Measuring Ported Speakers

[ilardi]

When I measure the near field response of a ported speaker, I follow the directions in D'Appolito's book and separately measure the response at the speaker and the port (adjusting for size) and add the responses together. According to Keele, this is only accurate up to about 1.6 Fb due to contamination of the port measurement with radiation from the diaphragm.

I have only used this method when the port and the speaker are on opposite sides of the box and it seems to work well (meaning it matched predicted response closely). So does this same 1.6 Fb limitation occur when the port and speaker are located on opposite sides of the box? If the port and speaker are on the same side could the measurement contamination be minimized using some sort of baffle –or will that alter the measurement from the measured diaphragm or port? What do others do improve the accuracy of measuring ported speakers?

Thanks.

Terry

 

[mdocod]

Hi Terry,

I could be wrong, however, I believe that the size of the baffle required to fully and effectively isolate the driver from the port would have to be prohibitively large and rigid as a standalone device. The way to achieve something "close enough IMO," would be to use an existing wall between rooms, or something to that effect. A test "window" built between 2 rooms that the speaker could be placed into (with a custom baffle board "insert" to match the box size) might just do the trick.

Regards,
Eric

 

[weltersys]

What do others do improve the accuracy of measuring ported speakers?

Measure outside in half space at a distance from 1 to 10 meters depending on the size of the speaker.

 

[ilardi]

weltersys:

Do I need to either bury the speaker or raise it on a stand well above ground level in order to do this accurately?

Does anyone have experience with ground plane measurements?

Terry

 

[weltersys]

weltersys:

Do I need to either bury the speaker or raise it on a stand well above ground level in order to do this accurately?

Does anyone have experience with ground plane measurements?

Terry

I have done extensive testing of low frequency speakers with ground plane measurements.
Low frequency measurements are accurate when the speaker and microphone are both on the ground. The response of a speaker does change with proximity to boundaries, but a half space measurement is fairly representative of most use regarding low frequencies.

Raising the speaker and microphone high above the ground (a wavelength or more at the lowest frequency of interest) gives a full space reading, appropriate for the high frequency range.

 

[a.wayne]

Measure outside in half space at a distance from 1 to 10 meters depending on the size of the speaker.

I have done extensive testing of low frequency speakers with ground plane measurements.
Low frequency measurements are accurate when the speaker and microphone are both on the ground. The response of a speaker does change with proximity to boundaries, but a half space measurement is fairly representative of most use regarding low frequencies.

Raising the speaker and microphone high above the ground (a wavelength or more at the lowest frequency of interest) gives a full space reading, appropriate for the high frequency range.

+10

Still don't understand why more people here don't drag their speakers outside for more accurate ground plane measurements .....

 

[ilardi]

+10

Still don't understand why more people here don't drag their speakers outside for more accurate ground plane measurements .....

Well for one-I do most of my speaker work fairly late at night and I don't really want to work in a cold, dark yard and more importantly my neighbors probably don't want to listen to me do it.

Also, it is kind of inconvenient. I don't really want to lug out the speaker, the mike, the amplifier, the laptop, etc. every time I want to test out a change in my design.

But, could I do these measurements in my garage? I have about 20 by 20 (feet) of empty floor space-would that work?

Terry

 

[pinkmouse]

+10

Still don't understand why more people here don't drag their speakers outside for more accurate ground plane measurements .....

'Cos my front room is bigger than my garden?

 

[weltersys]

But, could I do these measurements in my garage? I have about 20 by 20 (feet) of empty floor space-would that work?

Terry

The garage will be the same as measurements in your house, other than the room modes will be different.

The reason for outdoor measurement is simply to measure the speaker, rather than the speaker and room.

No need to lug out the speaker, the mike, the amplifier, the laptop, etc. every time you make a change in your design, you may want to design with your particular room and listening and speaker location in mind.

 

[ilardi]

The garage will be the same as measurements in your house, other than the room modes will be different.

The reason for outdoor measurement is simply to measure the speaker, rather than the speaker and room.

The reason I mention the garage is because I have more 'free' space there than any other place in the house. I can at least keep the mike 10 feet from walls or other objects. I can't manage that much anywhere else in my basement and testing in the living areas of my home is a non starter (for me).

Terry

 

[weltersys]

Originally Posted by a.wayne

Still don't understand why more people here don't drag their speakers outside for more accurate ground plane measurements .....

'Cos my front room is bigger than my garden?

You make a good point, "outdoor" measurements that are near walls or fences are still not half space.

Although I often test speakers only a couple meters outside my shop door, I am aware that that location still adds about 4 dB in the octave around 40 Hz, compared to another location that is about 50 feet from any buildings, but requires lugging the speaker another 80 feet...

I have gone to leaving the test gear in the shop, and just running speaker and mic cord outside, after wind blew my laptop off the amp rack, destroying the screen .

 

[ilardi]

The garage will be the same as measurements in your house, other than the room modes will be different.

The reason for outdoor measurement is simply to measure the speaker, rather than the speaker and room.

The reason I mention the garage is because I have more 'free' space there than any other place in the house. I can at least keep the mike 10 feet from walls or other objects. I can't manage that much anywhere else in my basement and testing in the living areas of my home is a non starter (for me).

Terry

 

[ernperkins]

If you put the speaker on the floor pointing up (1/2 space measurement), mic 3 feet above speaker and assume 10 feet is the nearest boundary (wall or celing):

Distance for 1st reflection = 2 X 10 = 20 feet.
Mic Distance = 3 feet.
Time until first reflection = (20 - 3)(12 in/ft) / 13500 (in/sec) = 30.22 msec
Freq = 1/ time = 1 / 30.22 msec = 66 Hz

A gated measurement (gate ~ 30 msec) should be good down to 66 Hz. Any longer gate (= lower frequency) and you'll start measuring the garage response.

Regards...

 

[ilardi]

A gated measurement (gate ~ 30 msec) should be good down to 66 Hz. Any longer gate (= lower frequency) and you'll start measuring the garage response.

Regards...

Thank you. That is helpful. I hadn't quite considered doing it that way. D'Appolitto suggests 2 meters mike to speaker, and about 5 times that for the distance from the nearest walls and I just don't have the space for that.

But your suggestion is helpful and gives me a solution. Keele states that the nearfield technique works on a ported speaker up to roughly 1.6 fb. For my planned speaker, that gets me to about 59 hz. I can use the nearfield technique to examine response below about 60 hz and the groundplane technique above that.

Terry

 

[a.wayne]

Originally Posted by a.wayne

Still don't understand why more people here don't drag their speakers outside for more accurate ground plane measurements .....

You make a good point, "outdoor" measurements that are near walls or fences are still not half space.

Although I often test speakers only a couple meters outside my shop door, I am aware that that location still adds about 4 dB in the octave around 40 Hz, compared to another location that is about 50 feet from any buildings, but requires lugging the speaker another 80 feet...

I have gone to leaving the test gear in the shop, and just running speaker and mic cord outside, after wind blew my laptop off the amp rack, destroying the screen .

What system are you using , I have observed that 30 feet away from the bldg to be sufficient and I have not noticed any difference going further

 

[a.wayne]

Thank you. That is helpful. I hadn't quite considered doing it that way. D'Appolitto suggests 2 meters mike to speaker, and about 5 times that for the distance from the nearest walls and I just don't have the space for that.

But your suggestion is helpful and gives me a solution. Keele states that the nearfield technique works on a ported speaker up to roughly 1.6 fb. For my planned speaker, that gets me to about 59 hz. I can use the nearfield technique to examine response below about 60 hz and the groundplane technique above that.

Terry

The reason being It' s very difficult to get accurate gated response when any closer. I would not trust gated responses below 500 hz when in doors ...