How can I find the SPL?

[zesla]

Hello Guys,

Sorry but I need some help....
I have several of these ultrasonic sensors:
http://www.prowave.com.tw/english/products/ut/open-type/400s160.htm

As you can see the said sensor has the below characteristic for the SPL:
"Transmitting Sound Pressure Level (at 40.0KHz; 0dB re 0.0002µbar per 10Vrms at 30cm) 120dB min. "

So by connecting 10Vrms sine wave across the sensor pins we'll get 120dB of sound pressure level at 30cm.
Now the questions I am dealing with are:
1: how can I find the SPL of the sensors at say 1m of distance or more?
2: how much of SPL I'll get if I increase the voltage to 20Vrms?
3: How much of SPL I'll get if I put 2 of these sensors in parallel and side by side while feeding them by 10Vrms at 30cm and at 1m?

The SPL formula is:
SPL= 20log (P2/P1)
where P is the sound pressure.

Thanks a bunch for any help

 

[canadaelk]

1: If the transducer is a point source, in free space: for every doubling of distance the SPL level is 1/4 or -6dB SPL (inverse square law)
2: Asuming the transducer is Ohmic (which it propably is not) it would be: E squared/R
3: If the transducers are side by side you will get lobing but generally +3dB SPL
E

 

[zesla]

1: If the transducer is a point source, in free space: for every doubling of distance the SPL level is 1/4 or -6dB SPL (inverse square law)
2: Asuming the transducer is Ohmic (which it propably is not) it would be: E squared/R
3: If the transducers are side by side you will get lobing but generally +3dB SPL
E

Hi and thanks,

1: Are you thinking that an ultrasonic sensor acts as a point source radiating the wave at all directions? it does not.
I guess that the SPL falls 3dB per every doubling the distance. Sorry I can not calculte the exact dB value at 1M can you please help?

2: the piezo ultrasonic sensors are not ohmic they are more capacitive but even if they were ohmic I dont think you could find the right SPL by calculating the Electrical power of them!!

3: as the sensors are connected in parallel and hence in phase so I think we'll get 6dB by putting them side by side not 3dB Which is for unphased transducers.

Any idea plz?

 

[Sceadwian]

I think we'll get 6dB by putting them side by side

This assumption is incorrect. You assume they're in phase, but they're only in phase electrically, placing them any distance apart will cause the actual acoustic waves in free space to be out of phase, this will cause both an increased directional effect and cause nulls to exist at various locations especially with reflections.

Calculating something like this is very difficult due to the large number of environmental variables, the math gets out of had VERY fast. If you really need to measure the SPL at a specific point, purchase an SPL meter that will respond at your frequency of interest, I think you'll be quiet surprised at the different measurements you'll get as you move the SPL meter around relative to the transducer, especially in an enclosed room with obsticals in it.

Using this calculator, if you punch in 40khz you'll find the wavelength of a 40khz audio wave is 0.8575 centimeters, so the exact center of the transducers will need to be very precisely aligned at a multiple of this value, and the directional/null effect and the shape of the lobes will change with each increase wavelength of separation of the transducers.

I'm attaching a PDF that contains a lot of good information about 40khz, including the radiation patterns of a typical disc based transducer (almost all ultrasonic transducers are piezo disc based)