measuring decibels

How do you measure the spl of headphones? I have a Radio Shack SPL Meter that I use to calibrate my home theater. Can it be used to establish safe listening levels-- I don't think I really like loud levels, but I indulge in a certain amount of paranoia. How does one ensure that the measurements are accurate enough to be useful? Where should the mike be placed and how should the weighting and response rate switches be set?

I used my Radio Shack SPL meter to measure the sound output of my headphones. To get a good acoustic seal with my Senn 650s, I use a firm piece of cardboard with a hole cut it in. I then insert the cylindrical-shaped mic "protrusion" of the SPL into the hole and lightly press the cardboard against the headphone's earpads to get a good seal for the mic. I used C-weighting.

What I found surprising is that what I perceive as "normal" and "comfortable" listening levels measures to be around 90dB, which I believe is well above the recommended "safe" levels for prolonged exposure.

85 is the safe level for prolonged exposure.

In the current issue of Rolling Stone there is an article on MP3 players and their contribution to hearing problems with todays youth. They've got a chart of safe listening levels for various activities including...

Rock Concert (with Earplugs) - 3 hours exposure @ 95db
Portable Music Player (60% Volume w/ Over the Ear Phones) - 1 hour @ 105db
Portable Music Player (60% Volume w/ Ear Buds) - 30 minutes @ 110db
Rock Concert (Without Earplugs) - 15 minutes @115db

They go on to say that there are ways to reduce risks, such as earplugs at concerts (they add a recommendation for Ety ER20's), Getting better headphones (they recommend Shure E4C's so you can turn down the volume due to the isolation the IEM's offer) and to give your ears a rest.

All in all it was a pretty interesting read...I haven't looked but it's possible that the article is on-line...

Quote:



rock concert without earplugs - 15 mins!!! Crap... wish I knew about that one years ago...

Quote:



You and I both...I'm sure anyone who has gone to a concert has come out and had ringing in the ears all night...Next time I go to a concert I'm going to order some of the Ety ear plugs before I go...

I'm using a similar setup with decibel meter and a cardboard cutout, but I realized that there's a subtle technical issue -- I wonder if anybody with some engineering knowledge can help me to figure it out.
 
According to this Web site, doubling the distance from the source of a sound results in a decrease in sound pressure of 6 dB.
http://www.sengpielaudio.com/calculator-distance.htm
 
This is not a big deal when the source is distant, but for measuring headphones, it becomes crucial. 
Furthermore, the place where the soundwaves cause damage to hearing is not the outer ear but the middle ear. 
But the big unknown is, how deep inside the sound pressure meter does the measurement take place? Maybe the actual physical measurment takes place deep in the guts of the SPL meter, not right where the sound enters the SPL meter

Let R1 = distance from the sound source (i.e. headphone drivers) to the middle ear when wearing the headphones
      R2 = the distance from the source to the part of the SPL meter where the measurement actually takes place
 

Really, we have to figure out the ratio R1/R2 and then use the formula from the hyperlink above together with the reading of the SPL meter to determine the actual decibel level of the headphones
 
I'm including a photo of my sound meter. It's a Radio Shack Realistic. The radio shack brand for SPL meters seems to be highly recommended on these forums. It's not the latest model, but rather a used one I got on eBay.
 

 
Quote:

kontai69 said:

I used my Radio Shack SPL meter to measure the sound output of my headphones. To get a good acoustic seal with my Senn 650s, I use a firm piece of cardboard with a hole cut it in. I then insert the cylindrical-shaped mic "protrusion" of the SPL into the hole and lightly press the cardboard against the headphone's earpads to get a good seal for the mic. I used C-weighting.

What I found surprising is that what I perceive as "normal" and "comfortable" listening levels measures to be around 90dB, which I believe is well above the recommended "safe" levels for prolonged exposure.

Click to expand...

All the "recommended" dB levels I've seen are specified in A-weighting. Use that.
 
85dBA 8 hours
90dBA 2,5 hours
etc
 
Old thread but noone mentioned.

It won't make a big difference, between A and C weighting, because C just weighs bass heavier. So unless the loudest part of your music is the bass, it won't really matter. Most of the noise safety charts do use A-weighting, though.

To answer my previous question about where the decibel measurement takes place, I talked to an engineer (but not one specializing in audio) and he guessed that it takes place in the front of the metal part where the sound enters the meter.