bass nulls

[JLC7]

Hi guys, I'm trying to troubleshoot bass issues with one of my speakers. Here's a picture of one of them. What I'm trying to do is figure out if this is a room/placement issue and if not, if it is an issue with the speakers themselves.




This is a 1m farfield measurement of two speakers in the same room. Room placement is similar. The better looking curve is from a small two way 6.5" bookshelf speaker I made. The red curve is from the transmission line floorstander I fixed up (recapped, but mostly a cosmetic makeover). The floorstanders have a somewhat odd design. The best I can describe them as is that they're a 2.5 way speaker. One 6.5 woofer with coaxial tweeter on the inner baffle, and another 8" woofer with coax tweeter on the outer baffle. They are ported on the front.



I've moved them along the wall they are placed on, but the bass response doesn't seem to change to my ear. (i.e kick drums are anemic due to the bass nulls, I don't need my mic to tell me that much.) I've moved around the room and the only places where the bass picks up to my ear are along the walls of the room and in the corners.

I have the baby brother speakers of the floorstanders (with a similar design but slightly smaller) and they do not appear to have this problem in this room.

Any suggestions?

[MaVo]

Cant be room modes when another speaker doesnt suffer from the same problem at the same location. I am not too familiar with TL speakers, but since they have some common ground with tapped horns, those nulls could be the speaker.

Apart from that i am amazed by the smooth bass response of the small speaker. My room is +/- 15 dB without EQ and multiple sub approach.

[Ron E]

Nulls are a feature of TL type resonant pipes. More stuffing density may improve things - try to model the speaker in MJK's mathcad sheets.

[JLC7]

That's interesting to know. So it might actually be characteristic of the speaker's design. Why is that? Does increased stuffing density move the nulls or does it actually lessen them?

Is it normal for the nulls to be octaves at a time though? Seems excessive to me.

mav, the bookshelf speakers were measured, modeled, and designed in this very room. Maybe that's why they integrate so well?

[nigelwright7557]

I would have said it was the speaker.

Having seen numerous pdf's of speakers I know some are full of peaks and troughs.

[MaVo]

Applying the concept of damping from room modes to speakers, i think stuffing could reduce the q of the nulls, making them less deep. It could also have a totally undesired effect. I agree with the "simulate the speaker" advice.

[gedlee]

Originally posted by JLC7
So it might actually be characteristic of the speaker's design. Why is that? Does increased stuffing density move the nulls or does it actually lessen them?

Is it normal for the nulls to be octaves at a time though? Seems excessive to me.

TL will have large peaks and dips if there is not a lot of sound absorption in the line. This is due to internal standing was in the TL. These are always assumed to be damped away, but they always exist, and what you show is classic.

[JLC7]

So can anything be done to alleviate the standing waves?

The speaker features a folded transmission line. It travels from the chamber behind the woofers down to the bottom of the speaker and then back up to the ports. The only damping material in the speaker is located in the chamber behind the woofers and down the transmission line until the point where it begins to fold back up. From that point on, there is no damping material. This is a good half of the TL without stuffing.

Would added damping material here help with the standing waves?

[gedlee]

Yes, as much damping as you can get without totally eliminating the back wave is what you want. This can be hard to guess at, but clearly NO damping in 1/2 of the line is not right.

[Scottmoose]

With respect, I'm not sure I'd agree that nulls are always present in TLs. Badly designed ones, certainly.

Anyway, as Earl points out, what you've got there are classic line harmonic modes. The problem with TLs is that the term itself has become a millstone -very few cabinets that are called TLs actually are transmission lines, and many of the characteristics claimed for them are incorrect.

In this case, you can increase the stuffing density to help absorb the unwanted line harmonics, but unfortunately, it will also start to absorb the cabinet 1st mode (fundamental resonance) too, weaking the LF gain. Technically, this will move it closer to actually being an acoustic transmission line, which are essentially extremely well damped, non-resonant cabinets, designed to provide the flattest possible impedance to the amplifier (the electrical TL is where the name comes from). But that's at odds with what many people think are TLs and therefore their reasons for using them. Definitions -they've always caused problems. If you want a compromise solution, I'd try stuffing the first half or 2/3 of the line with ~0.75lbs ft^3 of dacron or similar, which won't affect the fundamental as much & then adjust to suit yourself from there.