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Stadium Sound: Do the Math

2.6.2007
Posted by: Bradford Entertainment Inc

Minneapolis based Pro Audio Engineer, Bradford Thompson, began designing speaker enclosures at the age of 12. Through much trial and error, and hands on experience, Bradford has developed a new art form with his speaker designs. His level of experience and depth of knowledge are valuable in today’s sound community.

I love my sports, but I hate the live experience. One of the reasons for this is because I’m an audio engineer by trade, and I hate the sound in stadiums. It is inherently terrible, and it shouldn’t be. When I do go to a game, I sit through the entire thing struggling to understand the announcer, and calculating how to correct it. I can’t help but repeatedly bother the person next to me with statements like, “What did he say?” “Do you understand what he’s saying? Does the sound bother you?”

I’ve been designing custom speaker enclosures for 32 years, and spent 18 years of that time on the road, touring as an audio engineer with regional and national performing artists. I know what I know after years of trial and error and on-the-job experience. I don’t claim to have all of the technical terms perfected. However, when I started in this business, professional sound engineers were excited by the latest development in live sound engineering called the electronic crossover. We were still using a tape repeater called an Echoplex to create delay, and reverbs were spring loaded. We all had our share of blown Phase Linear power amps and moved from Shure VocalMaster column speakers to the new JBL 4560 Perkins bins with a JBL 2441 driver and defracted lens on top. If you were really hip you had the JBL 4550 BKA enclosures, but you had to be 6’2” and over 200 lbs. to even think about loading them into a truck. They didn’t have handles either. These days, during the week, I spend most of my time in front of a calculator or a computer, designing sound reinforcement speaker enclosures and systems.

I’ve asked myself countless times, why do public address systems in stadiums sound SO BAD??? I attribute this “Incoherent Sound Phenomenon” (“ISP”) to many things, most of which are NOT technology based. The technology of sound is just math, so if the math is done, and done correctly, the people will be able to hear, and understand, clearly. Large rooms are a challenge to sound designers because of reflective surfaces, distances, and the slow way that sound travels, but these challenges can be met, when you know what you’re doing. Think about it. The physics of sound do not change from year to year to year, no matter how large the room, or how many people are packed into it.

“ISP” can be attributed partly to salespeople from major sound companies. Over the years, I have reviewed many sound system proposals, which are based on the equipment readily available for sale, rather than the appropriate equipment for the venue. Since salespeople are usually not speaker designers, they have limited knowledge about what equipment is appropriate for a particular room. A “less is more” principal typically is not applied, meaning that if 80 of the wrong boxes aren’t enough to do the job, then 120 of the same wrong boxes must be. At least this way, the sound company sells more product, and makes more money, right?

This is sheer laziness. Designing a new enclosure that will meet the need of the facility to deliver clear and full range audio to the audience is not brain surgery, but it does require some thought. Just do the math.

Doing The Math: Low-Freq Design & Understanding

If you front-load a single 15” woofer into its appropriate vented enclosure, and the woofer has a sensitivity of 98db 1w/1m, assuming that the reasonable usable frequencies are 35hz (at –9db, and 0db at 45hz) with no great dips through 300hz, you will receive 98db 1w/1m. If you add an identical enclosure along side this enclosure with the exact same woofer, together they deliver 101db 1w/1m. In order to receive 104db 1w/1m it requires twice as many loaded enclosures, or in this case four single 15” vented enclosures. To receive 107db 1w/1m you will now need eight of these enclosures. 110db 1w/1m requires sixteen enclosures and so on.

The cubic feet, or space, of all of these enclosures is significant. These boxes would require an area of approximately 120”(h) x 80” (w) x 18”(d). (120”x 80” face-space).

The challenge is to design an enclosure that will deliver 110db 1w/1m of low frequency using half the woofers and half of the face-space. By designing a horn-loaded enclosure I gain 3db SPL right out of the gate. By retaining the allotted venting appropriate for this woofer I retain the speed and recovery of the sound of the combination of the original enclosure and the 15” component. This is unique in a horn-loaded enclosure. Most horn-loaded enclosure designs skimp on the volume of the enclosure. I won’t do this. It drastically changes the sound and raises the floor of the very low frequencies.

The BE/HL-4/3200 design incorporates four 15”, 98db 1w/1m woofers in a horn-loaded/vented combination enclosure. Each pair of woofers face each other at approximately 40-degrees creating a constant directivity lens for the woofers. The four woofers deliver 107db 1w/1m, 35hz-300hz. Two of these enclosures deliver 110db 1w/1m. The face-space of the combined enclosures is 45”(h) x 64”(w). The depth is only 30”.

The horn-loaded design launches the same great sound over 500’ before any significant loss in SPL. The enclosure reacts as if all four woofers are coupled with the ground (or floor). This creates an additional 3db gain naturally per enclosure. Another additional benefit that would wear out my single typing finger to explain is that the more BE/HL-4/3200 enclosures that you couple together the lower the enclosures go. This is typically not true of front-loaded enclosures.

Unlike almost all other horn-loaded enclosures, there are no false subsonic frequencies presented in the BE/HL-4/3200. I know that 5th harmonic resonance must exist in the enclosure however, they’re inaudible to the natural ear. Or at least this is my experience.

There is also a natural compression that exists in a horn-loaded/vented combination enclosure. This compression is created by the degree of off-axis centering of the woofers. The trick is to create a natural sounding launch of sound from the enclosure without over-compression.

In summary, I met the challenge of delivering 110db 1w/1m of reasonable usable frequencies of 35hz-300hz, cut the number of woofers in half, and reduced the face-space by 2/3. This means that half of the power amps are required, half of the actual power (A/C) is required, and about eight less trips to and from the truck. Plus you’ve got one heck of a pair of bass frequency enclosures that also perform in subsonic-world very nicely. No subwoofers required.

You might ask what all this has to do with stadium sound? A ton! The compression, focus, power, and delivery of the enclosures required for stadium use is key. If you can’t reach the audience before you hit the wall – you’ve lost the battle from the start. You’ve actually added noise to the room. This is the case of the Twin Cities Metrodome. The sound in that room hits everything before, or at the same time as, the audience and the sound waves virtually stand in the center of the venue for over 10 seconds. The source-sound from the enclosures does not reach the audience.

Also, by suspending front-loaded enclosures as they did you eliminate the opportunity for natural coupling. This reduces the sensitivity of the enclosure in half. Now you require twice as many enclosures. (And power, and power amps).

Now let’s talk about the client, who typically knows little or nothing about sound, and is forced to blindly follow the advice of the salesperson. After spending millions of dollars, and relaxing in the comfort of the brand name thrown at them, the sound system goes up, and sounds like crap! Then, one of two things will happen. Either the client believes it must sound good, because it’s brand new and it’s a brand name; or, the client realizes it doesn’t sound good, but is unwilling or unable to reinstall due to the expense, frustration, and lack of expertise available to do so effectively. So, like the Metrodome, they just leave it in, and deal with the complaints for decades. Consequently, after spending millions of dollars, the Metrodome is widely known as one of the worst sounding domes in the world.

When I go to the Metrodome, there is a 10-11 second standing wave of sound in the facility. It’s horrible! The installer of this system clearly didn’t understand enclosures, their purpose, or their design. If you’ll notice, all of the low-frequency and low-mid frequency enclosures are designed as front-loaded, meaning that the cone is mounted from the front of the cabinet, and there is no push (or directed-wave) design. The characteristic of this cabinet is that it will project 30 feet forward and drop off in its volume, SPL (sound pressure level). The simple math is that there is nobody hanging out in the middle of the airspace of the dome to receive that sound within its effective area. Maximum effectiveness of the enclosure design exists within 30 feet of these enclosures. In layman’s terms, beyond 30 feet of the cabinet, the sound then meanders around the building, trying to find a place to land. The sound waves are 100 degrees, and continue to bounce off of reflective surfaces within this 100 degree directivity, until they are absorbed by the crowd. Over the past decades, we’ve all been dissatisfied with the audibility of this failed design. The resolve is to direct as many of the sound waves as is physically possible to the audience, which is the absorbing surface. This is impossible with the existing enclosures.

An equally damaging part of the design of the Metrodome sound system is that while bringing the enclosures from the upper center of the dome forward towards the audience makes sense in its first thought, further thought reveals that they created an open, non-applicable reflective void in the arena. The enclosures hanging there now are not close enough to the audience to do any good, because of the reasons explained above, so while sound waves look for a place to land, they wander in the center of the dome between the back sides of the enclosures. This is where a majority of the delay in sound occurs. However, if they moved the enclosures forward to meet the audience they would create a larger standing void in the room. The only option is to suspend all enclosures from the center of the dome, and cluster them all together, called “point-source”. The enclosures would require a folded-horn, long-throw design for this purpose and positioning. Instead, they went right down the middle and caused a nightmare in audibility.

What I’ve reported here concerning the Metrodome sound system, the worst possible case that I’m aware of in the country, can be corrected. I would recommend a demonstration at the dome itself, because the dome is a specific arena, with its own specific problems. The most cost effective way of demonstrating would be to set-up this demo system in the center of the dome on the field. I would set it up inverted to its flown design on the ground. This point source, horn-loaded, or folded-horn design could then be turned on and observed over a period of about five hours, start to finish. You could then walk around the dome and hear the difference. This would be the most cost-effective way of proving that the sound problems can be resolved.

I would love to someday sit in the Metrodome and enjoy the game, hear every word, and celebrate the addition of an exciting new sound design. So would the people who sit next to me when I’m there.