When Measuring a Woofer's Response, Where Do You Place the Mic?

[toddalin]

I've been designing a set of speakers/crossovers and have been dealing with the woofer's response.

The woofers (10" JBL 2251J) are in ~1/2 cu ft ported cabinets. These are to be used as midrange speakers with an 18" for the woofer and a Heil AMT for the tweeter

I can set the crossover values to get the 2251s to better than +- 3 dBA from ~120 Hz - 4K, in my room, when the mic is placed <1" of the woofer dome, within the "confines of the cone" itself.

However, if I pull the mic back, even just a couple inches, it's a totally different story where the bass end fall off like a stone. And of course, the further back you measure, the more the room influences the results.

I wish I had an anechoic chamber, but that won't happen. I may be able to take it outside and lie it on its back raised a bit so the port would be open in the back.

But even then, how close should the mic be to the cone and where/how did JBL take the measurement??? http://www.audiokarma.org/forums/styles/default/xenforo/smilies/scratch2.gif

Thanks

[Ian Mackenzie]

Hi Tod

This is a normal problem with near field measures

One way around this is to do a measurement as you have done of the woofer and then put the mic over the port and splice the two measurement together

But due to the port radiating area done calculations are required to adjust the measurement for the real spl if hhs port.

One option is to do a simulation of the driver in BassBox of the ported enclosed. This is normally fairly accurate.

Alternatively, attempt a ground plane measurement on your driveway outside

Place the enclosure on its side on the driveway away from any buildings

Place the mic on the driveway 2 m from the enclosure

Tilt the enclosure so the centre of the cone is on axis with the position of the mic

You will get a 6 db increased measurement overall due to the ground acting as a reflection of the direct sound from the driver and this is normal

There are some limitations

The ground plane measurement is not the same as a half space measurement where impact of the baffle radiating area can influence the baffle step response

See this link for further discussion
https://www.prosoundtraining.com/2014/04/24/ground-plane-measurements/

http://www.mh-audio.nl/Groundplane.asp

Below is a link to a Jbl spec sheet of a typical driver

Refer to comments about measurement
http://www.jblpro.com/pub/obsolete/2118.pdf

[toddalin]

Thanks.

[ivica]

I've been designing a set of speakers/crossovers and have been dealing with the woofer's response.

The woofers (10" JBL 2251J) are in ~1/2 cu ft ported cabinets. These are to be used as midrange speakers with an 18" for the woofer and a Heil AMT for the tweeter

I can set the crossover values to get the 2251s to better than +- 3 dBA from ~120 Hz - 4K, in my room, when the mic is placed <1" of the woofer dome, within the "confines of the cone" itself.

However, if I pull the mic back, even just a couple inches, it's a totally different story where the bass end fall off like a stone. And of course, the further back you measure, the more the room influences the results.

I wish I had an anechoic chamber, but that won't happen. I may be able to take it outside and lie it on its back raised a bit so the port would be open in the back.

But even then, how close should the mic be to the cone and where/how did JBL take the measurement??? http://www.audiokarma.org/forums/styles/default/xenforo/smilies/scratch2.gif

Thanks






Hi todd,

As You have relative small box for the speaker, You can put the box on the shelf about in the middle of the wall and move back the mic 1~1.5m, and do the measurements. Use the gating time to reduce the reflections from the walls. to see the influences of the reflections on the LF, move the mic about 0,5m left-right-up-down and get the mean value of the measurements.

regards
ivica

[toddalin]

Could work but would have to be far enough so as to not influence the rear port.

[Ian Mackenzie]

http://www.audioheritage.org/vbulletin/showthread.php?17010-2251-Variants

Some more info above. A few have gone this road before

What are the dimensions of the front baffle of the mid dog box?

Is it a satellite or part of a larger system?

If it is just the driver dimensions then it might cause a step response but depending on where you actually mounted the enclosure would have an over riding influence on the low end

[toddalin]

Separate cabinets ported in rear, 11.9" x 11.9"

http://www.audioheritage.org/photopost/data//500/medium/DSC_00011.JPG
http://www.audioheritage.org/photopost/data//500/medium/DSC_00031.JPG

[Ian Mackenzie]

Looks like a B&W 801 on steroids..LOL

I would not worry too much.

The problem is that any thing nearby will impact on your measurements at those frequencies and mess it up. The near field is as good as you can do unless you can drag its outside. Gated measurements are both unreliable and somewhat impractical in the 100 hz area due to the distance required for a gated clean impulse.

[Ruediger]

From D.B. Keele's website:

6. "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement," J. Audio Eng. Soc., (April 1974).

A loudspeaker test technique is described which depends on nearfield pressure measurements
made in a nonanechoic environment. The technique allows extremely simple
measurements to be made of frequency response, power response, distortion, and
electroacoustical efficiency.

Ruediger