Confused about dB's, Sensitivity....

[Chuck]

Hi Jack,

I had only read the original post, not all the responses and sub-threads, but since you posted your reply in this sub-thread, I read some more of the posts. All of them either made good points or asked good questions, but it's clear that there is a lot of confusion. You seem to have a good understanding, but since you're the only person I've been chatting with in this topic-thread, You probably already know most if not all of what follows, but it seems appropriate to tie some of the information together for those who are still confused.

Sensitivity Ratings:

These ratings specify the sound pressure level a driver or loudspeaker will produce at a standard distance and input level. I won't go into the credability of manufacturers specifications other than to say that sometimes they are unreliable. Even so, they are rarely exaggerated so much that they can't be used as a guideline.

Measurements:

It's not enough to know that someone fed a loudspeaker 1 watt and measured 87dB SPL at one meter. All loudspeakers have peaks and dips in their response, as do all rooms. If I put a loudspeaker in a room, and find the largest peak in the frequency response, I can then feed one watt to the speaker and get a nice high "sensitivity number" to impress people. Anyone can spin measurements (and claims) to make products look more appealing. Still, I don't know of any loudspeakers that are so over-rated (in terms of sensitivity) as to make the number meaningless.

Distance and Room Size:

Sound intensity decreases with increasing distance. Unless you sit one meter from your loudspeakers, one watt will not actually produce the pressure level given in the sensitivity rating. At the same time, a loudspeaker that can reach a given output level outdoors will be able to reach a much higher output level once it is placed in a room. This is due to what is called "room support," and in general, the smaller the room (and all else being equal), the less power we need to reach a given output level.

Dynamics and Complex Waveforms:

For all of the reasons given above, the sensitivity ratings are ONLY a guideline. Loudspeakers with a sensitivity rating of 87dB/w/m across it's useful bandwidth might actually produce 87dB, more or less, at the listening position in a given room with only one watt available, but it is important to realize that this is the PEAK output we'll ever see if we drive the speaker with a one watt amplifier. The typical SPL meter doesn't actually measure the signal peaks, even when set in "peak mode." Most actually average the signal over a very short period of time, and the "peak" setting just reduces that time period. Music and natural sound can produce signal peaks that are only 1/2 cycle long, and the SPL meter will just average that big peak into the measurement. When using an SPL meter it becomes readily apparent that the dynamic range of music varies greatly from one recording to the next, in spite of the fact that the meter isn't really showing us the absolute signal peaks. Some recordings will show as little as 10dB difference between the loudest and softest passages, while others can show 20dB or more. 30dB is probably a reasonable amount of overhead (above the long-term average level) to take care of most if not all recordings. However, if we want to be sure that the transients that the SPL meter doesn't show us don't cause clipping issues. The effect of clipping on these momentary peaks varies widely depending on the amplifier design, and can range from hardly noticable in a tube amp to adding grain or brightness to the sound. In many cases it won't be recognized as distortion, but the sound may get irritating after a long listening session ("listener fatigue"). I could not even guess what the largest momentary peak ever recorded might be, but if one is after the highest fidelity they can manage, they have to allow for these peaks. If we are conservative and allow 10dB more for momentary peaks we're looking at a total of 40dB to allow for dynamics and crest factor.

Listening Levels and Preferences:

If audiophile A is listening to very dynamic music at an average level of 80dB, the peak demands will be at least 15dB higher, and as noted above, might require as much as 40dB above the average level. Given a typical recording though, 15dB is pretty common. Audiophile B, listening to rock music with limited dynamic range might be listening at an average level of 85dB, but the music might only demand only 10dB for the peak levels. In both cases the peak SPL requirement would be 95dB SPL, even though the average levels vary by 5dB (80dB vs 85dB). For many, 80 to 85dB averages seem pretty loud, even when the performance is live. For others, it doesn't "get real" until the peak levels exceed 100dB SPL. Given all of the above it is clear that power requirements vary greatly from person to person, room to room, loudspeaker to loudspeaker, and so on. When we are involved in "active listening" here, the average SPL is typically between 80 and 85 dB SPL. At those levels I turn on the SPL meter and keep an eye on the levels, becuase as our ears clamp, it is all too easy to end up listening at levels that are too high, for too long. With some rock music the averages here can exceed 90dB, but that's never going to happen for very long, because everyone here is paranoid about damaging their hearing. Those who want to reproduce both rock and classical music totally free of any clipping issues under any curcumstances should probably allow 40dB over the average level at which they listen to highly dynamic music, so for us that would mean that we need to reach momentary peak levels of 85dB plus 40dB, or 125dB. Most of that peak energy will be at lower frequencies, and all of the momentary peaks will be momentary, so it's not as loud as it might sound (and rarely will you actually find a recording that will make such extreme demands). A system that can reproduce peaks of 100 to 103dB is actually enough for most people. Use that number and the sensitivity rating to get a (very) rough estimate of power requirements. If the speaker is rated at 83dB at one watt, we need to reach a level 20dB higher to reach the 103dB peak target. Since one watt will produce about 83dB we know that 10 watts will produce 93dB and 100 watts will get us to our 103dB goal.

Power Demands:

Given all the variables it is impossible to make any blanket statements that cover every situation. In general it is not going to make much difference if we go from an 80 watt amp to a 100 watt amp (all else being equal). Depending on how we listen, both will be clipping on some momentary peaks unless we are using speakers with very high sensitivity, listening near-field, or have a very small room. In most cases the clipping won't be at all obvious, depending on the way the amp is designed. If it's a tube amp it is unlikely that the momentary clipping will do anything that anyone would notice without direct comparison to an amp that is free of clipping issues. With an SS amp there may be grain or veils that aren't noticable except in direct comparison to a tube amp or an amp with anough power to avoid the clipping issues. At higher average listening levels with peaks approaching or exceeding 100dB, and typical speaker sensitivity, there will be a difference between a 50 watt SS amp that is clipping every major transient and a 200 watt amp that rarely if ever clipps a transient, but since the momentary clipping almost always goes unnoticed (or unidentified) it is often hard to see where more power might be needed (usually this only becomes readily apparent when one upgrades from something like a 60wpc amp to a 200-250wpc amp (again, all else being equal). One reason tubes get such high marks with so many audiophiles is because they handle this momentary clipping in a far less objectionable way than most SS amps. Basically I'd have to say that if one's system plays loud enough, at an acceptable level of distortion, then a bigger amp probably isn't a must. However, increasing the power by a factor of four or more can often (if not always) yield some benefits. Increasing it by 50% is not likely to make much of a difference (though in some cases it might).

Bi/Tri-amping and Sub-woofers:

When a powered sub with a crossover is used, the system is technically bi-amped. This reduces the crest factor demands on both amps, and most subs have pretty strong amps. Even little subs tend to have 100 watts or more, and are often high sensitivity ported designs. Big high-performance subs often have over 1000 watts behind them. With HT we run the sub below 80 Hz., and may drop the crossover point to 60Hz. or less in a stereo "music only" system. A LOT of the high-energy bass now totally bypasses our main amp and gets handled by the sub amp, so the main amp can be smaller. A system with a 400 watt subwoofer, and a 200wpc amp driving the mains, if of any decent sensitivity at all, can produce a LOT of sound in most rooms (assuming that the drivers can handle the power well). Even cutting those numbers in half will probably provide enough output for most people, but the perfectionist audiophle wanting to get the cleanest possible sound is likely to want a little more power. The benefits might be small (or not), but for the perfectionist audiophile it's all about small benefits and incremental improvements. That's not the case with most people though.

DANGER LEVELS:

A constant level of 87dB (or less) at the wrong frequency over a long period of time will damage our hearing. Listening to any kind of music, long term, and levels above 90dB SPL will damage our hearing. Jack hammers, chain saws, leaf blowers, jet engines, and trips to the shooting range WILL damage our hearing if we don't use proper ear protection, and limiting exposure is prudent. Lower frequencies at high levels are less damaging than middle frequencies (where the ear is most sensitive) but it's always best to be cautious (IMO).

In General:

A speaker that can reach a peak output of 90dB with one watt in is not going to approach dangerous level when driven with only one watt, without being obvously overdriven (likely to sound very nasty, even with tubes). 10 watts MIGHT allow momentary peaks of 100dB, but the amp is not likely to handle this well, especially if it is an SS amp. Even the shortest transient demanding a little more will result in momentary clipping. Yes, if you listen at 90+dB average levels, long term, using a 10 watt amp and high sensitivity speakers, you could damage your hearing, but I doubt that you'd be able to tolerate the distortion products of the setup (unless you use tubes and tolerate dynamic compression well). At 100 watts the speaker with a sensitivity of 90dB/w/m will go pretty darn loud in an average room, and for many this may well be at or above the point of diminishing returns. Given all the variables and preferential issues it's just not possible to make blanket statements about power requirements.

Case In Point:

InnerSound's Eros is bi-amped. The original design came with a seperate bass-amp/crossover, and the bass amp produced 200 watts per channel. They sold a ton of the original design and I've never heard anyone complain about the bass performance. However, starting with their Series II Eros they changed upped the bass-amp output to 600 watts per channel (three times the original power). Owners (I know several) reported an improvement, and attributed it to improved dynamics. "The music has more impact now." In point of fact, the bigger bass amp does give the system more impact, beacuse it clips fewer (perhaps none) of the momentary peaks that the older/smaller amp couldn't handle. That's the only difference anyone reported when upgrading to the new more powerful amp/crossover, but the few who tried switching back to the older crossover after a few months were SHOCKED at what they heard. Going back to the smaller amp makes the mid-bass and bass both downright muddy in comparison, and it gets noticed instantly. The newer amp/crossover always goes right back into the system and people are left wondering how it was that they actually liked the bass before the upgrade. The difference is jaw-dropping, but it's most apparent when going back to the old amp/crossover after getting use to the bigger, improved amp/crossover. I thnk that this example shows that a lot depends on one's experiences and expectations. The 200 watt amp seemed to be sufficient, but in hindsight there was clearly room for improvement. Those who have never upgraded their crossover/amp are probably still perfectly satisfied with their setups and the bass performance, so for them, the 200wpc amp is obviously quite good enough.

It's All Relative:

Anyone who thinks they can drive my 97dB/w/m stats to "concert hall" levels in my (large) room with a 10 watt amplifier is simply wrong. A tube amp with less than 20 watts can generate "satisfying levels" at the expense of substantial dynamic compression (at higher levels), but a 20wpc SS amp would sound nasty due to the fact that it won't like the load it has to drive. Lots of people drive similar speakers to satisfying levels with amps in the 100wpc range, but they are in fact clipping the signal at times (current clipping due to the load rather than voltage clipping due to the output demands). That would drive me nuts, becuase I'm use to the sound of systems that never approach clipping, and really dynamic recordings played at higher levels just won't deliver what I've come to expect. It may be a subtle difference, but it's noticable, so to me, it matters. Past the point of diminishing returns? Absolutely! Excessive? I think that depends on so many factors that it's impossible to say for sure.

You Can Never Have Too Much Power:

Come on guys, this is a man thing. Fast cars, fast bikes, and audio systems can never really have too much power. They can all three easily hurt you if they have lots of power and you push them too hard. I've dinged more than one high-performance car in my day because I've been stupid and let the power get the best of me. Never anything serious, but I have dinged a few really nice cars because I've mismanaged the power (and been stupid). I'll probably do that again too, because I'm not so crazy as to totally throw away a high-performance vehicle, but I love to push the envelope, and sometimes unexpected things happen. In this area deer are a major danger, and we've had two minor dings as the result of deer and driving just a little too fast to deal with the sudden appearance. If you play, sometimes you have to pay, so never play with anything that might do irreversable damage. A ding in the car can be fixed, but once the ears go, they're gone. Guys, don't let our love of power in all things lead you to do anything that will do perminent damage to any part of your body. When you do things that involve risk, take sufficient precautions so that you don't do any perminent damage.

See ya,

Chuck

[Jimrod]

Hi, just a thought, in skimming over these comments I didn't see anyone mention the actual current output of amps as a factor, the reason a Sony 100watt amp will not drive speakers as well as most "high-end" 100watt amps is down to it's current limitations. Most people seem to think distortion = loud as in many demo's I have performed with speakers people keep saying "go louder" and don't realise they are having to shout very loudly to get their message across then the speaker hit's it's end-stops and they are all disappointed because they think the speaker was feeble because it didn't go all harsh and crappy sounding. This is where good amps (such as class-a and valves) outperform lower quality ones because they don't distort with their high current keeping the speaker under control. Next time you look at an amps power rating bear this in mind as this is usually the reason some 250watt amps dont work as well as some 50watt ones (liken it to torque and horsepower in cars... a Honda with a 2 litre engine pumping out 200hp and a car of a similar weight and hp but with a 5.7 V8 will both have advantages but you can bet the big V8 will cruise at 90mph a lot better than the Honda if the gearing was the same...)

So sometimes an amp with a low wattage will go much louder without distortion than a high wattage model, I know this is a little off topic but it is an often overlooked point that should be considered more often. You can have a speaker pumping out 90 decibels of horrible distortion which will not be pleasant to listen to at all or you could have one putting out a pure and clean 100 decibels which you could probably listen to for much longer...