Film-Tech Forum ARCHIVE: AMP POWER??

This is topic AMP POWER?? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=007588

Posted by Amanda Mundin (Member # 3386) on 11-05-2005, 09:28 AM:

What are peoples thoughts RE amp power, who uses Dolby Tech Guidelines? And if you do use them do you use them exactly or do you add any number of decibels to their ideal figures to oversize the amps for headroom?
Dolby Tech Guidelines state:
Stage speakers should reach 103db SPL @ 2/3rds back from screen
Subwoofer should reach 113db SPL @ 2/3rds back from screen
Surrounds should reach 100db SPL @ centre of house
I have read some suggest there be 6db headroom, but also read others work out their stage & sub power at half way back rather than 2/3rds back.
Does anyone agree/disagree with either of these methods or suggest any others?

Posted by Steve Guttag (Member # 268) on 11-05-2005, 10:04 AM:

Amanda,

Here are the facts...

Given that the studio will mix their film in a room where "7.0" on a Dolby fader will correspond to 85dBc with pink noise playing...and given that this signal is 20dB down from full (on digital)...aka -20dBFS...one needs to allow for really up to 105dBc...

...where Dolby got 103dB has never really been fully explained but perhaps there is a 2dB saftey margin to avoid ever hit digital clipping. Perhaps it has to do with the crest factor of pink noise versus real world signals. Remember, pink noise represents all frequencies at equal volume. I can't imagine the soundtrack that would need the "pink noise" playing at full-scale. So I guess Dolby allows for a couple of dB there.

So, if you run your fader to 7.0 (ever) then you HAVE to allow for the full level of the soundtrack.

The surround spec comes from the fact that the surround channel level was always based on the mono surround system that could have 103 for the entire system...100dB comes from just half the system playing (stereo surrounds) so if both Left AND Right surrounds are playing, then you achieve 103dBc like the stage channels.

The subwoofer channel for digital systems has 10dB of boost right out of the shoot and hence the 113dB spec (note this is a nominal level...dBc is a nonsense reference for a band liminted signal such as subwoofer)...instead it refers to the pass band of the subwoofer channel being 10dB higher than the pass band of any stage channel.

Next to issue is that in Dolby's Technical Guidlines, they state 113dB...BUT their nomograph actually is computed for 110dB!!! Where they justify this has yet to be explained or perhaps it was a typo. One might claim that since the subwoofer channel is so band limited it doesn't need as much power since the higher frequencies are not being reproduced and the 10dB is referenced only to the pass band...however it is the bass frequencies that constitutes the majority of the power. Another possible explaination is "room gain" where the build up of the sound in the room tends to artificially adds to the level and the mic or listener is picking up direct and reflected sound.

So, back to your question...if you run your fader at spec and your system is tuned to spec...you HAVE to have that much power and power handling capability...if you are running your faders at 4-5...you are going to get 6-10dB of leeway there.

With 3dB representing a doubling of power and 10dB representing 10-times the power...this is pretty significant.

The two areas I see most people under specing their system the most are the surrounds and subwoofers (both powerhandling and amp power)

Steve

Posted by Amanda Mundin (Member # 3386) on 11-06-2005, 05:43 AM:

Thanks for that Steve, you wouldn't happen to know the maths behind creating the nomograph would you?, in order to create 1 with the corrected figures, e.g. 105db SPL for stage speakers, or would dropping the sensitivity of said speaker by 2db achieve the increase from 103 to 105db?

Posted by Steve Guttag (Member # 268) on 11-06-2005, 09:07 AM:

Amanda, the maths are given in the Technical guidlines (math equations do not translate well into ASCII here so to place the equation in this post would require a jpg image of some sort so it would be understandable).

However...if you merely double the power listed on the nomograph (for subwoofers) then you will have the figure for 113dB. So, in the case of their example line for a subwoofer with a sensitivity of 98dB @ 1W/m in a 50-foot deep room, the nomograph lists the power requirement of 1600 watts. Double that to 3200-watts for a true 113dB output.

Posted by Amanda Mundin (Member # 3386) on 11-06-2005, 04:13 PM:

Thanks alot for that Steve

Posted by Gordon McLeod (Member # 33) on 11-07-2005, 09:06 AM:

Also one needs to watch out for continous compared to peak rateings on amps and speakers and also the specified impedance that the rateing was taken at compared to the load being used