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???Why The HUGE Discrepancy in Sensitivity Ratings???

I'm shortlisting speakers & have noticed a HUGE difference in the sensitivity ratings provided by manufacturers & what they actually measure.For example the Martin Logan Motion 35XT is specd at 92db.sens.@1 watt & yet actually measured at 87db!At least 4 different pair of speakers on my list are the same,specd at 90db or better @1 watt yet NONE measured better than 87db so what gives?
freediver
timlub
2,039 posts
 

The only fairly accurate way to measure is 1 watt 1 meter anechoic...

even then,  many speakers are  not adequately impedance compensated.  As posted above... 1 watt @ 8 ohms is 2.83 volts, 1 watt @ 4 ohms is 2.00 volts.... a big difference in power for rating a speaker... Many speakers are called 8 ohm that stray down to even 3 or 4 ohms and those measurements can be quite flawed....

Plus when put in them in a room setting, you know deal with peaks in the room, so someone could easily rate at its highest peak rather than 1k or over average frequency response. 

Not sure that there is anyway to win here.

These are just a few of the problems when quoting sensitivity...

If they are using off the shelf parts, like a seas, scan speak, audax, etc,  you could take a look of individual drivers and go from there... If manufactures are truly trying to give you a flat response,  the lowest sensitivity of any single driver should be very close to the sensitivity of the entire speaker,  but even that is flawed... Tough call.

Tim



erik_squires
15,871 posts
 
FYI, sensitivity and efficiency are two different things, and is rarely the same for any given speaker.

Efficiency is measured at 1 Watt, and is also called "Power Efficiency." Sensitivity is measured at 2.82 Volts and is also called "Voltage Sensitivity." If the speaker is purely resistive and measures exactly 8 Ohms, then these two figures will be the same.

Efficiency explains how much work is required to produce a certain volume level, while voltage sensitivity cares nothing for the amount of work, and only thinks about what happens when a certain voltage is applied. Take for instance a 1 Ohm speaker, remember the old Apogees?

At 2.83 Volts, an 8 Ohm speaker draws 1 Watt, a 4 Ohm speaker 2 Watts, 2 Ohm speaker 4 Watts, and a 1 Ohm speaker draws 8 watts.

Sensitivity could be accurately called 85 dB, but at 1 Ohm it would actually be drawing 8 Watts, so the efficiency is actually a lot lower, probably around 78 dB or so.  I've gotten quite forgetful about the precise math, but you get the idea.

This matters more to tube amplifiers, which have the same power output regardless of the transformer taps. For those with reasonably beefy solid state amplifiers, and speakers with impedance curves at 3 Ohms or higher voltage sensitivity matters more.



Best,


Erik
bombaywalla's avatar
bombaywalla
1,958 posts
 
freediver,
i'm looking at the M-L 35XT speaker manual & on the last page, the specs read:
Sensitivity = 92dB @ 2.83V / meter.
You can find the manual here:
http://www.martinlogan.com/pdf/manuals/manual_motion_35xt.pdf

The speaker is clearly noted as a 4 Ohms speaker.
If you input 2.83V into 4 Ohms (after you calculated V^2/R) you get an input of 2W.
Usually the standard is to input 1W into an 8 Ohm speaker. 
So, for the M-L Motion 35XT we have to compensate/normalize to bring Sensitivity number to an equivalent 8 Ohms so you can do an apples-apples comparison with other speakers.
We need to halve the number of input watts to 1W Or double the input speaker impedance to 8 Ohms (which will also yield an input of 1W).
In either case, the output SPL should drop by 3dB (assuming the speaker SPL response is linear in this region & is being fed by power well within its handling capacity. This should be the case as 1W is well within the power handling capacity of the speaker which also means its not distorting hence response should be linear).
So, in effect the M-L Motion 35XT Sensitivity is 92-3 = 89dB.
You say it's measuring 87dB ( I could not find a review that said this. Can you please provide a link? Thanks).
it's not out of the ordinary to lose 2dB due to measurement setup issues. And, like others wrote, the spec could be in-room & the measurement could be in an anechoic chamber. 
Anyway, you should view the M-L Motion 35XT as an 89dB sensitivity speaker; not 92dB. 

timlub
2,039 posts
 

I am very guilty of using the term sensitivity and efficiency interchangeably and really they are, a more efficient speaker will play louder than a less efficient speaker, but in the truest sense...

Loudspeaker efficiency is defined as the sound flux or sound power at output divided by the electrical power input...

Sensitivity is how loud a speaker plays with a given input.   I defer back to 1 watt of power measured 1 meter away anechoic. 

Tim

nonoise's avatar
nonoise
10,587 posts
 
These last posts from timlub, erik_squires and bombaywalla should be carved in stone.

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