Dbl ported bass reflx enclosure question:

[roddyama]

Roddy:

Well, that sure is complicated. But perhaps, with this particular speaker, we can un-complicate it.

My understanding of a vented box is as follows. From the high range down to an octave above the vent frequency, the vented box and the closed box are identical. Then the port action begins about an octave above the vent frquency and increases until the it hits the vent frequency itself, (Fb). Beneath the vent tuning frequency, the box begins to revert to a dipole, and lets the frequencies below the vent tuning "leak" outside the box.

This is theoretically correct, but Thiele and Small made some basic assumptions and simlifications to focus their analysis to the low frequency behavior. They ignored the rear radiated energy of the driver in the higher frequencies. Their studies do not extend above the effects of the box and port tuning frequencies.

There are issues not covered by Theile and Small and therefore thought by many to be non-issues, like the Hemholtz frequency for the port(s) which is different then the port tuning frequency. Also the rear radiation from the driver in the upper frequencies can and will be transmitted through the port.

It is the mass of the air in the port that resonates with the compliance of the air in the box at the tuning frequency. This is true, but that doesn't mean that sound cannot pass through the open port at the higher frequencies by normal sound propogation though air.

These phenomenon will interact with any standing waves in the box. Covering the walls with absorbing material will help in the upper frequencies (>500Hz), but the mid-bass carries a lot of energy and won't be so easily tamed.

Grahamn,

Don't let this debate detour you. Most of these effects are minor, but it doesn't hurt to know of their exisistence. However, you will want to line the walls of you box (upper and lower) to attenuate the upper frequencies as much as possible with out over stuffing it.

 

[kelticwizard]

This is theoretically correct, but Thiele and Small made some basic assumptions and simlifications to focus their analysis to the low frequency behavior. They ignored the rear radiated energy of the driver in the higher frequencies. Their studies do not extend above the effects of the box and port tuning frequencies.....

.....It is the mass of the air in the port that resonates with the compliance of the air in the box at the tuning frequency. This is true, but that doesn't mean that sound cannot pass through the open port at the higher frequencies by normal sound propogation though air.

These phenomenon will interact with any standing waves in the box.

Roddy:

Must disagree. Below is a series of actual measurements taken by DB Keele. This is from Keele's article in the Journal of The Audio Engineering Society in April,1974: "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement".The top graph is for the cone. The middle graph is for the vent. In the middle graph, a microphone was inserted at the middle of the vent, flush with the front board, and here is the result.

The box is tuned to 40 Hz.

At first, there appears to be an anomaly. Around 80 Hz-double the Fb- the vent appears to have a surge of output. However, Keelse explains that this is not the case.

The peak at 150 or 200 Hz or so is created by crosstalk between the port and the cone. That is, the microphone, being located flush with the front baffle, is picking up the output of the cone as well as the vent. Normally, the mic would not pick up the front cone. but as the output from the vent fades off, the output of the cone can be heard.

There is additional output farther up the line, but very suppressed. Any sound that is 12 dB lower than another, adds less than 1 dB to the total output. So if your sound is 12 dB or more under the middle level, you really are adding almost nothing.

This bears out my earlier statement that there is really little difference in output between the closed and vented box once you go an octave above Fb.

The top chamber of the speaker in question is tuned to 120 Hz. The bottom chamber is tuned to 53 Hz. The standing wave of the bottom chamber is about 270 Hz. That means that a 270 Hz frequency is attenuated 12 dB by the first vent, excites a standing wave in the bottom chamber, but is attenuated by 24 dB, (since it is over two octaves remoeved from Fb), before it hits the outside world. Therefore, the standing wave of the lower chamber is not a factor in this speaker. The standing wave is sharply attenuated notonce, but twice.

PS: I can scan the article and send it to anyone who wishes via Email.

 

[kelticwizard]

To further illustrate the point, here is a simulation from Subwoofer Simulator, a freeware program written by our own F4ier. This program hs many, many curves you can illustrate. However, you can choose to model just one or two if you choose. Downoadable here:
http://www.geocities.com/f4ier/speaker.htm

This simulation is for an Adire Shiva Mark III in a 4.4 Ft³, (125 L) vented box tuned to 20 Hz.

The blue line is the SPL output from the cone, the green line is the SPL output from the port.

This illustrates that the port in a reflex box is basically a narrow band pass filter with 12 dB/octave slopes on each side. In the double chamber reflex we are dealing with here, the output from the standing wave is attenuated by two of these filters, since it is near the peak frequency of neither of them.

 

[roddyama]

KW,

Keele is not taking into account the possibility of a standing wave at some upper frequency existing in the box. If the standing wave exist in the box, whether it is from the front-to-back dimension, side-to-side, top-to-bottom, in the port itself, -12db can quickly become +3db or worse.

 

[krishu]

Hello to everybody

did you take into account that a smaller front wall of the speaker will change its frequency response?

If this would not be the case, i could buy a diy speaker kit and totally change the dimensions of the suggested box (by keeping the internal volume, of course), including the placement of the drivers. can I?

Cheers
Christian.

 

[kelticwizard]

Okay, doing the math on this enclosrue, we see that the length of the new enclosure is going to be 4 times the depth or width. That is a pretty pipelike enclosure.

There probably will be a standing wave in it. How sharp is another matter.

However, it is the nature of the double chamber enclosure that this standing wave will be suppressed more than in the single chamber reflex.

The top chamber is tuned to 120 Hz. So the frequency that excites the standing wave in the lower chamber-270 Hz-is likely to be 8 dB down from what it would be if the box were a single chamber reflex. This helps to suppress the standing wave right there.

Plus, the lower box is tuned two octaves below the standing wave frequency. So the standing wave is probalby suppressed aobut 24 dB, plus the 8 dB from the top box. That is about 30 dB of standing wave suppression.

It's possible that a standing wave can cause an anomaly in the response, but this box has double filtering, (via two different ports) to make it more unlikely.

 

[krishu]

Hi,

BTW, the german diy speaker manufacturer Visaton (www.visaton.de) measured standing waves in rectangular and triangular boxes (say: parallel wands and non-parallel wands); when dampening material is put into the speaker, they found no difference! But others did or pretend to.

The author Bernd Timmermanns of the german diy magazine "Hobby Hifi" suggests internal Helmholtz resonators to kill standing waves.

What about the shape of the front baffle? (my post above)

Cheers
Christian.

 

[roddyama]

Hi Christian,

Hi,

BTW, the german diy speaker manufacturer Visaton (www.visaton.de) measured standing waves in rectangular and triangular boxes (say: parallel wands and non-parallel wands); when dampening material is put into the speaker, they found no difference! But others did or pretend to.

I looked for where they talk about this, but could not find it. It has to depend on how non-parallel the wall are.

The author Bernd Timmermanns of the german diy magazine "Hobby Hifi" suggests internal Helmholtz resonators to kill standing waves.

This would be a solution, aleit quite complicated.

What about the shape of the front baffle? (my post above)

Cheers
Christian.

This will definately change things. No doubt about that. Another issue to deal with. It tends to make you respect the designers that can consistantly come up with great sounding speakers.

 

[roddyama]

It's possible that a standing wave can cause an anomaly in the response, but this box has double filtering, (via two different ports) to make it more unlikely.

What's the matter KW. Didn't you sing in the shower pretending you were Robert Goulet.

 

[krishu]

Hi Rodd,

indeed, obviously they took it away. Maybe they were not sure about it or even wrong. (BTW, this is a company that always tells that the quality of a capacitor is not very important - they use lots of electrolytics...)

I do not think that those internal helmholtz absorbers are too difficult. Mr Timmermanns calculated and measured standing waves and then addes a separated volume which is coupled by a reflex port tube (in a certain diameter and length, of course) and the only slightly difficult thing is to put the right amount of dampening material into this absorber.

I already respect designers of any speakers 😉

I would like to know what will happen when i change the placement of the drivers (from tweeter - midrange - midrange to midrange - tweeter - midrange like D'Apollito) and making the front baffle four centimeters slimmer.

regards
Christian

 

[roddyama]

I do not think that those internal helmholtz absorbers are too difficult. Mr Timmermanns calculated and measured standing waves and then addes a separated volume which is coupled by a reflex port tube (in a certain diameter and length, of course) and the only slightly difficult thing is to put the right amount of dampening material into this absorber.

Does the volume of theair in the absorbers add or subtract from the volume of the box? If they subtract then you have to make the box bigger to accomodate them changing the tuning of the absorbers, etc, etc. It doesn't sound so straight forward.

I would like to know what will happen when i change the placement of the drivers (from tweeter - midrange - midrange to midrange - tweeter - midrange like D'Apollito) and making the front baffle four centimeters slimmer.

You should start a new thread for this issue, unless Grahamn has heard enough.😴

 

[krishu]

Hi Rodd,

of course, the absorber is added, which means that the internal volume of the box itself stays as it is. The external volume of the speaker grows then. Can you understand my bad English? I'm sorry for that.
This means, you cannot easily build in such an absorber into an already existing speaker. But when building it from zero it is almost no added work.
I did not find some pictures on www, so unfortunately i cannot show.
I guess there are commercial designs out there which have these absorbers.

The other issue: the change of front baffle width. When it is that complicated, i keep my fingers away from it.

Cheers
Christian.

 

[kelticwizard]

I don't think that the internal Helmholtz resonators would be added to box volume, as long as they were tuned appreciably above the lower Fb of the enclosure. In this case, the lower Fb is 53 Hz, so the Helmholtz resonators tuned to 270 Hz would not be a factor at the lower frequencies.

A Helmholtz resonator essentially reverts to being a dipole once it goes below Fb. If the Helmholtz resonator is tuned to 270 Hz, the Helmholtz resonator should be essentially invisible to frequencies 135 Hz and below.

Here is a thread dealing with Helmholtz resonators to kill standing waves:
http://www.diyaudio.com/forums/showthread.php?threadid=11533&perpage=15&pagenumber=1

 

[kelticwizard]

I would like to know what will happen when i change the placement of the drivers (from tweeter - midrange - midrange to midrange - tweeter - midrange like D'Apollito) and making the front baffle four centimeters slimmer.

regards
Christian

Well, the kit might or might not have some passive circuit for Baffle Step Compensation, that would have something to do with how wide the front baffle is. However, four centimeters really does not seem to be that much, and I don't think you'll be thrown off badly. I don't even know if your kit has Baffle Step Compensation anyway.

Even more likely is that the crossover is set for certain distances between drivers. Distance betwen the centers of the drivers definitely can affect frequency response. If nothing else, I would certainly keep the drivers the same distance from each other that the kit calls for.