Trying to understand speaker position...

[naturallmystic]

Hi folks, I am a certified lurker until today.

Background: I am about to undertake a speaker building project using the Greg Monfort (thanks Greg) Alpair 10 cabinet design. It seems straightforward enough and the design appeals to me. The driver I have chosen for this the MarkAudion Alpair 10 generation 2. The cabinets will be constructed of 3/4" baltic birch ply with a zebrawood veneer. I also intend to put a flared base to give the speakers a wider stance for stability reasons.

Question: As you can see by the attached drawing provided by Greg Monfort the driver is placed about 2/3 up from the bottom of the cabinet. I would like to have the driver placed just 1.5 inches from the top of the cabinet if, everything else being equal, it doesn't affect the sound/performance of the speaker.

This change is just for personal taste and if it makes a difference I will be driving these with the Elekit TU879 S, an 8.5 watt @ 8ohms tube amp.

Your thoughts kind members would be most appreciated.
Also I did look for related information here at diyaudio but have come up empty handed.

 

[naturallmystic]

Have I done something inappropriate ?

 

[Pano]

Give it time. The forum is slowest on the weekends.

 

[Keriwena]

Seeing as what you've got there is a transmission line, moving the speaker will impact the sound.

This is a good overview, and includes in-depth references:

Quarter_Wave_Resonators

 

[naturallmystic]

Keriwena, Thanks for some information. I am not sure I understand how to make adjustments though.

 

[zman01]

Nauralmystic, I am no expert, but I think it will have an impact - trust that Greg Monfort made that exact placement of driver becasue it was optimal for his design.

If you prefer the driver a bit higher from the floor, using spikes you will gain 0.5" on height.

Also are you sure that Alpair 10.2 will work well in this cabinet? AFAIK Alpair 10.2 is new (2nd Gen) and improved version of Alpair 10. The plans are for Alpair 10, and using Alpair 10.2 might need slightly different considerations. If you already have and answer to that, great. If not you can post the question at the Markaudio section of the forums.

 

[naturallmystic]

zman01, You make an excellent point. I had not considered that there are differences between the 10 and 10.2. I have posted a more general question about which driver to select in the MarkAudio section. Once I get some response to it I will address the cabinet design more specifically.

THanks to all who have taken their time to respond.

 

[AEIOU]

I bet that you can move the driver higher without completely impacting the design. Typically you just need to adjust the amount of stuffing.
Someone took a TABAQ and moved the driver higher and it worked just fine.
What you need though, is to model you design. Someone who has Martin Kings software could model it for you.

 

[planet10]

The driver offset is important to the design. It kills the 1st objectionable harmonic, allowing for less damping and a net gain in bass reinforcement. The offset is not ~2/3 but likely carefully placed to within fractions of an inch.

If you end load the driver you will need more damping and will end up with less bass or more ripple.

ML-TLs are typically fairly tolerent, A10.2 will probably work just fine (althou starting from scratch you would probably end up with something slightly different)

dave

 

[Squeak]

Consider doing a ML-TQWT if you want the driver at sitting down ear hight (1 to 1.10 metres, is a good compromise). A slanted wall within the cap will give you the driver displacement effect.

 

[naturallmystic]

AEIOU, planet 10, and squeak thanks for your posts. Though I am willing to consider many designs I don't actually have the knowledge to create one from scratch so I am bound to use designs other have taken the time to create and trouble shoot. Perhaps I should be less concerned with looks and find a proven design that appeals to me. I will post what I wanted to build originally before I started this thread once I draw it up in photoshop. Perhaps there is a member who would find it interesting and could model it to see if it is worth pursuing. THen again maybe modeling is a difficult process and I am asking too much. Be back with an image of some sort soon....

 

[Squeak]

Without being sure, you could probably find a ML-TQWT design that fits your needs and drive specs pretty closely. And pretty closely is good enough in most cases, because driver differences, room placement and stuffing will play a bigger part in sound than tight measure tolerances.

 

[naturallmystic]

I just looked at the pencil plans for the alpair 10 and 10.2 and zman01's point becomes clear, that being, "the plans are for Alpair 10, and using Alpair 10.2 might need slightly different considerations". The cabinet volume for the 10.2 is 4153.3 cubic inches. Even taking out the battens to allow the back to be removed it is 4104.5 cubic inches. The pencil plans for the alpair 10 has an internal volume of 2999.875 cubic inches. That's approximately an 1153 cubic inch difference.

 

[planet10]

Checking your math:
Pencil 10,2 extends the depth by 43mm to yield an extra 7.83 litres on the originals 59.88 litres. 13% increase in volume

(4153.3-2999.9)/2999.9 =38.4%

dave

 

[naturallmystic]

Thanks for the math check.