Bass Reflex port for Usher 701 kit

[nippontiger]

I am in the process of building the Usher 701 kit, however, here in Japan, I am unable to source the exact bass reflex ports specified in the kit. The kit specifies a 6 1/4 inch long port with a 2 inch internal diameter. However, this seems to be complicated by the fact that a significant part of this length is taken up by large internal and external flares. Here are the details:
http://www.partsexpress.com/projects...1/assembly.cfm
I have been able to source some 49 mm ID ports with an external 90 mm flare, no internal flare - this was the nearest I could get. Would anyone be able to suggest a suitable port length for this port to give me a similar tuning to the port in the kit?
Thanks in advance for any advice!

[infinia]

Hi

Flare port tutorial.
http://www.subwoofer-builder.com/port-flares.htm
Hope this helps.


BTW the Usher 701 is very nice system to build.
regards,

edit
see thread
Port calculations

I like Unibox (free DL) excel SS for modeling ports.

[nippontiger]

thank you for the reply.
I used the formula suggested here:
http://www.diysubwoofers.org/misc/portcal.htm
to calculate the length of port I will need to achieve the tuning frequency of 36.7 hz using one flare and one free end with a 48 mm ID port. I got the value of 15.46 cm.
My port may be a little bit thinner than the one used in the kit, but hopefully I wont get any port turbulence because I probably wont be listening at very high volumes.

[infinia]

Hi
It (Lv) should be slightly shorter than the stock 52 mm. If you have access to a router you could install the flared end inside the enclosure and end up with both ends flared. Flaring boths ends is more important than a (1 - 3/52) change in dia.
cheers,

[nippontiger]

I didnt realise the internal flare was so important. I should be able to buy another couple of flare parts and attach them to some tubing I managed to pick up from the DIY store today. Hopefully this will give me something pretty close to that supplied with the actual kit, with both internal and external flares. Im looking forward to putting it all together - all going well I should get my first listen in a week or two!

[infinia]

Hi
I'm sure you will be very happy with the result, I know I would, as it's my ideal 2-way DIY project.
Happy listening.

[Jay_WJ]

Originally posted by nippontiger
I am in the process of building the Usher 701 kit, however, here in Japan, I am unable to source the exact bass reflex ports specified in the kit. The kit specifies a 6 1/4 inch long port with a 2 inch internal diameter. However, this seems to be complicated by the fact that a significant part of this length is taken up by large internal and external flares. Here are the details:
http://www.partsexpress.com/projects...1/assembly.cfm
I have been able to source some 49 mm ID ports with an external 90 mm flare, no internal flare - this was the nearest I could get. Would anyone be able to suggest a suitable port length for this port to give me a similar tuning to the port in the kit?
Thanks in advance for any advice!

If you go from 2" (50.8 mm) ID to 48 mm ID port, you need to shorten its length by about 10 % to achieve the same tuning frequency. But 10% difference in port length will have a negligible effect. Note that the Usher 701 kit's port spec is not a single right one. You can freely experiment with its port length to find one that suits your taste. According to my Unibox sim, the kit's Fb is close to 40 Hz. I'd try lower Fb by using a longer port.

Also, depending on your preferred tonal balance and speaker placement in your room, the stock 701 kit crossover may sound bright. Take a look at my following web page:

http://www.geocities.com/woove99/Spkrbldg/Usher_701.htm

You may want to try the original crossover and then my suggested modification to choose the better one to your ear. The mod will be very easy.

-jAy

[nippontiger]

Thanks for the advice. I think I will try a slightly longer port initially and then shorten if necessary. I have a small room - acutually I think these speakers are a bit big for my room - and the speakers will be quite close to the wall, so I think I will probably need to cut down the bass response if anything. I will try the crossover mod if they sound bright - seems quite a simple modification.