Acoustic suspension vs bass reflex

[Cameron Glendin]

I find it amusing that no one has taken the coke bottle test as yet! It only takes a few minutes of ones life to demonstrate how a port actually works.

The person who told me about its time delay was someone who spent a lifetime in audio and film sound. He was the head of sound for very large institution, Neville Theale was employed as the head of video tape there at the same time. The two are friends. He is the sort of person that use to send his sound recordists to the school of the blind as part of there training.

So whether its 10, 15, or 42 miliseconds delay depending on driver/ port combo its fair to say it exists. Port noise also exists at frequencies up to 500 - 800 hz, Bass box pro will even include it in its graphs. Many use a passive radiator to overcome this problem.

Now I build bass reflex enclosures and will continue to build them.
I even like the sound of them. All speaker boxes have there pros and cons. To Iinor this and pretend that one is perfect, still seems pretty narrow minded to me.

[kelticwizard]

Originally posted by Cameron Glendin
[B]I find it amusing that no one has taken the coke bottle test as yet! It only takes a few minutes of ones life to demonstrate how a port actually works.

Tried it years ago, that is why I didn't repeat the experiment. Blow across the opening and it emits a tone, which will be the tuning frequency of the bottle.

Originally posted by Cameron Glendin
So whether its 10, 15, or 42 miliseconds delay depending on driver/ port combo its fair to say it exists.....[Ignore] this and pretend that one is perfect, still seems pretty narrow minded to me.

Nobody is ignoring anything. Just pointing out the ported's strengths, as you point out Infinite Baffle's. You did make a mistatement about the delay being a fixed number, when it varies with frequency and possibly Thiele-Small alignment. Nevertheless, I went to some real pains to show that when the tuning is 12 Hz, it meets your statement of full frame delay, and when the tuning is 22 Hz, it meets your later criteria of half frame delay.

You made it clear you outfit cinemas for a profession, and while I am not familiar with cinema equipment we all know movie soundtracks go way down. So essentially my effort was to show that while your 1/24 second claim as made was erroneous, down in the frequencies you likely work with in cinema sound it probably works out to be the case. When one makes that effort on behalf of one's "opponent" in a discussion, that hardly constitutes narrow-minedness.

Cameron, you have to learn how to take "Yes" for an answer.

[Cameron Glendin]

Sorry Kelticwizard

I must admit that your posts are very giving, I tend to take more from diy audio than give. Infact I'm a bit of a fan of your posts.

My grumpy mood is more from other issues today and I'm sorry, It is clear to me that I am being close minded!

The coke bottle test is more about Spl, I find no matter how hard I blow, the tone remains a fairly consistant in sound level. I imagine that this is also an issue with ported designs

The biggest regret I have was not trying the infinate baffle style of box earlier. Altec were the only company that suplied cinemas with this style of speaker, and only briefly before they folded. Cinemas have a reputation of big boomy bass, it for me was interesting to hear a cinema with a lean tight bass instead. Lower bass notes were re enforced by the placement of the speaker boxes into a double layered 30 ft x 14ft gyprock wall with sound dampening material on the front, so I did not find need for additional drivers to boost efficiency of the low end just a boost from the eq.

What I enjoyed about it was the added realism that was created this way. Atmosphere tracks and music were what benifited the most. Ironically the first film released after the upgrade was a jazz film called Calle 51 which featured a lot of double bass in its music! I even did a side by side comparison of a ported subwoofer and the sealed one. Was it a huge difference? No, but to me it was worth the effort. The sealed sub ultimatly stayed.

[Geoff H]

"The coke bottle test is more about Spl, I find no matter how hard I blow, the tone remains a fairly consistant in sound level. I imagine that this is also an issue with ported designs"

This could be why many sealed cabs appear (to me anyway) to have a better dynamic range than ported cabs. Not all though. A7s for example. Foam versus cloth surrounds come into the mix as well.

[auplater]

having run both sealed and vented designs in my larg-ish room (25' x 30' cathedral 2 story with open back walls and such) for over a decade, I can say with 100% certainty room placement makes a DRAMATIC effect on mid and loowww bass.

If I drive all modes with corner placement, I get HUGE boomy bass with all sorts of peaks and troughs thru the room, spaced pretty much as expected from simple geometrical calcs, and, while at some locations the sound is awesome (even separated the sheetrock seams alonfg the ceiling) at any normal listening location the sound quality is so-so.

By placing 2 subs midway along the shorter wall, I've reached a compromise with smooth even bass, well articulated on string and slap bass, with quality apparent over quantity, and a dramatic reduction in excited room mode irregularities.

This with either sealed or vented subs, so at least in my environment, the arguments about sound quality of one vs. the other seem a bit academic.

John L.