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Two sounds with equal spl, one "louder" than other

  1. May 23, 2014 #1
    According to my experience, a sound coming from just one source will sound and feel louder than a sound coming from all directions, with equal spl. Allow me to elaborate. On my smartphone I have a spl meter (because I'm a classical musician and I value my hearing). Let's say that a speaker is creating a sound of 83 decibels. This is roughly the spl of an aircraft cabin. In an airplane cabin, I can talk in a normal conversational tone to other people. However, when I'm in a room with the speaker, I have to raise my voice.

    Why is this? Sorry, I know it sounds like a stupid question, but this has been on my mind for quite some time, and I'd like to know the answer.
  2. jcsd
  3. May 23, 2014 #2


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    Your ears are a sonic navigation system - they "like" to locate the origin of a sound. Your brain interprets the slight delays from one ear to the other as a directional signal.

    As you grow older your ability to overcome background noise will diminish.
  4. May 23, 2014 #3

    D H

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    I suspect it's more a matter of signal (and type of signal) versus noise as opposed to directionality. That speaker is putting out a lot of signal. You have to compete with that signal to make yourself understood. The airplane noise is just that, noise. The sound is spread much more uniformly across the sound spectrum than is the sound from the speaker and it contains no information that your brain needs to process and sort out.

    Another factor is type of signal. It's easier to talk over instrumental background music than vocal background music, which in turn is easier to talk over than a TV commercial. The instrumental music isn't competing with the part of your brain that focuses on converting voice to meaning. The TV commercial: They've learned quite nicely that there's more to "loudness" than just loudness. Everyone but the machines that measure it agree that TV commercials are louder than the show.
    Last edited: May 23, 2014
  5. May 23, 2014 #4


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    Your smartphone app should specify which dB “weighting” it uses when it measures sound pressure and displays “dB”. This would help understand your perception of loudness and how the human ear reacts to various frequencies. From the Wiki entry for “Sound Pressure” at: http://en.wikipedia.org/wiki/Sound_pressure

    “Ears detect changes in sound pressure. Human hearing does not have a flat spectral sensitivity (frequency response) relative to frequency versus amplitude. Humans do not perceive low- and high-frequency sounds as well as they perceive sounds near 2,000 Hz, as shown in the equal-loudness contour. Because the frequency response of human hearing changes with amplitude, three weightings have been established for measuring sound pressure: A, B and C. A-weighting applies to sound pressures levels up to 55 dB, B-weighting applies to sound pressures levels between 55 and 85 dB, and C-weighting is for measuring sound pressure levels above 85 dB.

    In order to distinguish the different sound measures a suffix is used: A-weighted sound pressure level is written either as dBA or LA. B-weighted sound pressure level is written either as dBB or LB, and C-weighted sound pressure level is written either as dBC or LC. Unweighted sound pressure level is called "linear sound pressure level" and is often written as dBL or just L. Some sound measuring instruments use the letter "Z" as an indication of linear SPL.”

    Cheers, Bobbywhy
  6. May 24, 2014 #5
    Human ears do not have flat response. Their discrimination powers are nothing short of amazing but aside from frequency discrimination we also have emotional discrimination, one of the elements that makes up psychoacoustics. Some sounds alert our fight or flight responses and thus demand our attention, in effect diminishing our abilities to hear other simultaneous sounds deemed less important to survival. Some of our emotional discrimination is learned and local while others seem to be now encoded in our DNA. Subsonics are ominous and screechie harmonics are threatening at very primitive levels of our brains. Apparently we can learn to overcome or even enjoy some of these as evidenced by the use of dissonance and atonality in some forms of music, or even movie soundtracks.
  7. May 24, 2014 #6


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    Will you please cite some acceptable reference(s) for the above claims and assertions? Thank you.

  8. May 24, 2014 #7
    Sure. I'd be glad to. Incidentally there are patents as early as the 1930's that mention the term "psychoacoustics" and the studies of this phenomenon are responsible for such commonplace applications as bandpass filters on the telephone system, which reduced the amount of power needed (reduced bass response) while improving intelligibility (minimal high end harmonics even under distortion).

    You can start here Psychoacoustics. I can find more but it may take me some time as I have spent many years learning about and designing for psychoacoustic phenomena as well as instrumental voicing so I sometimes forget when and where I learned something as it is a continuum with my own experiences and experiments mixed in.

    I also worked rather extensively with high powered Public Address systems where either raw intelligibility and/or pleasing sound were requirements. I've read almost everything Don Davis ever wrote or was written about he and his family. Les Paul and TomDowd are heroes to me and I studied their ideas much of my life.

    Additionally, if you really want to get deep into the physics of electronic sound reproduction, and the nature of human hearing, google Transient Intermodulation Distortion (don't leave off the "transient" part) as the history of how it was discovered, how it is measured, and how to design systems with minimal amounts of it is just fascinating. I have read so many books, and subscribed to so many trade publications on this subject alone, as well as built systems around it and phase cancellations, I'm afraid the source is so manifold it is difficult to link just one.

    If you like me find this truly interesting and if it suits forum rules I'd be happy to answer any specifics I can.

    Edit - There are many good treatises on human hearing, it's strengths, weaknesses, and idiosyncrasies but here is an excerpt about our amazing frequency discrimination Frequency Discrimination
    Last edited: May 24, 2014
  9. May 24, 2014 #8


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    90125, Have you found the answer(s) to your questions using all the references provided in all the above posts?

    If yes, then Physics Forums has once again assisted a true searcher.

    If no, then please try asking your question(s) and try to be as specific as possible.

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