March 2003

The Evil Backwave

High-quality music reproduction relies on the playback system being able to reproduce the recording accurately without sounding either annoying or distorted. The role the backwave plays in distortion is not well understood or appreciated by the average audiophile. More people would be interested in owning loudspeakers that do a superior job of eliminating backwave energy if the backwave were recognized as a serious source of distortion. The backwave is all the energy that comes off the back of each driver. The front of each driver radiates sound into your room, but an equal amount of energy comes off the back of each driver and has to be dealt with.

I’m going to try to illustrate how mixing backwave energy with the "correct" forward-radiating sound produces distortion. I will ignore loudspeakers that intentionally radiate sound to the rear (dipole, bipole, or omni) -- those are a whole different discussion. I’m going to concentrate on loudspeakers with sealed boxes, transmission lines, ports and other speakers that "box" the drivers. This category accounts for a huge percentage of the market (not counting Bose, of course).

Let's use a recording that has a BOOM sound on it -- a nice forceful transient that has a sharp rise and not too much decay, a sound for which it is easy to hear the beginning, the end, and the "dwell." If this sound is reproduced on a "perfect" system, you’ll hear it exactly as it was recorded: BOOM. If there are internal reflections and little or no internal damping in the loudspeaker, you’ll hear something different, perhaps BOOMboom. The backwave may also be responsible for permutations like BbOoOoMm or BoboOMom -- the direct and delayed backwave sounds weave together. In some speakers, there is so much backwave energy flying around undamped that you could have all of the permutations of BOOMboom mixed together and spread out over a surprisingly large time interval. While BbOoOoMm may not sound as obviously bad as BOOMboom, BbOoOoMm is still something worth trying to minimize so you get closer to the original sound captured on the recording. Backwave energy bounces around inside the speaker, gets delayed in time from all the bouncing around, then recombines with the forward-radiating sound -- a distortion that arises from mishandling the backwave energy.

In a conventional loudspeaker with conventional cone drivers, you hear sound at the listening position at some reasonable listening level -- let’s say 85dB. If you place a microphone six inches from the cone of a driver, you might measure 100dB when you are experiencing 85dB at the listening position. We tend to forget what is happening inside the loudspeaker. If you put a measuring device inside the speaker, six inches from the back of the driver, you will also measure 100dB. That 100dB of acoustic energy is trapped inside the speaker. How each speaker manufacturer deals with that 100dB is a very large factor in the sound of various speakers. Some speakers do little or nothing to damp the backwave. Other speakers do as much as possible to eliminate the backwave, allowing sound to essentially radiate forward from drivers and not from anywhere else.

Undamped backwave energy can bounce around inside the speaker several times, eventually hitting the back of the driver’s cone again. When the backwave hits the back of the cone, the cone radiates some of that sound into the room as distortion. The backwave sound is delayed and decorellated. It is no longer related to the sound on the original recording; it has become distortion. The delayed backwave sound did not exist in the original recording, and this makes it distortion.

How much delayed energy might radiate back into the room? In our example, it certainly wouldn’t be 100dB, but it might be as much as 80dB close to the cone and maybe 65dB at the listening position. You’d hear BOOMboom or some variation of it, not the intended BOOM captured on the recording. That is the evil of the backwave -- it stretches out sounds, puts two peaks (or more) on transients instead of one. In short, it alters what you hear more than anything you could do to the electronic components or cables upstream of the loudspeakers. If your speaker has a lot of delayed energy getting out into the room, you may as well use a receiver, inexpensive wires and an $80 DVD player to play your CDs and LPs because upgrading all the electronics will not make any more of an improvement than eliminating the delayed energy leaking into the room from the loudspeakers.

A slight aside here. If your room adds delayed reflections to reproduced music, that’s not distortion. It is simply the characteristic "sound" of the room. Sometimes it is good, but many times it is bad in one way or another. There may be significant benefits from room-tuning products or techniques. This discussion is limited to what actually radiates into the room from the speaker.

Loudspeakers with exceptional internal damping remove close to 100% of backwave energy. On the other end of the scale are loudspeakers with essentially 0% reduction. Those have empty space behind the drivers and resonant enclosures. You may be surprised to know that there are a number of speaker manufacturers that intentionally put no damping behind the drivers and claim this technique makes their products sublime music-reproduction devices. I say, "Who cares?" Such sound is low fidelity. Those speakers generate sound with tons of time-delayed decorrellated energy mixed in with the actual sound that should exist on its own.

There are perhaps thousands of loudspeaker manufacturers in the world. You could go absolutely nuts trying to whittle the field down to those worth considering. To give yourself a break, why not just eliminate those speakers manufactured with guaranteed low-fidelity design elements before you even start? There will be plenty of choices left after knocking off the speakers that are the biggest offense to fidelity. You will also be rewarding manufacturers who work hard to design high-fidelity loudspeakers while still making them sound musical, magical, or any other audiophile descriptor you may wish to use instead.

Illustrating the backwave problem

Picture this: You have a pair of three-way sealed-enclosure loudspeakers in your listening room. The speakers have been modified so that there is a second midrange driver inside the midrange enclosure. It radiates sound only inside the speaker. There is no way for the sound to get out to the listener except through the cabinet walls or by hitting the back of the real midrange cone and getting radiated into the room. Same thing for the woofer -- a second woofer is inside the chamber behind the "real" woofer. For this experiment, the "real" forward-facing drivers will not be connected to the music signal. Only the extra midrange and woofer will be connected to the amplifier. The volume level is set so that there is a reading of 100dB inside the midrange enclosure and inside the woofer enclosure. Will you hear anything at the listening position? You certainly will hear sound from these internal speakers, especially if the midrange and woofer enclosures are undamped square or rectangular spaces. The cone thickness of the "real" drivers isn’t going to damp the backwave sound much. The undamped speaker cabinet will also radiate backwave sound into the room.

Next, let’s add typical damping materials to the chambers behind the midrange and woofer drivers. Appropriate products are glued to or painted on the internal surfaces to damp resonances. The open space in the chamber is filled with common loudspeaker damping fiber, not terribly different from the stuffing in pillows and furniture. Having another listen reveals that the amount of sound escaping from inside the speaker out into the room is vastly reduced. This design and level of performance are probably the most common found in high-end loudspeakers -- past and present.

But there are additional steps that can be taken. Suppose the special test speakers have a well-designed labyrinth system behind each driver that directs sound away from the "real" drivers. In addition, there is damping material in the labyrinth to further reduce sound levels inside the speaker. The labyrinth is designed in such a way that constrained-layer damping becomes an effective energy-absorbing construction element so that cabinet resonances are much lower than conventional flat cabinet panels. With these additional features, the backwave energy has to travel longer distances before it can reflect back toward the driver again. The longer distance and the damping material are rather effective at reducing the energy in the backwave. Furthermore, the convolutions in the labyrinth cause frequent internal reflections, and each reflection absorbs more energy from the backwave. Reflections also increase the distance the backwave energy has to travel to get back to one of the drivers. Would you still hear sound at the listening position when those special internal drivers are fired up? Perhaps, but it would be very low in level, perhaps just barely audible, rather than being fairly loud and obvious as it would with the undamped loudspeakers.

Examples

I have heard several speakers made by two different manufacturers continue to resonate audibly in a room for about a second and a half after pressing the pause button on the CD player. These were internally undamped loudspeakers. It was definitely the speakers and not the room -- putting an ear up to the speaker proved that. Do you really want all that time-delayed, amplitude-altered, frequency-modified energy smearing up your high-fidelity music? The problem we humans have is that when we hear the time-delayed, amplitude-altered, frequency-modified energy mixed in with the actual sound of the recording, a lot of us think it sounds great. I doubt many of us would buy speakers advertised as the lowest-fidelity $4000 speakers money can buy. But if a company sells the same $4000 pair of speakers with all sorts of flowery prose about how musical it is, people will pull out their checkbooks left and right, even if the speakers are quite obviously low fidelity just based on the design elements.

Hi-fi flies high

May I make a humble recommendation? Let the technical specifications and construction details of loudspeakers shorten your list of potential candidates. I can guarantee you that there will be plenty of differences in sound between speakers that are still in the running when you eliminate the poor engineering. You’ll still have a heck of a time selecting the exact speaker that’s right for you. I’m not a believer in the "trust your ears" platitude -- ears are too easily tricked. I am much more satisfied with the "trust your ears" philosophy if the field has been stripped of posers and tricksters that confuse and seduce. If your field is narrowed down to technically competent products, then "trust your ears" works for me.

Don’t mistake this recommendation to mean I advocate buying speakers based only on measurements or technical specifications. That’s not what I’m saying. What I‘m saying is to take low-fi products off your list before even considering them. Then make your selection by considering and listening to all the well-engineered products that are left. I’ll even make the radical suggestion that if a reviewer loved the sound of the speaker but the measurements aren’t up to snuff, take the speaker off your list and don’t even bother considering it. There are plenty other rave reviews for speakers with technically competent measurements. On the other hand, great measurements do not guarantee a great speaker. So be prepared to lop a lot more speakers off the list in spite of their good measurements.

As you whittle the list down from both directions, you’ll end up with a small number of speakers you really like that measure well. Those are the "worthy" speakers, the ones deserving of your money and a place in your audio system.

...Doug Blackburn
db@soundstage.com

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