Exhaust Fans: Volume(cfm)-NC(sones) relationship 2

[kraiva]

This is a newbie question, I am an EIT.

I am specifying Penn Zephyr bathroom fans on a job I am designing. If the performance data says 171 cfm @ 2.0 Sones and I specify a speed controller withing the fan casing to throttle it down to, say, 155cfm, what should my Sones value be? In particular,

1. Is there a linear relationship between the two variables?

2. When the contractor, goes in to buy this, how do they set this, if at all? Is done as part of the balancing?

3. Are there any rules of thumb on when to throttle down the volume? I am finding that sometimes I have calculated an exhaust volume at say 160cfm and the spec sheet has a fan at 170cfm do I throttle that down or is it within acceptable range?

Kind regards.

 

[friartuck]

1. No
2.Fit it and then measure the performance in situ. Slow the fan down till you get the right flow rate.
3.Clients requirements and codes may be different. In UK if the flow is at the required or up to +10% then it is acceptable...though if you have numerous fans running at 10% over duty, the wasted energy will be high.

PS halfing the fan speed reduces the noise level by 15dB.
I have a table produced by a fan manufacturer who did the lab tests and I can tell you what the normal reduction is. (If u want it)

note some speed controls (particularly electronic (not autotransformer or inverter) can INCREASE noise due to harmonics produced in winding by square wave forms in thyristor control circuits.

PS Never knew anyone specd fan noise in sones....a bit odd??

dBA at 3m or SWL sound power level is more appropriate.

suggest fan manufacturer produced 'proper' sound data

Friar Tuck of Sherwood

 

[imok2]

The sone is derived from psychophysical measurements which involved volunteers adjusting sounds until they judge them to be twice as loud. This allows one to relate perceived loudness to phons. A sone is defined to be equal to 40 phons. Experimentally it was found that a 10 dB increase in sound level corresponds approximately to a perceived doubling of loudness. So that approximation is used in the definition of the phon: 0.5 sone = 30 phon, 1 sone = 40 phon, 2 sone = 50 phon, 4 sone = 60 phon, etc.

Wouldn't it be great to be able to convert from dB (which can be measured by an instrument) to sones (which approximate loudness as perceived by people)? This is usually done using tables that you can find in acoustics handbooks. However, if you don't mind a rather crude approximation, you can say that the A weighting curve approximates the human frequency response at low to moderate sound levels, so dBA is very roughly the same as phons. Then use the logarithmic relation between sones and phons described above.

For a complete explanation g

http://www.phys.unsw.edu.au/~jw/dB.html#definitiono

here: