In actual fact, the performance of a Harbeth is uniform over a wide operating range - the speaker itself doesn't 'know' how loud it is playing, so it doesn't change its character with volume level. What does dramatically change with level is the human ear's perception of sound. You can read about it here: http://www.webervst.com/fm.htm Curves like these have to be normalised at some 'pivot' frequency, and since the ear is sensitive at 1000Hz, this was chosen as the reference frequency. The graph looks confusing I know, but all you need to grasp is that the left hand vertical scale is the loudness of sound ranging from 0dB (absolute and total silence) up to 110dB+ (rock concert levels) and that the horizontal scale is in Hertz, from low frequencies to high ones. The wiggly lines (the equal loudness contours) describe the sensitivity of the ear to particular frequencies versus listening level.Originally Posted by Casaross
Example: Let's see what happens if the loudness (defined at 1000Hz) is 60dB in our listening situation.
1. Find 1000Hz (1kHz) on the horizontal (x) axis and follow it up until it meets the 60dB rule on the y axis. Note that it touches a particular contour line. Keep an eye on that contour line alone: ignore all the others. I've coloured it for you.
2. Now find, say, 30Hz on the x axis and follow it up until it touches the same contour line we found in step 1. Read-off from the left scale the loudness at that point - I estimate that it is about 80dB. Agree?
Now the interesting bit. What can we deduce from this observation? We can make a statement which is intuitive but numerically rather shocking .... we can say "At a listening level of 60dB (normalised to 1000Hz), we would need to electrically boost the 30Hz region by [80dB - 60dB =] 20db for 30Hz to sound equally loud as the 1000Hz tone."
Note from the graph how as the contour lines flatten out at low frequencies as the overall listening level increases. That implies that as volume increased, less boost is needed to make those low frequencies sound equally loud as 1000Hz. That agrees with what we hear doesn't it. This explains why that when you turn the volume down on any hi-f the bass appears to drop away rapidly: it's not the speaker, it's our perception of sound balance. Also note the very interesting situation in the 3000-4000Hz region and at the top end too above 10kHz.
Appreciating the way the ear/brain works is what led to the invention of the bass and treble control and later the loudness contour switch. It's also the basis of Dolby A/B/C/S/SR/AC3 technology, and underpins MP3. In my humble opinion the deletion of these very useful and pragmatic tone shaping controls from audio amplifiers in the 1980s flies in the face of common sense and encourages the listener to play his hi-fi, TV, Walkman, car audio louder to hear the bass he wants. The consequence: needless volume, irritation to the neighbours, more room-resonances excited, wear and tear on the ears and hi-fi and noise pollution. I'm sure it does wonders for the sale of big beefy amplifiers sans tone controls though!
The great example of the art of tone shaping to match the ear's well documented characteristics is the Quad amplifier's tilt control which provides a good approximation to the optimum level-correction and is the same circuit solution that I have incorporated into the Monitor 40 Active. Peter Walker really did understand music, rooms and the ear.
Suggested Google search "Equal loudness contours". Note: These curves were developed to represent the situation with pure tones, and the test subjects were wearing headphones and presumably the subjects were in good health and with young ears: the results may not perfectly translate into music listening through loudspeakers. I suspect that the situation shown in this headphone-listening graph may somewhat overstate the boost needed at low frequencies when listening to speakers, but that a boost is needed is undeniable.
Thought: if you were a speaker designer attempting to make a natural-sounding speaker, how would you shape the frequency response curve? Clue: What may be 'right' for a diet of rock and roll may not be for chamber music. That's just one reason that there are countless thousands of speakers available to the consumer all serving a market niche and satisfying someone's tastes and needs.