Eminence Alpha 15" has bass in OB? - diyAudio
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Old 26th September 2012, 12:11 PM   #1
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Default Eminence Alpha 15" has bass in OB?

Hi,

I know that eminence alpha 15" is one of the most popular speaker for OB.

I was looking around for a potential woofer for a new OB project so why not start looking at the alphas.

When I look at the datasheet, I got a shock.

http://www.parts-express.com/pdf/290-407s.pdf

The bass rolls off at 100Hz and by the time it reaches 30Hz, it's already down by 20dB...

So how can a speaker like that be good for bass?

One can easily get a 6" woofer with better bass response than that, so what gives.

According to the datasheet, it should sound horrible as a woofer, much less an OB one.

Anybody care to comment on that?

Oon
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Old 26th September 2012, 03:26 PM   #2
kec is offline kec  United States
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Have you read MJK's white paper?

http://www.quarter-wave.com/OBs/OB_Design.pdf
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Old 26th September 2012, 09:05 PM   #3
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Things are more subtle and complicated.
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Old 27th September 2012, 12:53 AM   #4
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Before we get any further on this topic, do most people introduce a bass boost to the speaker either by passive means or active means by biamping?

From what i know in loudspeaker theory. A bass reflex will introduce a hump and can move the -3db point lower. Sealed box if done properly can introduce a steeper slope. Open baffle just cuts the bass. I believe the measurement is done in infinite baffle. So unlikely the bass performance in open baffle will be better.

Yet i am aware that many people have measured their OB system with this speaker with frequency response down to 40 hz or so. So what's the catch, seems to contradict manufacturer's datasheet...

Oon
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Old 27th September 2012, 01:22 AM   #5
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Quote:
Originally Posted by oon_the_kid View Post
When I look at the datasheet, I got a shock.

http://www.parts-express.com/pdf/290-407s.pdf

The bass rolls off at 100Hz and by the time it reaches 30Hz, it's already down by 20dB...

So how can a speaker like that be good for bass?

One can easily get a 6" woofer with better bass response than that, so what gives.

According to the datasheet, it should sound horrible as a woofer, much less an OB one.

Anybody care to comment on that?

Oon
A few things

- With varying baffle width you change the frequency response of a dipole.
- A 15" barely has to move to produce sound
- 97db/w/m
- qts = 1.26

NOw what does that mean?

It means

- This driver won't have to move. Less excursion can be better than more excursion, even if it takes more power to do it.
- the high qts will affect the frequency response even in an OB. So if there are dipole losses, they are "offset" by the high Q "gains"
- We don't need 97db @ 1m. Let's say you have an 85db midrange. Now when you low-pass the 97db/w/m woofer, you have 12db of headroom to "offset" the dipole losses again.

Remember, if it's 97db @ 200hz, and -12db @ 50hz in an infinite baffle - That's still 85db of IB efficiency you have @ 50hz. That's not bad for an OB, especially since higher frequencies have more bass content. Some other 15" drivers may look "flatter" but are less efficient 80hz to 200hz. That means more EQ boost is needed above 50hz. If you have the same efficiency at 50hz, and more efficiency above 50hz, i say keep it :P.

...Now consider using two in parallel.

Last edited by RockLeeEV; 27th September 2012 at 01:28 AM.
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Old 27th September 2012, 06:08 AM   #6
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Hi Rock Lee,

If you look at the graph from the link I put up in the first post, sensitivity is only about 73dB/W at 30Hz.... even though it is 92dB at 100Hz. That is why I can't get why it is possible to get any bass from that speaker.

Quite insufficient to match most "normal speakers".

MY other guess is that people might be driving it with valve amps, since there is a fairly high output impedance, the source become a little bit of a current source. I think the sharp rolloff is the high increase in impedance around Fs. Increasing to about 40 ohms. With a current source like amp, the increase in impedance is not felt so much. in other words the voltage will be automatically be boosted to compensate for it and hence producing more bass.

Any thoughts?

Oon
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Old 27th September 2012, 06:48 AM   #7
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Quote:
Originally Posted by oon_the_kid View Post
If you look at the graph from the link I put up in the first post, sensitivity is only about 73dB/W at 30Hz.... even though it is 92dB at 100Hz. That is why I can't get why it is possible to get any bass from that speaker.
That graph itself doesn't seem right to my eyes in the bass, which was probably incorrectly spliced around 300hz - that looks like a very smooth Q < .5 rolloff of a different driver.

A Q = 1.4 /Fs = 40hz woofer like the alpha 15 would have a rather different IB rolloff. Just run the ts paramters through any sealed box sim for an extremely large sealed box ie 15000L - and you'll see that response should peak near free air resonance 40hz not have a shallow rolloff.

Last edited by RockLeeEV; 27th September 2012 at 06:51 AM.
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Old 27th September 2012, 08:31 AM   #8
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Hi Rock Lee,

If I want a more accurate source, this is the direct one from Eminence..

http://www.eminence.com/pdf/Alpha_15A.pdf

I know this doesn't make any sense, which is why I started this thread, but this is the manufacturer's datasheet from their website.

The Deltapro 15 has a similar graphical response.... strange isn't it..

http://www.eminence.com/pdf/Delta_Pro_15A.pdf

So at this moment I am still tryoing to figure out why the world of difference between what the forum apparently observes and the supplied datasheet...

Oon
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Old 27th September 2012, 09:15 AM   #9
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by oon_the_kid View Post
So at this moment I am still tryoing to figure out why the world of difference between what the forum apparently observes and the supplied datasheet...
Look at these simulations which start from the published TSP: Open Baffle vs. Closed/Vented Box Speakers

From what I know they are more in line with reality than your deductions from the manufacturers data sheet. This will not keep many people from finding the remaining efficiency unacceptable.
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Old 27th September 2012, 09:35 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

Its best to assume all Eminence measurements below say 100Hz are inaccurate.

rgds, sreten.
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