HOW BAD IS THE PROBLEM IN PRACTICE?
It would be great if we had some kind of cool loudspeaker technology with vast amounts of high-frequency output, very high fidelity, and low weight, size, and cost. I’ll get back to you on that one.
In the mean time, how bad is the problem, really?
FACTORS THAT REDUCE THE HIGH&FREQUENCY REQUIREMENT
We may not need line with a full, high-frequency, power-bandwidth response. There are three factors that reduce high-frequency demand:
1) The General Nature of Music. The loudness of musical signals is less at high frequencies. This has been confirmed by several studies done over the last 20 years. Dennis Bohn, co-founder and technical architect at Rane Corporation, discusses this issue in an electronics application note (here) about analog long-line drivers.
He gives a conservative rule of thumb for high-frequency signal level as flat to 5 kHz, then falling at 6 dB per octave above that. I would add something to Dennis’s picture: Peaks.
The fact is that a 20-dB headroom for peaks has been the industry standard since the first mixers for broadcast in the 1920s and 30s. Hence the +4 dBu nominal and +24 dBu maximum level for most all pro audio gear.
The “10 dB rule” is probably a rule of thumb for rock n’ roll road dogs using limiters to make it as loud as possible. Over the years, I’ve seen lots of shows where the signal above, say, 7 kHz, has very big peaks.
If the sound system is properly set up, its limiters will kick in when it runs out of steam, and the high-frequency peaks will be removed with no damage to the loudspeakers and only a moderate negative effect on the sound. If the system does have enough high-frequency headroom for limiting not to occur, it will sound airier and more impactful on the high end.
So, in Figure 5 there are two curves – mine and Dennis’s. My curve has more allowance for peaks.
2) Style of Program Material. Some research studies have shown that maximum high-frequency level depends on the type of program. For musical programs, the usual results have shown that classical music, jazz, and pop have reduced high-frequency requirements. Rock, however, has a relatively flat maximum output spectrum, and therefore requires a power-bandwidth response that remain strong all the way to the top.
3) Compensating Effects of Human Hearing. In reverberant environments, where the listener is immersed in a sea of sound coming from all directions, the sound waves diffract around the human head in such a way as to produce a net boost of about 9 dB with a peak around 8 kHz.
The human hearing transfer function is described well in a classic Audio Engineering Society (AES) paper by the late Robert Schulein of Shure. Well-written and easy to read, the paper is called “In Situ Measurement and Equalization of Sound Reproduction Systems.”
Published in the Journal of the AES in April 1975, it describes a series of experiments that Schulein did to determine the reasons for the so-called “house curve” (aka “X-curve”), a 3 dB per octave roll-off above about 1,500 Hz that is traditionally applied to cinema and auditorium sound systems. The paper identifies the role of head diffraction in determining the listening experience in reverberant environments.
It’s an important piece, and I think that everyone involved in designing and tuning large sound systems should read it. It’s available for download at www.aes.org.
Figure 6 illustrates the curve that Schulein derived. It shows the transfer function of the ear when the head is immersed in a pure reverberant sound field, where the sound is coming equally from all directions.
Such fields are found at most of the seats in an arena concert.
As you can see, the ear helps the tweeters out quite a bit in reverberant environments.
On the other hand, in non-reverberant (anechoic) environments—outdoors, for example—this curve does not apply, and the tweeters have to do all their own work.