Film-Tech Forum ARCHIVE: JBL 3677 screen speakers: do you cut them (30hz or 50hz ), and do you

This is topic JBL 3677 screen speakers: do you cut them (30hz or 50hz ), and do you in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by David Baum (Member # 568) on 01-06-2001, 11:09 AM:

keep the foam on the air ducts ? I thought these air ducts should be free of any blocking/filtering material.
As for the cutting at low freq, I intend to use the QSC rear panels switches ( 30hz and 50hz selectable ). Subwoofers are receiving the LFE and the bass ( under 80hz) from the front channels so no problem "losing" those sub-30hz or sub-50hz information when cuttting.

thanks in advance
David

Posted by Darryl Spicer (Member # 711) on 01-06-2001, 11:17 AM:

I guess if I were to make a choice I would cut it at 50hz. You are already sending this to the subs. No need for excess bass that can drowned out much needed information such as dialaugue.

Posted by Stefan Scholz (Member # 147) on 01-07-2001, 06:53 AM:

This speaker's maximum acoustical power output is about 6 Watts. Do you use this in a theatre? Of what size? hopefully with analoge only.

Posted by David Baum (Member # 568) on 01-07-2001, 09:56 AM:

Hi
Based on your reply, you seem to say that these speakers are barely recommended for small, analog theaters only ( which is what JBL says too) but since it's my
home theater ( but 9,500 cubic feet )....
maybe I should use them for surround and get 3x 4670D

(kidding)
seems a cut at 50hz via the DCA amp will do ( with 80-100hz boost by the onboard high pass filter should do )
I use a Denon 4800 with option LFE+main ( ie, subs get the LFE + the bass from the small set speakers AND the large front ).
anyway, I wonder why JBL left those foams on the vent ports...
Btw, still stunned by what power to feed these: JBL says for carefully monitored sound ( such as studios encoding etc), 4xthe IEC rating (!!), for quality sound and general applications, 2x the IEC ( cinema sound fits this category ? this would mean an 2x500watt amp!) and for budget constraint systems or when clipping often occurs, IEC rating.

regards

Posted by Darryl Spicer (Member # 711) on 01-07-2001, 10:41 AM:

A couple of things you have to remember when it comes to amplifiers. It can be two hundred watts it can be 5OO watts so long as it is not fall below the recomended source. If you hook a watttage meter into your speaker conection and pump in 85 db of pink noise measured by a sound preasure meter at the listening area you will find that the amps are putting out no where near what you think it is. Thats because the bigger the room the more power is required to get the 85 db. And by cutting the bass at 50hz you will use even less power because it takes more power to power the lower frequencies than it does the highs. A rule of thumb is each channel should run at the same power requirements. if you are useing 200 watts to power your center channel you should also be useing 200 watts for left, right and surrounds. subs are different you should use twice the power if useing more than one sub. In cinema aplications the amps are turned all the way up. yet that does not mean you are feeding all those amps to the speakers. sound is governed by the volume controles on the processor, preamp cards and eq cards. same goes for home stereo. the internal amps are all the way up in recievers and the internal pre-amp feeds the volume. external amps same way. In a small area that sound preasure will become unbearable to your ears before that speaker blows.

Posted by David Baum (Member # 568) on 01-07-2001, 11:05 AM:

Darryl

Agree but then, what you usually feed the speakers, ie, what power amp do you use with JBL speakers ? example: a 4670D has an IEC of 600w and a continuous program capacity of 1200 w so you connect a 2x600watts (for 2 4670D) or a 2x1200 or the former bridged ?
what sound category is cinema sound ? studio, live concert etc ? seems JBL stresses deeply these categories in its FAQ. I'd say cinema sound is much closer to studio sound than live concerts where clipping is so common, as almost constant. As for subwoofers, JBL recommends adding subs rather than increasing the watts ( they recommend usually 800-1000 watts per 4645C/4645B). there's clear gain in adding subs. Talked to a net friend who is deep into live sounding, concerts etc and he almost always screams 1500-2000watts for this kind of subs.... I also agree that such powers would be EXCESSIVE in a home theater configuration... totally.

thanks a lot for your comments. appreciate.
PS: I realize I forgot you almost already answered me on this issue on a previous post...
maybe I should stop torturing myself and get those DCA amps ( 1222 for 3677 and 3422 for the subs , should be PLENTY enough with AWG 8..... ).

Posted by Darryl Spicer (Member # 711) on 01-07-2001, 11:54 AM:

David,

If I am reading you correctly. The recomended minimum power source for the speaker is 600 watts. don't go below 600watts on the amps. In a small instalation you could probably get away with useing 600watt amps for one speaker that would be the minimum. To be on the safe side I would talk to the dealer or manufactuer of the amps that you want to purchas and tell them the type of speaker and the type of instalation that you are going to be doing. That way they can recomend the type of amp you will need. Theaters are considered studio sound. That is why each channel in a sound processor has to be eq'd with a real time analyzer so the auditorium matches what is called the x curve wich the hollywood mixing studios are set to.

Posted by David Baum (Member # 568) on 01-07-2001, 12:20 PM:

Darryl

thanks again. but this gets more confusing! ( note: the 600w was related to the 4670D, which I don't have. I only have models 3677 and 4645C, and probably 8340 for surrounds later on).
what you then say is minimum IEC rating ( continuous pink noise or AES, as JBL states) thus if a speaker has an IEC of 250w, power it with an amp of minimum 250w per channel, right ?
But then you say cinema sound is considered as studio sound, thus , you should feed the speaker... hold on.... 4 times its IEC rating (and adjust level with volume on the processor)! That's what JBL says in its FAQ.
So, based on the few posts here and on other threads, there seem to be 2 schools among film handlers: some say go with the log curve and feed the necessary watts to reach 105dB as per the 2/3 distance of the room ( and all the SPL meters and tools used to measure SPL in-rooms levels), and another school might go with JBL's recommendations, which says 2 or 4 times the IEC ratings, no matter what the room size, and adjust the level via the volume on the processor. I hope this is clear
The second "school" would rule out the cost factor since this means beefier amps, many bridged btw. What strikes me still is the fact that most of these JBLs are very sensitive, some as high 107db/1W/1m and thus start producing high SPL level even with small amount of watts... so why going so sky high with their recommendations (2x the IEC is ok, but 4times!!)? is this solely due to headroom requirements and lowest possible THD ? peak transients capacity ?

here's what they say: (www.jblpro.com FAQ):
"How do I choose the right amplifier power for my speaker system?

Ideally you should pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. This means that a speaker with a "nominal impedance" of 8 ohms and a continuous IEC power rating of 350 watts will require an amplifier that can produce 700 watts into an 8 ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms.
A quality professional loudspeaker can handle transient peaks in excess of its rated power if the amplifier can deliver those peaks without distortion. Using an amp with some extra "headroom" will help assure that only clean, undistorted power gets to your speakers. Some professional amplifiers are designed so they have additional headroom. These amps can cleanly reproduce transient peaks that exceed the amplifier's rated power. In this case select a model with an output power rating equal to the continuous IEC power rating of the speaker. Consult the amplifier manufacturer or owner's manual to learn more.
In some applications, such as critical listening in a studio environment, it is important to maintain peak transient capability. For these applications, use an amplifier that can deliver 6db (or four times as much) more power than the continuous IEC power rating.
If budget restraints or legacy equipment force you to use an amplifier with less power, extreme care should be taken to see that the amplifier is not driven into clipping. It may surprise you to learn that low power can result in damage to your speaker or system. Download our Danger: Low Power (Adobe Acrobat, 204kB) tech note for more information. "

Posted by Kent Elliott (Member # 487) on 01-08-2001, 07:54 AM:

David, your question about power amplifier size is a good one, but I'll have to disagree with many replies. I've been an audio engineer for 20+ years, involved in live sound & studio sound. Speaker manufacturers today rate their power handling as high as they can get away with. The power handling specs are typically overated, and in any case are concerned with producing the maximum amount of sound pressure level possible.
The real answer to how much power you need on your speakers is "enough to keep from amplifier from clipping". If your current amplifier has clipping, or overload indicators, and is not clipping at your listening levels in your room, there is no need for a larger amplifier. In fact, there are some very good reasons for not increasing the amp size beyond your "real world" needs. The first is noise floor (background noise, hum & hiss). The noise floor of an amplifier is related to it's MAXIMUM POSSIBLE output level, not it's 'actually used' level. So if you really only ever use 50 watts per speaker, a 100 watt amplifier will have a lot less background noise than a 1000 watt amplifier that you never need. And this background noise can be very obvious in a home theater application.
The second argument against using a bigger-than needed amplifier relates to distortion. All pro or semi-pro equipment has very good distortion specifications these days. But again, the distortion specifications are taken at just under full output power. You might be surprised to find that some 500 watt amplifiers rated at .01% distortion (at 500 watts output), will actually produce over 1% distortion when run at an average 15 watt output power. This distortion is a particularly nasty sounding type of distortion called "crossover distortion". This type of distortion is very likely to show up in home theater applications, where the primary component of the sound is spoken word, and where the average power needed is not really that high.

Posted by Gordon McLeod (Member # 33) on 01-08-2001, 08:57 AM:

There is no point in going bigger than you need for the size of room volume
All you need to do is use the spl onewatt one meter and the maximum SPL of the format you want to use and calculate the amps size accordingly then I usually add 10%
The dynamic range of film soundtracks is a finite number

Posted by David Baum (Member # 568) on 01-08-2001, 09:41 AM:

thanks to both of you, learned more things today!

(especially the higer distortion/noise at low level)
i will start with a 2422H QSC on the subs and see how things go ( clipping or not, noise level etc) and 1222 for the 3677.
thanks a lot, it's getting clearer. a little voice told me to stick with JBL log charts indeed...

regards

Posted by Darryl Spicer (Member # 711) on 01-08-2001, 10:17 AM:

David,

OK, here it goes. If your minimum continuse power on the speaker is 250 watts multiply it by 2 and it gives you 500 watts an odd number. If you want you can get an amp that is 1200 watts and feed two channels 600 watts. just make sure that all channels are useing the same amount of power. The theater is considered studio because of the accustic factors associated with it. THings you do not get in live halls. Sound preasure levels are set at 85 db from 2/3 back from the screen. Volume is controled on the processor but you will find that you will not maintane 85 db because you will drop the level of sound from a setup point on the processor wich is 7 down to around 5. that is why it is a cuntiuos preasure level. the level that is stated on the speaker 107db/1w/1m is soundpreasure measuered at 1 meter from the speaker and it only took 1 watt of power to reach 107 db. It does boil down to the more power you have availabe the less distortion. A lot of that has to do with the fact that the bass requries more power than the highs. I know this gets confusing that is why I said to check with the dealer or manufacture of the amps that you want to get and explaine to them the type of speaker and how you are going to use them. I don't know if this helps any.

Posted by Gordon McLeod (Member # 33) on 01-08-2001, 03:49 PM:

Actually you have some confused numbers in use. The 85db is not part of this equation
If you are running A type optical only you must design the system for 91 db
If optical SR or SRD are used you have to design around 103 on each stage speaker
100db on the surround channels
and 113 db on the sub
Mag 70 can be slightly higher depending on the oxide
The other number is the efficiency of the speaker which is specified as db@1watt 1 meter.The rest is math

Posted by Darryl Spicer (Member # 711) on 01-08-2001, 04:43 PM:

So Gordon, what you are saying is that the speaker should be able to handle 113 db for digital based on the efficiancy of the speaker. because when we are setting up our speakers we set all front channels to 85db and surrounds set to 86db and subs set to 71 do to bleed over into other theaters when running dts. I guess I'm a little confused about the numbers. If you would write out the formula a little planer for me this is some interesting information.

Posted by Gordon McLeod (Member # 33) on 01-08-2001, 09:01 PM:

As I said some wrong numbers are being used
85dbc slow is the reference level of pinknoise with the fader at 7 50% modulated optical track.
The peak level is a 100% modulated track and that is the numbers I quoted and those are the ones you use to design the system
The 85db is to calibrate afterwards

Posted by Darryl Spicer (Member # 711) on 01-09-2001, 12:39 AM:

Thanks for clearing that up Gordon. I think I was confusing myself.

Posted by Mark Gulbrandsen (Member # 72) on 01-10-2001, 12:32 PM:

Personally I would cut these speakers at 60 to 70 hertz. If you do this,the system will have much lower harmonic distortion in the LF. This is assuming that you have a good subwoofer to take over the low end.
Mark

Posted by Gordon McLeod (Member # 33) on 01-10-2001, 01:58 PM:

I wouldn't be too worried about low frequency distortion as these speakers are designed for small room applications and they are also designed around the use of a baffle that will effectively lower the amount of energy required at extreme low frequencies and at the same time reduce the distortion

Posted by David Baum (Member # 568) on 01-11-2001, 09:04 AM:

thanks for the comments. currently cut at 80hz ( I have 2 4645C subs ) but will try with the QSC a cut at 50hz ( which gives a nice boost around 80-100hz) and see how they do. they are on a platform but maybe I'll try some sort of bafle wall.

thanks again!!!

Posted by John Pytlak (Member # 331) on 01-12-2001, 09:51 AM:

JBL has an on-line "Cinema Sound Systems Manual" that give guidelines on power requirements, which are dependent upon a variety of parameters including room size, speaker type, speaker placement, etc.:
http://www.jblpro.com/

JBL also has a professional "white paper" on "Theatre Systems: A Quick Guide to Specifying Cinema Loudspeakers" by John Eargle and Mark Gander.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

Posted by David Baum (Member # 568) on 01-12-2001, 03:55 PM:

thanks John!
I don't seem to see the second file you mention on their website. is this the "Product Application Guidelines" , which I have also ?

best
David

Posted by John Pytlak (Member # 331) on 01-13-2001, 07:33 PM:

The "white paper" was distributed at the presentation JBL gave at last week's ITEA Seminar in Los Angeles. Assume you could write them and ask for a copy.

Posted by Mark Mayfield (Member # 729) on 01-18-2001, 12:49 PM:

Couple comments. (Going back a ways) The JBL 3678 has foam in the port tubes to prevent light from the light-bulb protection circuit from leaking out to a darkened theatre, causing a visual distraction. It causes no impediment to low frequency transmission; you should leave it in. Also - I agree with previous comments that you should band pass ANY cinema screen system at 50 Hz. Let the subs do the VLF work, and minimize your screen LF amp clipping.

Posted by Gordon McLeod (Member # 33) on 01-18-2001, 09:44 PM:

There should be no amp clipping is the amp sizeing vs speaker efficiency are matched to the room volume vs the desired SPL. So the rolloff should be a mute point if they are installed in the baffle recomended and they are used in the room volume they were designed for

Posted by Mark Gulbrandsen (Member # 72) on 01-19-2001, 04:57 PM:

Gord,
Why push a speaker to put out low frequencys that it physically can't? Why not cut it off at a given low frequency and end up with much cleaner, lower distortion bottom end, baffle wall or not?
Small cabinets simply cannot produce extreme low frequencys at low distortion if pushed beyond the design limitations
On the THX systems that I have done they cut the mains at 50 hz, or didn't give them any boost at all. They let the sub take over and things stayed nice and clean. Pushing a cabinet beyond its design capabilities will only leads to listening fatigue, and a muddy bottom end.
Mark

Posted by Steve Guttag (Member # 268) on 01-19-2001, 10:51 PM:

Even worse...Once the frequency falls below the design of the cabinet/driver system, the driver will unload and slap itself silly since the enclosure isn't helping it out.

Since one of the LFE crossover points is 50Hz, let the subs roll in there so there isn't an overlapping of the two system causing an unnatural boominess.

Steve

P.S. Mark Mayfield, it is a real pleasure having you here! Anyone who doesn't know who Mark is should do some homework!

Posted by Mark Gulbrandsen (Member # 72) on 01-20-2001, 11:18 AM:

I can remember hearing one of Mark's talks at Showest way back when he was with E-V.
Mark @ GTS