Everest enclosure volums vs professional enclosure guide volume

Hi all,

According to this post, the original (DD55000) Everest had a bass enclosure volume of 180 litres + 27 litres = 207 litres, and an "effective volume" of 227 litres (8 cubic feet) when taking into account the ports.

Yet the JBL Professional enclosure guide states that the enclosure volume for the E145 (and several other JBL drivers, including the 2225) should be 4 cubic feet; i.e., half the actual volume used in the Everest.

Can someone please explain the discrepancy?

Apart from curiosity, the main reason I ask is that I am having a ported enclosure built (initially for a single 2225H, but I may upgrade to a different driver at a later time) and I am trying to determine the optimum enclosure volume to use...

Thanks in advance!

Richard

Quote:

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Originally Posted by rab

Hi all,

According to this post, the original (DD55000) Everest had a bass enclosure volume of 180 litres + 27 litres = 207 litres, and an "effective volume" of 227 litres (8 cubic feet) when taking into account the ports.

Yet the JBL Professional enclosure guide states that the enclosure volume for the E145 (and several other JBL drivers, including the 2225) should be 4 cubic feet; i.e., half the actual volume used in the Everest.

Can someone please explain the discrepancy?

Apart from curiosity, the main reason I ask is that I am having a ported enclosure built (initially for a single 2225H, but I may upgrade to a different driver at a later time) and I am trying to determine the optimum enclosure volume to use...

Thanks in advance!

Richard

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See http://www.ht-audio.com/pages/SpeakerBasics.html. Look for "Shaping the Low-Frequency Response".

4 cu feet is "portable" (size is an object), 8 cu feet is "stationary" (size does not matter).

If you want to upgrade at a later time, the TS (Thiele Small) Parameters fs, Qt (read: Qa, Qe, Re) , Vas, should be similar. The 2226 is similar to the 2225 in this respect.

Ruediger

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Originally Posted by Ruediger

4 cu feet is "portable" (size is an object), 8 cu feet is "stationary" (size does not matter).

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Thanks Ruediger. Your comments suggest: (i) that the JBL enclosure guide provides only minimum enclosure volumes for portable speakers, and (ii) that 8 cubic feet (226 litres) will be better in my situation, provided that the larger enclosure size is acceptable.

Here is a WinISD plot I generated, showing the 2225H driver in 4 (red line) and 8 (green line) cubic feet enclosures with 40Hz port tuning:

Attachment 64148

If this plot is correct, then doesn't the smaller enclosure provide a better frequency response? (Incidentally, I plan to add a sub at some stage to provide the ~20-40Hz region).

- Richard

Quote:

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Originally Posted by rab

Thanks Ruediger. Your comments suggest: (i) that the JBL enclosure guide provides only minimum enclosure volumes for portable speakers, and (ii) that 8 cubic feet (226 litres) will be better in my situation, provided that the larger enclosure size is acceptable.

Here is a WinISD plot I generated, showing the 2225H driver in 4 (red line) and 8 (green line) cubic feet enclosures with 40Hz port tuning:

Attachment 64148

If this plot is correct, then doesn't the smaller enclosure provide a better frequency response? (Incidentally, I plan to add a sub at some stage to provide the ~20-40Hz region).

- Richard

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Playback/portable (PA) should not be confused ..the latter is designed for flat OUT high power/vol.
and sounds good doing it....example.....I used JBL TR225 towers for rear playback (home stereo)
and one day ..I walked in on a school DJ thing .and I couldn't believe my eyes... to see those's
same TR225's on stage filling the whole gymnasium w/Supersound.....

Quote:

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Originally Posted by rab

Thanks Ruediger. Your comments suggest: (i) that the JBL enclosure guide provides only minimum enclosure volumes for portable speakers, and (ii) that 8 cubic feet (226 litres) will be better in my situation, provided that the larger enclosure size is acceptable.

Here is a WinISD plot I generated, showing the 2225H driver in 4 (red line) and 8 (green line) cubic feet enclosures with 40Hz port tuning:

Attachment 64148

If this plot is correct, then doesn't the smaller enclosure provide a better frequency response? (Incidentally, I plan to add a sub at some stage to provide the ~20-40Hz region).

- Richard

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At all frequencies below 60 Hz the green alignment (larger box) gives you more(!) bass than the other alignments. Note: I wrote "more", not "better".

Generally, when you inrease the volume, in most cases you will decrease the box tuning frequency. You don't double the volume and keep the tuning frequency.

Look for the thread "http://www.audioheritage.org/vbulletin/showthread.php?36008-JBL-2226-amp-Bi-Radial-Horn-DIY-Monitor", there look for the entry "confessions of a loudspeaker designer". There is a flowchart of a method which should give you some starting parameters.

BTW: wasn't there a discussion about the E145's published TS params being wrong?

Ruediger

As Ruediger said, move your port tuning down for the larger box. Try to get a smooth response curve, similar to the red curve, with no "hump". A hump in the response will tend to give boomy or "one-note" bass.

Francis

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Originally Posted by fpitas

As Ruediger said, move your port tuning down for the larger box. Try to get a smooth response curve, similar to the red curve, with no "hump". A hump in the response will tend to give boomy or "one-note" bass.

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OK, so here are WinISD plots for the 2225H in enclosures of 120, 140, 160, 180, and 200 litres (red, orange, green, blue, and violet lines), where the port tuning frequency was adjusted to be the highest frequency that did not give rise to any increase in output with decreasing frequency (these port frequencies being 44, 40, 35, 33, and 30 Hz, respectively).

What I take away from this exercise is that increases in enclosure volume certainly give you lower frequencies, but at the expense of increasingly falling response with decreasing frequency at frequencies less than 200 Hz.

For example, the response of a 200 litre enclosure is flat from about 32-50 Hz, but then rises approximately linearly with log(f) from about 50 Hz to 150Hz, rising by about 3dB. So presumably the low pass filter would have to compensate for this behaviour.

This effect becomes less pronounced with small enclosure volumes, but at the expense of a loss of low frequency extension. For example, the 120 litre enclosure gives an flatter response that is nearly 4dB higher than the 200 litre enclosure in its flat response region, but it extends only down to about 52Hz.

So based on these calculations, it seems to me that a larger enclosure:

• extends to lower frequencies, but at the expense of:
• lower output at lower frequencies, thus requiring more power, and
• a less flat frequency response requiring more complex low pass filtering.

Overall, I am thinking of using an enclosure volume of either 140, 160 or 180 litres.

I acknowledge that the above considers only frequency response, and there are other characteristics that change with enclosure volume (e.g., damping).

However, is my understand a reasonable summary? Comments anyone?

Attachment 64202

.. and here are the corresponding plots of phase and group delay. I'm not sure what to make of these...

Attachment 64203

Attachment 64204

Your summary is reasonable. There are a lot of aspects to lowering the tuning using a larger box. The drooping response isn't necessarily a bad thing, since the response of the speaker in a room near walls etc. can accentuate the lower frequencies. In any event, EQ may be applied to the system to raise the response, but only above the port tuning. Below the port tuning the speaker cone becomes unloaded, the port essentially acting like a large hole in the box. So, lowering the port tuning and using some judicious EQ can result in a significant amount of additional low bass extension for a given driver. Pushing the peak of the group delay curve lower in frequency is a good thing in general, since the increasing group delay then occurs below most musical notes. Finally, if you decide you want less volume in the future, partially filling the box with an inert material (wood, bricks etc) will easily accomplish that. Increasing the box volume is not so easy ;)

Francis

Quote:

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Originally Posted by fpitas

<snip>
1.) Your summary is reasonable.
<snip>
2.) Finally, if you decide you want less volume in the future, partially filling the box with an inert material (wood, bricks etc) will easily accomplish that.
<snip>
3.) Increasing the box volume is not so easy ;)
<snip>

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1.) You may want to study the Thiele paper. Forum: General Audio Discussion, Thread: Technical References, Entry: Thiele Paper.

2.) You can use hard styrofoam.

3.) If the box is only for low bass and not for mid bass, you can turn the driver so that the magnet is on the outside. Saves you the "volume displaced by the driver" and adds a bit volume in the cone. Helps the driver to dissipate heat. Minimizes the shift of TS parameters due to a heated voice coil. Requires swapped cables (+ <-> -).

4.) Study the Thiele Paper for the effect of Rg on Qt (Equation 70 in 2nd part of paper). The DC resistance of the bass coil in the Xover and the cables all contribute to Rg.

Ruediger

Thanks Francis and Ruediger, much appreciated.

Quote:

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Originally Posted by Ruediger

1.) You may want to study the Thiele paper. Forum: General Audio Discussion, Thread: Technical References, Entry: Thiele Paper.

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Found it: link.

Quote:

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Originally Posted by Ruediger

2.) You can use hard styrofoam.

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Thanks; I assume you mean to reduce the enclosure volume.

Quote:

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Originally Posted by Ruediger

3.) If the box is only for low bass and not for mid bass, you can turn the driver so that the magnet is on the outside. Saves you the "volume displaced by the driver" and adds a bit volume in the cone. Helps the driver to dissipate heat. Minimizes the shift of TS parameters due to a heated voice coil. Requires swapped cables (+ <-> -).

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I'll be using this up to about 700 Hz, a la Everest 55000

Thanks again. I have some reading to do! (Wow, i never realised that Thiele was Australian!)

Richard