D'Appolito design

[Nath]

OK, another newbie is trouble.

Looking at a D'Appolito design and looking at using two Peerless 830881 in a vented box.

But my problem is I'm not sure if what happens to the size and port if the two woofers spare the same enclosure?

For one woofer;

Volume: 4L
Port length: 19cm
Port Diameter: 5cm

For two woofers;

???


Befriend a newbie.

[Scottmoose]

Vas doubles, and so does driver surface area. If you wire the drivers in parallel, then impedence halves, if you wire them in series, it doubles. Sound Pressure Levels (SPL) +6db if wired in parallel, unchanged if in series.

OK, igoring the technical stuff, the practical upshot is you need to double your cabinet volume, and recalculate your port dimensions based on that, and the above-mentioned factors.

Best
Scott

[tcpip]

Though Scottmoose has just about closed the question with his answer, I'll add a couple of small bits. In case it isn't obvious from his answer, the Qts/Qes/Qms values remain unchanged.

Secondly, you may consider not trying to calculate "equivalent T/S parameters" for your pair. You may consider just using a good box modelling software (I use and strongly recommend Unibox), and then tell the program that you're using two drivers in series or parallel or whatever. In that case, you will have to enter the T/S parameters of just one driver, and the program will figure out what the equivalent is for the pair. And then you can just do your modelling in it and arrive at the box volume, duct dimensions, etc. That's what I did with my MTM design.

Thirdly, read Vance Dickason. He talks about what happens when you combine two drivers in one box. I learnt my basics there. His book also opens one's thinking about driver pairs because he talks about unusual combinations too, like isobarik enclosures, etc. (In that one, the Vas actually halves, not doubles.)

[Nath]

Don't worry peoples. Got a copy of Bassbox Pro 6 and I've answered my own question.

It's what I guessed. If you double the volume and leave the port the same, the curve is about the same.

Also have a copy of "The Loudspeaker Design Cookbook". A little appointed, don't feel it's worth what it costs.

Doesn't answer really beginner questions like why do you radius/chamber this inside of speaker port. To let the speaker "breath". Took a bit to find the answer.

Now it off to attack so MDF with the circular saw...

buahahahahahah

[Nath]

PS. To the moderator who will review this post. You guys really should add an edit function.

[Cal Weldon]

Hi Nath,

There is an edit function at the bottom of the window. It remains active for about 1/2 hour after your post.

[bjorno]

Originally posted by Nath
Don't worry peoples. Got a copy of Bassbox Pro 6 and I've answered my own question.

It's what I guessed. If you double the volume and leave the port the same, the curve is about the same.........

buahahahahahah

I believe with the given port length, if you double the volume to accommodate two drivers, you should also multiply the port area by two in order to preserve the maximum air velocity of the port (low duct noise). In your case, use a 7 cm port that is close enough.

I also recommend you not to use this D'Appolito speaker lower than about 150 Hz due to high group delay at about 70 Hz (15 mS) if you want squeeze out high performance and low distortion.

B

[Nath]

Thanks guys,

This is where I got the idea from.

http://www.hardwareanalysis.com/content/article/1815/

They're going to be dedicated surround sound speakers. Building separate 3-way stereo speakers.

I've thought about the velocity through the port. Read that it should be under 35m/s. Not sure how to calculate it though and I don't think BassBox Pro can.

But from what I've modelled in Bassbox, if you alter the size of the port you change the tuning regardless of whether there is one or two drivers.

Also if I use a corner frequency of 150Hz won't there be too much of a gap between them and the subwoofer?

[Scottmoose]

So long as it stays under the Mach is thould be OK. I doubt it'll be a problem, but possibly best rear-panel mounted to be sure, providing you've got about 6in room behind the speakers. Given that they're not going low, and they're hardly large speakers, activating room-modes shouldn't be an issue.

[bjorno]

Quote:But from what I've modelled in Bassbox, if you alter the size of the port you change the tuning regardless of whether there is one or two drivers.

Nath, First, excuse me, making a false assumption; I thought your speakers were meant for frontal use.
Now later, you told us they are mainly intended for surround sound.

This less critical use can obviously change the sonic appearance from the speakers if they are driven at a lower power.

The choice of drivers is of course yours, and I think you fell for the driver appearance, the promising FR curve, application data supplied from Peerless and that ‘HDS Exclusive’ really sounds appealing.

My assumption was based on maintaining Peerless good design and thus keeping the same margin to avoid port noise and other artefacts for a new port with otherwise unknown port design properties.

I calculated possible mid band SPL to be about 104 dB for two paralleled drivers and xmax for a frequency about one octave below box tuning frequency fb. Then I used the common practice to choose a Port-area/Sd equal or exceeding 1/3 for the 8 L box.

But if the same port diameter is kept, then this is equivalent to about a 3, 5 cm port for a single driver with a quotient of 1/5.4, which is considered a to low value, also obviously by Peerless who recommends a ratio of about 1/2.7 for one driver in a 4 L box.

Maybe most people don’t care and think this is over doing, but I always prefer to be on the safe side at least for home audio unless advance nozzles are used.
For example read www.haliaetus.com/AES_112th.pdf and www.haliaetus.com/AES_116th.pdf if you are interested to exceed 0.1 Mach as the maximum recommended air velocity for a minimum port seize.

‘Also if I use a corner frequency of 150Hz won't there be too much of a gap between them and the subwoofer?`

For your case, it’s only necessary if you incorporate a larger seized woofer in the same cabinet as in a three-way system for frontal use, but for surround speakers I don’t think it matters.

B