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Old 31st October 2009, 04:26 AM   #1
jamikl is offline jamikl  Australia
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Default Reverse of the old Loudness Control

Is it posible to make a filter, working off the volume control, which lowers the volume of frequencies below about 55Hz as the volume is turned up. This would allow me to use open baffles with boost on the low frequencies for quieter listening without using subs but If I listened at loud volumes I would turn on the subs and the boost on the open baffles would be reduced. I guess I am looking for a circuit to reduce boost with increasing volume but operated off the volume control so that two separate controls are not needed. This is to reduce xmax at high volumes. Thanks for any help, negative or positive.
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Old 31st October 2009, 07:39 AM   #2
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Why not just use a low cut filter in the pre-amp section?
Most pre-amps and integrated amps used to have this function so I am sure there would be a circuit diagram somewhere or an alternative could be a low frequency parametric filter centered on : say 40 Hz, but with my limited knowledge thats as much a guess as anything
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Old 31st October 2009, 07:51 AM   #3
Mooly is offline Mooly  United Kingdom
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True loudness compensation used tapped volume controls, something you never see these days. Are they still available ?

Failing that as Moondog says, you need a filter, and you need to decide on the rate of roll off etc that you need.
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Old 31st October 2009, 08:06 AM   #4
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Originally Posted by Mooly View Post
True loudness compensation used tapped volume controls, something you never see these days. Are they still available ?

Failing that as Moondog says, you need a filter, and you need to decide on the rate of roll off etc that you need.
An alternative is a four ganged pot. Use two gangs for the volume and the other two for your contour.

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Old 31st October 2009, 09:01 AM   #5
jamikl is offline jamikl  Australia
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Thanks for the replies. I think Phoenix358 has grasped what I would like to do. I am not looking for fixed filters but another way of putting it would be filters that increase slope as SPL increases.
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Old 31st October 2009, 09:17 AM   #6
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Originally Posted by jamikl View Post
Thanks for the replies. I think Phoenix358 has grasped what I would like to do. I am not looking for fixed filters but another way of putting it would be filters that increase slope as SPL increases.
jamikl
What you are after I think is called a shelving filter with it's knee at 55 Hz. If you want the effect to be 'in' at 55 you may need to set the filter knee at 60Hz or so. I don't know how to design these but at least you know the name!

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Old 31st October 2009, 09:32 AM   #7
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Try googling 'active shelving filter'

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Old 31st October 2009, 09:50 AM   #8
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In my opinion, loudness compensation should be standard in audio reproduction. A glance at the equal-loudness contours shows that the subjective loudness at bass frequencies is much more compressed than at mid frequencies. A musical work is normally balanced to sound "right" at whatever SPL the mastering engineer listens at. You need to play back at the same SPL in your room to achieve the same tonal balance. If you play back louder than that, the bass is accentuated. If you play back quieter, the perceived bass is attenuated (relative to the midrange). Dropping the overall level by 3 dB may result in a perceived 10 dB drop at 20 Hz. This is the reason for the classical "loudness control", which progressively boosts the bass as the level is lowered.

The problem with the classic loudness control is that it is fixed relative to the position of the volume control. It only works well if your system's overall gain structure provides a reference level at the right position of the control. If not, it provides too much or too little bass boost as you turn down the volume.

What is needed is a progressive bass boost/cut control linked to the volume control, along with a gain setting control that lets you calibrate your system to the standard.

It would work like this:
At a reference position of the volume control, maybe at the "12 o'clock" or half-way position, the bass boost/cut would be "flat" (no boost/cut).
You set your volume control to this position, then adjust the calibration control for the music you want to play so that it reaches a specific SPL setting at your listening position. From that point, turning the volume control up or down will also adjust the bass level to provide a consistent tonal response.

The mastering engineers set their reference level like this:
Play pink noise at -20dB ref to FS, one channel at a time.
Using an SPL meter at your listening position, adjust the playback level for a reading of 83 dB, C weighted.
Repeat for the other speaker. Don't try to balance between the speakers, just measure the level of each of them and set the playback signal to the average. If there is more than a couple of dB difference, your system or room is set up wrong.

Having set the level, mastering engineers may then deviate from it quite a bit - for example, they may turn the monitoring level down by up to 15 dB if mastering heavily compressed music such as that by the Red Hot Chili Peppers or Metallica. The intention is that the average level in the mastering room will be 83 dB SPL per speaker, regardless of whether the music is high dynamic (classical music, jazz etc) or ultra-compressed (heavy metal, most pop).

For reproduction, the process is less complex - if you set your volume control so that the music averages 83 dB SPL per speaker, at your listening position, then you will be hearing the music as the mastering engineer heard it. The required gain setting will vary with the music type - you may need to turn your calibration gain down by up to 15 dB when playing Metallica compared with a string quartet.

A system using a "linked volume / tone control" would have two "volume knobs" (calibration and volume) and would be used like this:
Set the main volume control to the reference position.
Play the music.
Adjust the calibration control to provide about 86 dB C weighted at your listening position.
Adjust the main volume up or down to taste.

With practice, you will find that you can often calibrate by ear - a lot of music has a "right" setting, where it sounds natural and "life size".
This is no accident - the mastering engineer has optimised the tonal balance etc for this SPL.

Finally, getting back to the specific application under discussion, this system provides automatic bass management. If you raise the overall volume by 10 dB, the bass level may only increase by about 3 dB. This is obviously good for systems with limited LF capabilities, such as OB designs.

To match the equal loudness contours, the bass boost/cut should follow the standard Baxandall type response. A second section could be added to provide additional cut below, say, 55 HZ for OB systems on volume increase only, to keep excursion under control.

Last edited by Don Hills; 31st October 2009 at 09:57 AM. Reason: Typos
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Old 31st October 2009, 10:09 AM   #9
jamikl is offline jamikl  Australia
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Thanks again, especially Don Hills. This is what I,d like to do and I'm sure that it would suit others with open baffles also. I would need to set the system for normal listening, whatever SPL that is and equalise the bass as much as possible
at that level. Turning the volume down would then give more headroom for equalising the bass in room and help the loudness contour. Turning it up louder than the set level would reduce the bass until the point came when the subs would have to be used.

I can"t afford the worlds best subs so I am looking for a way to get the best use I can out of the open baffle bass which will be two Eminence Beta 15s each side. Perhaps the T Bass circuit would help with this too but I don"t know enough to know if it will work for me. I think when the music becomes quite loud most times nobody will be too bothered about the purity of the bass so the subs should be OK.
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Old 31st October 2009, 10:19 AM   #10
jamikl is offline jamikl  Australia
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I seem to recall that many years ago a circuit was published for a Loudness control that varied with volume in perhaps Wireless World as it was then called. I don't know whose circuit it was, perhaps J. Lynsly Hood. This might well be what I need but I wouldn't have a clue where to find it now. Perhaps somebody out there remembers it.
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