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#138235 - 05/12/06 05:27 PM Maximum SPL at 1kHz
weekendwally Offline
newbie

Registered: 05/12/06
Posts: 2
I am an engineer developing a microphone system for a cockpit voice recorder. One of the tests required is microphone distortion at 1kHz at 120dB SPL. The distance from the loudspeaker source is not specified. I am not sure if your everyday commercial off the shelf loudspeaker would be capable of delivering this level of pressure at the microphone. If I had a typical sensitivity level of 92dB SPL at 1m what sort of level would one expect at 3" from the loudspeaker? At 1m I would need to drive 1000 watts into the speaker. Any suggestions as to the speaker to use? Must be under 5% distortion.

I also need a speaker that is relatively flat in response, 150Hz to 10kHz capable of producing 94dB SPL with about 2% distortion max.

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#138236 - 05/12/06 10:19 PM Re: Maximum SPL at 1kHz
pmbuko Offline
shareholder in the making

Registered: 04/02/03
Posts: 16227
Loc: Leesburg, Virginia
I'm pretty sure an Axiom M80 driven by a capable amp could easily hit 120db at 3" with low distortion. And, as luck would have it, an M80 can handle 1000 watts, too.

Seriously, the M80 is what you're looking for. Buy two, and keep them for music and movies after you're done with the microphone.
_________________________
"I wish I had documented more…" said nobody on their death bed, ever.

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#138237 - 05/12/06 10:43 PM Re: Maximum SPL at 1kHz
Ken.C Online   content
shareholder in the making

Registered: 05/03/03
Posts: 17699
Loc: NoVA
Peter, master of the questionable requisition.
_________________________
I didn't do it, no one saw me, you can't prove anything.

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#138238 - 05/14/06 12:03 AM Re: Maximum SPL at 1kHz
danmagicman7 Offline
connoisseur

Registered: 12/16/05
Posts: 1467
Axiom stress tests the M80's at over 1400 Watts.

I'm sure Ian or Alan can chime in and give you some serious statistics on how the axioms perform, distortion and soforth.

They have their own testing chamber, so they may even run a quick test just for you to verify what you are asking. :-)
_________________________

M22s|VP100|QS4s|HSU STF2

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#138239 - 05/14/06 11:50 AM Re: Maximum SPL at 1kHz
alan Offline
connoisseur

Registered: 01/29/02
Posts: 3178
Loc: Toronto/New York/Dwight
Hello weekendwally,

Tests of endurance, sensitivity and maximum output levels in dB SPL of domestic loudspeakers are not performed with single-frequency test tones, but with what's known as a "broadband noise signal" with weighting applied. It's a simulated program signal (much like pink noise with equal energy octave to octave) but intended to simulate the bulk of energy content of a complex music signal.

One example used for years during tests of loudspeakers at the Canadian National Research Council had a uniform output between 200 Hz and 1 kHz, rolling off above and below those points at rates that resulted in -10 dB points at 25 Hz and 9 kHz.

If you wanted to try using a fixed-tone 1-kHz signal at extremely high SPLs like 120 dB, I'd suggest you use a big horn-loaded system with very high sensitivity (over 100 dB SPL, 1 watt/ 1 meter). Such speakers exist for concert sound reinforcement. They do not have flat frequency responses (unless carefully equalized) but typically distortion is very low and they are designed to be indestructible at such high SPLs.

Using a fixed-tone test signal at 120 dB with domestic Hi-fi speakers would risk damaging individual drivers. I'll check with Ian, the owner and designer of Axiom speakers, on the broadband noise signal used for our tests of the M80. The specification for the broadband noise signal used at the NRC used weighting according to IEC 268-1 (International Electro-technical Commission).

Regards,
_________________________
Alan Lofft,
Axiom Resident Expert

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#138240 - 05/14/06 06:34 PM Re: Maximum SPL at 1kHz
weekendwally Offline
newbie

Registered: 05/12/06
Posts: 2
Thank you Alan,

The specific test I am trying to meet is an Eurocae standard ED-112. The microphone is to be used in a cockpit voice recorder aplication. ED-112 wants to ensure that the distortion at 120 dB does not exceed 10% at 1 kHz. It is probably not the most up to date standard in terms of testing, but it is what the FAA says I have to meet. Do you have suggestions for a manufacturer of large horn-loaded speakers? I have taken a look at Mackie as they have some impressive SPL numbers in a relatively small encosure, but I would prefer just to buy the horn and not the entire loudspeaker as I have to fit it in an environmental chamber.

Also, could you advise on an Axiom product that has the flattest frequency response between 150Hz and 10kHz? The SPL for the frequency response test is 60dB and 94dB.

Appreciate the input.

Thanks
Trevor

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#138241 - 05/14/06 09:41 PM Re: Maximum SPL at 1kHz
danmagicman7 Offline
connoisseur

Registered: 12/16/05
Posts: 1467
The m80 would have the flattest response for that range. Axiom posts frequency graphs for almost every speaker on the left side.

Here is the frequency graph for the M80:


_________________________

M22s|VP100|QS4s|HSU STF2

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#138242 - 05/14/06 10:32 PM Re: Maximum SPL at 1kHz
ratpack Offline
connoisseur

Registered: 04/30/05
Posts: 1155
Loc: Alabama
At 3" you are inside the diameter of some of the drivers. I'm not sure that the sound field would be fully developed so close in.
_________________________
The Rat. M80s, VP-150, QS8s, SVS PC 20-39+, OPPO, Onkyo 703s, Harmony 880 Sony 60" SXRD HDTV

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#138243 - 05/15/06 09:55 AM Re: Maximum SPL at 1kHz
alan Offline
connoisseur

Registered: 01/29/02
Posts: 3178
Loc: Toronto/New York/Dwight
Hi,

Ian will be doing some measurements with our M80 in Axiom's large anechoic chamber, so I should have some data for you in a day or so.

Of the Axiom line, the M80, M60, M22 and M2 are very linear, and the M60 and M80 would have very low distortion (under 2%) in the 94 dB SPL range.

My expertise is in domestic hi-fi speakers, not concert and pro sound reinforcement. Some well-known past brands include Meyer and JBL, and there may be new players on the block. Axiom does have a consultant on digital design who used to do the live on-tour concert mixes and setups for Led Zeppelin way back when, so I'll see if I can get some feedback from him.

Regards,

_________________________
Alan Lofft,
Axiom Resident Expert

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#138244 - 05/15/06 12:38 PM Re: Maximum SPL at 1kHz
MarkSJohnson Offline
shareholder in the making

Registered: 09/27/04
Posts: 10698
Loc: Central NH
In reply to:

Axiom does have a consultant on digital design who used to do the live on-tour concert mixes and setups for Led Zeppelin way back when




Cool!

He should definitely know a thing or two about volume!

I said....
"HE SHOULD DEFINITELY KNOW SOMETHING ABOUT VOLUME! No, VOLUME!!!!!
_________________________
::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::

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