Chapter 6: Why Chasing Peak SPL Ruins Home Theater Audio Quality
Framework 1:
The Headroom Illusion
The Headroom Illusion is the belief that unused SPL capability automatically improves playback quality.
Headroom (peak SPL capacity) is insurance. Insurance is valuable. But insurance only creates direct performance benefits when it is actually used. If a system never approaches its output limits, additional unused output capability is not a sound quality upgrade—it is merely additional safety margin. Margin is good. Maybe you like being tall, and one day you feel like wearing heels. You never know. Better to have it and not need it, than need it and not have it, as they say. But you pay for that. Maybe better to pay for what you plausibly might use, rather than for what you likely never would even want to?
Peak SPL is selling. Who’s Buying?
In every hobby there is a point where performance stops being the product and status becomes the product. In HT audio and private screening rooms, maximum SPL often sits right on that border. Nobody wants to admit it because numbers feel objective, but a lot of systems are purchased for the same reason people buy watches rated for depths they’ll never dive to and sports cars they’ll never drive flat out.
So, THX set up ‘Reference Level’ as a target of aspiration. It became, as with many things established by the THX program, a sales tool.
Now CEDIA might seem to be saying via RP22 that you should have even more output than that. And some people may interpret the higher ‘Performance Levels’ attaching higher recommended SPL capabilities with comprehensive sound quality capabilities. Standards have a funny habit of becoming aspirations, and aspirations have a funny habit of becoming insecurities. What begins as a recommendation quietly evolves into a hierarchy. Before long, people aren’t asking whether a system sounds good. They’re asking whether it qualifies.
In all honesty, I’m not really sure what they’re trying to imply with these output recommendations, because the current RP22 documentation doesn’t seem to explain the benefit of having more than you actually determine your ‘need’, aside from wanting a ‘Performance Level’ designation. Those recommendations are made without any specific explanation of why.
Getting there. Get a quote for the ticket before getting on that boat.
Manufacturer Perspective (Cody Hiebert). SPL seems to have become the generalized term for all kinds of auditory goals. Do you want more impact? Then you need more SPL. Do you want more immersion? Then you need more SPL. Rarely are the effects associated with dynamic capacity and complex loading capabilities, rather they are reduced to a set of SPL numbers assigned to a frequency range (or not assigned frequencies), and then equated to a driver diameter. This is generally how subwoofer sizing goes, even in professional integrator circles where “there is no replacement for displacement”. Simply because the verbiage to articulate what is actually happening has been literally thrown away by current measurement verification standards. Seriously. AES75 is the only one that gives you an indication of when problems arise, but it can’t tell you what they are or if they will be better or worse when the signal deviates from M Noise.
But the SPL recommendations are what they are, and if you want them, it would be good to understand what that requires. This is not a trivial thing that is going to be a cake walk for any quality product in any ‘normal’ listening environment.
It’s not a small thing. It’s not an easy thing. And if you’re going to achieve it by delivering good sound quality through the entire dynamic range, it’s not a cheap thing. In the context of really good sound quality, quality that most people can easily discern, you’re probably not going to actually pull it off with cheap PA equipment, even if tweaked for dispersion and frequency response requirements to qualify for what RP22 does spell out for loudspeaker behavior recommendations.
As Tom Vodhanel rightly pointed out, ‘Reference level’ is hard enough in a simple world.
For illustration, I’m going to do calculations using inverse square law, published (promised) loudspeaker voltage sensitivity, and voltage equivalent to rated power, similar to the RP22 example, but with more direct math, at different distances, to consider direct (un-reflected) sound, with no room ‘help’.
Making the distinction of direct sound is important, for two reasons. One, it’s the only sound we can count on, assuming a clear line of sight. Two, it is the sound that isn’t jumbled in time by the room. Direct sound is the core foundation of ‘intelligibility’.
Reflected sound may up your SPL at distance, and may sound cool and spacious when carefully applied, even slightly more ‘present’ or ‘detailed’, if you get lucky or know how to manipulate it. But in the range of dialogue and up, where audible details get critical to movie enjoyment (dialogue is most often crucial to story), good, intact, faithfully reproduced direct sound is really what you’re predominantly after.
If you like your reflected sound components, I’m not suggesting you change that, just take it as a bonus, because it’s free in terms of generated output. A ‘Recycling Benefit’ of sorts.
My point is also consistent with the RP22 offered estimated calculations and also consistent with observations and discussion related to the article in Residential Systems that David Mathews shared, as it relates to situations with lower proportions of direct sound needing higher SPL for similar subjective effect. Above the ‘transition’ frequency, it is the direct sound that dominates the experience, and if you’re talking about fidelity, intelligibility, etc. , it is the direct sound that we want to dominate. RP22 recommendations in the acoustics section largely validate this, even if at times inconsistently.
At the lowest frequencies the room itself becomes part of the impulse launch, the immediate boundaries somewhat blend into the baffle itself, and the first reflections may combine with the first launch to change the impulse response, but more like a waveguide with a kind of haphazardly loaded driver than with distinct separate first arrivals.
As frequency drops, there is less ‘purely direct’ sound in the mix from a standpoint of the strictest definition. Fortunately, because the wavelengths make that distinction less distinctive, maybe we care less. In a way, the initial reflected sound from the first launch, because it recombines cohesively, maintains first arrival behavior, to a degree, changed in shape, but perhaps not significantly degraded in terms of ‘transient response’. Reflected sound isn’t necessarily reverberation, nor a resonance, both of which are low frequency detail killers. In low frequencies, reflections with short-delay times CAN preserve temporal, ‘transient’ detail, even if it may be not quite the same as the source heard without reflections.
That’s a heck of a spin off topic, but I think the quick of it is, we’re not talking about that. For very low frequencies produced by the subwoofers in a home setting, we aren’t talking simple point source inverse square law propagation loss relationships.
Acknowledging all that, we’re going to shelve it, for the time-being. Back to the simple, horrifying point, talking about the ‘Main’ speakers, and reliably meeting that ‘Reference level 105 dB’ output level ceiling capability to ensure ‘headroom’ in content does not become a bumped skull.
And now, a reality check: Calculating what we need for Main Speakers to reach… ‘Reference Level’.
Originally published by Totally Hypeless
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