Volume, but in terms of signal level

[Serebus0126]

10/10/2014 at 1:45 pm

I was curious if anyone has ever measured the signal strength (dB) of the output of their speakers? I am using cheap PC speakers and used my phone with a free app to measure the output of the speakers (mostly to convince myself the silents were playing and the speakers weren’t blown).

I have to have the phone about 6″ from the speaker, in direct line of the speaker and the microphone pointed at the speaker to measure about -88dB. I played with the volume controls and got a max reading of -80dB so that is about as loud as I can get out of these speakers.

Just curious if others have made these measurements and can confirm this is indeed “loud” enough. The speaker is sitting on her nightstand pointed right at her head which is usually within 3′ of the speaker. Switching to a normal audio output they are certainly loud enough to hear.

[Fizbin]

10/11/2014 at 11:37 am

I haven’t done that particular test, but consider that your microphone’s response curve will feed into that as much as the speaker output. At these frequencies a lot of built in phone mics are way outside of their rated range (though so are lots of really cheap speakers).

For the most part, if your setup passes the other test you mention (clear and comfortable volume with a normal source), you should be loud enough to be effective.

[Serebus0126]

10/11/2014 at 7:12 pm

Ok. Because I found a discrepancy (differing opinions maybe) between the Audacity forum and here. On the Audacity forum the instructions did not include a step to use the amplify effect to reduce the signal by -44.2 dB. When I made a file based on those instructions the only obvious difference was signal strength (the speaker output was not audible to either myself or my wife for either file).

I have not been able to find a concrete answer that defines: “Ultrasonic waves need to be at X dB signal strength to be “heard” by the human ear”. This is what is driving my curiosity in this (I am an engineer for an RF communications company and have lots of audio technical expertise to rely on but in the ultrasonic range).

I agree what I am doing is probably good enough, but my engineering nature still wants to understand it and know all the numbers, facts, etc.

[trainer81]

10/12/2014 at 1:22 am

Ah, the step to use the amplify effect to reduce the signal by -44.2 dB, that’s of my own doing I came up with it to help conceal the subliminal messages better so they couldn’t be heard at the level of a whisper. And some beta testing went into it showing positive results.

-44.2 was the best match…

Trainer81

[Serebus0126]

10/14/2014 at 9:55 am

I borrowed an audio measurement device from work and made some measurements at home. Running with the -44.2 amplify effect I had to have the speakers turned up to around 90-100% to read -85dB at the speakers. With the measurement point where my wife’s head would normally be the measurements were around -120dB.

Without the -44.2 amplify and the speakers set at 50% I was measuring -70 dB at the speaker and even on my side of the bed (king size) was getting -90 dB. This also resulted in a wider cone of coverage space.

Agreed that with the louder files the speakers had to be turned down to avoid the squealing and such, but with the speakers turned all the way up to 90-100% there was a humming sound coming from the speakers that was removed when the speaker output was decreased to 50%. Also with the speakers at the higher output we were both getting a persistent ringing in the ears, with the speaker output reduced the ringing went away.

Keep in mind this is on my setup and my speakers (which don’t even have a label or silk-screen indicating mfg or model # and I know there were cheap ones from Target a couple years ago). I am sure the results would vary from setup to setup.

While I would use the -44.2 files with a head-set or ear-buds, I feel better about using the higher output files in the bedroom setup for the noise reduction factor as well as the broader coverage of the output.

I would also expect the results to vary from person to person. Depending on their level of hearing, historical ear damage, etc. So this is still an evolutionary thing, try one setup and vary from time to time to see if the results differ.

[Tap]

10/15/2014 at 4:39 am

I reduce my amp only to -17 and -23, making two versions of my files. I’ve found the -23 version to play well on my iphone. At full volume, you can hear a definite squeaking sound if it’s a foot a way from you. My wife seems to not notice it, even though I do. It makes me think that her hearing is not as good.

To me -44.2 is good for computers with speakers. At the highest volume, squeaking is barely heard. However, if you are using an old mp3 player with bad speakers, the sound may not be loud enough. I believe the number one reason why subliminals fail is the simple fact that the subject cannot hear the file clearly, caused by playing it at too low of an audible level.

I have started using a subliminal file for myself that I have at -40 amp. At this level, on the loudest volume on my iphone, the squeaking is not noticeable. I often times wear earbuds as well and don’t put the volume as high just in case to prevent hearing loss.

Experimenting with the different amp levels is a good idea based on what type of stereo output or device you are using and if she will use ear buds or headphones to hear it.