[Early B.]

...and there can be sonic downsides to putting a lot of wattage into a speaker.

Duke --

Can you elaborate on this?

I'm in the process of procuring a 98dB speaker and I have a 300wpc SS amp that I hadn't planned on replacing, so that's why I'm asking. I never play music loudly if that matters at all.

Thanks.

[Duke]

Duke --

Can you elaborate on this ["there can be sonic downsides to putting a lot of wattage into a speaker"] "?

I'm in the process of procuring a 98dB speaker and I have a 300wpc SS amp that I hadn't planned on replacing, so that's why I'm asking. I never play music loudly if that matters at all.

The most well-known power compression type is long-term thermal compression, which is primarily a function of the magnet heating up over time and temporarily losing strength.   If operated at 1/10th its rated power for a while, it's typical for a driver to lose about 1 dB to compression.   At 1/2 its rated power this rises to about 2.5 dB ballpark, and then about 4 dB at full power.  By "rated power", I mean its Audio Engineering Society protocol power rating, which is typically 1/2 its "music program" rating, which in turn is typically 1/2 its "peak power" rating.

A less well-known and well-documented phenomenon is "thermal modulation", which happens much more quickly, and which is primarily a function of rapid voice coil heating, which causes the voice coil's resistance to rise essentially instantaneously (the voice coil then cools down much more slowly as the heat is absorbed by the nearby motor structure and the air).   Stereophile did an article on the subject some years ago and concluded that the effects of voice coil heating are negligible, but several experts in the field have come to different conclusions.  One of them is Earl Geddes, who applied for a patent on an alloy for use as voice coil material that exhibited little or no rise in resistance with temperature.  I believe a patent on a functionally similar alloy has been granted to JBL.  And then not long ago I had an interesting conversation with Floyd Toole on the subject, and here is what he had to say:

"The audibility of power compression in its many variations probably could use some more research to define what is audible and what is tolerable. The magnet heating that you describe is important in pro audio sound reinforcement systems where the loudspeakers are required to work at or close to their design limits for long periods. Such heating and cooling has a very long time constant. This is not the case in most home systems. Although the modification of motor strength through magnet heating is a factor, most of the audible effects are from voice coil heating, which has a much shorter time constant. I just saw a test of a high-end audiophile speaker that in going from an average level of 70 dB (loud conversation, background music) to 90 dB (a moderate crescendo, or foreground rock listening) lost about 4 dB in output over about 3 octaves in the mid-high-frequency range. It became a different loudspeaker at different listening levels."

So one sonic downside of losing a few dB off of peaks due to thermal modulation from voice coil heating is, loss of emotional impact, because musicians use dynamic range to convey emotion.  Another possible downside is, changing tonal balance with change in sound pressure level, as Toole describes above.

EarlyB, you have nothing to worry about from driving a 98 dB speaker with a 300 watt amp.  I doubt you'll ever push that speaker into anything remotely approaching thermal compression or thermal modulation.  Your ears will throw in the towel first!

[a.wayne]

isn't that the same thing???...

Yeah like HP and TQ ..... 

[FullRangeMan]

isn't that the same thing???...

Efficiency refers to electrical conversion.
Sensitivity refers to dB, acoustic, sound.

[a.wayne]

Hello Duke,

Thermal compression is not really an issue with woofers in a domestic listening environment where RMS power sometimes barely exceeds 1 watt,  2-3 watts RMS is typically  considered high and 10 watts rms is alot , most of the amplification is usually  necessary for dynamic peaks ...


In Recording Studios  and pro sound  applications ,   yes , as you pointed out with Toole ...


Regards

[Duke]

Hello Duke,

Thermal compression is not really an issue with woofers in a domestic listening environment where RMS power sometimes barely exceeds 1 watt,  2-3 watts RMS is typically  considered high and 10 watts rms is alot , most of the amplification is usually  necessary for dynamic peaks ...


In Recording Studios  and pro sound  applications ,   yes , as you pointed out with Toole ...


Regards

Did you read the entire Toole quote?  He says that voice coil heating (one of "many variations" of power compression) is a source of "audible effects" in home audio, and then cites an example where 4 dB of compression occurs when going from 70 dB to 90 dB.   In other words, we don't have to get into magnet heating to run into audible power compression effects.  What we don't know is how widespread this is, but Toole tells us this was "a high-end audiophile speaker", presumably a three-way because these severe compression effects were localized to a three octave mid-high frequency band.

[a.wayne]

Hi Duke,

I'm familiar with Toole's work and yes dynamic compression is apart of all audio, not just speakers, but from under powered  amplifiers too. In regards to speakers, this compression is not only from heating ( as you stated , but we were discussing ) and is more common because of the single point source conductor of music, the  tweeter, more so  than the woofers, of course woofers  with poorly designed  xovers ( insertion  loss) low BL and not enough Xmax (there are other factors of course) will have issues too.

Have you ever measured for Dynamic compression ... ?

[Duke]

Have you ever measured for Dynamic compression ... ?

No, I haven't.  I usually use prosound drivers that are unlikely to see more than 10% of their AES rated power on peaks, and when that's not feasible, I use multiple drivers. 

If you've ever heard a speaker that needs to be turned up to "come to life", you may have heard one of the consequences of dissimilar power compression characteristics among drivers.   The "come to life" level may be where the relative loudness of the different drivers is correctly balanced, and at significantly higher or lower levels the tonal balance may be wrong.  One advantage of high efficiency and/or high thermal capacity drivers is, their thermal characteristics are less likely to diverge at levels normally encountered in home audio, so their tonal balance stays the same across a wider range of volume levels.  As a result, high efficiency speakers and high-thermal-capacity low to medium efficiency speakers usually still "come to life" at low volume levels. 

As an example of this issue, suppose we were tasked with "voicing" the speaker Toole describes above.   Let's say we wanted it to sound great at 90 dB so that it would impress people in a fairly loud demo.  So we voice the speaker to be "flat" at 90 dB, where the midrange driver is compressing by 4 dB more than the woofer and tweeter.  So down at 70 dB, the bass and treble would be 4 dB softer than the midrange, probably rendering the speaker boring and lifeless.  At still lower levels, if anything the discrepancy would be even worse. 

This points to an advantage of single-driver speakers - their tonal balance usually changes little as a result of thermal compression or thermal modulation (the short-time-constant effect Toole was talking about). 

[brother love]

Fascinating info re: single driver speakers & voicing general speakers for 90 dB. Thanks Duke for dropping your knowledge on this subject. 

[JCS]

Duke,

Much appreciate that info!  I have always felt that speakers that need a lot of power "to come to life" had something wrong with them.  Now I can explain it!

It also helps explain why I like certain speakers.  And why I mainly build single full range driver speakers.  (Plus I have issues w/ XOs, which I believe might be OK if they were essentially transient perfect--something almost impossible to do w/o use of DSP).

Cheers,  Jim

[ProSoundMan]

It's amazing just how distorted the term "reference" has become.  And it has absolutely nothing to do with sensitivity.  Look how many reviewers who are out there reviewing electronics on speakers that are not reference even though they call them reference.  And yes they are some of the big name manufacturers.  You can't trust an amp review from a reviewer who's "reference speakers" measure like crap.  I don't even think the reviewers know what a reference speaker is.

[JLM]

It's amazing just how distorted the term "reference" has become.  And it has absolutely nothing to do with sensitivity.  Look how many reviewers who are out there reviewing electronics on speakers that are not reference even though they call them reference.  And yes they are some of the big name manufacturers.  You can't trust an amp review from a reviewer who's "reference speakers" measure like crap.  I don't even think the reviewers know what a reference speaker is.

True, but how would a reference speaker in a mastering studio relate to what we have to listen through?  The term "my reference speakers" would be more accurate.  You really need a test bench to measure amps and knowledge to know what properties of the amp will or won't synergize with a given speaker. 

The best you can do is to find a reviewer (pro or otherwise) who's background you know, can write a good/clear/complete review, and taste you agree with that happens to be using speakers you are very familiar with in a similar room/setup to your own.  (Is that all?)   

I added that "good/clear/complete" bit because IMO the huge majority of reviewers abuse the English language and are better suited to write novels than do technical writing.  (rant off)

[Albert Von Schweikert]

Read text books on acoustics and electronics. Then look up Hofman's Iron Law of acoustics. There is a tradeoff between bandwidth (deep bass extension), enclosure size, and sensitivity. Due to modern decor and room size, combined with a desire for deep bass extension, sensitivity has been traded off by design. Most speaker designers feel that 100 watts per channel is a norm.

[a.wayne]

No, I haven't.  I usually use prosound drivers that are unlikely to see more than 10% of their AES rated power on peaks, and when that's not feasible, I use multiple drivers. 

If you've ever heard a speaker that needs to be turned up to "come to life", you may have heard one of the consequences of dissimilar power compression characteristics among drivers.   The "come to life" level may be where the relative loudness of the different drivers is correctly balanced, and at significantly higher or lower levels the tonal balance may be wrong.  One advantage of high efficiency and/or high thermal capacity drivers is, their thermal characteristics are less likely to diverge at levels normally encountered in home audio, so their tonal balance stays the same across a wider range of volume levels.  As a result, high efficiency speakers and high-thermal-capacity low to medium efficiency speakers usually still "come to life" at low volume levels. 

As an example of this issue, suppose we were tasked with "voicing" the speaker Toole describes above.   Let's say we wanted it to sound great at 90 dB so that it would impress people in a fairly loud demo.  So we voice the speaker to be "flat" at 90 dB, where the midrange driver is compressing by 4 dB more than the woofer and tweeter.  So down at 70 dB, the bass and treble would be 4 dB softer than the midrange, probably rendering the speaker boring and lifeless.  At still lower levels, if anything the discrepancy would be even worse. 

This points to an advantage of single-driver speakers - their tonal balance usually changes little as a result of thermal compression or thermal modulation (the short-time-constant effect Toole was talking about).

Cant agree here Duke, a full bandwidth speaker properly built and designed and not suffering from dynamic  compression via driver/xover/enclosure will suffer no such issues  at low listening levels , the transistion from low to high listening levels will only have the effect of going from the  30 th row to row 5.

BTW the only way to get proper Timbral  balance and high sensitivity  ( 92db +) is to have a large multi-driver or linesource type setup , no single point source two driver or 3 driver setup will deliver such and have a proper balance low to high with high sensitivity  ..


regards