Importance of impedance?

[Ethan Winer]

Folks,

Audiophile magazine reviews often include plots of speaker impedance versus frequency. I can see the importance of frequency response - both on and off axis - and THD and IM distortion, and ringing, etc. But impedance? I always assumed the impedance of a speaker simply is what it is - as long as the power amp that drives it can cope, then the specific impedance is of little relevance from the consumer's standpoint. Likewise for phase. Sure, phase is important to the designer because it affects a speaker's frequency response. But should a magazine reader care about these? Am I missing something?

--Ethan

[gullfo]

beside the impact on amplifier load, the resonant frequencies vs impedence is one of the trade-offs in speaker design. this is a good measurement to have since it will indicate what frequencies may be problematic from the speaker itself. the actual impedence/reactive values typically balance out, but at certain frequencies, the impedence will act capacitive resulting in high frequency filtering. same for speaker wires etc... low frequencies are generally less affected.

i'm no expert, so you'll want to search around. there is plenty of detailed theory and testing on this...

[Ethan Winer]

Glenn,

> this is a good measurement to have since it will indicate what frequencies may be problematic from the speaker itself. <

Yeah, but that would show up directly in a frequency response plot, as opposed to indirectly in a phase / impedance graph. So in that light, what does impedance or phase tell us that frequency response doesn't?

--Ethan

[gullfo]

I would think that impedance versus phase would have little impact if I'm understanding your point correctly.

here's some links I found related to this topic:
--

http://www.epanorama.net/documents/audi ... dance.html

At DC, the impedance is completely dominated by the DC resistance of the voice coil
As you increase in frequency towards the fundamantal mechanical resonance, the reflected motional impedance begins to dominate and is inductive in nature. However, the total phase angle of the impedance RARELY exceeds 45 degrees and thus the resistive and reactive (inductive) parts of the impedance are just about equal.
At fundamental resonance, the impedance is purely resistive, its phase angle is 0, and is determined by the effective series combination of the voice coil DC resistance and the reflected mechanical losses of (primarily) the suspension (Re + Res in standard Thiele/Small notation).
Above fundamental resonance, the impedance drops, has a negative phase angle (rarely exceeding 45 degrees) and is, surprise, capacitive in nature. The impedance drops until...
In the midrange, it approaches the DC resistance of the voice coil it is SLIGHTLY higher than that DC resistance for a variety of reasons, typically about 10-20% (and THIS is the region that is used by MOST reasonably responsible manufacturers for specifying the nominal impedance). The impedance at these frequencies is predominantly resistive in nature and is dominated by the DC resistance of the voice coil.
Above this region, the inductance of the voice coil begins to influence the impedance. However, it NEVER becomes purely inductive, or even remotely close. First, over the majority of the range of operation, the voice coil resistance still dominates. Second, eddy current losses in the pole piece (see Vaderkooy, et al) dominate quickly, such that the phase angle of the impedance asymtotically approaches about 45 degrees, and NEVER 90 degrees, which would be necessary if your assertioon that the impedance was almost a pure inductance were true.
Impedance of a speaker IS NOT ALMOST A PURE INDUCTANCE. It is NOWHERE NEAR a pure inductance. The impedance of a speaker is only a rough average of the impedance and that the the voice coil dc resistance of most normal cone type dynamic speaker is roughly 75% of its "rated" impedance as the industry rates impedance. Most 8 ohm speakers will measure somewhere around 6+ ohms dc give or take a bit. (When horn loaded, the impedance increases).

http://www.epanorama.net/links/audiospeakers.html

--

Amplifier Output Impedance Why It’s So Important
http://www.transcendentsound.com/amplif ... edance.htm

--

Damping Factor
http://www.trueaudio.com/post_013.htm

list of topics including speaker wire resistance impact
http://www.trueaudio.com/st_index.htm

--

interesting describing mathematical placement of speakers...
http://stereos.about.com/od/homestereot ... tech_p.htm

-- experiment

http://hyperphysics.phy-astr.gsu.edu/hb ... 06ex3.html

[Ethan Winer]

Glenn,

> I would think that impedance versus phase would have little impact if I'm understanding your point correctly. <

Yes, you understand my point. I know how impedance varies with frequency and why. The real question is if impedance and phase matter to consumers, and if so why. So far it seems there is no good reason for magazine reviews to graph impedance and phase. Of course, what they should be graphing, besides on-axis frequency response, is off-axis response and distortion.

--Ethan

[gullfo]

Ethan,

like most things, the impedance vs phase probably only matters to the 1% of people who also spend large quantities of money on exotic power cables and digital-safe vibration mounts...

[Don T]

Ethan:
I use the impedance curve to identify resonances in the cone/driver assy. It's real handy to check your box / port / horn tuning. Also, if you are using a constant voltage X-over, you need to trap the impedance rise of the woofer in the high frequencies. I have also designed impedance traps to smooth out the ever present response peak in a soft dome tweeter (ProAc & 20-20's comes to mind). I have seen some X-over designs that become almost a dead short at a given frequency. Power amps will perform miserably in those conditions.
But, I design (or fix) speaker systems and the plots in a magazine (fz or imp) with 1 octave smooting tells very little about the speaker. Maybe it's so the magazine looks or sounds like a pro.

[gullfo]

it seems the impedance vs phase is not useful to consumers since its primarily of interest to designers when creating the speaker and x-overs. whereas the distortion and resonant frequencies probably would be of interest to consumers but probably tend to be scarey looking...

[Ethan Winer]

Glenn and Don,

Thanks guys, so it looks like we all agree that:

its primarily of interest to designers when creating the speaker and x-overs.

>the distortion and resonant frequencies ... tend to be scarey looking. <

Indeed. It amazes me that people - audio pros and audiophiles alike - will obsess over jitter that's 120 dB below the music, distortion (in their gear) that at 0.001 percent coud not possibly be audible, yet they ignore the hugely skewed frequency response (and ringing) caused by room acoustics. And they don't even know how much THD and IM distortion their loudspeakers have. Worse, the magazines either are either clueless too (my guess) or don't want readers to know. So they show a bunch of impressive graphs (phase and impedance) that are mostly meaningless.

--Ethan

[Don T]

Ethan:
I agree, room acoustics affect mixes way more than minor speaker phase deficiencies. I tell my students that $500 spent on acoustics will improve their recordings many fold compared to any $2k - $4k piece of equipment. I think it's because acoustics are not as easily quantifiable as THD etc. It may be the old salesman need for "point of sale" perceived value.