US1730531A - Acoustic transformer - Google Patents

Description

Oct 8 1929. P. A. RoBBlNs 1,730,531

ACOUSTIC TRANSFORIER Filed Nov. 30, 1928 Patented oct. s, .1929

rEncY A. Romanas,- oF HIGHLANp PARK, ILLINOIS ACOUSTIC TRANSFORMER -Application led November 30, 1928. Serial No. 322,898.

When a loud-speaker diaphragm is" vibrated, Work is expended upon it. This work air partlcles in contact with the diaphragm. i

is divided between accelerating and retarding the movement of the mass of the dia hragm, bending the material of which it 1s made against the internal forces lwhich give the material rigidity and elasticity,'and inagitating The useful effort is the agitation ofthe air particles, but the eficiencyof the diaphragm in performing this useful work is limited by factors which depend uponnecessary rigidity, elasticity, durability and weight of the diaphragm itself.

The power inpulses are usually applied at or near the center of al diaphragm'and vthe diaphragm must be strong enough and stiff 1 enough to spread the impulses over a large part of its area without undesirable distortion of its shape.

number of vibrations per second and each vibration of the reproducing diaphragm must be made within the time interval between soundwaves or impulses. In case of reproducing the sounds of an orchestra the diaphragm must vibrate slowly,'say to the notes of-the bass-drum, and at the same time it must vibrate rapidly to` the shrill notes of a violin or flute. i

Thus the diaphragm must be stiff enough to carry the vibrations of the drum over its entire surface and at the same time be flexible enough to break up linto nodes and antinodes over its entire surface to accommodate the shrill pitch vibrations. Thus vthe diaphragm must be a compromise between the requirements for all ranges of pitch ,and sha es of sound waves.

hese conditions require a delicate adjustment between mass, internal strength, elasticity, durability7 and dimensions. Y Loudness of tone depends upon the ampli.-

' tude of vibrations at the ear of the listener,

and the loudness of tone emitted by a dial phragm depends upon the number of air par;

ticles acted upon by the diaphragm and the degree of violence with which the air particles are vibrated. Loudness may therefore be increased by increasing the varea of the diabrations For any given tone or pitch there is a given i mg ratthng or Screechmg To guard agamst phragm and maintaining a ,XQd amplitude of vibration: by retaining a fixed area but increasing the amplitude of vibration.: by crowding a greater number of air particlesv into contact with the diaphragm: or by providing a gas heavier than air to receive the vibrations from the diaphragm.

Diaphragms of large size are sometimes used, but their increased sizeinvolves greater thickness and weight to make them proof '0 against distortion and their mass becomes so great that'they vibrate feebly or not at'all inresponse to shrill notes and overtones. If a diaphragm is retained of a small size and vibrated with a greatly increased amplitude of vibration theaccelerating force must be great-- y increased and the application of this force may distort the diaphragm or break its vi- 'up intovundesirabl'e overtones causthis the diaphragm must be made stiffer and therefore less responsive. A Various attempts have been made to maintain the advantage of alight flexible diaphragm by-subjecting it to a tension stress by means of a distendlng gas pressure applied' to'one side of the diaphragm. A characterl .istic-of a diaphragm must' be elasticity, and

any application of gas pressure to one side distorts and stretches the diaphragm and interferes with the delicate ladjustment between it and its actuating magnet ofstylus. f

I overcome these 'objections-by' immersing the diaphragm completely in a dense gaseous medium, and no matter what variations may be made in the pressure of the immersing gas,

there 'is no distortion or` stretching of the Any diaphragm yhas fixed lune-chemical strength, elastic characteristics, weight and in the inertia stresses Within the structure 100 of the diaphragm or in its elastic character# I istics or in its mass.

Owing to the greater number of air particles being agitated, the amplitude of vibration will be less than previously, but the total amount of work done will be the same as before, but in the dense atmosphere more work will be donc upon the air particles and less upon the diaphragm, and thus the efiiciency of the diaphragm will .have lbeenin-y creased.

This may be expressed by symbols thus:

If W isvthe total .work done at atmospheric pressure and ais the workdone upon the air, and d the work done upon-the diaphragm,

then W=al d. If the amplitude of vibration of the diaphragm Ibe reduced the work done upon .it vwill be less and may be repre-v than d it follows that a is greater than a.

Thus the efiiciency of the diaphragm has been increased.

y' If we now increase until the amplitude of Vibration of the diaphragm is the same in the dense air as it was in the freeair, the

'amount of work expended upon the diaphragm will still be el, as neither the mass nor other characteristics or' the diaphragm have been changed.

The equation then becomes:

V'Wilincrease= (aN-increase) lal and thus the entire increase in total work is 4expended Aupon the air particles, and nonev of the increased work is wasted upon moving the diaphragm. This is only broadly true as a slightly different set of internal stressesi would be involved within the structure of the 45 diaphragm 'when vibrating in a dense atmosphere owing to the increased load of air particles. These forces would vary according to the formula M122, where M is the mass of air particles and u is the velocity of acceleration. The velocity has not been changed so lv is constant, and hence the stresses will only be increased by an increment due to the increase in the mass of the air.

Manufacturing and ordinary handling and usage require that the diaphragm shall have an excess of'mechanical strength and in the combined mass of diaphragm and air particles, the weight of air is very small compared to the weight of the diaphragm, so even a considerable increase in the mass of the air will impose but as'mall increment upon the stresses within the diaphragm, and the result shown by the above equation will be approximately true.

The ability to transform a greatly increased amount of mechanical or lelectrical energy into sound energy or air vibration, in the 'caseof any. diaphragm, is a very desirable accomplishment. The increased efficiency indicated is not all gained however, for the sounds propagated through the dense'atmosphere must in turn` pass through the second.

or outer diaphragm with aloss of energy due to reflection and work done upon the second diaphragm.

The second diaphragm may be made very large in 'comparison with the first diaphragm and may be made of very thin metal, quartz glass, paper, or other suitable material. The force required to overcome. the mass of the second diaphragm is equivalent to Mrz-where M is the mass and a the rate ot acceleration. The totalfforce propagated through the dense atmosphere to the second diaphragm will therefore be reduced by whatever amount is reiulired to vibrate the secondvdiaphragm.

Owing to the large 'area of the second diaphragm it will have a comparatively short amplitudel of vibration when transmitting lthe sound-waves, and this short amplitude will result in the rate of acceleration being verv small.,` With a spherical shape the second diaphragm may be made very thin as rit has to resist tension only, and thus its mass will be quite small. With a low value for the acceleration factor a, and a low value for the mass factor M in the equation, the value ofthe force necessary to do" the work on the diaphragm must necessarilybe very small. y

The invention is hereinafter more fully described and is illustrated inthe accompanying drawings in whiche 1 Fig. 1 is a longitudinal section, more or less diagrammatic, of one embodiment of my invention; and

Fig. 2 is a similar view of a modiiication.

yIn Fig. 1', 1 is a' conical diaphragm receiving its power from the driving electro-magnet 2 which receives electrical impulses over the conductors 3, 3, which lead to terminals 28 extending through the wall of the chamber. 4 is an air-tight casing which has the annular ring 55 supported on brackets within it. The diaphragm 1 is held to the ringv 5 by the junk ring 6. and the screws 7 7 in the usual manner.

An extension of the casing 4 has the annular flange 8 to which a conveXly curved diaphragm 9 is held by the junk ring 10. and the bolts 11. The part-of the casing 4 at the back of the diaphragm 1 is lined with felt or other sound absorbing material. Gas under pressure from any suitable source'is supplied through the pipe 12 by opening the valve 13 and the entire casing 4 becomes filled with the gaseous medium under pressure.

A brauch pipe 15 is controlled by the valve 16 which admits gas under pressure into the cylinder 17 which contains the close fitting i force the piston 18 back tothe end of the shape.

cylinder 17. 'The piston rod v21. has an electrical contact brush 24' supported on its end to which the electrical conductor 23, leading to one of the terminals 28, is attached. As the piston 18 is, forced' forward, the contact brush 24 slides along the resistance coil 25` cutting out resistance as it advances.

Current for operating the cone diaphragm 1 flows along the conductor 26 leading to the second terminal 28 and 27 leading to the variable resistance 25 and as the amount of resistance 25 is varied the flow of current is varied and the intensity of actionof the dia-` hra m therefore varies beiniy reater when the pressure Within the chamber' 4 is greatest,

and least when the pressure Within the chamber 4 is least. I

Thus as the number of gas particles in contact with the diaphragm increases, the vibrations of the dlaphragmare increased 1n 1n-y tensity and greatly increased sound energy is propagated to the second diaphragm 9 which in turn transmits the vibrations to the surrounding atmosphere.

Figure 2 shows an arrangement in which a flat disc l*1 is used for a diaphragm, and the second diaphragm 9a is made'spheric'al in In other vrespects the constructions are similar.

lVhat I claim as my invention is:

1. An acoustic transformer com arising a closed chamber containing a gas aving a greaterdensity than air at atmospheric pressure; a diaphragm immersed insaid gaseous medium; and a second diaphragm exposed at one side to the atmosphere and at the other to the gas pressure'wlthin the chamber.

42. An acoustic transformer comprising a closed chamber containing a gas atmore than `atmospheric pressure a diaphragm immersed in said gaseous medium; and a second diaphragm exposed at one side to the atmosphere and at the other to the gas pressure Within the chamber.

3. An acoustic` transformer comprising a closed chamber containing gas having a greater density than air and at more than atmospheric pressure; a diaphragm immersed in said gaseous me ium; and a second diaphragm exposed at one side to the atmosphere and at the other to the gas pressure Within the chamber.

4. An acoustic transformer comprising aclosed chamber of lenticular form filled With a gas having a greater density than air at atmospheric pressure, one Wall of thel chamber being formed, in part at least, of a vibratable diaphragm immersed in said gaseous medium; and a second diaphragm, opposite greater density than air at atmospheric pres sure; an electro-magnetically operated diaphragm immersed in said gaseous medium; a second diaphragm exposed at one side to the atmosphere and at the other to the gas pressure within the chamber; means for varying the gas pressure in the chamber; and means for varying the energy input to the electromagnetic diaphragm as the gas pressure varies.

7. An acoustic transformer comprising a closed chamber containing a gaseous medium having a greater power transmitting capacity than air at atmospheric pressure; an electromagnetically operated diaphragm immersed in said gaseous medium; a second diaphragm of larger size closing an opening in said chamber; means for varying the gas pressure in the chamber; and means for varying the energy input to the electro-magnetic diaphragm as the gas pressure varies.

Signed at Highland Park, Ill., this 15 day l of November, 1928.

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