Sunday, November 30, 2008

Motor Effects of Dynamic Loudspeakers: Stifling Sound Quality?

Most can say that progress in dynamic loudspeaker technology had been moving sideways for over 30 years now. Given that dynamic speakers operate like electrical motors, does this make them flawed in sound quality terms?

By: Vanessa Uy

Probably 97% - or more – of the loudspeakers being sold on the market today have inner workings based on dynamic loudspeaker operating principles. But since dynamic loudspeakers can be thought of as an electrical motor that moves back and fourth in order to produce sound, does this make then “inherently flawed” when faced the task of converting an electrical signal from a recording (analog or digital) or a live mike feed into sound. Not only just “intelligible” sound, but a sound whose quality is indistinguishable from the “captured” real event.

To test this out for yourself, get a small motor, or better yet a very small low-powered motor that would rotate slowly when connected to the “X1” part of your analog multimeter in it’s ohmmeter mode / resistance measuring mode. If your multimeter’s batteries are fresh, the small motor is now being rotated via 150 milliamperes or milliamps or mA of current. If you look at the multimeters display, it is now probably reading between 15 to 20 (ohms) or so as the motor rotates freely. If you try to touch gently the tip of the small motors rotating shaft to slow it down slightly / loading the motor, the measured resistance across the motor’s terminals would fall to between 10 or 8 or so. If you’ll hold the shaft to make the motor completely stop rotating, the resistance reading across the motor’s terminals would now read 5 or a bit lower. Don’t worry about burning out the motor because the ohmmeter function of your multimeter / multitester produces only a miniscule level of current as noted previously. But don’t do it for too long because it unnecessarily depletes the multimeter’s battery.

You might now from our experiment the explanation on why a loudspeaker system's measured impedance across the audio band varies widely like those snazzy graphs and tables published on leading hi-fi magazines. For example, a speaker system’s impedance is rated at 8 or so, It’s measured impedance can vary widely from as low as 2.7 ohms to as high as 40 ohms across the audio band – i.e. from 20 Hz to 20,000 Hz. Thus speakers whose impedance dips to very low levels – approaching 1 ohm – coupled with a nasty phase angle usually spell bad news for tube-based amplifier designs.

But the “motor effects” inherent in dynamic speaker operation is not the only problem plaguing our quest for hi-fidelity. Materials used in the speaker’s cone construction can have an affect on sound quality too. Paper has been a de rigeur material for cones for a very long time because it “flatters” the inherent tonal qualities of the majority of musical instruments that hi-fi enthusiasts listen to. Like Stradivarius violins for example. Or a Gibson Les Paul played through a Marshall Amplifier stack in anger can really sound lifelike on a well-constructed paper cone speaker. When plastic cone – or more properly known as mineral filled polypropylene - speakers were first marketed during the 1970’s due to their “attractive” measured responses and high power handling, they have an “unintentional” benefit of flattering / beautifying a cello recording’s tonal properties.

But other materials did become fashionable in speaker cone use during the end of the 1980’s, other exotica like aerogel – an aerospace material so light that Saddam Hussein coveted it for Scud Missile “tweaking” in the run-up to Operation Desert Storm – came into use. Followed by carbon fiber and cones made of ultra-lightweight magnesium metal composites all for the quest of better sound quality. Despite it’s inherent limitations, dynamic loudspeaker design did make significant progress since the 1970’s. It’s just that good sound don’t come cheap and your local discount bin is very unlikely to stock quality hi-fi speakers, even those entry-level value-for-money models priced under 400 US dollars or so.

1 comment:

Letiche said...

The Audio Engineering Society or AES has since noted this problem of widely varying speaker impedance and the back EMF - so called motor effects - as very taxing for whatever power amplifier you have at hand. Conventional box monopole loudspeakers that are representative of the over 95% of commercially manufactured and sold hi-fi loudspeakers - in my humble opinion - has enjoyed decades of rigorous mathematical and / or laboratory scrutiny and analysis. During which enormous amount of theory and computing power - like the Thiel-Small parameters and what have you - have been applied to "tame" what you described as the so-called motor-effects of moving coil loudspeakers. And yet, the motor-effects that you have stated in this particular blog had - to date - been only minimized of it's negative effects on the final sound quality and not fully solved.
Maybe major hi-fi manufacturing firms should be reintroducing the open-baffle dipole loudspeaker concepts which they popularized during the "Golden Age of Hi-Fi" of the 1950's and 1960's.
I too agree that better cone materials like the high-definition aerogel introduced during the 1990's or carbon fiber as loudspeaker cone material is certainly a way forward when it comes to sound quality.