Bass Horn FAQ
[Paul Spencer]
<mike.e/mikee12345>
Which parameters are important for a bass horn and which values are considered best?
For narrow bandwidth horns, it is more a question of matching the horn to the driver than finding an ideal driver.
Not a direct quote, but gleaned from a post by Bill Fitzmaurice on his forum:
Bass drivers for direct radiator applications typically have a low fs (lower than fb), medium Qts and fairly high BL. Suitable bass horn drivers, by contrast, fs is higher than fc, Qts lower than 0.35 and higher BL.
Gleaned from a post by Tom Danley regarding substituting other drivers in the Lab12 subwoofer:
"Other drivers could be used if they are similar in acoustic density [SD and mms] and motor strength [BL]."
He also states:
- Greater motor strength increases the efficiency and upper cutoff and may increase ripples.
- Greater inductance lowers the high cutoff
- Greater density lowers the high cutoff but may increase LF efficiency.
- Less density results in more response ripples and less LF output.
What is the maximum acoustic gain one can expect from horn loading?
10db increase in efficiency is common
Typical direct radiators are 1% efficient, but with horn loading, 50% can be achieved.
What are the comparative pros and cons of back loaded and front loaded horns?
A back loaded horn (BLH) may be used for a high sensitivity wide range or full range driver which has an efficiency which drops off in the bass. Horn loading is used to increase the efficiency of the bass and extend the useable bandwidth. Hence a BLH may have a much wider bandwith, whereas a front loaded horn (FLH) will have a much more limited top end, since it does not have direct radiator component. BLHs are often referred to as "bins" and are fairly common in pro audio. Some midbass drivers are actually designed to be used in a BLH as the emphasis with regard to efficiency is placed on the upper end; horn loading in a BLH in this case will increase the efficiency of the bottom end. Instead of having an efficiency of 95 db useable down to 50 Hz, such a driver may have an efficiency of 100 db in the midrange, dropping down toward the lower midrange and bass.
The compression chamber in a FLH equalises the pressure on both sides of the driver, in effect forcing it to operate in a more linear fashion. Hence distortion is further reduced in a FLH. Tom Danley states a preference for using a fairly small rear chamber volume, stating that compressed air is more linear in its behaviour than the syspension system of a driver.
What is the purpose of the compression chamber?
In a BLH, it acts as a low pass filter and increase LF slightly when large.
In a FLH, it restricts cone excursion due to the added air spring. It equalises the pressure on either side of the cone and thus forces the driver to perform in a more linear fashion, thereby reducing distortion.
What is the upper limit of useable bandwitdh of a folded bass horn?
This depends on the particular horn, and how effectively it minimises anomalies
caused by parallel walls and sharp bends in a folded horn.
Definately less than 200 Hz, in many bass horns 120 Hz sounds 'funny' to most
Can a horn be crossed higher if equalised flat?
Most likely not. Reasonable compromises can result in a non flat response which can be equalised. However, trying to extend the top end this way is likely to be difficult if not impossible. Sharp high Q peaks and dips are likely.
What is the power handling of a horn
Power handling = non-loaded power handling x 1/(1-horn efficiency)
Hence for a horn with 50% efficiency, the power handling will double.
Gleaned from a post by Tom Danley:
Horn loading increases the electrical impedance compared to a direct radiator,
which in turn reduces current and increases thermally limited power handling.
Hence, a stack of four lab horns can achieve a power handling of 2kw each.
As direct radiators, the two combined drivers handle 800w (less if xmax limited).
Note: Below the LF cut-off, the power handling will be the same
as that of the driver without horn loading.
What is reactance annulling?
[Audio Asylum post:Paul Eizik]
Throat reactance is the phenomenom in bass horns where the mass of air in the horn itself begins interfering with the ability of the horn mouth being able to acoustically load the lowest/longest wavelengths. The physical size of the horn mouth should predict the size of the longest wavelengths it can affect, but the reactance with the air mass inside the entire horn will interfere with this near the predicted low freq. cutoff. I had never been happy with most of the explainations I had read of the "why" of reactance, or what is reacting with what. A conversation once with Bruce Edgar cleared this up. Excellent observations in the posts below on the "how" of reactance annulling.
[Bill Fitzmaurice]
Reactance annulling is a fancy way of saying that you tune the closed box rear chamber so that the driver/rear chamber resonance is at or very close to the horn Fh, and it does work, maximizing the horn SPL to the Fh, but results also in somewhat of a 'brickwall' drop in response below Fh. It works on whatever horn length you use. Predicting the size chamber that you'll need to achieve reactance annuling is a bit dicey, and most formulas tend to make the chamber too large, requiring adding bricks as Bruce did on the Showhorn to correct the fault. The reason is that most formulas try to predict the rear chamber/driver resonance based only upon the driver Fs and chamber size and fail to account for the lowering of that resonance when the chamber/driver are mated to a horn.
[More from Bill]
I've recently found that the longer the horn the less requirement there is for reactance annulling, as the system resonance becomes less determined by rear chamber volume and more determined by the horn air mass. I haven't found McBean
? to be accurate in all cases as to how small the rear chamber must be. My results point to the existing math being inadequate to accurately predict the necessary rear chamber volume. Seat of the pants seems to remain the best procedure here.
[Audio Asylum:toxicport.e]
You can think of a horn as a transformer which transformsthe low impedence air load into a high impedence that the driver sees at the throat
At the flare frequency,the throat reactance peaks,where as the throat resistance is zero and rises to its maxium value above the flare frequency (fig 1) Theoretically,a basshorn should give response down to the flare frequency,but the throat reactanve will choke off any response near the flare frequency.
Over the years,wente and thuras at Bell Labs and Klipsh independantly found you could cancel out the throat reactance by using a sealed back chamber.This technique,which Plach termed 'reactance annulling' allows for bass response right down to the flare frequency.
Leach showed that for a number of exponential horn examples reactance annulling does not occur at the flare,but at a higher frequency.In a follow up letter Leach concluded that reactance annulling works best with hyperbolic-exponential horn as discovered by Salmon (US patent #2,338,262)
