Modifying Speaker/headphone drivers

[murphyslabrat]

If the volume of a speaker is a function of the length of the voice-coil wire, average diameter of the coil, and strength of the magnet, would attaching another magnet to the existing assembly increase the volume output of the unit. That is, would it do so without changing the coil at all.

If the answer is yes, would the increase be a linear ratio of the strength of the two magnets, or would it be affected by the coil. Further, would this negatively affect the performance in any way, or modify the frequency response?

Again, assuming a "yes" would a misalignment (the additional magnet not being in the dead-center) affect the performance of the speaker (performance being defined in any metric), and if so, how much of a misalignment would matter.

The reason for asking is that I have some speakers that have a good frequency response arc, but only reach a moderate volume.

 

[CycloWizard]

The answer is no, adding an additional magnet will not increase the volume of your speakers. However, replacing an iron magnetic with a rare earth magnet (e.g. NdFeB) could make it louder. The simple reason is that the residual magnetic flux density of the magnet is independent of the magnet length. By adding an additional magnet, you are essentially increasing the length of the magnet.

Probably the most linear way (i.e. the way that adds the least distortion) to increase volume would be to increase the current through the system. If the current becomes very large, this could have strange effects as the reluctance force (I^2/2*dL/dx, where I is current, L is impedance, and x is the position along the axis of the voice coil) begins to become important.

 

[f95toli]

The maximum SPL ("volume") of a system depends on two things: The sensitivity of the speakers (usually rated in in dB/W at a distance of 1m) and how much power the amplifier can deliver.
The only thing that limits the SPL from the speaker is how much power it can handle before things start to melt (voicecoil) or it breaks for some other reason. This is what the power handling number that is usually written on the speaker tells you.
This means that inder normal circumstances (playing at home) you are hardly ever limited by the speakers: it is the amplifier that sets the limit.
If the sensitivity of your speakers (which btw usually depends more on the design of the box and filters than on the elementsm so modyfing them won't help) is very low- which might be the case here- you will simply need a bigger amplifier to drive them.

 

[Rubycon]

Increasing sensitivity via increasing magnetic flux mechanically is no different than increasing amplifier power - excursion increases and distortion will rise significantly. This is why professional reinforcement speakers are always horn loaded to maximize power handling and efficiency while reducing diaphragm excursion within the design frequency range.

If you have a driver with a low sensitivity of 84dB (1W/1M) and another that's 94dB, the former will need 10W input to be as loud as the latter.

If your speakers sound good at a particular volume level and sound distressed at higher levels you must only play them within their range. This is most likely a mechanical function and increasing BL just means they will reach the point of distressed with less voltage input.

 

[murphyslabrat]

regarding the response from CycloWizard, what if the additional magnet is of a higher grade than the original? Would this increase in the average (whatever you call it) increase the magnetic field affecting the voice-coil?

Or, what if you added additional mass, of equal or greater grade, around the existing magnet?

Now, addressing RubyCon's answer. I was aware of harmonic resonance creating additional tones at related frequencies and is a property of the ambient stress and stiffness of the membrane, but I was under the impression that clipping was a function of limitations on the voice-coil. So, which type of distortion does your comment address? And, if I am incorrect, how does that work?

--EDIT--
Oh, and increasing the applied current is not an option. I am using my laptop's on-board audio for these headphones, and the audio is only livable at or near max volume. So, I figured that this would be a good learning experience.

 

[Rubycon]

Yes it will reach Xmech sooner whether BL increases or you increase voltage. Excursion will be extremely limited with tiny notebook speakers.

Increasing BL - even if you can successfully do this with a stronger PM assembly will just mean the speakers will be as loud on 6 as they were previously on 10 and at 10 you would have lots of distortion, for example. Fun to experiment with, yes but not very practical.

You may be able to get a different driver. Lower frequency cutoff and playback sensitivity (below 200Hz) really depends on the cavity size that's available. If this is a compact notebook it's not going to be very big.

Getting a little OT...

I must say I'm impressed with the speaker on the iphone! The opening is TINY yet a lot of sound comes out of there with absolutely respectable fidelity given its size. The seal is also pretty good because a finger pressed firmly over the aperture results in nearly muting the sound!

 

[KIAman]

Think of it like this, a magnet inside a speaker is just like a kinetic spring. You need more force to "coil" it but in return, you get better "responsive" force. Typically, a speaker with strong magnets are not as efficient(requires more force to spring) as one with weaker magnets, but can handle a much higher power load without "bottoming" out. That's the SPL side of it.

Just keep in mind, a speaker is just mechanically emulating an en electric sine wave. The bigger the peaks and troughs, the louder. Thinking about that, to get a loud speaker, you need good suspension material that allows long movement and thick enough VC to handle the high power and finally, strong enough magnet to return the driver to its normal position before the next wave comes.

 

[Howard]

Originally posted by: Rubycon
Increasing sensitivity via increasing magnetic flux mechanically is no different than increasing amplifier power - excursion increases and distortion will rise significantly.

Why will distortion rise significantly?

Originally posted by: KIAman
Think of it like this, a magnet inside a speaker is just like a kinetic spring. You need more force to "coil" it but in return, you get better "responsive" force. Typically, a speaker with strong magnets are not as efficient(requires more force to spring) as one with weaker magnets, but can handle a much higher power load without "bottoming" out. That's the SPL side of it.

No, not really.

Thinking about that, to get a loud speaker, you need good suspension material that allows long movement and thick enough VC to handle the high power and finally, strong enough magnet to return the driver to its normal position before the next wave comes.

Once the current through the voice coil disappears (zero crossing point, for example) the only significant forces on the diaphragm in a conventional electrodynamic driver are from the spider.

 

[Rubycon]

Originally posted by: Howard
Why will distortion rise significantly?

Increasing excursion into non linearity whether from increasing voltage input or as a result of increased BL at the same voltage gives the same result.

 

[Howard]

Originally posted by: Rubycon

Originally posted by: Howard
Why will distortion rise significantly?

Increasing excursion into non linearity whether from increasing voltage input or as a result of increased BL at the same voltage gives the same result.

Excursion does give rise to significant distortion factors, mainly BL non-uniformity, but it is not the only factor. Flux modulation, for example, is dependent on the current flowing through the coil, and generally causes non-linear distortion (IIRC). Power compression, a form of linear distortion (under voltage drive), is also dependent on the current through the coil, and may have effects that are beyond simple sensitivity loss.

 

[bobsmith1492]

If the sound output is simply way too quiet, I doubt mechanical over-excursion will be a problem, eh?

Since mechanical movement occurs because of the interaction of the two magnetic fields (fixed and electromagnetic), a STRONGER magnet should increase the sensitivity of the speaker (not necessarily just another one, like CycloWizard said).

 

[heymrdj]

Unless you have some cheap headphones (my 14$ headphones from radioshack or more than capable of defeaning levels) you have A: a problem with your computer not outputting enough power, very possible that your systems amp that powers its headphones are at their max, B: your software on your computer isn't set right to deliver volume, or D: You're Deaf.

 

[CycloWizard]

Originally posted by: Howard
Excursion does give rise to significant distortion factors, mainly BL non-uniformity, but it is not the only factor. Flux modulation, for example, is dependent on the current flowing through the coil, and generally causes non-linear distortion (IIRC). Power compression, a form of linear distortion (under voltage drive), is also dependent on the current through the coil, and may have effects that are beyond simple sensitivity loss.

Larger excursions move the coil into regions where BL is non-uniform, as well as inducing potential material nonlinearities in the suspension. How much each one matters depends on the design, of course.

 

[Howard]

Yes

 

[murphyslabrat]

So I actually tried it, and, lo and behold, it didn't help. It actually exacerbated the issue by drastically reducing the maximum output.