gagging, chest wall vibrations, and perceptible visual field vibration.https://www.audioholics.com/room-acoustics/bass-the-physical-sensation-of-sound
123db is right there I guess...I always look at data like this as an average. Meaning that some could be more or less sensitive There were times during the movie that I could feel the bass through the air and I felt like like it wasn't playing that loud....
Maybe the longer reverb time could do this. I need access to the IMAX theater, so I can experiment for about 4 hours.
but sealed gives more distortion and less spl…
group delay are more audible then distortion?
Group delay caused by bass reflex reacts to the room acoustic, right?
in a well treated environment, say with huge bass traps in the room, would group delay be more or less audible? id love to know!
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