Newbie's Question

[Jay_WJ]

Hello,

I came up with a question when I was reading Ray Alden's "Speaker Building 201" and Vance Dickerson's LDC (7th Edition). This might be too simple a question for experts here, but I simply can't understand the answer the books provide.

Why does the sensitivity (db/2.83V) stay the same (i.e., 0 db change) in a double woofer system with woofers connected in a series, compared to a single woofer system?

Why not -3 db loss? The single woofer system uses 1W when the input voltage is 2.83V if the woofer's impedance is 8 ohm. According to my calculation, this implies that there is a current flow of 0.3534 amp. In a serial double woofer system, the current decreases to half of this due to the doubled impedance at the same 2.83V, and in turn the total watts consumed by the two woofers also decrease to a half (by the formula: W=V*I). Then, this turns into 10*log(0.5/1) = -3 db.

Am I wrong?

[Davey]

You are correct that current flow is cut in half (3db reduction in power output) and thereby the excursion of each driver relative to a single woofer. But remember, you now have double the radiating area so you get the 3db "back" for a net change in SPL of 0db.

This holds true when the source is small relative to the wavelengths produced and they are located close to each other and effectively couple.

Cheers,

Davey.

[Rudy Jakubin]

It's not a bad thing. It becomes useful in designs like these in the picture.
4 - 8 ohm 7" woofers in series parallel. Let's say 86 db each for a total of 92 db (there's baffle step losses to consider 'bout 3-4 db).
The swept volume of 4 - 7 inchers = a 12" woofer approximately.
The benefit comes from the small light weight cones vs. a large heavy cone.
The subjective feel & sound seems faster. I call it slam!

[ThomasW]

The benefit comes from the small light weight cones vs. a large heavy cone.
The subjective feel & sound seems faster. I call it slam!

....... :huh:

[cotdt]

is it true...? smaller cones more bass slam?

[Davey]

No, smaller coned fast/quick bass is an audiophile urban legend.....or wishful thinking....or subjective evaluation trumping the laws of physics.

Recall the basic equation Force = Mass X Acceleration. If you've got a larger/heavier cone you can still maintain the same acceleration....you just need more force.

Also, Rudy's example of swept volume is not necessarily correct since he was referring to cone area and not incorporating the excursion capability of the drivers. Area times excursion yields the SPL capability of a particular driver.

Cheers,

Davey.

[AJINFLA]

is it true...? smaller cones more bass slam?

LOL. Yes cotdt, haven't you been to a rock concert yet? You know, where they have giant stacks of 6" woofers slammin' away.... :W

cheers,

AJ

[AJINFLA]

I should also add that "slam" is the accurate recreation of "speaker box" sound.
If you want the sound created by an amplified speaker box - as you would hear at a rock concert, stoplight, etc., then a box speaker is the way to go in terms of accuracy.
If however, you go to listen to live stand up bass or drum set, unamplified, you hear nothing that resembles a speaker box. There is no "slam" or "boom".
This is where the open baffle bass concept reflects greater accuracy. When you are trying to recreate real instruments, rather than an amplified speaker box "slam" sound.
Different strokes for different folks I suppose.

Cheers,

AJ

[Jim Holtz]

Hmmmm....

I should also add that "slam" is the accurate recreation of "speaker box" sound.
If you want the sound created by an amplified speaker box - as you would hear at a rock concert, stoplight, etc., then a box speaker is the way to go in terms of accuracy.
If however, you go to listen to live stand up bass or drum set, unamplified, you hear nothing that resembles a speaker box. There is no "slam" or "boom".
This is where the open baffle bass concept reflects greater accuracy. When you are trying to recreate real instruments, rather than an amplified speaker box "slam" sound.
Different strokes for different folks I suppose.

Cheers,

AJ

AJ

Interesting comments. If I understand you correctly, in your *opinion* any speaker based on a normal cabinet is inaccurate and only di-poles can accurately recreate a live performance?

Am I understanding your opinion correctly? This is your opinion and not based on factual information, correct? :roll:

Cheers back at ya...

Jim

[AJINFLA]

Nope. I said box loudspeakers will more accurately reproduce what you hear at a rock concert - which is very much a "live" performance. "Slam". And what are you hearing at a rock concert? Amplified sound. Playing through...speaker boxes. Surely you've been to a concert Jim. You know the sound.
Now strip away the amplifier and the box. Listen to a stand up acoustic bass from 10ft away. Listen to a drum set.
The open baffle/dipole becomes the more accurate reproducer of this. That's what I said. Again :W .
Keep in mind, I've always liked box bass. Below my dipoles .

cheers,

AJ

p.s. what did Dr. Geddes use to demo his (box) Summas? :W

[Jim Holtz]

Nope. I said box loudspeakers will more accurately reproduce what you hear at a rock concert - which is very much "live". "Slam". And what are you hearing at a rock concert? Amplified sound. Playing through...speaker boxes. Surely you've been to a concert Jim. You know the sound.
Now strip away the amplifier and the box. Listen to a stand up acoustic bass from 10ft away. Listen to a drum set.
The open baffle/dipole becomes the more accurate reproducer of this. That's what I said. Again :W .
Keep in mind, I've always liked box bass. Below my dipoles .

cheers,

AJ

AJ,

Yep, I've been out of the house a time or two and managed to make it to a few concerts in my life. BTW, I never use concerts as a reference because of the reasons you mentioned.

The reason for my response, was that you presented your comments as fact rather than opinion. I was simply trying to get you to agree that it's your opinion and present it in that manner so the folks that are newer to the hobbie will understand that you were just presenting your view.

I can not agree with you that the only speakers that accurately reproduce unamplified stand up bass or drums are di-poles. I like di-poles and would agree that they have many fine qualities. That's as far as my agreement goes, however. They also have a number of short comings inherent to the design that box speakers do much better. Di-poles my be the best to your ears, but not mine. I'm not willing to give up the many benefits of a well designed box speaker (line arrays). This is my opinion of course.

I think John K. said it best. Di-poles bring the performance to you. Line source speakers take to to the performance. Different strokes for different folks I guess. What sounds more accurate to you is a personal decision, which seems to becoming more and more like religion and politics.

Best regards,

Jim

[Rudy Jakubin]

Why not -3 db loss?

I didn't mean to hijaak Jay_WJ thread but he didn't get the correct answer.
Baffle Step Correction has to be equated to Davey's reply.

I liked Davey's reply to my slam comment.

Recall the basic equation Force = Mass X Acceleration. If you've got a larger/heavier cone you can still maintain the same acceleration....you just need more force.

Also, Rudy's example of swept volume is not necessarily correct since he was referring to cone area and not incorporating the excursion capability of the drivers. Area times excursion yields the SPL capability of a particular driver.

Sensitivity does increase with more drivers and vertically aligned drivers refocus the polar resonse to that of a line array.
Slam comes in the frequency area of 80 to 120 hz and this is where larger coned drivers 10" and above don't impress me much.
Listening to standup bass and lower piano keys the reverberation from the sound boards are augmented by the harmonics, hence the feel.
AJINFLA is probably right. A dipole would recreate the standup bass more realistically.

[Davey]

It's interesting......I actually see it the opposite of Jim and John K. I think dipoles do a better job of taking you to the performance and monkey coffins might do a better job of bringing the performance to your room. My opinion of course.

We're now completely off the topic of Jay's original question.....which I hoped I answered. He actually didn't mention anything about baffle-step correction or line-array "focusing" in his original question. It was just a simple question regarding the SPL of two drivers in series....which I answered, correctly.


Anyways, you don't have to believe me.....Here's a quote from SL's website:

"Note that piston velocity and displacement are proportional to each other and both are directly related to the current through the voice coil. With two identical drivers connected in series, piston displacement and velocity decrease to 1/2, but the piston area doubles, which leaves the sound pressure and radiated power unchanged (0 dB) compared to a single driver connected to the same voltage. Electrical power dissipation is now 1/2 (-3 dB) and again the power conversion efficiency has doubled."

http://www.linkwitzlab.com/faq.htm#Q21

Cheers,

Davey.

[AJINFLA]

Jim

for the sake of clarity, let me quote myself, then your translation.

I should also add that "slam" is the accurate recreation of "speaker box" sound.
If you want the sound created by an amplified speaker box - as you would hear at a rock concert, stoplight, etc., then a box speaker is the way to go in terms of accuracy.
If however, you go to listen to live stand up bass or drum set, unamplified, you hear nothing that resembles a speaker box. There is no "slam" or "boom".
This is where the open baffle bass concept reflects greater accuracy. When you are trying to recreate real instruments, rather than an amplified speaker box "slam" sound.

JH: If I understand you correctly, in your *opinion* any speaker based on a normal cabinet is inaccurate and only di-poles can accurately recreate a live performance?

Note that Jim has translated and simplified my words into "box inaccurate", "dipoles accurate". I guess thats what he wanted to read.

I said box loudspeakers will more accurately reproduce what you hear at a rock concert - which is very much a "live" performance. Now strip away the amplifier and the box. Listen to a stand up acoustic bass from 10ft away. Listen to a drum set.
The open baffle/dipole becomes the more accurate reproducer of this. That's what I said. Again.

JH: I can not agree with you that the only speakers that accurately reproduce unamplified stand up bass or drums are di-poles.

See the problem? That's why I'm not religious. I don't like reading only I want to see.

Cheers,

AJ

[Dennis H]

Efficiency formulas

Base efficiency (1 driver, 1W, 1m) = Base sensitivity (1 driver, 2.83V, 1m) + 10*log(nominal impedance / 8 ohms)

The effective efficiency and voltage sensitivity gains from running multiple drivers can be calculated as:

Efficiency gain = 10*log(Number Drivers)

Sensitivity gain = 10*log(8 ohms / nominal impedance of series-parallel combo)

Net efficiency of system = base efficiency + efficiency gain

Net sensitivity of system = base efficiency + efficiency gain + sensitivity gain


We now return you to your regularly scheduled political/religious debate.

[Jay_WJ]

Thanks, Dennis.
This is exactly what I wanted! Now I see how to calculate a multi-woofer system's sensitivity. If I simplify the formula with respect to a single driver's given sensitivity:

Net sensitivity of multi-driver system = base sensitivity (1 driver, 2.83V, 1m) + 10*log(Number of Drivers) + 10*log(driver nominal impedance / system nominal impedance)

This is clear now! Of course, this formula gives a theoretical number assuming no baffle step compensation, as Davey mentioned. Right?

[Jay_WJ]

BTW, is there a formula for baffle step compensation?

[HeatMiser]

BTW, is there a formula for baffle step compensation?

You can calculate the frequency as a function of the baffle width:

f3 = 115/baffle width (in meters)

or 380/baffle width (in feet)

http://www.trueaudio.com/st_diff1.htm

The classic solution asks for 6dB of compensation, but some prefer to allow for some expected bass reinforcement from the back wall (depending on the intended placement) and roll it into the same circuit, ending up with anywhere between 3 and 6 dB net. Matter of personal preference I suppose, although it seems many people would rather avoid a "thin" sounding speaker (not enough BSC) and might not mind a slightly "fat" sounding speaker (full BSC, to heck with boundary reinforcement).