Room Temperature affects sound mix

Discussion in 'Location Recording' started by mdb, Dec 23, 2009.

  1. mdb

    mdb Active Member

    Hi all. I am new here and have a question regarding temperature and the mixing process. It is because during training a sound engineer has encouraged the idea of "zeroing" the sound board after every use; claiming the reason is because room temperatures fluctuate day-to-day and affect the overall mix (wavelength changes).

    What I have found is this:

    v=f*(lambda)

    v = sound velocity (m/s)
    f = frequency (Hz (cycles/s))
    lambda = wavelength (m)

    v=331.4 m/s + 0.6(Tc)

    v = sound velocity (m/s)
    Tc = temperature (ºC)
    331.4 m/s = sound velocity in dry air @ 0ºC

    Example:

    Using a room temp. Tc = 20ºC, v = 331.4 m/s + 0.6(20ºC) : v = 343.4 m/s

    Middle C = 261.63 Hz Therefore,

    343.4 m/s = 261.63 Hz(lambda) : lambda = 1.31 m

    Middle C at room temperature (20ºC) has a frequency of 261.63 Hz and a wavelength of 1.31 m.

    This is all wonderful, but if the temperature of the room happens to be 10ºC at venue time then:

    Tc = 10ºC, v = 331.4 m/s + 0.6(10ºC) : v = 337.4 m/s

    Therefore, 337.4 m/s = f*(lambda)

    The BIG QUESTION/THOUGHT:

    Readjusting the frequency at the source is done by proper tuning of an instrument and the instrument should be re-tuned in temperature changes anyway. An instrument re-tuned to Middle C may sound off when the sound comes out of the speaker as the wave travels through the air at a slower or faster rate of speed, but no EQ or volume adjustment will correct the frequency shift. I think a person would need a pitch shifter or a pitch correction plug-in/ hardware for this purpose. Am I wrong? What am I not seeing or understanding and is there a logical reason to "clear the board" and start over?

    I'm new to this so thanks for your enlightenment on this subject. :wink:
     
  2. pr0gr4m

    pr0gr4m Well-Known Member

    When the sound engineer said that temperature affects the mix, did he mean it because of the way sound travels through the air OR did he mean because the electronic components within the board may operate differently at different temperatures?

    The temperature and sound through the air thing should really be the last concern. Get everything else right, and set the AC to a comfortable temp, and it won't matter.
     
  3. hueseph

    hueseph Well-Known Member

    The reason for Zeroing the board after your session has nothing to do with temperature. It has to do with the fact that the guy who comes in after your session has to do it if you don't.

    If you are attending a school this is even more true. How many people are in that studio during the course of a day? Can you imagine doing a session, wondering what the hell is wrong with your mic technique and why does it sound so tinny only to find that someone forgot to zero the board before they left?

    There was a time when you were charged $1 per pot or fader not zeroed at the end of your session. Of course if you block out the week this makes your job easier and you only have to zero the board once but if you are bound to doing five, four-hour long sessions over the course of the week, you've got your work cut out for you. That is where meticulous documentation come in to play.

    Incidentally, I think humidity has more effect on your perception of the mix than temperature ever will.

    At the microtonic level of change that you would get from such variations it would be much easier to just adjust your listening. At any rate does the temperature vary so much over the course of a week that you would notice such changes? I doubt it. Unless you are recording a single project over the course of a year, I wouldn't worry about it. Zero the console out of respect for the next client. Leave it at that.

    This is the kind of information that bothers me about these schools. What are they really teaching you?
     
  4. Boswell

    Boswell Moderator Distinguished Member

    Lots of things wrong here. Hueseph has dealt with the board zeroing, but you are confusing velocity of propagation of a sound with the original pitch of that sound. If you reproduce a middle C on a loudspeaker (or any other instrument that is itself not affected by temperature), it will still appear to a stationary listener as a middle C, no matter what the speed of sound is. The pitch is the rate at which compressions and rarefactions happen in the longitudinal sound wave and is independent of the velocity of propagation. To take another example, the speed of sound in wood is very different to that in air, yet wooden loudspeaker cabinets do not emit different frequencies directly to air and from the wooden parts.

    Where there is a temperature effect is in wind instruments that rely on the velocity of sound remaining constant in order to be in tune with themselves. The instrument can usually be tuned by lengthening or shortening a sliding joint, but this will only be accurate at for one pitch (e.g. A440) at one temperature. At high and low temperatures, wind players have difficulty in pitching correctly.

    I hope you get good marks for this question.
     
  5. mdb

    mdb Active Member

    He alluded to the way sound travels through the air. It does change because of temperature. The speed of sound is not a constant because it depends on the media it is traveling through and it's temperature as well as humidity. Apparently the effect of humidity is so small that it is ignored.

    Yah, the frequency isn't going to change so much as to make a B1 sound like a C2, but it could potentially sound slightly out of tune (which isn't our responsibility).

    I don't know about that. This is what I think is going on (again, I could be wrong)...

    • practice is on Friday @ 20C
      venue is on Monday @ 25C
      musicians re-tune instruments using a plug-in digital tuner (not relying on their ears, but on the frequencies of the notes played into the tuner) - of course the pianist is at the mercy of the temperature since you're not going to tune a piano every day. :D

    The sound coming out of the speakers travels through the air at a rate faster than they would @ 20C. If the wavelength remains consistent with the note of "A" (tuning note), because that same wavelength has increased in velocity, the frequency (cycles of wavelength per second) jumps by 4.1 Hz. In essence, creating a "Doppler Effect" like an ambulance siren moving towards you sounds of higher frequency than it's actual frequency and sounds lower when it moves away.

    http://en.wikipedia.org/wiki/Doppler_effect

    Even so, what I'm getting at is that I don't see how a change in temperature in itself would constitute having to clear the board. there may be other reasons to do so, but not this reason.

    Any more thoughts?
     
  6. Boswell

    Boswell Moderator Distinguished Member

    The wavelength does not remain constant. When the velocity of sound changes due to a change in the physical properties of the propagation medium (density, humidity etc), the wavelengths change in the same ratio. By applying the first equation that you quote: f = v/lambda, the frequency remains constant. There is no Doppler effect, at least, not due to this reason.

    And yes, you would re-tune the piano for a concert.

    You clear the board for the reasons Hueseph and others gave.
     
  7. mdb

    mdb Active Member

    Thanks for all your input. I'm trying to wrap my head around this thing.
    I guess in a professional situation a person would tune the piano every time - for the enthusiast or home owner, probably not.

    Boswell, what's your background: Sound Engineering schooling, Physics, Long time recording artist? You seem to know your stuff. I only started looking into sound engineering about a month ago and I've read all kinds of different opinions on this subject of sound velocity/ frequencies.

    Interesting stuff. I think I'm going to like it here.
     
  8. EricIndecisive

    EricIndecisive Active Member

    hmm...

    set up mics
    press record
    play
    ...??

    profit!


    interesting reading, but this is way over my head.. i don't think i could ever hear a difference of 4.1 hz in any situation though
     
  9. natural

    natural Active Member

    I'm with Boswell and Hueseph.
    This is strictly a lab exercise.
    In the real world, the temp would change over time as people fill the space. What then?
    And what happens in the event of a breeze, AC turning on and off, (in most halls, the AC rarely works consistently well anyway) and all the other anomalies of life. How much methane do people produce during a concert?

    Zero the board so the next user doesn't have to clean up your sloppy work. Common sense and courtesy.
     
  10. sheet

    sheet Well-Known Member

    Profit? What world are you in? LOL
     
  11. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Temperature won't change frequency. It will affect the accuracy of delays used to electronically align physically separated speakers.

    bouldersoundguy.com
     
  12. sheet

    sheet Well-Known Member

    Here is my experience. When I was mixing for Dicoveryland's "Oklahoma" (an outdoor theather production of "Oklahoma"), the weather would change dramatically. The ampitheater is nestled in the woods, on a hill. The stage floor is a blacktop with a stone facade. There was a permanent set onstage (houses, barns, bridge over a stream) with natural woods for a backdrop. Eight PCC-160s across the front of the stage captured all of the vocals, dancing, tree frogs, crickets, etc. We did use tracks for reinforcement on choral sections. During the afternoon, the temps on stage could reach 90-100 depending on the day. At night it would get chilly. I would have a tight mix happening in the heat, with the highs well in control. As it cooled, the volume would increase and the highs would take off.

    I notced that the difference in temperature and barometric pressure did change the frequcny response of the PA. If we had a hot humid day and then a cool drier night, maintaining good gain before feedback was a bear. Hot air has high pressure. Cool air has low pressure. The air temps and pressure also means that there is a change in the amount of molecules available to vibrate. We all know that lower temps with drier air lead to more friction. The instances of static increase. The speed of the sound increases because there are more molecules, more friction. more vibrations. Higher temperatures with high humidity yield stagnant. thinner air. There are less molecules vibrating slower with less friction. If temperature and pressure has an affect on the speed of sound, and it does (why else would the speed of sound be listed with the qualifiers of altitude and temperature? Read more here: http://en.wikipedia.org/wiki/Speed_of_sound), then it will change frequencies. Frequencies have different cycles per second and wavelengths.

    More proof is radio. When it is cold, scan the dial and note how many stations you are able to receive from other markets/states that aren't normally received.
     
  13. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Doing an outdoor gig throughout the day and into the evening is always a challenge. I remember one year here I was mixing a regional act during the day. No problems in the Florida heat and humidity. A couple of members of Spyro Gyra sat in, their sound guy was fine with the set. Then as the headliner (Spyro Gyra) came on laterthat evening, the monitor mixer was pulling his hair out. The air cooled and the humidity got much worse, almost a fog.F-f-f-feedback city. Humidity down here seperates the men from the boys in situations like this...:)
     
  14. Boswell

    Boswell Moderator Distinguished Member

    Not so on both counts.

    As discussed earlier in the thread, the frequency of the source is propagated unchanged to the listener whatever the velocity of sound in the medium that conveys the waves.

    Radio waves are transverse electromagnetic waves propagating in the ether. They are subject to reflections off the ionosphere dependent on temperature and other factors such as incident sunlight. It is these reflections that enable you to receive more distant stations at night at frequencies up to the HF band. Sound waves are longitudinal waves propagating in air or other tangible medium. They are not related to radio waves.
     
  15. sheet

    sheet Well-Known Member

    Yes, the frequency remains the same, but the speed of travel does not. Temperature will not affect all frequencies uniformly. Therefore, the signal may be "smeared" with delays and a non-linear response (assuming it was linear to begin with).

    For those that wish to play with a calculator:
    Speed of sound in air and the temperature calculator air pressure - table density of air calculation acoustic impedance air density sea level velocity ideal gas 20 degrees or 21 degrees Celsius C - sengpielaudio Sengpiel Berlin

    This site disagrees with the pressure factor. There are smarter people than me that say it is a factor. I dunno. Everyone agrees that temperature does change the speed of sound.

    FM and TV waves are basically line of sight with the physical limit of 50 miles. ABC claims on their site that pressure and temperature affect radio reception:
    Common Radio Reception Problems - ABC Reception Advice

    Yes, there is the Reflectivity Index to consider. But WHY is there a reflection? Why are there better reflections than others? This still proves the point. Temperature, pressure, etc affect the airmovement.
     
  16. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    So, uh, what's phase 2?

    The difference between 50Hz and 54.1Hz is pretty audible. The difference between 5.0000kHz and 5.0041kHz isn't.

    bouldersoundguy.com
     

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