Low Frequencies

[Gene Howe]

Anybody out there know of any instrument to measure the dB of sound below 20 HZ?
Thanks,
Gene

[Myk Rian]

That could be a tough one. All the ones I've seen are for 31.5Hz and up.

[Dennis McGarry]

Sound below 20hz, ie infrasound will be tough to measure without a scope and special software.

i have not seen anything short of that to do it..

[Mike Henderson]

Just for interest, what's your application? That is, what situation (or environment) are you measuring below 20 Hz?

Mike

[Ken Fitzgerald]

There are spectrum analysers out there that will measure to 1 Hz.

[Edwood Ferrari]

A frequency analyzer preferably a FFT type would be a good choice. There are software tools available. But the real problem is the sensor or microphone. Most microphones will ether not work at all at such low frequencies or would have no calibration. There are some available but for big bucks. A second problem is getting a pre-amp with low enough bandwidth and noise. A good FFT analyzer would be able to handle signals around -100dBm however. Since you are asking to measure the levels in dB you are needed a calibrated system.

If you just need to measure frequencies just below 20 Hz there are a lot of SPL meters that will easily work down to 20 Hz and probably a bit below. But to be accurate you would need a known calibrated source and the know how to calibrate the meter. Scantek makes one that works in the infrasonic range but I can only imagine the cost.

Good luck

[Derek Gilmer]

A radio shack spl meter will measure down to around 10hz I believe. You just need a correction chart because it is slightly off. Here is a chart: http://forum.audiogon.com/cgi-bin/fr.pl?htech&983682086&openflup&1&4

[Gene Howe]

Just for interest, what's your application? That is, what situation (or environment) are you measuring below 20 Hz?

Mike

Hi Mike,

A developer is applying to our county for a permit to install 300 large wind turbines (450' high). As you may know, they produce low frequency sound as the blades pass by the tower. Low frequency sound, at 35 dB +- causes vertigo, spatial disorientation and several psychological phenomena.

In order to adequately protect people living close to these proposed turbine sites, setbacks need to be established. There are several of these turbines operating already in a different and remote location. we would be taking readings on those turbines. The environment for testing would be out side, in winds of up to 25 mph.

Money is no object, as it would fall to the developer to perform the testing, or hire it done. We only need to know what sort of instrumentation to require of the testers.

If a manufacturer and model can be identified, we can obtain the specs and require that those specs be used in selecting the instrument.

Thanks,

Gene

[Derek Gilmer]

A developer is applying to our county for a permit to install 300 large wind turbines (450' high). As you may know, they produce low frequency sound as the blades pass by the tower. Low frequency sound, at 35 dB +- causes vertigo, spatial disorientation and several psychological phenomena.


Yes it does. I mess with home theater stuff quite a bit and doing sine wave sweeps down to 12 hz at 110db is an oddddd feeling. Almost like my eyeballs are vibrating :D

[Gene Howe]

Yes it does. I mess with home theater stuff quite a bit and doing sine wave sweeps down to 12 hz at 110db is an oddddd feeling. Almost like my eyeballs are vibrating :D

Almost like squirting cold water into your ear....completely disrupts the balance.
Don't try it if you're prone to motion sickness:eek::eek:

[Ken Garlock]

Get your hands of a Tektronix oscilloscope. Being a precision instrument you can expect to pay in the 2K and up neighborhood. Refurbished are less expensive.

A friend once designed an amplifier that was flat from DC to light and the customer still complained about the frequency response. :eek::rolleyes:

[Dennis McGarry]

Note sure if it would work for you, but look into the low freq dub mics for drums..

http://www.shure.com/ProAudio/Products/WiredMicrophones/us_pro_SM81-LC_content

Type Condenser (electret bias) Frequency Response 20 to 20,000 Hz http://www.sawmillcreek.org/stellent/groups/public/@gms_gmi_web_us/documents/web_resource/site_img_us_rc_sm81.gif view larger (http://www.sawmillcreek.org/stellent/groups/public/@gms_gmi_web_us/documents/web_resource/site_img_us_rc_sm81_large.gif) Polar Pattern Cardioid (unidirectional) response-uniform with frequency, symmetrical about axis Output Impedance Rated at 150 ohms (85 ohms actual) Recommended minimum load impedance: 800 ohms (May be used with loads as low as 150 ohms with reduced clipping level) Output Configuration and Connector Balanced, transformer-coupled output; male XLR connector Sensitivity (at 1,000 Hz) Open Circuit Voltage: -45 dBV/Pascal (5.6 mV
(1 Pascal = 94 dB SPL) Clipping Level (at 1,000 Hz) 800 ohm Load: -4 dBV (0.63 V) 150 ohm Load: -15 dBV (0.18 V) Total Harmonic Distortion Less than 0.5% (131 dB SPL at 250 Hz into 800 ohm load) Maximum SPL (at 1,000 Hz) 800 ohm load: 136 dB (attenuator at 0) 146 dB (attenuator at -10) 150 ohm load: 128 dB (attenuator at 0) 138 dB (attenuator at -10) Hum Pickup -3 dB equivalent SPL in a 1 mOe field (60 Hz) Self-Noise (equivalent sound pressure levels; measured with true rms voltmeter) 16 dB typical, A-weighted 19 dB typical, weighted per DIN 45 405 Signal-to-Noise Ratio 78 dB (IEC 651)* at 94 dB SPL *S/N ratio is difference between 94 dB SPL and product's equivalent self noise (SPL A-weighted). Overvoltage and Reverse Polarity Protection Max. external voltage applied to pins 2 and 3 with respect to pin 1: +52 Vdc Reverse polarity protection: 200 mA max. (diode-clamped) Polarity Positive pressure on diaphragm produces positive voltage on pin 2 relative to pin 3 Cartridge Capacitance 54 pF Low Frequency Response Switch Positions Flat; -6 dB/octave below 100 Hz; -18 dB/octave below 80 Hz Attenuator Switch Positions (Lockable) 0 or -10 dB Power Supply Voltage: 11 to 52 Vdc, positive, pins 2 and 3 Current Drain: 1.2 mA max.