Hypothetical question on multiple Small woofers vs few larger woofers - diyAudio
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Old 3rd June 2010, 07:15 AM   #1
navin is offline navin  India
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Default Hypothetical question on multiple Small woofers vs few larger woofers

Of late I have had discussions with friends about moving a lot of air gently vs moving a lot of air vigourously. This stemmed from comparing modern small (10" and less) high BL and Mms woofers with huge motors and Xmax (12mm+) to the older larger (15" and more) woofers that did not have such large Xmax or motors (lower Le) but often sound more musical.

The arguement put forward by the high BL, Mms, Xmax camp was that the smaller woofers had better WAF. Point taken. WAF (we agreed) for apartment dwellers means a floorstanding loudspeaker that is less than 12" wide and occupies less than 1.5 sq. ft. of floor space (plinth and feet included).

We sorta also agreed that this WAF speaker's internal box volume (after bracing, etc..) would not exceed 4 cu. ft. If we were to fit a 3 way into this box the woofer system would have about 3 cu. ft.

So we compared 8", 10" and 15" woofers. A 10" being as big a woofer one can fit into a box that is 12" wide (atleast on the front baffle) and a 15" being the biggest woofer we can fit on a box 18" deep. Any wider or deeper and WAF would be compromised.

The most recent system we had built was the Zaphaudio SB12.3. John Krutke of Zaph Audio seems to like SB Acoustics so we decided to see how much bass we can produce from 3 cu. ft. using simple woofers from SB Acoustics. Obviously we did not have access to these woofers (we live in India).

Would the bass from a 15" woofer be more musical than from 2 10" or 3 8" woofers? The Sd of a 15" woofer is about the same as that of 2 10" or 3 8" woofers (650cm2).

Has anyone compared the 15", 10" or 8" woofers from SB Acoustics?
Madisound lists an F3 of 45Hz for the SB42, a F3 of 48Hz from the SB29 (in less than half the volume of the SB15) a similar F3 from the SB23 is also possible. Room gain would push the in room F3 to the mid 30s - adequate for almost all music at sane SPLS save some large scale organ works and maybe hip-hop.

SB Acoustics SB42FHC75-6 15" Woofer from Madisound

SB Acoustics SB29NRX75-6 10" Woofer from Madisound

SB Acoustics SB23NRXS45-8, 8" Woofer from Madisound

Now we are not related to SB Acoustics, I am sure SEAS's Prestige series or ScanSpeak's Discovery series woofers would perform similarly (neither SEAS nor ScanSpeak have a 15" in their portfolio though). We just chose SB Acoustics so that our discussions were based on real world drivers and not any hypothetical drivers.
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Last edited by navin; 3rd June 2010 at 07:18 AM.
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Old 3rd June 2010, 11:22 AM   #2
Loren42 is offline Loren42  United States
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Have a read from this link.

Pay attention to the section on polar dispersion and response. He goes through a pretty compelling explanation as to why larger is better at these frequencies.
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Old 3rd June 2010, 11:56 AM   #3
navin is offline navin  India
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Quote:
Originally Posted by Loren42 View Post
Have a read from this link.

Pay attention to the section on polar dispersion and response. He goes through a pretty compelling explanation as to why larger is better at these frequencies.
While that article does discuss the polar patterns and how it changes and even why we need 4 drivers to effectively and effciently cover 10 octaves it does not discuss if, how, and why a single woofer having an Sd of say 600cm2 should sound different from 3 similar woofers each having an Sd of say 200cm2 (total Sd being the same). After all with even mm of movement (Xmax) one is moving the same volume of air with 3 x 8" woofers as one is with 1 15" woofer right?

That said I think the author forgot about modern wide range drivers (Fostex, Feasterx, PHY, Visaton, and Lowther) as well as smaller 'fullranges' (Jordan, CSS, Alpair, etc..) when he discussed the '3 octave rule'. Many of these wide/full range drivers can be linear over 5 octaves (200-6kHz) needing only a subwoofer (3 octaves) and helper tweeter (2 octaves).
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Old 3rd June 2010, 12:41 PM   #4
Loren42 is offline Loren42  United States
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For one thing you assume that all three smaller woofers move the same. Theoretically, they might, but driver-to-driver variation will produce somewhat different results out of each. Ideally, they should be in independent chambers to minimize effects from each other.

Polar dispersion, both horizontally and vertically, is not something you can just throw off the table, either. It impacts power response.

When you add multiple drivers they interact with each other's dispersion pattern, which causes lobing.

You also introduce different diffraction patterns from multiple drivers and different baffle sizes, so you really do not have an apple to apples comparison.

Lastly, the large woofer is a different driver than the smaller one. Again, you are not comparing apples to apples, just combined cone area, which is just one attribute of very many for a loudspeaker.
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Old 3rd June 2010, 12:46 PM   #5
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I'm interested too by this question. I can say about multiply drivers
Pro
- Faster transient
- Better high frequency

Con
- Price
- surface used
- higher resonance
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Old 3rd June 2010, 01:01 PM   #6
SY is offline SY  United States
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Quote:
Originally Posted by Loren42 View Post
...driver-to-driver variation will produce somewhat different results out of each. Ideally, they should be in independent chambers to minimize effects from each other.
Why? Wouldn't having them in the same chamber couple them more closely and reduce the differences in the output (assuming that's even a problem)?
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Old 3rd June 2010, 01:06 PM   #7
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There's a very interesting thread about this by Geddes.
Loosely speaking, he finds that multiple subs at different positions sort of 'average out' the room modes leading to more balanced reproduction at any listening position. Although, beyond 4 woofers in a room, you get into diminishing returns.

jd
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Old 3rd June 2010, 01:07 PM   #8
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Quote:
Originally Posted by jerome69 View Post
I'm interested too by this question. I can say about multiply drivers
Pro
- Faster transient
[snip]
This is unlikely. The transient part of the music is reproduced by the tweeter. I don't see how the woofer can have any impact on that.

jd
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Old 3rd June 2010, 01:11 PM   #9
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by janneman View Post
There's a very interesting thread about this by Geddes.
Loosely speaking, he finds that multiple subs at different positions sort of 'average out' the room modes leading to more balanced reproduction at any listening position. Although, beyond 4 woofers in a room, you get into diminishing returns.

jd
If I am reading the original question correctly, it looks like this is multiple drivers in the same full-range enclosure, not subwoofers.

I agree with the Dr. Geddes on the multiple sub approach, but that wouldn't apply here.
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Old 3rd June 2010, 01:17 PM   #10
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by jerome69 View Post
I'm interested too by this question. I can say about multiply drivers
Pro
- Faster transient
- Better high frequency

Con
- Price
- surface used
- higher resonance
I think that the transient response is really a non issue for lower bass. You might loose some mid bass transient response with a large woofer, but that was one of the points for adapting a 4-way approach as pointed out by Lenard Audio.

You hit one of the reasons why a multiple bass driver approach would be preferred in the first post. That is aesthetics. It just depends where your requirements lie and what you are willing to trade off.
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