While reading "Loudspeaker design cookbook" by Vance Dickason, I came across following " traditionaly enclosure damping, to suppress standing waves in a vented-box system, consists of lining one of each opposite side with 1"-2" of fiberglass. It is recommended, however, that you cover all surfaces directly behind, and adjacent to, woofer. Colloms recommends that such damping material be placed with the volume or open area, not on the box walls. "
Does the author mean to say, the damping material should be stuffed like that of closed enclosure design. taking care that vent is not obstructed? That is not just lining the walls of vented enclosure.
Does the author mean to say, the damping material should be stuffed like that of closed enclosure design. taking care that vent is not obstructed? That is not just lining the walls of vented enclosure.
He means, if you're going to use 1 or 2 inch thick material, hang it in the middle of the box instead of against the walls. I't'll be more effective. Note that you can fill the whole box if you want to, just leave a bit of space around the port mouth. Several box modeller programs such as Unibox allow using damping material in the enclosure design.
Thank you Don and chris.
Chris EPS site is my important bookmark. It had always been a big source of knowledge.
Do you know any site which discuss deep about this with some calculation.
With random surfing, I could understand that, because the stuffing absorbs sound and minimizes reflecting it back avoiding standing waves, this makes it look like a bigger enclosure. but the physical volume of enclosure gets smaller, so the compression ratio changes.
But mathematically how can this be related and calculated just don't know.
Chris EPS site is my important bookmark. It had always been a big source of knowledge.
Do you know any site which discuss deep about this with some calculation.
With random surfing, I could understand that, because the stuffing absorbs sound and minimizes reflecting it back avoiding standing waves, this makes it look like a bigger enclosure. but the physical volume of enclosure gets smaller, so the compression ratio changes.
But mathematically how can this be related and calculated just don't know.
In actuality the biggest problems with sub enclosures is inadequate panel bracing. Lack of sufficient panel bracing allows LF resonances to build up within the enclosure and this is what many refer to as a sub having a boomy character.
Most sub enclosures are just not big enough to allow standing waves of LF to build up, given the wavelength of low frequencies
Fiberglass, Dacron, carpet, etc have virtually no effect on controlling or reducing low frequencies, although they do help somewhat in controlling upper harmonics of the LF content.
You might be surprised to learn that many of the best sounding ported subs have very little to no stuffing/lining but are extremely well engineered to be very "dead" resonance wise by using panel bracing and double panel thickness with a constraining layer between the two panels.
Most sub enclosures are just not big enough to allow standing waves of LF to build up, given the wavelength of low frequencies
Fiberglass, Dacron, carpet, etc have virtually no effect on controlling or reducing low frequencies, although they do help somewhat in controlling upper harmonics of the LF content.
You might be surprised to learn that many of the best sounding ported subs have very little to no stuffing/lining but are extremely well engineered to be very "dead" resonance wise by using panel bracing and double panel thickness with a constraining layer between the two panels.
The only thing that stuffing a ported enclosure does is it helps a bit with absorption, (Qa in WinISD) to lower the Q of a undersized(high Q) enclosure - stuffing/lining really has very little to no effect on controlling any standing waves within a enclosure.
If you are concerned(and you should not be with sub enclosures) about standing waves simply make sure no two inner surfaces of the enclosure are completely parallel to each other, that way wave frequencies inside the enclosure can never build and form a standing wave.
If you are concerned(and you should not be with sub enclosures) about standing waves simply make sure no two inner surfaces of the enclosure are completely parallel to each other, that way wave frequencies inside the enclosure can never build and form a standing wave.
Damping inside a bassreflex has the same effect as a muffler to a exhaust-system, it reduces the soundlevel and ruins soundquality. The right way to go would be to look at 1/4 wave horns or do like I`ve done; asymetric cabinet w. diffractors + aerodynamic ports pointing forward paralell w. floor.
ditto's
Acoustic Absorbent material is of NO USE and might actually be a detriment in a sub only enclosure. I have built a 3 cu foot enclosure of equal dimensions HWD for a 15" sub driver in a very solid built box. The results were a very natural low frequency playback quality, crossing below 80Hz.
The need for Acoustic Absorbent material is relevent for a typical bass reflex design where the crossovers for a 3 way system are typically 700 & 5K Hz.
Acoustic Absorbent material is of NO USE and might actually be a detriment in a sub only enclosure. I have built a 3 cu foot enclosure of equal dimensions HWD for a 15" sub driver in a very solid built box. The results were a very natural low frequency playback quality, crossing below 80Hz.
The need for Acoustic Absorbent material is relevent for a typical bass reflex design where the crossovers for a 3 way system are typically 700 & 5K Hz.
you mean Acoustic absorbent material is not needed when you are working with subwoofer that is crossing at 160Hz. did I get you right?
crossing subs
I'm sorry, I do not know where you got 160 Hz. IMO 160 Hz is much too high a crossover for subs. In my example I mentioned 80 Hz. Here's a link where you can look at the effectivness of various materials vs absorbtion co-efficients.
http://www.bobgolds.com/AbsorptionCoefficients.htm
Hope that helps !!
I'm sorry, I do not know where you got 160 Hz. IMO 160 Hz is much too high a crossover for subs. In my example I mentioned 80 Hz. Here's a link where you can look at the effectivness of various materials vs absorbtion co-efficients.
http://www.bobgolds.com/AbsorptionCoefficients.htm
Hope that helps !!
Thanks,
Scott & Coke, after I saw the chart on the link provided by Scott. It was a easy read that the absorbent material has virtually no effect on Low frequencies. I still have a question, why must JBL have used it in their SUB? I mean any hidden reason that is unpublished.
About 160Hz. Dolby recommends sub crossover at 120Hz as their LFE is @ 120Hz. The JBL scs140 has crossover @ 160Hz (-6db). this was the reason why I decided to cross over @ 160Hz.
Scott & Coke, after I saw the chart on the link provided by Scott. It was a easy read that the absorbent material has virtually no effect on Low frequencies. I still have a question, why must JBL have used it in their SUB? I mean any hidden reason that is unpublished.
About 160Hz. Dolby recommends sub crossover at 120Hz as their LFE is @ 120Hz. The JBL scs140 has crossover @ 160Hz (-6db). this was the reason why I decided to cross over @ 160Hz.
After looking at the specs on that JBL subwoofer, the size if the enclosure appears to be too small to meet their spec. This results in a box with a very high Q (and probably sounds boomy) - they added absorbent material to help lower the Q of the box and make it less boomy - the down side of doing this is that by doing so you also lower the LF output levels and all the LF sounds mushy and blurred.
No you have that incorrect - Dolby specifically states that their LFE crossover point is 80 Hz
http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/4_Multichannel_Music_Mixing.pdf
http://www.dolby.com/uploadedFiles/...lish_PDFs/Professional/L.mn.0002.5.1guide.pdf
That JBL system is typical of many off the shelf underpowered HT systems currently on the market - if you look at their specs, the sub on that system is -6db @ 35Hz ( not very low in the real world of HT systems) and they fail to mention what the effective operating SPL are - I would suspect not much over 102db with <1% distortion.
The bottom line is that the JBL scs140 was designed and built to a price point - not as a good standard for HT systems
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Thanks Coke for the documents.
The documents too say Dolby recommends LFE of 3Hz to 120Hz.
Here are some cuts from documents.
Document title : L.mn.0002.5.1guide.pdf
Chapter : 1.2 Dolby Digital and 5.1-Channel Audio
“Dolby Digital bitstreams deliver full frequency bandwidth main channels, from 3 Hz to 20 kHz, and a limited frequency bandwidth LFE channel, from 3 Hz to 120 Hz.”
Chapter: 3.3 Bass Management
“In the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz. To properly hear the LFE content, a sixth or seventh order 120-Hz low-pass filter must be included in the monitor chain. It is advisable to include this filter in the console output before the monitor such that both the recorded information and the heard information are band-limited. Failure to include this filter will result in hearing substantial bass information above 120 Hz in the mix that will not be present in the Dolby Digital encoded version. 120 Hz is the proper crossover frequency for theatrical film applications.”
Chapter : 3.4.6 Subwoofer Calibration
“Ideally, the test noise used for subwoofer calibration should be band-limited pink noise,low-pass filtered at 120 Hz. ”
Document title : 4_Multichannel_Music_Mixing.pdf
Chapter: 3.2.2 Subwoofers
“The frequency response of the subwoofer should be flat, ±3 dB between 20 Hz and 120 Hz. ”
“Because the LFE channel for cinema applications contains content up to 120 Hz,
however, the subwoofer itself should be capable of reproducing up to 120 Hz. ”
-------------------------------------------------------------------------------------------------------------------
Then what about crossover @ 80Hz. Well its explained in bass management.
These days consumer equipment has 80Hz crossover at LFE. But this 80Hz is sliced from the original 120Hz and the bass from LFE that is above 80Hz is mixed with the chanells.
Here is a cut supporting it.
Document title : L.mn.0002.5.1guide.pdf
Chapter :3.3 Bass Management
“For consumer applications such as DVD and Digital TV, the consumer decoders add a slight twist to the equation. Consumer decoders take the LFE signal and add any channels in need of bass management, as determined either by product design or user selection. The five main channels are then high-pass filtered at either a fixed frequency of 80 Hz or a selectable frequency of 80, 100, or 120 Hz. The summation of the LFE and any other channels is low-pass filtered at the same frequency. If the crossover frequency is fixed at 80 Hz, as is standard in lower priced decoders, information in the LFE channel between 80 Hz and 120 Hz will be reproduced at a lower level than it is recorded at. To replicate what the consumer will hear, a third order 80 Hz filter in the LFE audio signal path to the recorder is recommended.”
But I have a deep feeling that this Bass management must have been done to get THX certification.
The documents too say Dolby recommends LFE of 3Hz to 120Hz.
Here are some cuts from documents.
Document title : L.mn.0002.5.1guide.pdf
Chapter : 1.2 Dolby Digital and 5.1-Channel Audio
“Dolby Digital bitstreams deliver full frequency bandwidth main channels, from 3 Hz to 20 kHz, and a limited frequency bandwidth LFE channel, from 3 Hz to 120 Hz.”
Chapter: 3.3 Bass Management
“In the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz. To properly hear the LFE content, a sixth or seventh order 120-Hz low-pass filter must be included in the monitor chain. It is advisable to include this filter in the console output before the monitor such that both the recorded information and the heard information are band-limited. Failure to include this filter will result in hearing substantial bass information above 120 Hz in the mix that will not be present in the Dolby Digital encoded version. 120 Hz is the proper crossover frequency for theatrical film applications.”
Chapter : 3.4.6 Subwoofer Calibration
“Ideally, the test noise used for subwoofer calibration should be band-limited pink noise,low-pass filtered at 120 Hz. ”
Document title : 4_Multichannel_Music_Mixing.pdf
Chapter: 3.2.2 Subwoofers
“The frequency response of the subwoofer should be flat, ±3 dB between 20 Hz and 120 Hz. ”
“Because the LFE channel for cinema applications contains content up to 120 Hz,
however, the subwoofer itself should be capable of reproducing up to 120 Hz. ”
-------------------------------------------------------------------------------------------------------------------
Then what about crossover @ 80Hz. Well its explained in bass management.
These days consumer equipment has 80Hz crossover at LFE. But this 80Hz is sliced from the original 120Hz and the bass from LFE that is above 80Hz is mixed with the chanells.
Here is a cut supporting it.
Document title : L.mn.0002.5.1guide.pdf
Chapter :3.3 Bass Management
“For consumer applications such as DVD and Digital TV, the consumer decoders add a slight twist to the equation. Consumer decoders take the LFE signal and add any channels in need of bass management, as determined either by product design or user selection. The five main channels are then high-pass filtered at either a fixed frequency of 80 Hz or a selectable frequency of 80, 100, or 120 Hz. The summation of the LFE and any other channels is low-pass filtered at the same frequency. If the crossover frequency is fixed at 80 Hz, as is standard in lower priced decoders, information in the LFE channel between 80 Hz and 120 Hz will be reproduced at a lower level than it is recorded at. To replicate what the consumer will hear, a third order 80 Hz filter in the LFE audio signal path to the recorder is recommended.”
But I have a deep feeling that this Bass management must have been done to get THX certification.
You also have to understand that crossovers are not like a light switch - all on or all off at a given frequency - they are filters.
Filters have slopes - typical good quality crossovers have slopes that are 12, 18 or 24db per octave. The 80Hz crossover means that the center frequency of the crossover is at 80Hz and that above that point LF will be attenuated at a rate of 12, 18, or 24db per octave. (btw - one octave is double or half of the original frequency)
For instance of you had a 12db per octave crossover set to 80Hz the -12db(one octave) frequency would be 160Hz, So yes, even at a 80Hz crossover point the subwoofers will still receive and be expected to reproduce frequencies at 120Hz and 160Hz - but at lower levels.
Finally you should not confuse the technical requirements about the production requirements for Dolby encoding with the requirements for the home playback side of things.
Filters have slopes - typical good quality crossovers have slopes that are 12, 18 or 24db per octave. The 80Hz crossover means that the center frequency of the crossover is at 80Hz and that above that point LF will be attenuated at a rate of 12, 18, or 24db per octave. (btw - one octave is double or half of the original frequency)
For instance of you had a 12db per octave crossover set to 80Hz the -12db(one octave) frequency would be 160Hz, So yes, even at a 80Hz crossover point the subwoofers will still receive and be expected to reproduce frequencies at 120Hz and 160Hz - but at lower levels.
Finally you should not confuse the technical requirements about the production requirements for Dolby encoding with the requirements for the home playback side of things.
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Try 80Hz
I might very well have an effect at 160Hz, but typically subs are crossed over at 80Hz or below where standing waves are too long to be a problem.
I might very well have an effect at 160Hz, but typically subs are crossed over at 80Hz or below where standing waves are too long to be a problem.
On the LFE channel there is no information above 120hz. During recording there is a "brickwall" filter at 120.
“In the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz. To properly hear the LFE content, a sixth or seventh order 120-Hz low-pass filter must be included in the monitor chain"
“In the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz. To properly hear the LFE content, a sixth or seventh order 120-Hz low-pass filter must be included in the monitor chain"
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Well, not really recommends. That stuff is more like the SPEC of the LFE channel, not a playback recommendation. The playback recommendation came from research at THX (IIRC) that showed 80 Hz was "undetectable." Of course some skimpier systems cross over higher than that in order to use ultra teeny satellites.
However, feel free to cross over your subs at 3 Hz if you like. Tom Danley will be glad to build you something for that application.
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