DIY transmission lines - thin upper bass?

[audio_tony]

I have a pair of DIY built transmission lines ( 2 way ), based on Audax drive units.

I didn't build these myself, however I have changed the crossovers from the originals, which were generic off the shelf 2 way crossovers.

I have an issue that the upper bass seems a little thin.

I note on the frequency response graphs I produced, that there is a large 'suck out' in the 150hz region, which based on comparisons with the same drivers in a ported cabinet, does not exist.

Attached are graphs with the final graph from a Wharfedale 9.1 for reference.

The first graph is in the TL cabinet, the second graph is the same drivers and x-over in a conventional ported reflex cabinet (Proac 2.5 clone), the final graph is a Wharfedale 9.1 front ported standmounter.

I didn't use any real sophisticated measurement techniques.

I used "FuzzMeasure" to make these graphs, with a fairly decent microphone, and a Tapco firewire mic amp / digitizer.

So to my question!

How can I cure the "suck out"? I have experimented a little with damping in the transmission line, and also around the (rear) outlet.

I also realise that my room (I just measured in my lounge!) could have an effect on the lower frequency registers, but I would have thought that would only effect below 100hz or so?

I did move the speakers into an open space to perform the measurements - they were all well away from the wall and other objects.

Cabinet dimensions are:

H) 88cm
W) 21cm
D) 30cm

Rear 'port' (TL outlet)

H) 9cm
W) 13cm

Height from top of cab: 12cm

Woofer center is 64cm from bottom of cab.

[planet10]

The suck-out is likely caused by insufficient attenuation of the 1st unwanted line harmonic. These days ne usually uses a driver offset to kill this. Otherwise dampng needs to be increased (loosing extension).

Pictures or drawing?

dave

[audio_tony]

I've not had time to draw the enclosure as yet.

Here are some pics of the outside if it helps?

From what I have been able to ascertain (from feeling around the cabinet!) is described in the attached image.

This is something I have just "knocked up" in 15 minutes.

Removing the drive units doesn't give me any further access, and the cabinets are completely sealed.

I have been led to believe this is an Audax kit from the mid 80's but I'm not 100% certain.

[Michael Chua]

Originally Posted by audio_tony

I did move the speakers into an open space to perform the measurements - they were all well away from the wall and other objects.

Do a "Ground Plane Measurement".

[audio_tony]

Originally Posted by Michael Chua

Do a "Ground Plane Measurement".

I had to research that one - thanks to Google for the answer :-)

I'll try this at the weekend.

Do you expect the Ground plane measurement to stop the 'suckout'?

The problem for me is, I can hear this as a distinct 'thinness' to bass guitar and other instruments around that frequency.

[Michael Chua]

The "Ground Plane" measurement is not a treatment. It's simply another approach to taking measurements. Others are "Nearfield" and "Pit". All these methods have their own advantages.

The "Ground Plane" measurement is a good start to find out the cause of the "suck out". Give it a try, it's worth the effort.

I know what you mean about that "thiness". I find it very tiring to listen to music with that "suck out".