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81ST4 8" Subterranean Subwoofer (4 ohm)

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The 81ST4 is the second in our line of mini landscape subwoofers designed for local zone bass performance. It can be configured with 1 satellite speaker as a mono channel, 2 satellite speakers as a 2.1 stereo or with multiple satellite speakers in small patio areas. James’ subterranean subwoofer’s direct burial, 8” compression band pass design creates unrivaled low end frequency response. External hooded port, aircraft/marine grade aluminum enclosure and stainless steel hardware provide for increased protection in extreme weather. This unit is also available in an 8 Ohm or 70V version. Made in the USA

Specifications
Frequency Response

28-180Hz +/-3dB

Nominal Impedance

4 ohms

Sensitivity

87dB 2.83V/1m

Recommended Power Range

50 - 350W

Woofer

(1) 8" (203mm) Aluminum

Enclosure

Aircraft Marine Grade Aluminum

Finishes

Black

Height

10.95 in (278.13 mm)

Width

10.5 in (266.70 mm)

Depth

10.5 in (266.70 mm)

Net Weight

20 lbs (9.07 kg)

Product Documents

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Category FAQs
Answer:

Traditional 70V speakers will have a transformer as the load.  Because tranformers react differently to DC and AC signals, an ohm meter reading will measure the DC resistance (DCR) of the transformer and will not provide any info on the actual rating of the speaker when passing audio, and might even read near 4 or 8 ohms which might indicate incorrectly, it is a 4 or 8 ohm speaker.  In the audio band, the impedance presented by the transformer will be much greater than the DC resistance.

Unlike traditional 70V speakers, James Landscape sats, the AT series, also feature an additional low frequency roll off via a built-in network to protect the speaker as well as allow running the sats on the same line as a subwoofer.  These features block all DC signals, making an ohm-meter useless for testing as it will measure "open circuit" on both 4 ohm and 70V speakers.

So the speaker must be measured with an 70V IMPEDANCE METER which characterises the IMPEDANCE at 1kHz, not at DC RESISTANCE.  Examples follow the table.

Below is a table which shows impedance and DCR reading to be expected on AT sats and ST subwoofers.

Table1 - AT and ST loudspeaker impedance data
MODEL 1kHz DCR NOTES
32AT4 9 ohms open  
32AT70 300 ohms open  
42AT4 7 ohms open  
42AT70 150 ohms open  
62AT4 11 ohms open  
62AT70 105 ohms open  
81ST4     Bi-amp only
81ST70     Bi-amp only
81ST70-X     Crossover for fullrange systems
101ST4     Bi-amp only
101ST70     Bi-amp only
101ST70-X     Crossover for fullrange systems
121ST4     Bi-amp only
121ST70     Bi-amp only
121ST70-X     Crossover for fullrange systems
151ST4     Bi-amp only
151ST70    

Bi-amp only

Fig 1 shows three examples of 70V impedance measurement tools; the TOA  ZM-104A, the TENMA 72-6948, and the Dayton Audio DATS2 speaker testing software.  The two impedance meters test at only 1kHz which can give you readings which may appear out of spec, where as DATS2 gives you a full 20Hz to 20kHz impedance curve so you see all the data and read the frequency of interest.  DATS2 is our favorite as it works with a laptop, is small and also the cheapest of the options.

Fig 1 - 70V mesurement tools

To show the versatility of DATS2, Fig 2 below shows a DATS2 curve run on a 32AT70 loudspeaker.  To get the same reading as an impedance meter, you would read the impedance at 1kHz o the graph, but as you can see, there is a wealth of information in the curve.  In this example, DATS2 is calling out about 281 ohms at 1 kHz, close to the 300 the impedance meters will read.

For those with an understanding of the math, the power at 1kHz will be 5000/281 = 18W.  The lowest impedance is around 450 Hz at about 190 ohms, giving 5000/190 = 26W.  So as you can see, even though the transformer is rated at 30W at 4 ohms, when used with a speaker the real power is quite different. 

When this same transformer is loaded with a 4 ohm resisitive load, the reading is 140 ohms which is 5000/140 =3 5W, typical for a 30W rating.  So all contant voltage transformers, regardless if they are 25V, 70V or 100V, only have a nomial power rating at a nominal load impedance - results will vary in use!

 

Fig 2 - DATS2 curve of 32AT70

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