Speaker Impedance, and its Effect on Sound

[spaceman5]

Hi

Sometimes when you want to buy a certain speaker, the manufacturer made more than 1 version of it, differing in Impedance.

Sometimes you even have 4 options:
2Ω, 4Ω, 8Ω, 16Ω

Assume that all other variables of the speaker are the same, like Diameter, Diaphragm material, etc..

In that case, when you are offered 4 versions, differing in Impedance,
what (in general) can you expect from different impedances, regarding its effect on the sound that you will get?

To sharpen the question a bit more, allow me to define this scenario:

Imagine that you bought all 4 impedances,
connected each one to an amplifier of its own (of course all 4 amplifiers are identical),
and you set the Volume knob on each, till you get the same Volume Level from all 4 speakers..

What will be the difference in Sound between them?


Thank you

 

[Windforce85]

Until amplifier had significant output impedance (measured in Ohms rather than miliOhms) the difference in perceived sound would be non-existent. Assuming that this amplifier is perfect voltage source electrically.

Highish amplifier output impedance will result in decrease of SPL in frequency ranges where amp output/driver impedance ratio is the highest.

Driver impedance has the largest impact on passive crossover elements values and on box tuning, becouse Re is a part of T-S parameters bundle and cannot be isolated from influence on driver's overall performance.

 

[spaceman5]

Hi Windforce

You mentioned Amplifier Output Impedance,
where I talk about Speaker Impedance..

 

[Windforce85]

Let me put it in another way:

There will be no difference between drivers when connected to amplifier assuming that:

- amplifier has near-zero output impedance
- amplifier is perfect voltage source (has perfectly load-independent frequency response)
- drivers are connected directly to the output and suspended in free air

 

[planet10]

The effect of loudspeaker impedance is largely in how it affects th eperformance of the amplifier.

Higher impedance means that the amplifier does not have to deliver as much current. This usually means that it is easier on the amplifier.

In a pure voltage amp (most amps) at higher impedance the amp delivers less power. It is the opposite with a pure current amplifier (rare).

Affects of a speaker impedance that is not flat has to be considered if one is using a current amp.

dave

 

[adason]

spaceman5, speaker impedance, even listed by manufacturer as one number, is just rough aproximation, because in reality speakers impedance varies wildly with frequency, and for instance can be reported as 4 ohm speaker, may have impedance anywhere between 3.5 to 50 ohm depending on frequency

Such speaker will sound differently if driven by voltage amplifier or current amplifier, that is if the amp has low output impedance or high output impedance

 

[TMM]

Basically no difference if your amp is half decent. Do note that the T/S parameters can vary slightly between different impedance versions of the same speaker. For instance, a certain manufacturer I like has consistently lower Fs and higher Qts for their 4ohm speakers compared to 8ohm. This can cause a slight difference in the amount of bass.

 

[head_unit]

...difference in perceived sound would be non-existent...Re is a part of T-S parameters

There will be no difference between drivers when connected to amplifier assuming that...

Basically no difference if your amp is half decent. Do note that the T/S parameters can vary slightly between different impedance versions of the same speaker...

Having designed a number of raw drivers, I'd say it is quite difficult to make different impedances have the SAME response and parameters. To get different impedance, the wire gauge has to differ, which usually affects BL. Mmmm IIRC the moving mass generally changes some. So even if a spec sheet shows the parameters and response are the same, I would not believe it without measuring it myself. Now, how much difference? Maybe quite a bit, maybe not a lot-it depends very much on the driver and the enclosure.

 

[kevinkr]

I suspect voice coil inductance might have a small but measurable/audible influence on the HF performance of an otherwise identical driver..

 

[head_unit]

I suspect voice coil inductance might have a small but measurable/audible influence on the HF performance of an otherwise identical driver..

Ah yes, that is a great point! The voice COIL will be physically different at the various impedances. Now speakers don't really have an inductance, the effect is due to eddy currents in the pole piece, but the physical coil also contributes.

 

[JMFahey]

I design and build my own speakers,obviously designing and winding my own voice coils, and, same as head unit said above, for the same magnetic circuit, meaning same gap, same flux density, same total flux, etc., lower impedance means not only thicker wire and less turns, but more moving mass.

Now higher moving mass at the voice coil, means less acceleration both ways, speeding up and speeding dwen, because of Inertia, so voice coil movement becomes "sluggish", compared to different , lighter coils, high frequency response suffers, same with "sharpness", transient attack.

Effect *is* noticeable if compared side by side, specilly in Guitar speakers which are used on their own.

This effect is masked in multi way Hi Fi (or PA) systems because midrange and tweeter speakers cover that range.

 

[head_unit]

Effect *is* noticeable if compared side by side, specilly in Guitar speakers which are used on their own...

Great example! I remember slaving over a transducer for Peavey, who wanted a sound that was "somewhat thin" (or some such phrase). Basically this needed a rising response to like 7 kHz, then dropping like a rock. Different coil windings made a big difference.

 

[abraxalito]

I design and build my own speakers,obviously designing and winding my own voice coils, and, same as head unit said above, for the same magnetic circuit, meaning same gap, same flux density, same total flux, etc., lower impedance means not only thicker wire and less turns, but more moving mass.

I can't yet see how using thicker wire makes for higher mass, unless its because a thicker wire coil will have less proportion of insulation to a thin wire coil. Ultimately the gap remains the same hence the total amount of copper remains the same, to a first order. This is of course neglecting whether a particular target impedance maps to an even number of layers of wire.

 

[TMM]

Compare these drivers:
Silver Flute W17RC38-04 ohm 6-1/2" Wool Cone
Silver Flute W17RC38-08 6-1/2" Wool Cone
Silver Flute W17RC38-16 6.5" Woofer Wool Cone - 16 ohm

For practical purposes, when a lower voicecoil impedance is used, Mms increases slightly causing Fs, Qes to lower, decreasing sensitivity (per Watt) but increasing potential bass extension. There is no significant change to the frequency response, at least for a driver with an already well damped cone.