Re: [Phys-l] speakers

[Bill Nettles <bnettles@uu.edu>]

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Anthony Lapinski
> Sent: Tuesday, April 26, 2011 9:31 AM
> To: phys-l@carnot.physics.buffalo.edu; tap-l@lists.ncsu.edu
> Subject: [Phys-l] speakers
>
> I have an old Fisher speaker system. Remove the grill and it shows a
> woofer, midrange, and a tweeter. Red/black connectors in back. Works
> well
> with a Radio Shack amp.
>
> On the back is a sticker with information about this unit:
>
> impedance = 8 ohms

[Bill Nettles] Impedence is the AC analog to resistance. Strictly speaking, it will vary with frequency, but will be close to 8 ohms over most of the audio range.  This is the external load that the speaker presents to the amplifier circuit which could be modeled as a voltage source in series with an internal resistance (see Thevenin and Norton equivalents).  The speaker is then connected in series with this equivalent. If the equivalent internal load is different from the speaker load, you will have less than maximum possible power transfer to the speakers, meaning that either the power supply is underutilized or the internal load is absorbing (wasting) as lot of power.  In the tube amp days (especially Class A amps), this was extremely important because an impedance mismatch could end up destroying your amplifier.  Solid-state amps are much more flexible and can handle varying speaker loads.

> input power = 5 - 50 watts
[Bill Nettles] The low number usually refers to the minimum power required to overcome "frictional" forces in the cone and produce an audible signal. The upper is usually the maximum continuous power the speaker coils can handle without overheating, or without causing the speaker cones to mechanically be destroyed by excessive movement.

> crossover = 1.5/6 kHz
[Bill Nettles] Crossovers can range from very simple RC or RL networks, to very complicated multiply connected R's L's and C's. The frequency quoted is generally the frequency at which half the speaker power is feed to the "lower" element and half goes to the "upper". So at 1.5 kHz, half the power goes to the woofer and half the power goes to the mid/tweeter, most of it to the mid.  At 6 kHz, half the power goes to the tweeter and half to the mid/woofer, most of it to the mid.  BTW, half power points are also 3dB decreases from full.  Typical humans barely notice changes of 1 dB, so although half-power sounds like a lot, the actual volume/loudness change, while noticeable, isn't huge.
> I have a few questions.
>
> 1. Does the impedance of a speaker affect the sound quality/loudness?

[Bill Nettles] Audiophiles will say "yes" to both, but most people don't have the room setup or amplifier to notice much of a difference of quality. Loudness, yes there will be a difference in a 4 ohm and an 8 ohm all other things being equal.  Between brands and designs, however, there are differences in speaker efficiency (how much sound is produced at 1 m at 1 kHz with 1 W consumed) due to mechanical properties. Even if the impedences are the same, the efficiencies aren't.

> So
> for a given power, you need a certain "ideal" impedance (resistance).
> And
> if you use a speaker with a higher impedance, the power output will
> decrease (P = V2/R)?
[Bill Nettles] You can still get the power (subject to amp limitations), but you might have to change the volume knob.  Again, the impendance is for matching the output loading design of the amplifier for optimum power transfer and amplifier protection.

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