Say What?

You’re having a gala awards dinner for 1,000. The décor looks fabulous. The food is exquisite. You’ve hired the best audio company in town and now it’s time for the awards to begin. What could possibly go wrong?

Well, let’s see. After two awards the rear half of the audience begins to complain that they can’t understand what the presenters and winners are saying. Thinking it may be only a volume problem, you approach the audio engineer to ask for more volume — only to be told there is nothing he can do about it because of the room’s acoustics.

Surprise! You’re now faced with 500 potentially dissatisfied clients because of something completely out of your control. And they are going to be looking to you for an explanation. What to do?

What’s in a Sound?

To understand why the spoken word (or music) can either be well heard or completely unintelligible, it helps to understand what sound is.

Sound is produced by vibrating objects such as musical instruments, loudspeakers and human vocal cords. These vibrations in turn produce pressure variations in the air that cause a “wave” effect of high and low pressure, much like a wave in water. This sound wave radiates outwards in all directions from the sound source in alternating regions of rarefaction (a region of densely compacted air molecules) and compression (a region of sparsely compacted air molecules). The physical distance between the start of one region of compression to the start of the next region of compression is called the wavelength, which is related to the frequency, or number of pressure changes (cycles) per second.

For example, the frequency of a bass drum or electric bass is about 50 Hz (cycles per second) and the wavelength is about 23 feet, whereas the frequency of a female voice in the upper range is about 1,000 Hz and the wavelength about 1 foot — considerably shorter than the bass sounds.

Friend or Foe Phenomena

Now these sound waves can be altered by their environment as they travel, by means of reflection, absorption, diffraction and refraction — the phenomena that make a venue good or bad acoustically.

Diagram A: Large, hard surfaces such as ceilings and walls generally reflect high- and mid-frequency sound waves, whereas soft material such as carpeting, drapery or clothing generally absorbs these frequencies.

With reflection (see Diagram A), a sound wave can be reflected by a surface or other object if the object is physically larger than the wavelength of the sound. Therefore, in a room with many large, hard surfaces (e.g. high ceilings and walls), architectural elements such as support columns, stages, and even dining tables and chairs (which are larger than the wavelength of most voice frequencies), there will be many reflections.

These many reflections cause what is known as reverberation. For event venues, particularly large ones like conference centers, this has the effect of making listeners feel as if they were inside a giant cavern.

The second phenomenon, absorption, has to do with materials. Some materials absorb sound rather than reflecting it. The amount and efficiency of absorption also depend on the frequency of the sound. Thin material such as acoustic ceiling tiles or clothing on humans will absorb middle to high frequencies, while thicker material such as pile carpets, padded furniture and heavy velour drape will absorb lower frequencies.

Absorption is your friend: It helps to control reverberation in an event space. (It’s also the reason why the sound in a room will be completely different during a sound check than during the event, when the room is filled with people.)

Diagram B: Because bass frequency wavelengths are large, they tend to diffract (i.e. transmit) around most objects, including people, tables, chairs, architectural elements and décor.

Diffraction is most often associated with lower or bass-type frequencies (those with a long wavelength). A sound wave will typically bend around obstacles in its path that are smaller than its wavelength. The resulting effect is for low-frequency sounds to fill a room, while high frequencies tend to be blocked by most small objects, particularly people (see Diagram B).

This is why audio engineers tend to place large bass speakers for low frequencies (aka subwoofers) on the floor, as they will be heard everywhere in the room, even with people sitting or standing in front of them.

Likewise, the middle- and high-frequency speakers will be placed either on top of the bass speakers or, ideally, flown from the ceiling so that they reach the entire audience without these frequencies having to go through the audience.

Diagram C: Sound waves tend to be refracted away from fairly sudden changes (gradients) in air temperature or wind velocity, which can cause changes in sound quality, particularly outdoors.

The final phenomenon, refraction (see Diagram C), is the bending of a sound wave when it passes through a change in air density, often causing a momentary decrease or increase of volume. This is most apparent outdoors, particularly at large distances, when the sound from loudspeakers passes through temperature and wind gradients.

Problem Solved

Now that you know this, how can you turn down the volume on the grumbles that are currently drowning out your awards ceremony?
Ideally, your “best audio company in town” would have draped 12 or 16 feet up the venue’s walls with sound-absorbing material, and possibly added carpeting and even ceiling treatment, according to Mark Fisher of Proshow Audiovisual Broadcast in Vancouver, Canada. This would have eliminated much of the reverberation.

Doesn’t fit your theme or budget? You could also have mounted full-range (i.e. all frequencies) speakers known as “line arrays” above the audience at the front of the venue (i.e. “flying” them from trussing), according to Fisher, so that minimal sound would be lost through absorption by the audience and echoes off walls would be minimized.

But it’s go time, and the coulda-woulda-shouldas aren’t helping you right now. Your best bet at this point is to use a system of “distributed speakers” on floor-mounted stands along the sides of the audience. Place the speakers during an intermission period if the audio company is on the ball and has spare speakers on-site.

No spares? Request that the audience keep quiet during presentations, and have the audio engineer try to equalize (or “EQ”) the room frequencies to eliminate those causing the most problems with reverberation.

If this all fails, there’s always option five: give yourself time for a long talk with your audio supplier — and your client your best mea culpa.