Axiom Home Page Forums General Discussion Stereo M80 Maximum SPL

I will attempt to condense this one down to the Coles notes (Cliff notes to some I believe) for the purpose of this post. But we should think about doing a mini series in our newsletters about it some time. There are many different types of distortion and they are all audible at different levels down from the fundamental depending on frequency and type of distortion. For instance harmonically related distortion is very hard to detect if it is the second harmonic but gets progressively easier as you head towards the fifth harmonic at which time it is as audible as straight noise. Low frequency noise distortion is very hard to hear where as high frequency is easy. Getting to the original question here; speakers, if properly designed and this is a big if out there, can have very low audible distortion at high power. They will exhibit compression which is comparable to just running out of volume or perhaps soft clipping. Inevitably this compression does not happen equally at all frequencies so you have a level below which there is no compression and then a level at which the compression would start to become worth noting which should be considered the max level the speaker can play to. A single M80 in an anechoic chamber starts to exhibit compression at about 118 dB. You can keep pumping more power in but you won’t be getting all frequencies back out in a linear fashion. The other very important fact to keep in mind is that even though 118 dB sounds well above any sort sane listening level; music and movies are dynamic. So if you want really clean reproduction at moderately high listening levels, or normal levels in a large room, you have to consider those peaks. This where lots of power and lots of speaker output without compression becomes heavenly.

thanks for the explanation I think this explains what I was trying to get at the best as far as the distortion is concerned:

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you have a level below which there is no compression and then a level at which the compression would start to become worth noting which should be considered the max level the speaker can play.

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This would be an intersting spec to publish on the website for all the speakers, even though it seems like something you would usually see on a white van speaker box, I think it is actually a useful spec especially when you consider the peaks in music and how loud people listen. One of the satellite HT systems on the KEF site is spec'd to max out at 104db which I dont think would cut it for alot of us.

JohnK,

I don't understand how to read that curve. For example, at 60Hz I see a peak of 69dB. What does this mean?

Mo, it's how far below the top curve representing the speaker output that the bottom curve representing measured distortion is. At the 60Hz frequency you mention the difference is roughly 21-22dB. Distortion down that amount is about 8-9%.

Ok, I get it. Thanks.

Wow! That means at the levels I listen to music (about 85 to 90dB), I experience about 1.5% THD max (room acoustics excluded)...and that's over a very narrow range of frequencies. Over the majority of the range, THD is about 0.5%. That's incredibly impressive actually. It would be even more impressive if it's all second harmonics .

I'd be curious to know how THD varies at lower SPLs.

Well, as to distortion at lower levels, the usual 90dB NRC graph is also shown and the distortion is low enough that at even lower dB levels it can be assumed that it wouldn't be audibly significant. Of course, note in the NRC measurements description that they point out that "90dB measured anechoically is very loud and considered far beyond normal listening levels..."

Incidentally, the formula for calculating the dB down distortion level is 20log(distortion percentage/100).