Why is the sound quality of onboard public address so bad?

Question

While not a frequent flyer, I did fly common airliners quite a lot, and each time the (very) low sound quality of onboard public adress system has always struck me.

We all know technology makes it possible to have a "HiFi" sound quality almost everywhere, so why don't major manufacturers (Boeing and Airbus) provide better sound quality systems?

While this is of course not mandatory to make an airplane fly, it is related to passenger safety. Better sound quality would allow crew instructions to be better understood during the pre-takeoff briefing and in the event of an emergency.

I have flown with several airlines, and there are already enough other factors that make it difficult to understand what's being said on PA:

• On board noise: air conditioning, running engines, neighbors talking, kids crying, ...
• Strong or difficult english accent when either the crew or the passenger are not english-native speakers.
• Crew speaking really fast.

The airplane companies can't do much on the above items, but they can improve the technical aspects. Of course, I do understand that this would require increasing weight (loudspeakers are heavy...) thus reducing commercial load.

Side question: How do US/EU regulations handle this point? Do they have any specs on sound quality requirements?

Note: I haven't tried recent planes such as A380, so maybe it has been improved.

Answer 1

For the regulatory portion of this question.

14 CFR 121.318

No person may operate an airplane with a seating capacity of more than 19 passengers unless it is equipped with a public address system which—

(a) Is capable of operation independent of the crewmember interphone system required by§ 121.319, except for handsets, headsets, microphones, selector switches, and signaling devices;

(b) Is approved in accordance with§ 21.305 of this chapter;

(c) Is accessible for immediate use from each of two flight crewmember stations in the pilot compartment;

(d) For each required floor-level passenger emergency exit which has an adjacent flight attendant seat, has a microphone which is readily accessible to the seated flight attendant, except that one microphone may serve more than one exit, provided the proximity of the exits allows unassisted verbal communication between seated flight attendants;

(e) Is capable of operation within 10 seconds by a flight attendant at each of those stations in the passenger compartment from which its use is accessible;

(f) Is audible at all passenger seats, lavatories, and flight attendant seats and work stations; and

(g) For transport category airplanes manufactured on or after November 27, 1990, meets the requirements of§ 25.1423 of this chapter.

[Doc. No. 24995, 54 FR 43926, Oct. 27, 1989]

14 CFR 25.1423

A public address system required by this chapter must—

(a) Be powerable when the aircraft is in flight or stopped on the ground, after the shutdown or failure of all engines and auxiliary power units, or the disconnection or failure of all power sources dependent on their continued operation, for—

(1) A time duration of at least 10 minutes, including an aggregate time duration of at least 5 minutes of announcements made by flight and cabin crewmembers, considering all other loads which may remain powered by the same source when all other power sources are inoperative; and

(2) An additional time duration in its standby state appropriate or required for any other loads that are powered by the same source and that are essential to safety of flight or required during emergency conditions.

(b) Be capable of operation within 3 seconds from the time a microphone is removed from its stowage.

(c) Be intelligible at all passenger seats, lavatories, and flight attendant seats and work stations.

(d) Be designed so that no unused, unstowed microphone will render the system inoperative.

(e) Be capable of functioning independently of any required crewmember interphone system.

(f) Be accessible for immediate use from each of two flight crewmember stations in the pilot compartment.

(g) For each required floor-level passenger emergency exit which has an adjacent flight attendant seat, have a microphone which is readily accessible to the seated flight attendant, except that one microphone may serve more than one exit, provided the proximity of the exits allows unassisted verbal communication between seated flight attendants.

[Doc. No. 26003, 58 FR 45229, Aug. 26, 1993, as amended by Amdt. 25-115, 69 FR 40527, July 2, 2004]

There are systems in place to increase volume in certain instances, eg, on the 767 both engines running gives a 6 dB increase and O2 masks dropped gives an additional 3 dB gain in volume.

Answer 2

The PA (Public Address) system is composed by a band-pass filter between a range of frequencies $f \in [f_i , f_f]$

The quality of the sound is affected because the voice frequencies may exceed this range and thus, the band-pass filter rejects these peaks, that are exceeding the filter range.

Remember how a band-pass filter works:

The frequencies below $f_i$ are not allowed nor the frequencies above $f_f$. Nevertheless, when the frequencies are outside of the bandwidth range (determined by the $\Delta f=f_2-f_1$ at which the gain has a value of $\mathrm{gain_{max}}-3$ $\mathrm{dB}$), the quality of the sound gets worse and therefore, from the cabin we are listening to a dimmed sound of the flight crew voice.

Some of the newer aircraft PA systems are done by using DSP (Digital Signal Processing) systems, also within the limited range, but the quality of the sound is more improved.

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References: Dimov Stojce Ilcev, "Global Aeronautical Communications, Navigation and Surveillance (CNS)", June, 2013.

Answer 3

The microphones used in aviation headsets are simple electret microphones, with high vocal clarity but low response at high frequencies. They provide a quality similar to standard telephone service. Since the pilots use these microphones to speak on the internal PA, there's no reason to install a high fidelity PA fed by a low fidelity microphone.

The PA's purpose is clear transmission of information, same as the radio. High voice quality is a very low priority, and it's also difficult to control without a custom microphone setup for each individual pilot.

It's also worth noting that your basic assumption is that a higher fidelity system would improve the passengers' ability to understand the crew. This is not actually true. Low-bandwidth systems such as telephone operate on the principle that speech can be understood as well (and sometimes better) with its high frequency components removed. This offers the additional advantage of requiring lower channel capacity per user of the system (in both telephone and radio comms).

A high quality audio address system, such as that used by a radio DJ, requires a quiet environment, a high-quality, highly-sensitive microphone, and a speaker with a consistent voice, who maintains a consistent distance from the microphone. None of these are practical in an aircraft cockpit.

A slightly lower-level system, such as that used by a speaker on a stage, would use a microphone on an earpiece in a similar location to that of the pilot's microphone. However these are condenser microphones, which are non-directional and highly sensitive. Such a microphone would be overwhelmed by background noise in an aircraft cockpit.

In both the radio setting and that of the speaker on a stage, there is also a dedicated sound engineer who is monitoring the level of the speaker and making adjustments to the microphone output. Equalizers, compressors, and reverb are also employed among other effects to improve the quality of the speaker's voice.