Date: Wed, 28 Mar 90 06:56:59 CST From: steve@ncsc.navy.mil (Mahan) Subject: Speaker magnets And now for a few words on the physics involved in a dynamic loudspeaker. The force experienced by a current carrying conductor in a magnetic field is the product of the magnitude of the current, the strength of the field, the length of the conductor in the field, and the sine of the angle between the field and the conductor. The force is perpendicular to the plane formed by the intersection of the current direction and the field direction. This means that with a stronger magnetic field in order to experience the same force it is possible to reduce either the current or the length of the conductor. Using the same mass of copper to form fewer turns reduces the resistance of the coil and therefore the losses due to coil heating. A coil moving in a magnetic field generates an electromotive force. This force is the product of the length of the coil, the strength of the field, the sine of the angle between the two, and the velocity perpendicular to the plane formed by the first two quantities. This electromotive force multiplied by the current in the coil is equal to the kinetic energy transferred into or out of the moving mass (including the coil mass). Nothing here implies anything about the impedence of the speaker, the length of travel of the cone, the efficiency of the speaker, or the sound quality. Well, a stronger magnet MAY imply higher efficiency, all other things being equal. This discussion reminds me of the old controversy of belt drive vs direct drive turntables. You have to ignore generalities and get down to cases. Which speakers sound better? That is the most important parameter. After this we get down to budget, compatibility, appearance, size, and other secondary parameters. And if the weather holds up I'm going to go see how my regulator sounds out in the Gulf today. More rantings from: Stephen Mahan steve@ncsc.navy.mil Naval Coastal Systems Center Panama City, FL 32408 Date: Wed, 28 Mar 90 09:05 EST From: Len Moskowitz Subject: Driver Efficiency Gary Korenek writes: > Question: > > Is it true that speakers w/ big magnets are less efficient? Don't > you need more power to overcome the stronger magnetic field? It's my impression that if you take a speaker with a small magnet and give it a bigger magnet you will increase its efficiency. The stronger magnetic field provides more force to the cone in reaction to a given current in the voice coil. But at the same time a stronger magnet reduces a driver's low end frequency response by raising the driver's free air resonance parameter, a key factor in low end performance. I work (or more accurately, play) with both home audio and pro musical instrument drivers. The pro drivers with their large magnets are typically 6 to 10 dB more efficient than their audio counterparts. But unequalized, they rarely have comparable low end performance when loaded in home-sized enclosures. Of course since they are designed to handle higher power you can often equalize their response to go deeper than the box/driver combination by itself would allow.