"Power hungry" speakers

Some distortions are not "additive" in nature i.e. they aren't creating / adding additional output on top of what the original signal had. This means that losing some ( any portion ) of the signal also equals to distortion. Since more power = more heat and more heat = thermal loss, you end up with more distortion from the speaker.

On top of that, lower sensitivity designs have to move more air to produce the same amount of air that a higher sensitivity design does. In order to do that, the driver has to make longer excursions. The longer the excursion that a driver takes, the more distortion that it will generate. Another side effect / drawback is that a longer excursion will also generate more reflected EMF ( electrical "back-pressure" ), making it harder for the amp to control & load into the speaker.

Besides those primary factors, the electrical characteristics of a speaker change as the power levels are raised. Depending on what power level / spl range a speaker was tuned or "voiced" at, one can end up running the speaker in a range where it is less accurate / running out of the linear pass-band of operation. Then again, the reverse is also true of a speaker that was tuned to operate at higher spl's being run at / not working as well at lower spl's.

As a side note, vented systems will suffer from what becomes a "roving resonance" depending on the transfer efficiency of the port itself. While ALL speakers end up raising their frequency of resonance as you drive them harder, "straight" ports or ports that are only flared on the exit side are much more sensitive to this phenomena than a double-flared port or passive radiator design. Passive radiators have their own problems though as they have the moving mass of the drone cone itself to overcome. This problem becomes more apparent / harder to deal with as spl's are raised. Since greater mass is harder to accelerate and / or stop rapidly, trying to do so with the longer excursions / more inertial momentum compounds the problem. This situation also occurs with sealed designs, but not as severely. That's because the pressure or "air spring" inside of a sealed box remains consistent regardless of the drive levels applied.

Other than that, most all of my speakers are sealed and of lower efficiency. This means that i need pretty sizable amounts of power to obtain good performance. On top of that, most are also low impedance designs, meaning that i need even more power to get them moving / keep them under control. Even with all of the above in mind and much like Muralman, the aforementioned trade-offs still aren't enough to make me want to get rid of them and move over to more efficient, but typically sloppier designs. The high efficiency designs that i do have ( 96 dB's and 104 dB's ) are both sealed. Since you can't get something for nothing and maintain linearity, the efficiency of the 104 dB design is achieved via horn loading. Due to the length and size of the horn used, low frequency extension is reduced. In order to obtain ultra high efficiency and maintain good extension, you have to go to a BIG horn like the ones that Mike aka Magnetar builds and uses. You can see what i'm talking about here on his "horny" website. The three subwoofer horns are the huge cabinets behind the mains ( they take up the WHOLE front wall ) and look like wood trim surrounding black centers. Yes, Mike is a "little" crazy, but he's a real nice guy. You just wouldn't want to live next door to him : )

The little 96 dB units that i have are horn loaded for the mids and tweeters, but not the woofers. As mentioned above though, you can't get something for nothing, so these too are also limited in terms of low frequency bandwidth.

By the way, the rear wave of a driver can be sealed or vented when using some type of front loaded horn. My larger horns are sealed and as far as i know, i think that Mike's MEGA horn's are too. Sean
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Everything you had to say, Sean, left the science behind my speakers (.8ohm/76db) unscathed. The prodigious energy required of my amps (class D) goes into creating the huge force fields needed to move large surface area ribbon drivers. The ribbons don't move to the naked eye. Even the great bass panels merely shimmer under loud 30hz passages. All the negatives you list don't apply.

Your discussion about speakers being driven harder than they were designed for hit home. For my speakers, there have been few amps in the past that could take them past 70db.

I have briefly driven my speakers to a median 95db, leaving plenty of room for huge transients. Never have they lost their composure. When these speakers were built, such levels weren't possible.

It is the nature of large surfaces utilized to produce sound levels that enable the speakers to go far beyond their designer's target. it also allows this speaker to side step drawbacks that other drivers encounter.

I assume that you're talking about Apogee's based on your impedances and mention of ribbons. Having said that, you state that the speakers are 76 dB's yet only a few amps could drive them beyond 70 dB's. How is that possible? It would require less than 1 watt to do this based on your own figures. Was it due to "protection" circuitry kicking in? If so, you need an amp without protection circuitry.

Other than that, i agree that a low bias output stage / high efficiency amp works very well with loads like this. Due to the lack of duty cycle that the amp reproduces / reduced current & heat involved, some of these newer designs truly are an "answer to audio prayers".

As far as 95 dB's go, i consider that to be a "moderate" listening level. While some recordings ( acoustic based music, etc.. ) sounds very good ( even "loud" ) at that level, it just wouldn't cut it for large scale Classical or hard Rock music. Bare in mind i'm talking about spl's as measured at the seated listening position, not one meter from the speaker.

Sean, I generally listen to my Scintillas at levels around 75db. Maybe I'm paranoid, but I believe my hearing will be better preserved. I've attended scores of live events, from intimate acoustic to stadium extravaganzas. I'm comfortable with my listening levels. I reserve 90 db and above for the likes of Led Zeplin once in a very great while. My seating a orchestral events is generally a third back, or balcony. 80 to 90db approximates the gestalt to my memory.

As for my speaker's ratings, you say, "1 watt should do." Am I listing it's specs wrong? Or are you being fecicious? ;) I am aware of successful powering of the Scinny with an 85 watt home made expressly for the Scinny. I had dissapointing results trying respectable amps of five times more power rating. Amp design is very important when pushing difficult loads.

Brand new class D amps are unfazed by the load. I'm thrilled by the results. The speaker playing field has been greatly leveled. I like to describe the sound as tubes on steroids.

Getting back to the question of efficiency, do you not agree large driver surfaces, resulting in minute travel, is a valid solution to speaker distortion? I can put my ear right to the ribbons, and hear the music as clear as fifteen feet away. There isn't any of that low level high frequency fuzz I've heard on domes and cones.

You mentioned an spl rating of 76 dB's. At 70 dB's, this would allow you 6 dB's of headroom before you reached 1 watt of input. As mentioned though, i didn't know if you were measuring at 1 meter ( which is useless except for sake of comparisons to reviews ) or at the seated listening position. Given that most rock music only offers appr 5 - 6 dB's of dynamic range, you would be using 1 watt of power to produce the full dynamic range of such a recording based on the above information. This isn't to say that i was recommending such an installation or that it would work as well as theory dictates, i was just spouting off figures based on the math that "acoustic theoriticians" would tell us was sufficient. We all know better than that.

I was also wondering by what you meant by the statement that very few amps would drive them beyond 70 dB's??? Obviously, most any amp could generate 1 watt of electrical output relatively easily, so what's the fuss? As mentioned, the only thing i can see coming into play here is the low impedance / protection circuitry kicking in.

As to why the Class D amps work better, that has to do with the reduced duty cycle that the amp sees, the lack of sag in the power supply and lower levels of reflected EMF generated by the speaker. As you reduce the duty cycle of the amplifier, you also reduce the amount of drive applied to the speaker. Less drive equates to lower levels of reactance, which gives us less reflected power to deal with. This in turn allows the amp to load up more efficiently, which is just more icing on the cake. If the switching frequency is high enough, the power is delivered in very short and fast pulses, giving the power supply ample time to recover from the small amount of power drawn from during those bursts. If the switching frequency is too low, you can hear the "pulsing" of signal and it sounds fuzzy, choppy and lacks cohesiveness. A higher switching frequency limits the duration between pulses and the energy in the gaps is somewhat "filled in" by what is called "the flywheel effect". In English, the overshoot of energy initially applied keeps the forward or reverse momentum going until the next pulse is delivered. The potential for distortion with such a design is quite high, but with newer technology and MUCH higher switching rates, they are finding ways to get around this. Sean
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