Questions about speaker sensitivity/frequency response

[Deleted User]

Hi folks, I'm trying to wrap my head around what makes a loudspeaker either an easy or difficult load to drive. At a basic level, obviously sensitivity combined with impedance determines how much amplifier power is required to play music to normal listening levels. And dropping from 8ohm to 4ohm impedance draws approximately twice the current, placing more stress on the amp's psu.

Am I correct in saying that an Impedance curve is a measurement of the current/voltage coming through the crossover? Also how does an Impedance curve differ from a normal FR curve?

I'm also a bit confused about phase angles. The Infinity Kappa 9 is a classic example of the complexity of what makes a speaker an easy or difficult load; It has a nominal impedance of 4 ohms, but drops below 1 ohm twice in it's impedance curve, with a sharply rising phase angle 40-50Hz (see attached graph) (mind you, that measurement was taken with a stereo amp; not bi-amped, and with the bass extension switched off), yet it is rated at 102db efficiency. The Kappa 9 has a well earned reputation for being an amp-killer. Is the main offender here the impedance dips below 1 ohm, the nasty phase angle, or all of the above?

For comparison, you can view Martin Colloms measurements of the S5 Mk1 here, and the NRC Canada's measurements for S5 Mk1 here.



On the same subject, what is the greatest contributing factor to a loudspeaker's ability to dig down in the low bass region? The amount of surface area of the bass drivers, cabinet volume or something else? It's obviously not black and white as the Magico S7 has 1 extra 10" bass driver and greater cabinet volume, yet the S5 Mk2 achieves the same Frequency response.

Cheers!

Jim Shearer

Hi Bodhi,

I think what you need to do is pick up some books on loudspeaker design! Here is a link to help with some of the basic terminology and concepts. Note that the highlighted words & phrases provide links to articles on those specific topics:
https://en.wikipedia.org/wiki/Loudspeaker

As to a speaker's low freq performance: two quick 'rule of thumb' helps:

1) there is no replacement for displacement; to get deep, loud bass you need to move a lot of air!

2) Hoffmann's Iron Law: low freq extension/ small size/ efficiency; choose the two most important to you; physics prevents you from getting all three.

Cheers, Jim

[Deleted User]

Hi Jim,

Thanks for posting the suggestion & posting the link. Apparently the Magico S5 Mk2's twin 10" bass drivers are quite different from the S7's 3 10's, and they're also 1db less efficient, suggesting Magico designed thier twin 10" bass drivers to move more air with slightly reduced cabinet volume, thereby sacrificing some efficiency according to Hoffman's law. Interesting.

The Wiki page looks like it provides a good general overview of loudspeaker types and design. From there I can delve into more technical articles on subjects like Ohm's law, impedance, electrical phase, efficiency etc on sites like Audioholics. Think I might have opened a can of worms!

halo

Jim Shearer wrote: »

2) Hoffmann's Iron Law: low freq extension/ small size/ efficiency; choose the two most important to you; physics prevents you from getting all three.

Cheers, Jim

I've been thinking about this as well. When speaking with Andrew Jones at AXPONA, he mentioned that in order to reproduce low frequencies from the 5 1/4" driver he had to have low impedance @ 4 ohm. However, my current speakers (Energy RC10) feature a 5 1/4" driver @ 8 ohm and the bass response is very close to the ELAC UB5. The dimensions of the cabinets are very similar as well. So, how did Energy accomplish this with an 8 ohm load and decent efficiency whereas ELAC has a 4 ohm load and decreased efficiency?

mhardy6647

Impedance has (DC) resistance, capacitive and inductive components; all (but the DC resistance) depend on the AC frequency of the waveform (music!) and are fundamentally the electrical properties of the loudspeaker drivers (and their acoustic loading in the cabinet) and the crossover components. Actually, the room acoustics will also have some effect on the load that the amplifier "feels" from a given loudspeaker.

Sensitivity is a measure of how much sound (SPL, in decibels) one gets out for a given amount of power put in, and doesn't necessarily reflect on the "amplifier friendliness" of the loudspeaker impedance curve.

An "easy to drive" loudspeaker would have a relatively flat impedance curve without excessive capacitive or inductive components. The flattest impedance curves you'll see in relatively common loudspeakers would be the Magneplanars -- which are essentially purely resistive loads. They're not very sensitive, though... and conventional wisdom would not consider them "easy to drive" loudspeakers.

[Deleted User]

@mhardy6647, thanks for that helpful info. I'll keep coming back to it until it gels. I might need to do some more reading first. Interesting stuff And yes, I agree there are always exceptions like the Kappa 9 & Maggies.

mhardy6647

History has provided us with some famously amplifier-hostile loudspeakers. The original Quad ESL-57 is one of my personal favorites. Low sensitivity, low power handling capability and a really interesting impedance curve -- virtually a short-circuit at high frequencies. Solid state amplifiers with poorly implemented negative feedback (to broaden frequency response, lower output impedance, and reduce THD) would immolate when faced with such a load. The high capacitance (if memory serves) Polk Audio "Cobra Cables" were another famous destroyer-of-amplifiers.



The sound of the Quad ESLs, however, trumps all of their vagaries, as far as I am concerned.

Paramours and Quada 112413 by Mark Hardy, on Flickr

Jim Shearer

halo wrote: »

I've been thinking about this as well. When speaking with Andrew Jones at AXPONA, he mentioned that in order to reproduce low frequencies from the 5 1/4" driver he had to have low impedance @ 4 ohm. However, my current speakers (Energy RC10) feature a 5 1/4" driver @ 8 ohm and the bass response is very close to the ELAC UB5. The dimensions of the cabinets are very similar as well. So, how did Energy accomplish this with an 8 ohm load and decent efficiency whereas ELAC has a 4 ohm load and decreased efficiency?

In order to know the answer to your question, we would need to know the T/S parameters for both drivers, along with the enclosure designs and XO details. And note that even with all the technical details, including SPL and impedance plots, all we could do is explain how the speakers managed to produce the low freq performance. We would not have any idea whether we would enjoy the sound they produce, nor could we we be certain how they would interact with specific amplifiers. It's possible that Andrew Jones was looking for a certain sound AND compatibility with modestly price electronics which he was unable to achieve with a more sensitive driver.

Cheers, Jim