Frequency sweep

Discussion in 'Speakers' started by Wayde_R, Sep 22, 2004.

  1. Wayde_R

    Wayde_R Stunt Coordinator

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    I've been working on sweeping my system frequencies, measuring how close my system comes to 75db for a wide range of frequencies put out by the Avia DVD measured by a R.S. SPL meter. I have read a handy guide on the THX webiste on how to conduct a low frequency sweep and can see its value for setting crossover on the sub and finding where any low freq anomalies lie in my room or speaker positions etc. My question/problem is in mid to high frequencies. First, I know that high freq >10kHz aren't as readable so the SPL meter isn't as reliable at this. The mid ranges in between I can measure but what can I really do about it? There is no crossover to set, or phase switch and outside volume control I can't do much. Furthermore I can't find much information on conducting a freq sweep of the mid ranges, THX seems to say nothing about it (unless I'm just looking in the wrong places). Does anyone find measuring/charting their system's freq response useful? Did you get noticeable gains with an equalizer? Is there anything else beside go out and buy new speakers or an EQ you can do about fluctuations in freq response?
     
  2. Edward J M

    Edward J M Cinematographer

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    The combination of room treatments (traps, furniture, curtains, etc.) and a parametric EQ can be most effective in dealing with nearly all midrange glitches.

    There are guidelines as to how "live" a room should be and how long it should take for boundary reflections to settle down (this can be measured with MLS software). Too much room treatment and it will sound dead like an anechoic chamber. Too little treatment and the room will be overly bright and reflective, and imaging and soundstage will suffer accordingly.

    If your room is very live and it takes too long for reflections to decay, then treatments or traps should be strategically placed at reflective boundaries. A good placement guideline is to have a helper place a mirror at various locations on walls/ceiling while you sit at the sweet spot. Place the treatments or traps where ever you can see the speaker(s) in the mirror.
     
  3. BruceD

    BruceD Screenwriter

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    Wayde,

    As Edward indicated, various room furnishings and acoustic treatments will alter the sound characteristics of any speaker/room combination.

    When considering low frequencies (typically sub frequencies), the key to getting good sound is reducing the large peaks and/or nulls typically caused by the interaction of the low frequencies and the room as well as ensuring a smooth transition (crossover) from mains to sub.

    There are lots of ways to proceed. You can use predictive computer programs like RPG's progam or the CARA program. Or you can use measurements like you are indicating with test tones and an SPL meter. You can also use the RCA output on the SPL meter as input to computer programs like TrueRTA and ETF5 for analysis on the computer.

    A couple of recommendations I can make. If you want to stay with simple test tones and the RS SPL meter.

    1) Use warble tones (like those on Stereophile CDs) at various frequencies to check the crossover between mains and sub (say from 20Hz below to 20Hz above the actual xover frequency). You want the C weighting of the RS SPL meter (with frequency/SPL corrections applied) to be relatively stable across the crossover frequency. Warble tones will minimize the effect of room modes on the SPL meter.

    2) Identify bass (sub) room modes (by calculating from room dimensions first, if you have a rectangular room) and then test to see if they are a problem at your listening position with a test tone CD (preferrably one with short single frequency test tones at 1/12th octave steps or 1Hz steps if you can find it).

    This will tell you if you need acoustic adjustments in the bass frequencies. This is where many of us actually use a Parametric EQ on the sub to reduce modal frequency peaks (also known as boomy bass).
     
  4. MikeTz

    MikeTz Stunt Coordinator

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    Bruce:

    Do you happen to know a source for test tone CDs that have warble tones at finer resolution than 1/3 octave? Same question for short tones that sweep the audio frequency band or step through it at 1/12 or 1/24 octave steps.

    Thanks

    MT
     
  5. BruceD

    BruceD Screenwriter

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    Mike,

    I think this is the same Stryke CD that used to have the test tones you asked for.

    Test CD
     

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