[net.audio] The physics of good sound a Bose speakers

pmr@drutx.UUCP (Rastocny) (04/03/85)

Subject: How/why do Bose speakers work?

"It's all done with mirrors!  And microprocessors use micromirrors."

Before my terminal died, I began ramblings on why Bose gets bass from a 4.5"
driver.  Someone said that they use long-throw drivers, and they do.  But to
achieve any kind of acoustic efficiency, you need a large piston area (the area
of the piston in square inches that is performing the electrical-to-mechanical
conversion).  Rule of thumb: the lower a driver's resonant frequency (Fs), the
lower the note it can reproduce (not always true, but let us assume that it is
for this explanation without getting into why).

Bose uses nine drivers to achieve an effective piston area equivalent to that
of about a standard 10" acoustic suspension driver.  In order to make a 4.5"
driver possess a low Fs, you can do two things: 1) increase the compliance of
the driver's suspension (make the suspension less stiff), and 2) increase the
mass of the cone.  This is fine, but you know you never get anything for
nothing!

Let's see what happens in each case.  When you increase the driver's compliance
you must also change the physical characteristics of the linear motor.  You
can increase the size of the magnetic gap where the voice coil operates.  This
is the best but most expen$ive solution and is therefore rarely used.

   \<---------- cone ---------->/ 
     \                        /	
       \                    /
	 \                /
	   \            /
	     \________/
	      |      | <------ Voice Coil and Former (bobbin)
	      |      |
	 ___  | ____ |   ___ 
	|\\\|.||\\\\||. |\\\|
	|\\\|.||\\\\||. |\\\|<----- Magnetic focusing gap
	|\\\|  |\\\\|   |\\\|
	|\|     |\\|      |\|
	|\|     |\\|      |\|
	|\|_____|\\|______|\|
	|\\\\\\\\\\\\\\\\\\\|
	|\\\\\\\\\\\\\\\\\\\|
	|\\\\\\\\\\\\\\\\\\\|
	|_______magnet______|

The other way around this dilemma is to make the voice coil longer so at least
some of it always is within the magnetic gap.  This is the most common solution
but it also reduces the efficiency of the driver by creating a magnetic field
outside of the magnetic gap.

   \<---------- cone ---------->/ 
     \                        /	
       \                    /
	 \                /
	   \            /
	     \________/
	     .|      |.<------ Voice Coil and Former (bobbin)
	 ___ .| ____ |.  ___
	|\\\|.||\\\\||. |\\\|<----- Magnetic focusing gap
	|\|  .| |\\| |.   |\|
	|\|     |\\|      |\|
	|\|     |\\|      |\|
	|\|_____|\\|______|\|
	|\\\\\\\\\\\\\\\\\\\|
	|_______magnet______|

Cone pistons also possess a physical limitation of reproducing wavelengths
shorter than their diameters.  (That's why good sounding tweeters have small
diameter pistons).  As mentioned in another article, as the wavelength of the
signal increases and approaches the diameter of the piston, the dispersion
pattern of the driver changes from a hemispherical pattern (well dispersed) to
a cardioid pattern (poorly dispersed).

			     ^
          ....               |
       .          .          |
   / .               .       |
||/ .                 .      |  dispersion pattern of wavelengths much larger
||\ .                 .      |	than the diameter of the piston
   \ .               .       |
       .          .          |
          ....               |
			     v


	        
   /   .  ........  .        ^
||/ .                   .    |  Dispersion pattern of wavelengths much smaller
||\ .                   .    |  than the diameter of the piston
   \   .  ........  .        v


Since there is a practical limit to the throw of a driver (especially a small
diameter driver), you must also increase its mass to again reduce the Fs.
Increasing its mass reduces the drivers transient response (f=ma).  (This is why
the best sounding tweeters are also low mass.)  Increasing the mass also
increases the power handling requirement of the voice coil.  Usually, increasing
the diameter of the voice coil (VC) is this solution (larger heat sink) but this
also increases the VC inductance and reduces its high-frequency reproducing
ability.  (This is why the best sounding tweeters have small VCs.)

Without getting into a detailed discussion of woofer alignments, basically Bose
compromised and chose not to reproduce deep base and settle for a roll-off
somewhere around 40Hz, and keep the driver's mass low for reasonable transient
response in the bass/midbass regions.

The trick Bose used to get around these problems is by building a speaker-
specific equalizer that electrically boosts the signal for the low and high
frequency roll-offs.  Again, there is a practical limit to the amount of
equalization you can apply to a driver in the power-handling limit of the VC.

scales: SPL (vertical);   Freq. (horiz)

      .................		SPL curve of a Bose speaker (no EQ)
    .                   .
  .                       .
.                           .





.                           .
  .                       .
    .                   .
      .................		EQ curve of Bose compensation unit




  .........................	Resulting SPL curve of Bose EQ on Bose speaker
.                           .


Dispersion is still a major problem with a 4.5" driver reproducing very high
frequencies.  Bose simply aimed eight of the drivers rearward and used the
natural surface reflections of the room to improve dispersion.


These are the major topics Bose speakers address.  There are many many more
interrelated and intricate problems in properly designing loudspeakers.  As
the song goes, "We've only just begun.."

		Yours for higher fidelity,
		Phil Rastocny
		AT&T-ISL
		ihnp4!drutx!pmr

don@umd5.UUCP (04/05/85)

> Subject: How/why do Bose speakers work?
> "It's all done with mirrors!  And microprocessors use micromirrors."
>
--   [ after reading what follows, I tend to believe it !]
> 
> Bose uses nine drivers to achieve an effective piston area equivalent to that
> of about a standard 10" acoustic suspension driver.
>
--   [ nice speaker graphics in his posting ]
> 
> Cone pistons also possess a physical limitation of reproducing wavelengths
> shorter than their diameters.  (That's why good sounding tweeters have small
> diameter pistons).  As mentioned in another article, as the wavelength of the
> signal increases and approaches the diameter of the piston, the dispersion
> pattern of the driver changes from a hemispherical pattern (well dispersed) to
> a cardioid pattern (poorly dispersed).
> 
> -Phil Rastocny

[]
OK, now just hold on here a cotton-picking minute!

   The only formula in Phil's posting is F=ma, and I buy his statement that
for a small mass the transient response of the speaker is better than for
a speaker with a large mass.
   This wavelength stuff he talks about has got me confused ...
A wavelength shorter than the speaker's effective 10 inch diameter is the
speaker's physical limitation you say?
   OK, Let us calculate the wavelength of a 20kHz wave for starters --
   (300,000 km per second) divided by (frequency in kHz) = 15,000 meters
This is approximately 9.32 MILES. Somehow I don't think this figures into
Phil's explanation very well, but let's go with it still --
   The limiting frequency at a 10 inch wavelength is:
   (300,000 km/s) divided by (0.254 m) = 1,180,000 kHz  (about 1.18 GHz !!)
Something is still wrong here! If Phil's explanation was correct, Bose 
speakers would either have to have a 9.32 mile diameter, or they would
still be good tranducers at 1.18 GHz (even your dog couldn't hear that!!).
Well, maybe I haven't punched holes in the explanation big enough to sail
the Nimitz through just yet -- How about waveguides?
If I model the 10 inch speaker as a circular waveguide, the cutoff wavelength
is 3.41 times the radius of the waveguide. Now to calculate:
   (300,000 km/s) divided by (3.41 times 0.127 m) = 693 MHz
Closer to 20 kHz, but still 30,000 times 20 kHz ... Now I don't profess to
know exactly what this dispersion mechanism that Phil is talking about really
is, but I seriously doubt the explanation that he posted.
   Any takers ?

-- 
-----------------------------------------------------------------------------
"Space, the final frontier .." Final, hell! It's the frontier of frontiers !!
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-==- IDIC -==-                                           (Thanks Bob!)

SPOKEN: Chris Sylvain
  ARPA: don@umd5.ARPA
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 CSNET: don@umd5
  UUCP: {seismo, rlgvax, allegra, brl-bmd, nrl-css}!umcp-cs!cvl!umd5!don

herbie@watdcsu.UUCP (Herb Chong [DCS]) (04/07/85)

In article <461@umd5.UUCP> don@umd5.UUCP writes:
>   OK, Let us calculate the wavelength of a 20kHz wave for starters --
>   (300,000 km per second) divided by (frequency in kHz) = 15,000 meters
>This is approximately 9.32 MILES. Somehow I don't think this figures into
>Phil's explanation very well, but let's go with it still --
try about 300 m/s.

>SPOKEN: Chris Sylvain

crandell@ut-sally.UUCP (Jim Crandell) (04/10/85)

> A wavelength shorter than the speaker's effective 10 inch diameter is the
> speaker's physical limitation you say?
>    OK, Let us calculate the wavelength of a 20kHz wave for starters --
>    (300,000 km per second) divided by (frequency in kHz) = 15,000 meters
> This is approximately 9.32 MILES.

I'm sorry that I've never visited the planet where you live, and I'm
not absolutely sure I really want to, but I really would like to see
some of the musical instruments you use.  Or do you measure audio
frequencies in GHz there?
-- 

    Jim Crandell, C. S. Dept., The University of Texas at Austin
               {ihnp4,seismo,ctvax}!ut-sally!crandell