Can someone explain db sensitivity?

Note to Herman: A difference of 3 db is twice the power, not twice the loudness. In order to double the loudness you need 10 times the power which is 10db. You need to send twice as much power to the 87 db speaker to achive the same loudness as the 90db speaker.

Note to Aragain. I beg to differ. I believe you are confusing percieved loudness as measured in dB with amplifier power, which is also measured in dB although they are two different things. The 90dB versus 87dB is the output loudness from the speaker, not the input power to the speaker from the amplifier.

The general rule, as you correctly state, is that to double the loudness from a particular speaker, you need to provide it with 10 times more power. Example: you are listening to an amplifier that is delivering one watt to the speaker. To make it sound twice as loud, it would have to deliver ten watts.

Twice as loud is a change of 3dB in loudness, not in amplifier input power. You need to send ten times (10dB) more power to the 87dB speaker to get the same loudness as the 90dB, not twice as much as you state.

Ok, Herman - I'm willing to retract my post although I have never seen it explained as you have stated.

So, if I understand what you are saying, you would need 1000 times the power to an 87db speaker to get the same loudness as a 96db speaker? This means it would take a 3000 watt ss amp on an 87db speaker to match the volume of a 3 watt 2A3 tube on the 96db speaker. Is this correct?

Thanks
- A

For every 3 dB increase in sound pressure level, the power output must double. For example, to drive an 87db rated (@1 watt, one meter) speaker at 108db will require 128 watts. To drive a 90dB rated speaker at 108 db takes 64 watts. This rule of thumb helps ya pick an amplifier with enough stability to drive your speakers.

Yeesh, i can see how people can get "confused" ( Kelly would be "corn-fused" ) about something like this.

While it is a general assumption that a speaker that is more efficient ( 93 db's / 1w / 1 m ) would play progressively louder than a less efficient speaker ( 87 db's / 1w / 1m ), that is not necessarily so. While both speakers SHOULD increase volume at the same appr rate when the same amount of power is fed into both of them, this is only true so long as they remain "linear" or "non-compressed". Depending on the design of the raw speaker "drivers" (woofer, tweeter, mid, etc..), the crossover network, etc... it is possible for the "low efficiency" speaker to "catch up to" and actually surpass the output of the more efficient design.

A good example of this would be comparing a more efficient "mini-monitor" type speaker to a large less efficient multi driver speaker. The monitor just doesn't have the surface area to move enough air or dissipate as much power in the long run. As such, its' maximum sustainable output level might be quite a few db's below that of the "quieter" floor stander. This is ONLY true if you have the "amplifier muscle" to make the large speaker "sing". In this respect and as Herman mentioned, there ARE advantages to starting off with more efficient speakers.

Something else to take into consideration when looking at speakers / sensitivity / volume capability is the size of the room and your listening position. Due to differences in dispersion patterns ( how sound is projected out of the speaker ), it is possible for one speaker to be "louder" at the 1 meter "test distance" but "quieter" at the actual listening position. Larger speakers with more drivers will "focus" at a greater listening distance whereas a smaller speaker will "focus" much closer.

The effect of the speakers' "focus" becomes even more noticeable when the drivers are spaced out quite a bit in some type of vertical array, i.e. something like the large Dunlavy's, etc... where the woofers, mids, etc.. are spaced several feet apart. This is why people say "big room = big speakers, little room = little speakers".

If sitting pretty close to the speakers, you would need speakers that can sustain a good image at that distance. This usually dictates drivers that are placed together closely i.e. a "small" speaker. On the other hand, sitting at a good distance would normally require a larger array. Since distance equals time in terms of the signal travelling to your ears, the larger speakers' "multiple array" of drivers can "unfold" in time, making it sound more coherent further away than it does up close. Large speakers used "nearfield" (up close) typically sound "blurred", "less coherent" and just don't "gel" all that well when compared with more compact designs.

As to which is better in terms of "high" or "low" efficiency, there really is NO "right" answer. It is all based on the specifics of the situation ( type of music, volume levels required, size of room, amount of power available, etc...) that will dictate what will work best for you. I have speakers that range from 82 db's @ 2 ohms to 104 db's @ 8 ohms. Obviously, the two would require VERY different types of amplification and are at opposite ends of the "SPL / amplifier load" spectrum. While my 30 wpc Marantz 8 works great on my 104 db horns, it will NOT do justice to the other speakers mentioned. As such, i have built the systems around what works best in those specific situations. Just keep in mind that the more "specialized" or "exotic" the speakers get, so do the amplifier requirements and the money it takes to make them perform at their best.

My suggestion is that if you can find something that sounds good to you, will work in your room and is pretty efficient ( 90+ db's ) with a reasonable impedance ( 6 - 8 ohms ), you would be time and money ahead. This will make amplifier selection a little easier and far less critical in the long run. Just don't be fooled into thinking that "louder" is necessarily "better". It MIGHT be, but only if all of the other criteria is taken into account first. Sean
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