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#81
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Re: Loudness and room-size was: TV-Mix for Theatre
I was not that much worried about perceived loudness, so our tests are trying to achieve a much simpler goal - we're trying to find how wrong is the use of pink noise/RTA system. We believe that it is already proven that RTA measurement is far from perfect and we concentrate our efforts to find a measurable error related to room size or RT60, or to whatever these experiments will reveal.
Essentially, our experiment is to align as many as possible different rooms using the conventional (RTA) method, then measure the differences using different test material. Our test programme consists of pink noise followed by several excerpts from dialog, music and combined film soundtracks. We expect to see differences (at the moment we focus our attention to amplitude differences) in playback of the material and to be able to draw some conclusions. Recordings (of this test material) from various studios will be normalized using the pink noise as reference. Further analysis will bring, we hope, some useful results. If you want to participate, email me for further instructions. Branko |
#82
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Re: Loudness and room-size was: TV-Mix for Theatre
Hi Branko,
I down loaded the files and tried the experiment... I'll email you the results early next week. But basically we hit almost exactly the same level as the Warner dub stage guys. ( i think it was the warner dub stage..) cheers geo
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georgia hilton CAS MPSE MPE Hilton Media Management Film Doctors http://www.filmdoctors.com Me... http://georgiahilton.webs.com/ Stage 32 http://www.stage32.com/profile/6569/georgia-hilton My Production Company http://www.hiltonmm.com CREDITS (partial) http://www.imdb.com/name/nm0385255/resume MEMBER: IATSE LOCAL 700 |
#83
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Re: Loudness and room-size was: TV-Mix for Theatre
Thanks, we´re having 7 recordings so far. Btw, what Warner stage are you referring to? It would be nice to have some tests done there too...
Branko |
#84
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Re: Loudness and room-size was: TV-Mix for Theatre
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The main thing is that any number such as 85dbSPLC as read by an SPL meter is in a way "subjective" to the overall frequency response of the speakers reproducing the Pink test tone. For example, this is what happens and you can try this yourself: if you were to take a very small speaker that has a very limited frequency response at the lower end -for example the Tannoy Reveal: 6.5” with frequency response of 60Hz – 30kHz and you use that speaker to calibrate a room at 85dbSPLC, you would get a "wrong or miss-leading reading of the SPL even it the meter says 85dbsplC. This is why: The Pink noise being reproduced by the limited response speaker is being affected, in essence the low end of the audio spectrum where a lot of the power exists, is being truncated by the inability of the speaker to reproduce the low end at normalized power. So, when you adjust the level of the amp to read 85dbsplC, the meter is reading/reacting/measuring the sound MINUS the acoustic power of the low end of the spectrum (because the speaker can not reproduce it correctly). Therefore, for the meter that is design to "over all acoustic power metering regardless of frequency distribution", to actually meassure 85dbsplC, you have to deliver a higher sound power. So at this point, once the meter is reading 85dbsplc people thinnk, great now I am aligned like a theatre NOT TRUE. If you then just unplug that speaker and replace it with a full range speaker (frequency response from 40 HZ and up) you will see the SPL meter jump up about 4 dbs. So really, the meter now says that you are at 89dbsplC *which is absolutely true*. In other words, the room when aligned with the small speaker was "miss-aligned" by 4db TOO HOT. Therefore, if you now mix in the misaligned room, your mixes will translate to the theatre about 4db too soft! There is another very important consideration that does not have to do with the frequency response of the speakers but rather the positioning. It has t do that in a small room, most people use near fields (or mid fields at best). These speakers are located by definition to approximate radiating into full space, meaning they are away from walls and floors and ceilings. Theatre/dub stage speakers are located to radiate into 1/2 space, they are usually either in a soffit or mounted flat on very large walls. When a speaker radiates into 1/2 space (meaning the have a huge wall behind them), they are affected by SBIR (speaker boundary interference response) which essentially re-directs the lower end of the audio spectrum towards the front as an immediate reflection from the rear wall. Because the wall is only a few feet away from the rear of the speaker and low frequencies have very long waveforms, the reflection from the wall become a positive additive to the source wavefront, therefore re-enforcing the low end of the spectrum. The result of this SBIR is very similar to my example above from the little speaker but of course in reverse. The pink noise at -20dbfs played in a theatre or alignment suffers from "extra" bass content as a result of SBIR and therefore the meter reading 85dbsplc is reading a little higher than it should (I don't know how many dbs higher and I really don't want to do the math because it is very complex because it has t be done by frequencies similar to figuring out room modes). So, the combination of these above 2 examples can yield a very large error in the room alignment. Examlple, the room aligned with the small speaker would sound may be close to 6 db lower in volume in a theatre (I am mentally combining/guessing at these numbers). So this is why, people say: in a small room mix at 79dbsplc. If the small room has full frequency speakers (same as the theatre), then the missalignemet will occur from the SBIR that occurs at the theatre. There are several other factors which I will only name: the slew rate of the room which is the time it takes for a large room to become fully energize and achieve steady state (one can think of this as the opposite of Reverb time because it is on the onset of sound), the rate of the decay of sound in non-free space which is frequency dependent, air absorption which I would actually think of as "free field sound dispersion" of high frequencies (which behave like more like ray acoustics) and 1/2 field sound dispersion of low frequencies (which behave more like wave acoustics). To try a test: 1_.-Take a very small speaker (auratone like) and put Pink and meassure SPL. 2.- Replace small speaker with better frequency response unit and meassure SPL. 3.- Replace speaker with full frequency and meassure spl. To test for SBIR: 1).-Take full frequency response speaker and place flat onto large wall. Run low frequency tone (Limited Pink noise is fine) and meassure SPL. 2).-Move speaker 10 feet ahead of wall (suspend on mid air using thin stand) and meassure SPL. Now, these SPL variations that you will see are for 1 (one) speaker, now you can just imagine what happens when you multiply the error by 5 speaker + the sub. Again, we are dealing with acoustic power and therefore spl reading as decibels can not be mathematically added, they need to be combined by logs but just by experience I guess thatif you have a 3 db spl error on 1 (one) speaker, the entire system will probably have a 4 to 4.5 db general error. G |
#85
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Re: Loudness and room-size was: TV-Mix for Theatre
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Another problem of aligning a small room for flat response with an RTA before pinking for 85dbsplC, is that in a small room wall reflections "significantly" contribute to the overall response of the room (much, much more that a theatre) and therefore, RT (and especially its relative frequency distribution) becomes of paramount importance. In my opinion, it is often better to "know" your room empirically, and get used to how it sounds, than to try to "correct it" by means that border the laws of physics. An experienced sound person (that knows his/her room) automatically mentally averages and corrects the sound so that the mix will sound good anywhere (of course the room needs to be at least descent and not a marble washroom cube and with good equipment). |
#86
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Re: Loudness and room-size was: TV-Mix for Theatre
Hi garnoil, while everything you say is true there is more to the puzzle. Accoustic energy of the low frequencies do indeed affect simple SPL measurements in a big way. This is why you always address room bass trapping, speaker placement, and speaker eq before setting overall speaker level.
Pink noise with restricted bandwidth (high passed at 300hz for example) should mostly overcome the issues you write about, but it does not. Dolby has used a limited bandwidth pink noise for a very long time. In an "ideal" room, the full bandwidth and limited bandwidth noise (with makeup gain applied to produce an equal dbC reading), should measure the same or nearly the same. If the speakers are not capable of the lowest one or two octaves of response, then the limited bandwidth pink alone should be used to set playback SPL. The full bandwidth noise is used with an RTA to check for frequency linearity problems. "Node" issues, those nasty spikes and dips that are usually invisible on 1/3 octave RTA's, are a non-issue for SPL calibration using pink noise. They are included in the SPL reading. Node issues are much more of a problem with musical tones than with pink noise, and while they do account for poor sound I am not at all certain they explain the problem or even come close to doing so. We still have a problem of volume perception difference in a small room especially with speakers only a few feet away vs. large room. I've read some theories about energy buildup along the walls, early reflections altering our perception, psychoaccoustic babble about how small rooms increase our awareness through fear, and more. Whatever the causes, we're left with the same problem, which means that either we need a much more accurate way to measure this stuff or for now our standards need to take room size into account. The ATSC's recommendations (soon to be CALM law in the US) do that in a rough way. Maybe future measurement techniques will take these factors, whatever they are, into account so we don't need to make subjective changes based on room volume. We could all set our dials to "XX" and hear perceive the same volume. That's not how it works today, as you well know! |
#87
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Re: Loudness and room-size was: TV-Mix for Theatre
Hi Postman,
""Node" issues, those nasty spikes and dips that are usually invisible on 1/3 octave RTA's, are a non-issue for SPL calibration using pink noise. They are included in the SPL reading. Node issues are much more of a problem with musical tones than with pink noise, and while they do account for poor sound I am not at all certain they explain the problem or even come close to doing so." I agree 100% but I was referring to was, what happens in a "typical small size" pre-mix room that has not been RTA flattened (bass traps, electroacustics, geometry) when one simply says play pink noise and set spl to 85 (and then one wonders why the levels do not translate well to a theatre). Specifically I was trying to point out: If one plays non-limited Pink Tone in a room with non controlled modal response, and simply takes an SPL reading, the SPL reading will not be accurate in as much as the entire lower end of the pink noise is being artificially "boosted" by the modal resonances in the room. Therefore, the SPL reading "while it is accurate for the specific room" it is not equal to a reading taken on a large room where no artificial boosting of the lower en occurs (even if both readings say 85dbsplc) |
#88
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Re: Loudness and room-size was: TV-Mix for Theatre
Sorry, I thought you were saying that the whole issue of perceived level in small rooms vs. large rooms is due to bad SPL readings due to room nodes and limited bass response of small speakers. If you meant to say that, I do not agree. If you meant to say those factors are important to be aware of when aligning a room, then I completely agree!
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#89
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Re: Loudness and room-size was: TV-Mix for Theatre
I had that very problem just three weeks ago, as we mixed a short for theatrical presentation. In a test screening, in which the playback chain was set to Dolby standards(7), the whole mix was 2-3 dB more quiet than in our somewhat small room we mixed it in. I'd already tried to compensate for the roomsize by using 82dB as a reference point, but it seems for the bandlimited pink noise (Bluesky file) and even Dolby noise this was not enough. I had to go to 79 dB SPL to get a comparable response in the theater we screened at. Several factors could and probably will have skewed the perceptions. The accoustics in both the theater and mixing room, as well as the speaker placement in the mixing room.
There's always a lot to check.
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"My ego comes pre-shrunk" - Randy Thom |
#90
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Re: Loudness and room-size was: TV-Mix for Theatre
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