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How is "flat response" measured?

Discussion in 'Amps and Cabs [BG]' started by Bryan Hassing, Nov 13, 2003.

  1. I have been looking at the Euphonic Audio web site lately. I see EU,and others, claiming their cabinets/amps/whatever produce "uncolored" or "flat response" to the instrument that is input. Just how can one tell whether an amp or cabinet is coloring the sound of a bass? I can sort of tell the character of my two basses when they're unplugged. But, being unamplified, it's sort of hard to get a good feel for their sound. I couldn't tell whether my amps color my basses' sound or not. Are there empirical measures to determine whether or not a piece of equipment colors the sound of an instrument?
  2. metron


    Sep 12, 2003
    Unity gain across the audio frequency spectrum is technically defined as a flat frequency response.
  3. Thanks Metron (that from Star Trek?): Would you say that unity gain across the audio frequency spectrum = lack of coloration?
  4. metron


    Sep 12, 2003
    In my experience yes. Im talking specifically about the Acme speakers which I did not find to be to my liking.
  5. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL
    for me, its my home recording setup that consists of a Mackie 1202VLZ, Stewart World 250, Event 20:20, and my Audio Technica monitoring headphones (cant remember the model number).

    plugging my Sadowskys into them is pretty much what i hear thru my Acme bass cabs.

    the EA's are incredibly close, but even they too have a slight kick in the upper highs for an open airy kinda sound.

    prolly the reason why alot of guys like the EA's over the "dead" sounding Acme's, which i happen to adore.
  6. Ah, but it's not that simple;). Flat frequency response is one factor, but the phase response of a cab is also very important in the amount of colouration. That's why two cabs with almost identical frequency response can sound totally different. It's a lot harder to get a flat phase response, especially in a ported multidriver cab with crossovers. An excellent phase response would be +/- 35 degrees throughout the spectrum. That's been acheived in very high end PA cabs with tons of processing and measurement equipment available. I'd hazard that even the 'flattest' bass cabs aren't anywhere near that...
    Even the 'flattest' responding speaker is still quite coloured.
  7. metron


    Sep 12, 2003
    Thanx Mark!
  8. Mike Dimin

    Mike Dimin Inactive

    Dec 11, 1999
  9. Hey Mike, does EA publish phase plots for their cabs? I'd be interested in looking at the phase responses of some actual transmission line cabs. I'm researching TL designs and considering developing some TL PA subs...
  10. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Good thought on the phase response. That's certainly an important factor. The frequency response of a bass cab is exceedingly difficult to measure. For three reasons. One is, the usual caveat that measurement devices that are truly "flat" across the audio spectrum are almost impossible to find, and if you can find one they're enormously expensive. Second, bass frequencies tend to travel through the ground more than high frequencies, this is due to the increased absorptivity of most materials at low frequencies, so a speaker baffle that works fine at 1 kHz can turn into a porous sieve at 31 Hz (which makes it really hard to measure with a microphone). And finally, the phase response of "any" speaker cabinet is frequency dependent, and it's difficult to separate what you're hearing into its components. Personally, I consider the concept of a "flat" speaker cab to be pretty useless. I'm not even convinced that such a thing exists in the real world. The best I've ever been able to do is to use my ear, I trust it a lot more than I trust anyone's external system or method for measuring a cab's response. If the cab makes me smile, it's a good cab. Also, I don't think there's necessarily anything "bad" about a cab that colors the sound, as long as it's colored the right way!
  11. Mik Walker

    Mik Walker Supporting Member

    Dec 26, 1999
    Have to agree wholeheatedly with nonsqtr's post. A speaker (or indeed an entire amplification system, which is everything from the end of the jack cord that the bass is plugged into) is bound to provide colouration to whatever instrument is connected to it.

    A "flat" or "uncoloured" speaker would be one that reproduces all frequencies with equal efficiency throughout its quoted operating bandwidth. To measure the response of this you would need to set up critically controlled test conditions with some form of reference microphone in an anechoic chamber and deliver white noise (a signal equal in frequency content from 0Hz or 20Hz to 20KHz, I forget which) or pink noise (same principle but with a higher upper frequency limit) through an amp with known colouration characteristics. If you know the response of the chamber, the amp and the mic, you can work out a pretty good response curve for the speaker, using a spectrum analyser.

    As has been suggested, phase changes will create anomalies in the response (and you will get phase changes between drivers in multi-driver cabs) plus temperature and humidity will affect system performance.

    Then remove the known system and stick it into a room, like a gig for example. The room has its own characteristics too and will resonate when sound is delivered into it so, even if you started with a perfectly "flat" cabinet you'd be hearing all sorts of corruptions to it by the time you were using it in any form of user situation.

    By the way, as bassplayers, we should be very careful about bandying about phrases like "flat response". I've toyed with fairly large conceert systems in the past and seen and heard much work done in attempts to obtain flat-response systems. Given the sheer impossibility of actually achieving such a state in reality, I must say that the closer you get to such a desparately-craved state of "perfection" the less and less bass is noticeable. We are conditioned to listen to and enjoy wildly coloured sound systems. A "theoretically" flat sound system sounds quite tame. Great for clarity and definition in mixing, but definitely light on the bottom!

    As for the character of your basses unamplified versus amplified, there are so many factors affecting the sound. The pickups, the woods, the construction methods, the components in the preamp, the mass of the instrument, neck joint type, scale length, string gauge, bridge mass, etc ad inf....

    The whole ethos of a "flat" sounding component in a setup would be for it to faithfully reproduce whatever appears before it in the signal chain. How much use that might therefore be would depend on what has happened prior to that point in the signal chain. How do you know your bass is producing a flat output? I'd wager most of them are nowhere near. If you use any kind of onboard or outboard preamp you've added yet more colouration. Even cabling introduces losses, and the losses are not linear as far as frequency goes. The list is alnost endless and, indeed, the colourations of these elements are many of the things we might otherwise term as desirable "character" of an instrument or component.

    There is a huge amount of subjectivity and personal preference, not to mention personal perception in all this and I guess it's a question of having the luck to be able to hear and try many different setups and the good fortune to be able to hear and afford the ones that sound "right" to you.
  12. Interesting reading, perhaps reasonably flat is a better description and objective. Although I must say my Acme 2X10 sounds to me like a big studio monitor.

    Here's my two cents. With my reasonably flat setup what comes out of the Acme sounds pretty much just like what comes out of a studio monitor when I record using the DI (post) of my Studio 220.
    The output of the Studio 220 sounds pretty much like my bass going into a good passive DI direct to the board (with a little top end sparkle that seems to be the color of the SWR).
    The sound of my EMG equipped bass direct to the board sounds a lot like the sound of the bass when I press my ear and the top of my jawbone against the top of the body (the color the pickups add seems to be in the highs and sound of the strings).

    Your ears and mileage may vary.
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