I bought a SPL meter the other day and have been able to determine that the THX/studio reference SPL of 85dB per speaker (not counting LFE) is pretty close to what I like at my listening position. The reference level is supposed to have around 20dB headroom too, which seems reasonable. But when attempting to design for 105dB I find a lot of good mid drivers don't reach this without exceeding their nominal power ratings.
Is the THX headroom just to account for short term peaks? If so then perhaps I would presumably still need to design for 105dB in terms of Xmax (and amp headroom) but perhaps just need to reach 85dB in terms of sustained driver power?
Cheers
Kev
Is the THX headroom just to account for short term peaks? If so then perhaps I would presumably still need to design for 105dB in terms of Xmax (and amp headroom) but perhaps just need to reach 85dB in terms of sustained driver power?
Cheers
Kev
The midranges should be able to handle peak levels well above their average power ratings. How much above are you taking about?? 3db, 10db??
I have a system set-up to handle 115db peaks, 30db above 85, but my midranges are 10" JBL 2123's with 101db @ 1 watt. What is the average sensitivity numbers of the drivers you are looking at??
Rob🙂
I have a system set-up to handle 115db peaks, 30db above 85, but my midranges are 10" JBL 2123's with 101db @ 1 watt. What is the average sensitivity numbers of the drivers you are looking at??
Rob🙂
Hi Rob,
Yes thats exactly what I'm getting at - is the +20dB headroom in THX just for peak SPL levels that could be achieved above the driver's average power rating, or is it to be 'real' sustained power with peaks on top of that too?
The total with headroom would be 105dB; there are drivers that'll do this within their nominal ratings, but there are also quite a lot of very nice ones which can't. For instance, I keep getting the Scanspeak F10 kindly recommended to me, it has a sensitivity of 86.6dB and would easily do 85dB all day every day, but needs 4x its rated power to reach 105dB - which would be a problem 'if' that is supposed to be sustainable for long periods.
Thanks
Kev
Yes thats exactly what I'm getting at - is the +20dB headroom in THX just for peak SPL levels that could be achieved above the driver's average power rating, or is it to be 'real' sustained power with peaks on top of that too?
The total with headroom would be 105dB; there are drivers that'll do this within their nominal ratings, but there are also quite a lot of very nice ones which can't. For instance, I keep getting the Scanspeak F10 kindly recommended to me, it has a sensitivity of 86.6dB and would easily do 85dB all day every day, but needs 4x its rated power to reach 105dB - which would be a problem 'if' that is supposed to be sustainable for long periods.
Thanks
Kev
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This is a SMPTE guideline that was adopted by THX.
The calibration test tones are meant to be on the digital channel at a level that is exactly 20 dB below digital clipping (sine wave assumed). If you calibrate playback level that way then you can theoretically have 105 dB from each channel at the listening position. How loud the calibrated system plays is then down to the movie producer, as they don't have to run the recording up to zero.
In our theaters with a calibrated playback our loud soundtracks will peak out around 95 (standard SPL meter on "fast" and probably under reading the peaks by a few dB).
For home theater I would ideally design for 105 pks short term and not worry about long term. The program can not peak louder than 105 per channel. I would also note that I seldom listen at home with a calibrated system. I set the volume I like, rather than being a slave to the producer's wishes. If the dialog is at a natural level and the action sequences are loud without being unbearable, then that is the right level!
David S
The calibration test tones are meant to be on the digital channel at a level that is exactly 20 dB below digital clipping (sine wave assumed). If you calibrate playback level that way then you can theoretically have 105 dB from each channel at the listening position. How loud the calibrated system plays is then down to the movie producer, as they don't have to run the recording up to zero.
In our theaters with a calibrated playback our loud soundtracks will peak out around 95 (standard SPL meter on "fast" and probably under reading the peaks by a few dB).
For home theater I would ideally design for 105 pks short term and not worry about long term. The program can not peak louder than 105 per channel. I would also note that I seldom listen at home with a calibrated system. I set the volume I like, rather than being a slave to the producer's wishes. If the dialog is at a natural level and the action sequences are loud without being unbearable, then that is the right level!
David S
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Superb, many thanks Dave S. With 105dB as the peak before clipping then (assuming a fairly typical crest factor) I'd be happy with noticeably lower power handling. That makes the driver selection 'much' wider 🙂
Thanks very much,
Kev
Thanks very much,
Kev
Hello Kev
It's only rated at 25 watts?? With 100 watts you would get 106+ with that sensitivity rating @ 1 meter. That seems low power wise. Are taking your seated distance into account when you determined that or just using the 1 watt @ 1 meter numbers??
That's only a 6db peak it should be able to handle that, you certainly won't be doing 105db for any length of time. That is loud!
I agree with David I set for comfortable volume on the dialog and the rest just slaves along. I can see on my DVD what the levels are set at. I see them in the dialog box right before a movie begins. Some are set above "0" level and it will tell you by how many db.
Your center channel is what's going to get a work out in the midrange more than anything else.
Rob🙂
Using a SPL-meter gave me a wtf?-moment. Even with dynamic recordings I find that I rarely exceed 85dB(C) peak in listening position. I becomes even stranger, since I've bought 4x18" Precision Devices woofers, a JBL 4675C pro-cinema system and the whole shebang needed for an active setup. Oh well: Headroom/crazyness it is then... 😀
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Hello Rojoh
That's bizzare?? I hit 105 on a regular basis mostly LFE and can hit peaks close to 115db in the LFE channel on certain disks like the train wreck in Super 8, Cloverfield or the New Tron as examples. You doing Fast using peak hold or the record function on the meter?? I have a digital meter you using an analog??
Rob🙂
Hello. I've got a digital meter, and use fast/peak hold. But: I only play music, and the low end is probably down 10dB@30Hz. I'm not running the described system yet, so my initial description is a bit misleading. My measurements were taken using one 18"/dual 15"/side HPed@100Hz via analog active crossover.
But yes: I was a bit surpriced over my rather modest SPL-requirements. But that's ok. I might fancy playing a bit louder if/when I get the big system up & running 🙂
But yes: I was a bit surpriced over my rather modest SPL-requirements. But that's ok. I might fancy playing a bit louder if/when I get the big system up & running 🙂
Overkill be damned, now you'll intrinsically under drive everything thus lowering distortion and maintain pristine dynamics. Then when you listen to something that truely has a wide dynamic range, it still shines without showing unnecessary stress when doing so
Thanks Rob,
Yeah, its actually only rated to 15W but WinISD calculates the sensitivity a tad differently than their data sheet for some reason, it says more like 87.75dB@1w/1m. Based on that my normal 85dB is < 1 watt, and I can get to about 99.5dB within the 15w rated power. So the peak to reach the 105dB headroom will be only about 5.5dB - although it takes 4x the rated power to get there, which would have been a problem if it had to be sustained but presumably not an issue for peaks.
Thats without listening distance factored in; my meter suggests I'm losing roughly around 3dB (in my room) between 1m and the listening position. But (ignoring the tweeter for the mo) each speaker would actually have two power drivers (mid and a bass), so I'm thinking that would help offset the 3dB distance, at least with music that shares itself between the two frequency ranges.
I've been doing some more testing, and I've discovered that I'm equally happy with somewhat lower SPL if I use the 'loudness' option to maintain the perceptual frequency response. IIRC that leaves the mid range more or less alone, whilst raising power to the bass and treble; could be useful if I found my mid-range was holding back the SPL.
Cheers
Kev
Thanks to others for the SPL discussion too - yes the SPL requirements surprised me a bit as well, especially when I realised the THX levels were per speaker not total. So I'm using it as an upper limit for my plans, though hopefully a limit that it will reach before sounding stressed, not one it can barely reach. In practice I'm going to be listening to music at rather lower levels normally, but it probably will get cranked up for action films.
Cheers
kev
Cheers
kev
Seems the vast majority of hifi midbass drivers are still being designed around the philosophy of building a stereo pair of music speakers that are intended for use without a sub. Most 5-7" hifi midbass drivers have an EBP that optimizes bandwidth to 50hz or lower at the expense of efficiency. 2-4 drivers per channel are required to achieve reference levels in most configurations. Not surprising that so many "hifi" speakers marketed in the home theater rooms at electronic stores are crammed full of so many drivers.
When designing a system with "theater" in mind. The 80hz crossover target is all the mid-bass really needs to be able to reach. Having more extension here will tend to come with other tradeoffs that are, from my perspective, not worth it. An EBP/displacement optimized design will be a fraction of the size of "hifi" equivalent to reach the same output levels. The savings in material, driver, and footprint cost can all be moved towards building more potent subwoofer.
Regards,
Eric
When designing a system with "theater" in mind. The 80hz crossover target is all the mid-bass really needs to be able to reach. Having more extension here will tend to come with other tradeoffs that are, from my perspective, not worth it. An EBP/displacement optimized design will be a fraction of the size of "hifi" equivalent to reach the same output levels. The savings in material, driver, and footprint cost can all be moved towards building more potent subwoofer.
Regards,
Eric
Remember to check the bandwidth over which the driver's power rating is specified.
Here's a rather nice graph from one of Rod Elliott's articles:
I'd suggest that it isn't too accurate at LF: huge peak power can be needed at LF, so, when it comes to subs, I go by the "too much power is almost enough" idea, for headroom.
Chris
Here's a rather nice graph from one of Rod Elliott's articles:
An externally hosted image should be here but it was not working when we last tested it.
I'd suggest that it isn't too accurate at LF: huge peak power can be needed at LF, so, when it comes to subs, I go by the "too much power is almost enough" idea, for headroom.
Chris
Thanks Chris and Eric, most interesting. I'm thinking of building active 3-ways, which I can see would be wasteful should I later progress to a sub setup, but at least the mid will be crossed over at something like 300Hz so there shouldn't be any SPL compromises in trying to make it reach 80Hz.
That chart looks reassuring, too - by coincidence I'm using Rod's mosfet amp so I was poking around his site, and read the artical which suggests my mid's crossover point should mean at least half of (typical) music power will be going to the bass driver.
I did some simulations in WinISD and with the ss 10F I seem to need just over half a watt to reach 85dB, and its nominal 15watt rating takes me to 99dB which should be plenty even from my listening distance. I believe the IEC test involves a vaguely realistic crest factor already, so anything beyond that really should be short term peaks, so hopefully it will manage 105dB (briefly around 60w) without being harmed.
However, I haven't checked what bandwidth the power rating was obtained at - its sold as a full range driver but good point Chris, I would be wrong to assume thats how it was measured...
Cheers
Kev
That chart looks reassuring, too - by coincidence I'm using Rod's mosfet amp so I was poking around his site, and read the artical which suggests my mid's crossover point should mean at least half of (typical) music power will be going to the bass driver.
I did some simulations in WinISD and with the ss 10F I seem to need just over half a watt to reach 85dB, and its nominal 15watt rating takes me to 99dB which should be plenty even from my listening distance. I believe the IEC test involves a vaguely realistic crest factor already, so anything beyond that really should be short term peaks, so hopefully it will manage 105dB (briefly around 60w) without being harmed.
However, I haven't checked what bandwidth the power rating was obtained at - its sold as a full range driver but good point Chris, I would be wrong to assume thats how it was measured...
Cheers
Kev
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