FR for the M80's/M60's

Posted by: beef

FR for the M80's/M60's - 09/24/05 10:50 AM

Hi,

I again apologize for my newbiness...I'm sure this question has been asked before. I'm trying to decide which Axiom speakers might be best suited for my situation. But there is something I don't understand about the M60 and M80 specs.

The M60 spec says that the frequency response is +/-3db down to 37Hz. But the graph clearly shows it rolling off 3db at around 60-70Hz. There is a similar situation with the M80's. Can someone let me know what I'm not seeing...I figure I'm looking at this wrong...but can't figure it out. Many thanks.
Posted by: bridgman

Re: FR for the M80's/M60's - 09/24/05 11:08 AM

My understanding is that the graphs were measured in the NRC anechoic chamber, which is accurate down to 80 Hz but rolls off below that. Apparently this is a function of the size of the room; bigger chambers are accurate to lower frequencies. Axiom combines the NRC anechoic chamber measurements with outdoor "speaker up on a pole" tests to get the overall frequency response.

My understanding is that the dip starting around 80 hz is largely an artifact of the chamber size but I'm not 100% sure about that.

EDIT - here are some comments from Ian about this :

http://www.axiomaudio.com/boards/showflat.php?Cat=&Board=tech&Number=89253&page=&view=&sb=&o=&vc=1
Posted by: beef

Re: FR for the M80's/M60's - 09/24/05 02:56 PM

Thanks for the post....but it still doesn't make sense to me. Anechoic...is theoretically "non-echo" in an open environment...like outdoors on a pole, as you mentioned. So rolloff due to chamber size makes no sense for anechoic measurements. The rolloff appears to be strictly related to the speaker performance. And it's down 3db at perhaps 70Hz in the M60 (as measured anechoically per Axiom specs).

Anyone else want to take a shot at this. Maybe I should contact tech support?

Thanks and good cheer. Still scratching my head.....

Beef
Posted by: SirQuack

Re: FR for the M80's/M60's - 09/24/05 06:57 PM

I'm sure Alan from Axiom will jump in here and give you the answer your needing, sorry I can't help, I just like the sound
Posted by: Wid

Re: FR for the M80's/M60's - 09/24/05 09:38 PM

I do believe that when it is stated as +/- 3 db in the specs that you have a 6 db swing, that accounts for what you see in the graph.Both the M60s and M80s are right on spec when you account for this.
Posted by: Wid

Re: FR for the M80's/M60's - 09/24/05 10:49 PM

In reply to:

I do believe that when it is stated as +/- 3 db in the specs that you have a 6 db swing




I meant to say you can have a 6 db swing and still be within stated specs.
Posted by: bridgman

Re: FR for the M80's/M60's - 09/24/05 11:12 PM

>>Anechoic...is theoretically "non-echo" in an open environment...like outdoors on a pole, as you mentioned. So rolloff due to chamber size makes no sense for anechoic measurements.

Ahh, but that's the whole point. The low frequency cutoff is where the anechoic chamber stops being anechoic. This was the first link I found that talks about it in more detail :

"An anechoic wedge is designed to provide a normal-incidence sound absorption coefficient greater than 0.99 for all frequencies down to its design cutoff frequency. Reflected sound from such a wedge is attenuated 20 dB or more. An anechoic wedge is typically one-quarter wavelength long at its cutoff frequency. For example, a 100 Hz cutoff wedge is usually approximately 36 inches long."

http://zone.ni.com/devzone/nidzgloss.nsf/webmain/62E64F4028A2D77C862568C6005E4B84?OpenDocument
Posted by: JohnK

Re: FR for the M80's/M60's - 09/24/05 11:55 PM

Beef, it's a combination of two things, first, as Rick pointed out and as Ian and I have discussed here in the past, +/- 3dB means a 6dB "window" or "swing", not a 3dB down point. Looking at the uncorrected graph, the M60 would be about 6dB below the 90dB high point(at around 95Hz)at 55Hz. Then, the low frequency correction Ian discusses in John B's link apparently shows that the 6dB down point measured anechoically is actually at 37Hz rather than 55Hz. Of course, in-room reinforcement changes the real-world result.
Posted by: beef

Re: FR for the M80's/M60's - 09/25/05 12:51 AM

Thanks, JohnK and others, for the education.

I STILL want to hear them! ;-))
Posted by: dmn23

Re: FR for the M80's/M60's - 09/25/05 09:54 AM

Hey, I always wondered about it myself but I was too shy to ask.
Posted by: bray

Re: FR for the M80's/M60's - 09/25/05 11:34 AM

Beef
BTW: WELCOME to the Axiom forums.
Like I said, (over there)" For the most part we're a pretty nice bunch".